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1
Kaskaoutis, D. G., H. D. Kambezidis, N. Hatzianastassiou, P. G. Kosmopoulos, and K. V. S. Badarinath. "Aerosol climatology: dependence of the Angstrom exponent on wavelength over four AERONET sites." Atmospheric Chemistry and Physics Discussions 7, no. 3 (May 30, 2007): 7347–97. http://dx.doi.org/10.5194/acpd-7-7347-2007.
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Abstract. The Ångström exponent, α, is often used as a qualitative indicator of aerosol particle size. In this study, aerosol optical depth (AOD) and Ångström exponent (α) data were analyzed to obtain information about the adequacy of the simple use of the Ångström exponent for characterizing aerosols, and for exploring possibilities for a more efficient characterization of aerosols. This was made possible by taking advantage of the spectral variation of α, the so-called curvature. The data were taken from four selected AERONET stations, which are representative of four aerosol types, i.e. biomass burning, pollution, desert dust and maritime. Using the least-squares method, the Ångström-α was calculated in the spectral interval 340–870 nm, along with the coefficients α1 and α2 of the second order polynomial fit to the plotted logarithm of AOD versus the logarithm of wavelength, and the second derivative of α. The results show that the spectral curvature can provide important additional information about the different aerosol types, and can be effectively used to discriminate between them, since the fine-mode particles exhibit negative curvature, while the coarse-mode aerosols positive. In addition, the curvature has always to be taken into account in the computations of Ångström exponent values in the spectral intervals 380–440 nm and 675–870 nm, since fine-mode aerosols exhibit larger α675–870 than α380–440 values, and vice-versa for coarse-mode particles. A second-order polynomial fit simulates the spectral dependence of the AODs very well, while the associated constant term varies proportionally to the aerosol type. The correlation between the coefficients α1 and α2 of the second-order polynomial fit and the Ångström exponent α, and the atmospheric turbidity, is further investigated. The obtained results reveal important features, which can be used for better discriminating between different aerosol types.
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Russell, P. B., R. W. Bergstrom, Y. Shinozuka, A. D. Clarke, P. F. DeCarlo, J. L. Jimenez, J. M. Livingston, et al. "Absorption Angstrom Exponent in AERONET and related data as an indicator of aerosol composition." Atmospheric Chemistry and Physics Discussions 9, no. 5 (October 16, 2009): 21785–817. http://dx.doi.org/10.5194/acpd-9-21785-2009.
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Abstract. Recent results from diverse air, ground, and laboratory studies using both radiometric and in situ techniques show that the fractions of black carbon, organic matter, and mineral dust in atmospheric aerosols determine the wavelength dependence of absorption (expressed as Absorption Angstrom Exponent, or AAE). Taken together, these results hold promise of improving information on aerosol composition from remote measurements. The purpose of this paper is to show that AAE values for Aerosol Robotic Network (AERONET) retrievals from Sun-sky measurements describing the full aerosol vertical column are also strongly correlated with aerosol composition or type. In particular, we find AAE values near 1 (the theoretical value for black carbon) for AERONET-measured aerosol columns dominated by urban-industrial aerosol, larger AAE values for biomass burning aerosols, and the largest AAE values for Sahara dust aerosols. Ambiguities in aerosol composition or mixtures thereof, resulting from intermediate AAE values, can be reduced via cluster analyses that supplement AAE with other variables, for example Extinction Angstrom Exponent (EAE), which is an indicator of particle size. Together with previous results, these results strengthen prospects for determining aerosol composition from space, for example using the Glory Aerosol Polarimetry Sensor (APS), which promises retrievals of multiwavelength single-scattering albedo (SSA) and aerosol optical depth (and therefore aerosol absorption optical depth (AAOD) and AAE), as well as shape and other aerosol properties. Cluster analyses promise additional information content, for example by using the Ozone Monitoring Instrument (OMI) to add AAOD in the near ultraviolet and CALIPSO aerosol layer heights to reduce height-absorption ambiguity.
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Russell, P. B., R. W. Bergstrom, Y. Shinozuka, A. D. Clarke, P. F. DeCarlo, J. L. Jimenez, J. M. Livingston, J. Redemann, O. Dubovik, and A. Strawa. "Absorption Angstrom Exponent in AERONET and related data as an indicator of aerosol composition." Atmospheric Chemistry and Physics 10, no. 3 (February 3, 2010): 1155–69. http://dx.doi.org/10.5194/acp-10-1155-2010.
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Abstract. Recent results from diverse air, ground, and laboratory studies using both radiometric and in situ techniques show that the fractions of black carbon, organic matter, and mineral dust in atmospheric aerosols determine the wavelength dependence of absorption (often expressed as Absorption Angstrom Exponent, or AAE). Taken together, these results hold promise of improving information on aerosol composition from remote measurements. The main purpose of this paper is to show that AAE values for an Aerosol Robotic Network (AERONET) set of retrievals from Sun-sky measurements describing full aerosol vertical columns are also strongly correlated with aerosol composition or type. In particular, we find AAE values near 1 (the theoretical value for black carbon) for AERONET-measured aerosol columns dominated by urban-industrial aerosol, larger AAE values for biomass burning aerosols, and the largest AAE values for Sahara dust aerosols. These AERONET results are consistent with results from other, very different, techniques, including solar flux-aerosol optical depth (AOD) analyses and airborne in situ analyses examined in this paper, as well as many other previous results. Ambiguities in aerosol composition or mixtures thereof, resulting from intermediate AAE values, can be reduced via cluster analyses that supplement AAE with other variables, for example Extinction Angstrom Exponent (EAE), which is an indicator of particle size. Together with previous results, these results strengthen prospects for determining aerosol composition from space, for example using the Glory Aerosol Polarimetry Sensor (APS), which seeks to provide retrievals of multiwavelength single-scattering albedo (SSA) and aerosol optical depth (and therefore aerosol absorption optical depth (AAOD) and AAE), as well as shape and other aerosol properties. Multidimensional cluster analyses promise additional information content, for example by using the Ozone Monitoring Instrument (OMI) to add AAOD in the near ultraviolet and CALIPSO aerosol layer heights to reduce height-absorption ambiguity.
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Nicolae, Victor, Camelia Talianu, Simona Andrei, Bogdan Antonescu, Dragoș Ene, Doina Nicolae, Alexandru Dandocsi, et al. "Multiyear Typology of Long-Range Transported Aerosols over Europe." Atmosphere 10, no. 9 (August 22, 2019): 482. http://dx.doi.org/10.3390/atmos10090482.
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In this study, AERONET (Aerosol Robotic Network) and EARLINET (European Aerosol Research Lidar Network) data from 17 collocated lidar and sun photometer stations were used to characterize the optical properties of aerosol and their types for the 2008–2018 period in various regions of Europe. The analysis was done on six cluster domains defined using circulation types around each station and their common circulation features. As concluded from the lidar photometer measurements, the typical aerosol particles observed during 2008–2018 over Europe were medium-sized, medium absorbing particles with low spectral dependence. The highest mean values for the lidar ratio at 532 nm were recorded over Northeastern Europe and were associated with Smoke particles, while the lowest mean values for the Angstrom exponent were identified over the Southwest cluster and were associated with Dust and Marine particles. Smoke (37%) and Continental (25%) aerosol types were the predominant aerosol types in Europe, followed by Continental Polluted (17%), Dust (10%), and Marine/Cloud (10%) types. The seasonal variability was insignificant at the continental scale, showing a small increase in the percentage of Smoke during spring and a small increase of Dust during autumn. The aerosol optical depth (AOD) slightly decreased with time, while the Angstrom exponent oscillated between “hot and smoky” years (2011–2015) on the one hand and “dusty” years (2008–2010) and “wet” years (2017–2018) on the other hand. The high variability from year to year showed that aerosol transport in the troposphere became more and more important in the overall balance of the columnar aerosol load.
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Kumar, Pradeep, Arti Choudhary, Abhay Kumar Singh, Rajendra Prasad, and Anuradha Shukla. "Aerosol Parameters During Winter and Summer Seasons and Meteorological Implications." EPJ Web of Conferences 237 (2020): 02003. http://dx.doi.org/10.1051/epjconf/202023702003.
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The MICROTOPS II aerosol optical depth (AOD) and Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua AOD and angstrom exponent (AE) were collected during December 2016 to May 2017. Higher AOD values were recorded during winter (December - February) and summer (March - May) Months. These values were observed by MICROTOPS II (0.35 - 1.279) and MODIS (0.222 - 1.904) during winter season. During summer AOD values were recorded by MICROTOPS II (0.272 – 1.744) and MODIS (0.227 – 1.33). Whereas MODIS AE (0.218 – 1.799) values were found high during winter season indicates about the dominance of fine particulates. During summer months MODIS AE (.001 – 1.648) values are indicating about the mixing of the aerosol particles. The relative humidity values during December and January months were found to be high, while its values were found decreasing during late February due to transition phase. Relative humidity values were recorded low during summer (36 – 86%) months in compare to the winter (50 - 100%) months.
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Chaubey, Jai Prakash, K. Krishna Moorthy, S. Suresh Babu, and Vijayakumar S. Nair. "The optical and physical properties of atmospheric aerosols over the Indian Antarctic stations during southern hemispheric summer of the International Polar Year 2007–2008." Annales Geophysicae 29, no. 1 (January 11, 2011): 109–21. http://dx.doi.org/10.5194/angeo-29-109-2011.
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Abstract. The properties of background aerosols and their dependence on meteorological, geographical and human influence are examined using measured spectral aerosol optical depth (AOD), total mass concentration (MT) and derived number size distribution (NSD) over two distinct coastal locations of Antarctica; Maitri (70° S, 12° E, 123 m m.s.l.) and Larsemann Hills (LH; 69° S, 77° E, 48 m m.s.l.) during southern hemispheric summer of 2007–2008 as a part of the 27th Indian Scientific Expedition to Antarctica (ISEA) during International Polar Year (IPY). Our investigations showed comparable values for the mean columnar AOD at 500 nm over Maitri (0.034±0.005) and LH (0.032±0.006) indicating good spatial homogeneity in the columnar aerosol properties over the coastal Antarctica. Estimation of Angstrom exponent α showed accumulation mode dominance at Maitri (α~1.2±0.3) and coarse mode dominance at LH (0.7±0.2). On the other hand, mass concentration (MT) of ambient aerosols showed relatively high values (≈8.25±2.87 μg m−3) at Maitri in comparison to LH (6.03±1.33 μg m−3).
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Dundar, Cihan, Ayse Gokcen Isik, and Kahraman Oguz. "Temporal analysis of Sand and Dust Storms (SDS) between the years 2003 and 2017 in the Central Asia." E3S Web of Conferences 99 (2019): 02004. http://dx.doi.org/10.1051/e3sconf/20199902004.
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Mineral dust particles play a vital role in climate and the Earth's energy budget and can have impact on weather systems. It has both direct (dust-radiation effect) and indirect (dust-microphysical effect) impacts on the energy budget effect. The most important sources of dust aerosols are located in the Northern Hemisphere, primarily over the Sahara in North Africa, the Middle East, Central and South Asia respectively and Central Asia is under the influence of mineral dust. The objective of this study to carry out intensity and frequency analysis of sand and dust storm in Central Asia for the period 2003-2017 and compare the results with global values as well as the values of the Middle East region. The AOD and AE parameters can be used to differentiate between coarse and fine particles of aerosols. To investigate average annual and monthly AOD (aerosol optical depth) and AE (angstrom exponent) for the period 2003-2017, AOD and AE data of MODIS Aqua is obtained from Giovanni website. In summary, for the last years (2013-2017), annual mean AOD is comparably lower than the other periods while the values are the highest between 2008 and 2012 for both Central Asia and Middle East. The results point out that there is no increasing trend in AOD values for the recent years and annual Central Asia AOD values show a similar trend with the Middle East AOD values.
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Simha, C. P., P. C. S. Devara, S. K. Saha, K. N. Babu, and A. K. Shukla. "Spatiotemporal Spectral Variations of AOT in India’s EEZ over Arabian Sea: Validation of OCM-II." International Journal of Oceanography 2012 (December 2, 2012): 1–13. http://dx.doi.org/10.1155/2012/473162.
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We report the results of sun-photometric measurements of Aerosol Optical Thickness (AOT) in India’s Exclusive Economic Zone (EEZ) over the Arabian Sea along with synchronous Ocean Color Monitor (OCM-II) derived AOT estimates during December 12, 2009–January 10, 2010. Relatively higher values of Angstrom exponent (α) around 1.2 near coast and 0.2–0.8 in the India’s EEZ, observed during the cruise period, indicate the presence of smaller particles near the coast due to anthropogenic activities; and larger particles in the India’s EEZ due to advection of pollutants from Indian subcontinent via long-range transport. Results related to α and its derivative reveal four different aerosol types (urban-industrial, desert-dust, clean-marine, and mixed-type) with varying fraction during the study period. Surface radiative forcing due to aerosols is found to be 20 W/m2 over India’s EEZ. OCM-derived AOTs showed good corroboration with in situ measurements with a correlation coefficient of about 0.95. A reasonably good correlation was also observed between AOT and wind speed (R = 0.6); AOT and relative humidity (R = 0.58). The concurrent MODIS AOT data also agree well with those observed by the OCEANSAT (OCM-II) satellite during the campaign period.
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Popovicheva, Olga, Alexey Ivanov, and Michal Vojtisek. "Functional Factors of Biomass Burning Contribution to Spring Aerosol Composition in a Megacity: Combined FTIR-PCA Analyses." Atmosphere 11, no. 4 (March 25, 2020): 319. http://dx.doi.org/10.3390/atmos11040319.
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Whether the spring season brings additional pollution to the urban environment remains questionable for a megacity. Aerosol sampling and characterization was performed in the urban background of the Moscow megacity in spring 2017, in a period of a significant impact of mass advection from surrounding fire regions. Parametrization of Angstrom absorption exponent (AAE) on low and high values provides periods dominated by fossil fuel (FF) combustion and affected by biomass burning (BB), respectively. The period identification is supported by air mass transportation from the south of Russia through the regions where a number of fires were observed. Functionalities in entire aerosol composition, assigned to classes of organic, ionic compounds, and dust, are inferred by diffusion refection infrared Fourier transmission (FTIR) spectroscopy. Functional markers of urban transport emissions relate to modern engine technology and driving cycles. Regional BB functionalities indicate the fire impacts to the spring aerosol composition. The development of the advanced source apportionment for a megacity is performed by means of combined ambient FTIR data and statistical PCA analysis. PCA of FTIR spectral data differentiate daily aerosol chemistry by low and high AAE values, related to FF- and BB-affected spectral features. PC loadings of 58%, 21%, and 11% of variability reveal the functional factors of transport, biomass burning, biogenic, dust, and secondary aerosol spring source impacts.
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Liu, Jiemei, Wenxiang Shen, Yuan Yuan, and Shikui Dong. "Optical Characteristics and Radiative Properties of Aerosols in Harbin, Heilongjiang Province during 2017." Atmosphere 12, no. 4 (April 7, 2021): 463. http://dx.doi.org/10.3390/atmos12040463.
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This study considers aerosol optical properties and direct radiative forcing over Harbin (126.63° E, 45.75° N), the highest latitude city in Northeast China, during 2017. Observations based on the CE-318 sun-photometer show that the annual mean values of the aerosol optical depth (AOD) at 500 nm and the Angstrom exponent (AE) at 440–870 nm over Harbin are respectively 0.26 ± 0.20 and 1.36 ± 0.26. Aerosol loading is the highest in the spring followed by winter, and the lowest loading is in autumn. AE440–870 is the highest in summer, second highest in winter, and lowest in autumn. The Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model is used to estimate the shortwave aerosol radiative forcing at the top of the atmosphere, on the Earth’s surface and in the atmosphere, and the annual mean values are −16.36 ± 18.42 Wm−2, −71.01 ± 27.37 Wm−2 and 54.65 ± 30.62 Wm−2, respectively, which indicate that aerosols cause climate effects of cooling the earth-atmosphere system, cooling the earth’s surface and heating the atmosphere. Four main aerosol types in Harbin are classified via AOD and AE. Specifically, clean continental, mixed type, biomass burning and urban industry, and desert dust aerosols accounted for 51%, 38%, 9%, and 2% of the total, respectively. Aerosol radiative forcing varies greatly in different seasons, and the aerosol load and type from different emission sources have an important influence on the seasonal variation of radiative forcing.
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Corr, C. A., S. R. Hall, K. Ullmann, B. E. Anderson, A. J. Beyersdorf, K. L. Thornhill, M. J. Cubison, J. L. Jimenez, A. Wisthaler, and J. E. Dibb. "Spectral absorption of biomass burning aerosol determined from retrieved single scattering albedo during ARCTAS." Atmospheric Chemistry and Physics Discussions 12, no. 6 (June 5, 2012): 13967–4008. http://dx.doi.org/10.5194/acpd-12-13967-2012.
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Abstract. Actinic flux, as well as aerosol chemical and optical properties, were measured aboard the NASA DC-8 aircraft during the ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) mission in Spring and Summer 2008. These measurements were used in a radiative transfer code to retrieve spectral (350–550 nm) aerosol single scattering albedo (SSA) for biomass burning plumes encountered on 17 April and 29 June. Retrieved SSA values were subsequently used to calculate the absorption Angstrom exponent (AAE) over the 350–500 nm range. Both plumes exhibited enhanced spectral absorption with AAE values that exceeded 1 (6.78 for 17 April and 3.34 for 29 June). This enhanced absorption was primarily due to organic aerosol (OA) which contributed significantly to total absorption at all wavelengths for both 17 April (57.7%) and 29 June (56.2%). OA contributions to absorption were greater at UV wavelengths than at visible wavelengths for both cases. Differences in AAE values between the two cases were attributed to differences in plume age and thus to differences in the ratio of OA and black carbon (BC) concentrations. However, notable differences between AAE values calculated for the OA (AAEOA) for 17 April (9.59) and 29 June (4.94) suggested differences in the plume AAE values might also be due to differences in organic aerosol composition. The 17 April OA was much more oxidized than the 29 June OA as denoted by a higher oxidation state value for 17 April (+0.16 versus −0.32). Differences in the AAEOA, as well as the overall AAE, were thus also likely due to oxidation of biomass burning primary organic aerosol in the 17 April plume that resulted in the formation of OA with a greater spectral-dependence of absorption.
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Corr, C. A., S. R. Hall, K. Ullmann, B. E. Anderson, A. J. Beyersdorf, K. L. Thornhill, M. J. Cubison, J. L. Jimenez, A. Wisthaler, and J. E. Dibb. "Spectral absorption of biomass burning aerosol determined from retrieved single scattering albedo during ARCTAS." Atmospheric Chemistry and Physics 12, no. 21 (November 12, 2012): 10505–18. http://dx.doi.org/10.5194/acp-12-10505-2012.
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Abstract. Actinic flux, as well as aerosol chemical and optical properties, were measured aboard the NASA DC-8 aircraft during the ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) mission in Spring and Summer 2008. These measurements were used in a radiative transfer code to retrieve spectral (350–550 nm) aerosol single scattering albedo (SSA) for biomass burning plumes encountered on 17 April and 29 June. Retrieved SSA values were subsequently used to calculate the absorption Angstrom exponent (AAE) over the 350–500 nm range. Both plumes exhibited enhanced spectral absorption with AAE values that exceeded 1 (6.78 ± 0.38 for 17 April and 3.34 ± 0.11 for 29 June). This enhanced absorption was primarily due to organic aerosol (OA) which contributed significantly to total absorption at all wavelengths for both 17 April (57.7%) and 29 June (56.2%). OA contributions to absorption were greater at UV wavelengths than at visible wavelengths for both cases. Differences in AAE values between the two cases were attributed to differences in plume age and thus to differences in the ratio of OA and black carbon (BC) concentrations. However, notable differences between AAE values calculated for the OA (AAEOA) for 17 April (11.15 ± 0.59) and 29 June (4.94 ± 0.19) suggested differences in the plume AAE values might also be due to differences in organic aerosol composition. The 17 April OA was much more oxidized than the 29 June OA as denoted by a higher oxidation state value for 17 April (+0.16 vs. −0.32). Differences in the AAEOA, as well as the overall AAE, were thus also possibly due to oxidation of biomass burning primary organic aerosol in the 17 April plume that resulted in the formation of OA with a greater spectral-dependence of absorption.
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Chubarova, N. Y. "Seasonal distribution of aerosol properties over Europe and their impact on UV irradiance." Atmospheric Measurement Techniques 2, no. 2 (October 26, 2009): 593–608. http://dx.doi.org/10.5194/amt-2-593-2009.
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Abstract. Using the aerosol optical thickness at 550 nm (τ550) from MODIS (collection 5) for the 2000–2008 period combined with the aerosol products from the ground-based AERONET network since 1996, monthly mean values of key aerosol parameters have been obtained with 1 degree resolution over Europe. Additional tests have revealed a satisfactory quality of the MODIS data, except in a few cases. Quality assured AERONET data are used for evaluating the Angstrom exponent, single scattering albedo and asymmetry factor, and for validating the final aerosol optical thickness in the UV spectral region. A method for extrapolating the aerosol parameters into the UV spectral region is discussed. The aerosol optical thickness distributions are considered together with meteorological fields from NOAA_NCEP_CPC_CAMS_ OPI climatology. The τ340 is shown to vary significantly from approximately 0.01 to 0.9 depending on the season and location. Permanent elevated aerosol loading over several industrial areas is observed, which agrees with the output of chemical transport models. Using radiative transfer modeling, monthly mean UV loss due to aerosol was estimated. The absolute decrease in UV indices varies from less than 0.1 to 1.5. The relative UV attenuation has large spatial and temporal variations (−1%–−17%) with a minimum towards the northwest and maxima over several southern local areas (Northern Italy, etc.) during the warm period.
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Scarnato, B. V., S. Vahidinia, D. T. Richard, and T. W. Kirchstetter. "Effects of internal mixing and aggregate morphology on optical properties of black carbon using a discrete dipole approximation model." Atmospheric Chemistry and Physics 13, no. 10 (May 16, 2013): 5089–101. http://dx.doi.org/10.5194/acp-13-5089-2013.
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Abstract. According to recent studies, internal mixing of black carbon (BC) with other aerosol materials in the atmosphere alters its aggregate shape, absorption of solar radiation, and radiative forcing. These mixing state effects are not yet fully understood. In this study, we characterize the morphology and mixing state of bare BC and BC internally mixed with sodium chloride (NaCl) using electron microscopy and examine the sensitivity of optical properties to BC mixing state and aggregate morphology using a discrete dipole approximation model (DDSCAT). DDSCAT is flexible in simulating the geometry and refractive index of particle aggregates. DDSCAT predicts a higher mass absorption coefficient (MAC), lower single scattering albedo (SSA), and higher absorption Angstrom exponent (AAE) for bare BC aggregates that are lacy rather than compact. Predicted values of SSA at 550 nm range between 0.16 and 0.27 for lacy and compact aggregates, respectively, in agreement with reported experimental values of 0.25 ± 0.05. The variation in absorption with wavelength does not adhere precisely to a power law relationship over the 200 to 1000 nm range. Consequently, AAE values depend on the wavelength region over which they are computed. The MAC of BC (averaged over the 200–1000 nm range) is amplified when internally mixed with NaCl (100–300 nm in radius) by factors ranging from 1.0 for lacy BC aggregates partially immersed in NaCl to 2.2 for compact BC aggregates fully immersed in NaCl. The SSA of BC internally mixed with NaCl is higher than for bare BC and increases with the embedding in the NaCl. Internally mixed BC SSA values decrease in the 200–400 nm wavelength range, a feature also common to the optical properties of dust and organics. Linear polarization features are also predicted in DDSCAT and are dependent on particle size and morphology. This study shows that DDSCAT predicts complex morphology and mixing state dependent aerosol optical properties that have been reported previously and are relevant to radiative transfer, climate modeling, and interpretation of remote sensing measurements.
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Yuan, J. F., X. F. Huang, L. M. Cao, J. Cui, Q. Zhu, C. N. Huang, Z. J. Lan, and L. Y. He. "Light absorption of brown carbon aerosol in the PRD region of China." Atmospheric Chemistry and Physics Discussions 15, no. 20 (October 21, 2015): 28453–82. http://dx.doi.org/10.5194/acpd-15-28453-2015.
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Abstract. The strong spectral dependence of light absorption of brown carbon (BrC) aerosol is regarded to influence aerosol's radiative forcing significantly. The Absorption Angstrom Exponent (AAE) method was widely used in previous studies to attribute light absorption of BrC at shorter wavelengths for ambient aerosol, with a theoretical assumption that the AAE of "pure" black carbon (BC) aerosol equals to 1.0. In this study, the previous AAE method was improved by statistical analysis and applied in both urban and rural environments in the Pearl River Delta (PRD) region of China. A three-wavelength photo-acoustic soot spectrometer (PASS-3) and aerosol mass spectrometers (AMS) were used to explore the relationship between the measured AAE and the relative abundance of organic aerosol to BC. The regression and extrapolation analysis revealed that the more realistic AAE values for "pure" BC aerosol were 0.86, 0.82, and 1.02 at 405 nm, and 0.70, 0.71, and 0.86 at 532 nm, in the campaigns of urban_winter, urban_fall, and rural_fall, respectively. Roadway tunnel experiments were also conducted, and the results further supported the representativeness of the obtained AAE values for "pure" BC aerosol in the urban environments. Finally, the average aerosol light absorption contribution of BrC was quantified to be 11.7, 6.3, and 12.1 % (with relative uncertainties of 4, 4, and 7 %) at 405 nm, and 10.0, 4.1, and 5.5 % (with relative uncertainties of 2, 2, and 5 %) at 532 nm, in the campaigns of urban_winter, urban_fall, and rural_fall, respectively. The relatively higher BrC absorption contribution at 405 nm in the rural_fall campaign was likely a result of the biomass burning events nearby, which was supported by the biomass burning simulation experiments performed in this study. The results of this paper indicate that the brown carbon contribution to aerosol light absorption at shorter wavelengths is not negligible in the highly urbanized and industrialized PRD region.
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Kharol, S. K., K. V. S. Badarinath, D. G. Kaskaoutis, A. R. Sharma, and B. Gharai. "Influence of continental advection on aerosol characteristics over Bay of Bengal (BoB) in winter: results from W-ICARB cruise experiment." Annales Geophysicae 29, no. 8 (August 24, 2011): 1423–38. http://dx.doi.org/10.5194/angeo-29-1423-2011.
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Abstract. The transport of aerosols and pollutants from continental India to the adjoining oceanic areas is a major topic of concern and several experimental campaigns have been conducted over the region focusing on aerosol characteristics and their climate implications. The present study analyzes the spectral aerosol optical depth (AOD) variations over Bay of Bengal (BoB) during Winter-Integrated Campaign for Aerosols, gases and Radiation Budget (W-ICARB) from 27 December 2008 to 30 January 2009 and investigates the influence of the adjoining landmass to the marine aerosol field. High AOD500 values (>0.7) occurred over northern BoB due to outflow of aerosols and pollutants from the densely populated Indo-Gangetic Plains (IGP); low AOD500 (0.1–0.2) was observed in central and southern BoB, far away from the mainland. The Angstrom exponent "α" was observed to be high (>1.2) near coastal waters, indicating relative abundance of accumulation-mode continental aerosols. On the other hand, over southern BoB its values dropped below ~0.7. National Center for Environmental Prediction (NCEP) reanalysis data on winds at 850 and 700 hPa, along with air-mass trajectories calculated using Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model, suggested transport of continental aerosols from central and northern India over the BoB. On the other hand, when the ship was crossing the eastern BoB, the aerosol loading was strongly affected by air-masses originating from Southeast Asia, causing an increase in AOD and α. Biomass-burning episodes over the region played an important role in the observed aerosol properties. Terra/Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) AOD550 and cruise measured AOD550 showed good agreement (R2 = 0.86 and 0.77, respectively) over BoB, exhibiting similar AOD and α spatio-temporal variation.
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Wang, Haoyue, Chunyang Zhang, Ke Yu, Xiao Tang, Huizheng Che, Jianchun Bian, Shanshan Wang, et al. "Aerosol Optical Radiation Properties in Kunming (the Low–Latitude Plateau of China) and Their Relationship to the Monsoon Circulation Index." Remote Sensing 11, no. 24 (December 5, 2019): 2911. http://dx.doi.org/10.3390/rs11242911.
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Based on the Langley method and the EuroSkyRad (ESR) pack retrieval scheme, we carried out the retrieval of the aerosol properties for the CE–318 sunphotometer observation data from March 2012 to February 2014 in Kunming, China, and we explored the possible mechanisms of the seasonal variations. The seasonal variation of the aerosol optical depth (AOD) was unimodal and reached a maximum in summer. The retrieval analysis of the Angstrom exponent (α) showed the aerosol types were continental, biomass burning (BB), and urban/industrial (UI); the content of the desert dust (DD) was low, and it may have contained a sea–salt (SS) aerosol due to the influence of the summer monsoon. All the aerosol particle spectra in different seasons showed a bimodal structure. The maximum and submaximal values were located near 0.2 μm and 4 μm, respectively, and the concentration of the aerosol volume was the highest in summer. In summer, aerosol particles have a strong scattering power but a weak absorption power; this pattern is the opposite in winter. The synergistic effect of the East Asian monsoon and the South Asian monsoon seasonal oscillations can have an important impact on the variation of the aerosol properties. The oscillation variation characteristic of the total vertical columnar water vapor (CWV) and the monsoon index was completely consistent. The aerosol types and sources in the Yunnan–Kweichow Plateau and the optical radiation properties were closely related to the monsoon circulation activities during different seasons and were different from other regions in China.
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Milinevsky, G., V. Danylevsky, V. Bovchaliuk, A. Bovchaliuk, Ph Goloub, O. Dubovik, V. Kabashnikov, A. Chaikovsky, M. Mishchenko, and M. Sosonkin. "Aerosol seasonal variations over urban sites in Ukraine and Belarus according to AERONET and POLDER measurements." Atmospheric Measurement Techniques Discussions 6, no. 6 (December 13, 2013): 10731–59. http://dx.doi.org/10.5194/amtd-6-10731-2013.
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Abstract. The paper presents an investigation of aerosol seasonal variations in several urban sites in the East European region. Our analysis of seasonal variations of optical and physical aerosol parameters is based on the sun-photometer 2008–2012 data from three urban ground-based AERONET sites in Ukraine (Kyiv, Kyiv-AO, and Lugansk) and one site in Belarus (Minsk), as well as on satellite POLDER instrument data for urban areas in Ukraine. Aerosol amount and optical thickness values exhibit peaks in the spring (April–May) and late summer (August), whereas minimum values are seen in late autumn over the Kyiv and Minsk sites. The results show that aerosol fine mode particles are most frequently detected during the spring and late summer seasons. The seasonal variation similarity in the two regions points to the resemblance in basic aerosol sources which are closely related to properties of aerosol particles. However the aerosol amount and properties change noticeably from year to year and from region to region. The analysis of seasonal aerosol optical thickness variations over the urban sites in the eastern and western parts of Ukraine according to both ground-based and POLDER data exhibits the same traits. In particular, over Kyiv, the values of the Angstrom exponent are lower in April of 2011 than in 2009 and 2010, while aerosol optical thickness values are almost the same, which can be explained by an increase in the amount of coarse mode particles in the atmosphere, such as Saharan dust. Moreover, the coarse mode particles prevailed over suburbs and the center of Kyiv during a third of all available days of observation in 2012. In general, the fine and coarse mode particles' modal radii averaged over 2008–2012 range from 0.1 to 0.2 μm and 2 to 5 μm, respectively, during the period from April to September. The single scattering albedo and refractive index values of these particles correspond to a mix of urban-industrial, biomass burning, and dust aerosols. In addition, strongly absorbing particles were observed in the period from October to March, and the modal radius of fine and coarse mode particles changed from month to month widely.
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Chubarova, N. Y., M. A. Sviridenkov, A. Smirnov, and B. N. Holben. "Assessments of urban aerosol pollution in Moscow and its radiative effects." Atmospheric Measurement Techniques 4, no. 2 (February 28, 2011): 367–78. http://dx.doi.org/10.5194/amt-4-367-2011.
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Abstract. Simultaneous measurements by the collocated AERONET CIMEL sun/sky photometers at the Moscow State University Meteorological Observatory (MSU MO) and at the Zvenigorod Scientific Station (ZSS) of the A. M. Obukhov Institute of Atmospheric Physics during September 2006–April 2009 provide the estimates of the effects of urban pollution on various aerosol properties in different seasons. The average difference in aerosol optical thickness between MO MSU and ZSS, which can characterize the effect of aerosol pollution, has been estimated to be about dAOT = 0.02 in visible spectral region. The most pronounced difference is observed in winter conditions when relative AOT difference can reach 26%. The high correlation of the AOT's, the Angstrom exponent values and the effective radii between the sites confirms that natural processes are the dominating factor in the changes of the aerosol properties even over the Moscow megacity area. The existence of positive correlation between dAOT and difference in water vapor content explains many cases with large dAOT between the sites by the time lag in the airmass advection. However, after excluding the difference due to this factor, AOT in Moscow remains higher even in a larger number of cases (more than 75%) with the same mean dAOT = 0.02. Due to the negative average difference in aerosol radiative forcing at the TOA of about dARFTOA = −0.9 W m−2, the aerosol urban pollution provides a distinct cooling effect of the atmosphere. The PAR and UV irradiance reaching the ground is only 2–3% lower in Moscow due to the pollution effects, though in some situations the attenuation can reach 13% in visible and more than 20% in UV spectral region.
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Chubarova, N. Ye, M. A. Sviridenkov, A. Smirnov, and B. N. Holben. "Assessments of urban aerosol pollution in Moscow and its radiative effects." Atmospheric Measurement Techniques Discussions 3, no. 6 (December 3, 2010): 5469–98. http://dx.doi.org/10.5194/amtd-3-5469-2010.
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Abstract. Simultaneous long-term measurements by the collocated AERONET CIMEL sun/sky photometers at the Moscow State University Meteorological Observatory (MSU MO) and at the Zvenigorod Scientific Station (ZSS) of the A. M. Obukhov Institute of Atmospheric Physics during September 2006–April 2009 provide the estimates of the effects of urban pollution on different aerosol properties in different seasons. The average difference in aerosol optical thickness between MO MSU and ZSS, which can characterize the effect of aerosol pollution, has been estimated to be about dAOT = 0.02 in visible spectral region. The most pronounced difference is observed in winter conditions when relative AOT difference can reach 30%. The high correlation of the AOT's, the Angstrom exponent values and the effective radii between the sites confirms that natural processes are the dominating factor in the changes of the aerosol properties even over the Moscow megacity area. The existence of positive correlation between dAOT and difference in water vapor content explains many cases with large dAOT between the sites by the time lag in the airmass advection. However, after excluding the difference due to this factor, AOT in Moscow remains higher even in more number of cases (more than 75%) with the same mean dAOT = 0.02. Due to the negative average difference in aerosol radiative forcing at the TOA of about dARF = −0.9 W/m2, the aerosol urban pollution provides a distinct cooling effect of the atmosphere. Due to the pollution effects, the PAR and UV irradiance reaching the ground is only 2–3% lower, though in some situations the attenuation can reach 13% in visible and more than 20% in UV spectral region.
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Hammer, M. S., R. V. Martin, A. van Donkelaar, V. Buchard, O. Torres, D. A. Ridley, and R. J. D. Spurr. "Interpreting the Ultraviolet Aerosol Index observed with the OMI satellite instrument to understand absorption by organic aerosols: implications for atmospheric oxidation and direct radiative effects." Atmospheric Chemistry and Physics Discussions 15, no. 19 (October 12, 2015): 27405–47. http://dx.doi.org/10.5194/acpd-15-27405-2015.
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Abstract. Satellite observations of the Ultraviolet Aerosol Index (UVAI) are sensitive to absorption of solar radiation by aerosols; this absorption affects photolysis frequencies and radiative forcing. We develop a global simulation of the UVAI using the 3-D chemical transport model GEOS-Chem coupled with the Vector Linearized Discrete Ordinate Radiative Transfer model (VLIDORT). The simulation is applied to interpret UVAI observations from the Ozone Monitoring Instrument (OMI) for the year 2007. Simulated and observed values are highly consistent in regions where mineral dust dominates the UVAI, but a large negative bias (−0.32 to −0.97) exists between simulated and observed values in biomass burning regions. We determine effective optical properties for absorbing organic aerosol, known as brown carbon (BrC), and implement them into GEOS-Chem to better represent observed UVAI values over biomass burning regions. The addition of absorbing BrC decreases the mean bias between simulated and OMI UVAI values from −0.57 to −0.09 over West Africa in January, from −0.32 to +0.0002 over South Asia in April, from −0.97 to −0.22 over southern Africa in July, and from −0.50 to +0.33 over South America in September. The spectral dependence of absorption after adding BrC to the model is broadly consistent with reported observations for biomass burning aerosol, with Absorbing Angstrom Exponent (AAE) values ranging from 2.9 in the ultraviolet (UV) to 1.3 across the UV-Near IR spectrum. We assess the effect of the additional UV absorption by BrC on atmospheric photochemistry by examining tropospheric hydroxyl radical (OH) concentrations in GEOS-Chem. The inclusion of BrC decreases OH by up to 35 % over South America in September, up to 25 % over southern Africa in July, and up to 20 % over other biomass burning regions. Global annual mean OH concentrations in GEOS-Chem decrease due to the presence of absorbing BrC, increasing the methyl chloroform lifetime from 5.62 to 5.68 years, thus reducing the bias against observed values. We calculate the direct radiative effect (DRE) of BrC using GEOS-Chem coupled with the radiative transfer model RRTMG (GC-RT). Treating organic aerosol as containing absorbing BrC rather than as primarily scattering changes global annual mean all-sky top of atmosphere (TOA) DRE by +0.05 W m-2 and all-sky surface DRE by −0.06 W m-2. Regional changes of up to +0.5 W m-2 at TOA and down to −1 W m-2 at the surface are found over major biomass burning regions.
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Yuan, J. F., X. F. Huang, L. M. Cao, J. Cui, Q. Zhu, C. N. Huang, Z. J. Lan, and L. Y. He. "Light absorption of brown carbon aerosol in the PRD region of China." Atmospheric Chemistry and Physics 16, no. 3 (February 9, 2016): 1433–43. http://dx.doi.org/10.5194/acp-16-1433-2016.
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Abstract. The strong spectral dependence of light absorption of brown carbon (BrC) aerosol is regarded to influence aerosol's radiative forcing significantly. The Absorption Angstrom Exponent (AAE) method has been widely used in previous studies to attribute light absorption of BrC at shorter wavelengths for ambient aerosols, with a theoretical assumption that the AAE of "pure" black carbon (BC) aerosol equals to 1.0. In this study, the AAE method was applied to both urban and rural environments in the Pearl River Delta (PRD) region of China, with an improvement of constraining the realistic AAE of "pure" BC through statistical analysis of on-line measurement data. A three-wavelength photo-acoustic soot spectrometer (PASS-3) and aerosol mass spectrometers (AMS) were used to explore the relationship between the measured AAE and the relative abundance of organic aerosol to BC. The regression and extrapolation analysis revealed that more realistic AAE values for "pure" BC aerosol (AAEBC) were 0.86, 0.82, and 1.02 between 405 and 781 nm, and 0.70, 0.71, and 0.86 between 532 and 781 nm, in the campaigns of urbanwinter, urbanfall, and ruralfall, respectively. Roadway tunnel experiments were conducted and the results further confirmed the representativeness of the obtained AAEBC values for the urban environment. Finally, the average light absorption contributions of BrC (± relative uncertainties) at 405 nm were quantified to be 11.7 % (±5 %), 6.3 % (±4 %), and 12.1 % (±7 %) in the campaigns of urbanwinter, urbanfall, and ruralfall, respectively, and those at 532 nm were 10.0 % (±2 %), 4.1 % (±3 %), and 5.5 % (±5 %), respectively. The relatively higher BrC absorption contribution at 405 nm in the ruralfall campaign could be reasonably attributed to the biomass burning events nearby, which was then directly supported by the biomass burning simulation experiments performed in this study. This paper indicates that the BrC contribution to total aerosol light absorption at shorter wavelengths is not negligible in the highly urbanized and industrialized PRD region.
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Scarnato, B., S. Vahidinia, D. T. Richard, and T. W. Kirchstetter. "Effects of internal mixing and aggregate morphology on optical properties of black carbon using a discrete dipole approximation model." Atmospheric Chemistry and Physics Discussions 12, no. 10 (October 5, 2012): 26401–34. http://dx.doi.org/10.5194/acpd-12-26401-2012.
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Abstract. According to recent studies, internal mixing of black carbon (BC) with other aerosol materials in the atmosphere alters its aggregate shape, absorption of solar radiation, and radiative forcing. These mixing state effects are not yet fully understood. In this study, we characterize the morphology and mixing state of bare BC and BC internally mixed with sodium chloride (NaCl) using electron microscopy and examine the sensitivity of optical properties to BC mixing state and aggregate morphology using a discrete dipole approximation model (DDSCAT). DDSCAT predicts a higher mass absorption coefficient, lower single scattering albedo (SSA), and higher absorption Angstrom exponent (AAE) for bare BC aggregates that are lacy rather than compact. Predicted values of SSA at 550 nm range between 0.18 and 0.27 for lacy and compact aggregates, respectively, in agreement with reported experimental values of 0.25 ± 0.05. The variation in absorption with wavelength does not adhere precisely to a power law relationship over the 200 to 1000 nm range. Consequently, AAE values depend on the wavelength region over which they are computed. In the 300 to 550 nm range, AAE values ranged in this study from 0.70 for compact to 0.95 for lacy aggregates. The SSA of BC internally mixed with NaCl (100–300 nm in radius) is higher than for bare BC and increases with the embedding in the NaCl. Internally mixed BC SSA values decrease in the 200–400 nm wavelength range, a feature also common to the optical properties of dust and organics. Linear polarization features are also predicted in DDSCAT and are dependent on particle morphology. The bare BC (with a radius of 80 nm) presents in the linear polarization a bell shape feature, which is a characteristic of the Rayleigh regime (for particles smaller than the wavelength of incident radiation). When BC is internally mixed with NaCl (100–300 nm in radius), strong depolarization features for near-VIS incident radiation are evident, such as a decrease in the intensity and multiple modes at different angles corresponding to different mixing states. DDSCAT, being flexible on the geometry and refractive index of the particle, can be used to study the effect of mixing state and complex morphology on optical properties of realistic BC aggregates. This study shows that DDSCAT predicts morphology and mixing state dependent optical properties that have been reported previously and are relevant to radiative transfer and climate modeling and interpretation of remote sensing measurements.
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Fernandes, Alnilam, Aleksander Pietruczuk, Artur Szkop, and Janusz Krzyścin. "Aerosol Layering in the Free Troposphere over the Industrial City of Raciborz in Southwest Poland and Its Influence on Surface UV Radiation." Atmosphere 12, no. 7 (June 24, 2021): 812. http://dx.doi.org/10.3390/atmos12070812.
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Atmospheric aerosol and ultraviolet index (UVI) measurements performed in Racibórz (50.08° N, 18.19° E) were analyzed for the period June–September 2019. Results of the following observations were taken into account: columnar characteristics of the aerosols (aerosol thickness, Angstrom exponent, single scattering albedo, asymmetry factor) obtained from standard CIMEL sun-photometer observations and parameters of aerosol layers (ALs) in the free troposphere (the number of layers and altitudes of the base and top) derived from continuous monitoring by a CHM-15k ceilometer. Three categories of ALs were defined: residues from the daily evolution of the planetary boundary layer (PBL) aerosols, from the PBL-adjacent layer, and from the elevated layer above the PBL. Total column ozone measurements taken by the Ozone-Monitoring Instrument on board NASA’s Aura satellite completed the list of variables used to model UVI variability under clear-sky conditions. The aim was to present a hybrid model (radiative transfer model combined with a regression model) for determining ALs’ impact on the observed UVI series. First, a radiative transfer model, the Tropospheric Ultraviolet–Visible (TUV) model, which uses typical columnar characteristics to describe UV attenuation in the atmosphere, was applied to calculate hypothetical surface UVI values under clear-sky conditions. These modeled values were used to normalize the measured UVI data obtained during cloudless conditions. Next, a regression of the normalized UVI values was made using the AL characteristics. Random forest (RF) regression was chosen to search for an AL signal in the measured data. This explained about 55% of the variance in the normalized UVI series under clear-sky conditions. Finally, the UVI values were calculated as the product of the RF regression and the relevant UVIs by the columnar TUV model. The root mean square error and mean absolute error of the hybrid model were 1.86% and 1.25%, respectively, about 1 percentage point lower than corresponding values derived from the columnar TUV model. The 5th–95th percentile ranges of the observation/model differences were [−2.5%, 2.8%] and [−3.0%, 5.3%] for the hybrid model and columnar TUV model, respectively. Therefore, the impact of ALs on measured surface UV radiation could be demonstrated using the proposed AL characteristics. The statistical analysis of the UVI differences between the models allowed us to identify specific AL configuration responsible for these differences.
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Nikonovas, T., P. R. J. North, and S. H. Doerr. "Smoke aerosol properties and ageing effects for Northern temperate and boreal regions derived from AERONET source and age attribution." Atmospheric Chemistry and Physics Discussions 15, no. 5 (March 5, 2015): 6445–79. http://dx.doi.org/10.5194/acpd-15-6445-2015.
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Abstract. Particulate emissions from wildfires impact human health and have a large but uncertain effect on climate. Modelling schemes depend on information about emission factors, emitted particle microphysical and optical properties and ageing effects, while satellite retrieval algorithms make use of characteristic aerosol models to improve retrieval. Ground based remote sensing provides detailed aerosol characterisation, but does not contain information on source. Here, a method is presented to estimate plume origin land cover type and age for AERONET aerosol observations, employing trajectory modelling using the HYSPLIT model, and satellite active fire and aerosol optical thickness (AOT) observations from MODIS and AATSR. It is applied to AERONET stations located in or near Northern temperate and boreal forests, for the period 2002–2013. The results from 629 fire attributions indicate significant differences in size distributions and particle optical properties between different land cover types. Smallest fine mode median radius are attributed to plumes from cropland – natural vegetation mosaic (0.143 μm) and grasslands (0.147 μm) fires. Evergreen needleleaf forest emissions show a significantly smaller fine mode median radius (0.164 μm) than plumes from woody savannas (0.184 μm) and mixed forest (0.193 μm) fires. Smoke plumes are predominantly scattering for all of the classes with median single scattering albedo at 440 nm (SSA(440)) values close to 0.95 except the cropland emissions which have a SSA(440) value of 0.9. Overall fine mode volume median radius increase rate is 0.0095 μm per day for the first 4 days of ageing and 0.0084 μm per day for seven days of ageing. Changes in size were consistent with a decrease in Angstrom Exponent and increase in Asymmetry parameter. No significant changes in SSA(λ) with ageing were found. These estimates have implications for improved modelling of aerosol radiative effects, relevant to both climate modelling and satellite aerosol retrieval schemes.
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Wang, Y., J. Xin, Z. Li, S. Wang, P. Wang, W. M. Hao, B. L. Nordgren, H. Chen, L. Wang, and Y. Sun. "Seasonal variations in aerosol optical properties over China." Atmospheric Chemistry and Physics Discussions 8, no. 3 (May 7, 2008): 8431–53. http://dx.doi.org/10.5194/acpd-8-8431-2008.
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Abstract. The seasonal variations in background aerosol optical depth (AOD) and aerosol type are investigated over various ecosystems in China based upon three years' worth of meteorological data and data collected by the Chinese Sun Hazemeter Network. In most parts of China, AODs are at a maximum in spring or summer and at a minimum in autumn or winter. Minimum values (0.10~0.20) of annual mean AOD at 500 nm are found in the Qinghai-Tibetan Plateau, which is located in the remote northeast corner of China, the northern forest ecosystems and Hainan Island. Annual mean AOD ranges from 0.25 to 0.30 over desert and oasis areas as well as the desertification grasslands in northern China; the annual mean AOD over the Loess Plateau is moderately high at 0.36. Regions where the highest density of agricultural and industrial activities are located and where anthropogenic sulphate aerosol and soil aerosol emissions are consistently high throughout the whole year (e.g. the central-eastern, southern and eastern coastal regions of China) experience annual mean AODs ranging from 0.50~0.80. Remarkable seasonal changes in the main types of aerosol over northern China (characterized by the Angstrom exponent, α) are seen. Due to biomass and fossil fuel burning from extensive agricultural practices in northern rural areas, concentrations of smoke and soot aerosols rise dramatically during autumn and winter (high α), while the main types of aerosol during spring and summer are dust and soil aerosols (low α). Over southeast Asia, biomass burning during the spring leads to increases in smoke and soot emissions. Over the Tibetan Plateau and Hainan Island where the atmosphere is pristine, the main types of aerosol are dust and sea salt, respectively.
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Pérez-Ramírez, D., H. Lyamani, F. J. Olmo, D. N. Whiteman, and L. Alados-Arboledas. "Columnar aerosol properties from sun-and-star photometry: statistical comparisons and day-to-night dynamic." Atmospheric Chemistry and Physics 12, no. 20 (October 25, 2012): 9719–38. http://dx.doi.org/10.5194/acp-12-9719-2012.
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Abstract. This work presents the first analysis of long-term correlative day-to-night columnar aerosol optical properties. The aim is to better understand columnar aerosol dynamic from ground-based observations, which are poorly studied until now. To this end we have used a combination of sun-and-star photometry measurements acquired in the city of Granada (37.16° N, 3.60° W, 680 m a.s.l.; South-East of Spain) from 2007 to 2010. For the whole study period, mean aerosol optical depth (AOD) around 440 nm (± standard deviation) is 0.18 ± 0.10 and 0.19 ± 0.11 for daytime and nighttime, respectively, while the mean Angström exponent (α) is 1.0 ± 0.4 and 0.9 ± 0.4 for daytime and nighttime. The ANOVA statistical tests reveal that there are no significant differences between AOD and α obtained at daytime and those at nighttime. Additionally, the mean daytime values of AOD and α obtained during this study period are coherent with the values obtained in the surrounding AERONET stations. On the other hand, AOD around 440 nm present evident seasonal patterns characterised by large values in summer (mean value of 0.20 ± 0.10 both at daytime and nighttime) and low values in winter (mean value of 0.15 ± 0.09 at daytime and 0.17 ± 0.10 at nighttime). The Angström exponents also present seasonal patterns, but with low values in summer (mean values of 0.8 ± 0.4 and 0.9 ± 0.4 at day- and night-time) and relatively large values in winter (mean values of 1.2 ± 0.4 and 1.0 ± 0.3 at daytime and nighttime). These seasonal patterns are explained by the differences in the meteorological conditions and by the differences in the strength of the aerosol sources. To take more insight about the changes in aerosol particles between day and night, the spectral differences of the Angström exponent as function of the Angström exponent are also studied. These analyses reveal increases of the fine mode radius and of the fine mode contribution to AOD during nighttime, being more remarkable in the summer seasons. These variations are explained by the changes of the local aerosol sources and by the meteorological conditions between daytime and nighttime, as well as aerosol aging processes. Case studies during summer and winter for different aerosol loads and types are also presented to clearly illustrate these findings.
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Córdoba-Jabonero, C., M. Sorribas, J. L. Guerrero-Rascado, J. A. Adame, Y. Hernández, H. Lyamani, V. Cachorro, et al. "Synergetic monitoring of Saharan dust plumes and potential impact on surface: a case study of dust transport from Canary Islands to Iberian Peninsula." Atmospheric Chemistry and Physics Discussions 10, no. 11 (November 9, 2010): 27015–74. http://dx.doi.org/10.5194/acpd-10-27015-2010.
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Abstract. Synergetic use of meteorological information, remote sensing both ground-based active (lidar) and passive (sun-photometry) techniques together with backtrajectory analysis and in situ measurements is carried out for the characterization of dust intrusions. A case study of air masses advected from Saharan region to the Canary Islands and the Iberian Peninsula, relatively located close and far away from the dust sources, respectively, was monitored from 11 to 19 March 2008. The observations were performed over three Spanish geographically strategic within the dust-influenced area stations along a common dust plume pathway. A 4-day long dust event (13–16 March) over the Santa Cruz de Tenerife Observatory (SCO), and a linked short 1-day dust episode (14 March) in the Southern Iberian Peninsula over both the Atmospheric Sounding Station "El Arenosillo" (ARN) and the Granada station (GRA) were detected. Meteorological situation favoured the dust plume transport over the area under study. Backtrajectory analysis clearly showed the Saharan origin of the dust intrusion. Under the Saharan air masses influence, AERONET Aerosol Optical Depth at 500 nm (AOD500) ranged from 0.3 to 0.6 and Angstrom Exponent at 440/675 nm wavelength pair (AE440/675) was lower than 0.5, indicating a high loading and predominance of coarse particles during those dusty events. Lidar observations characterized their vertical layering structure, identifying different aerosol contributions depending on altitude. In particular, the 3-km height layer observed over ARN and GRA stations corresponds to that dust plume transported from Saharan region after crossing through Canary Islands at 3 km height as observed over SCO site as well. No significant differences were found in the lidar (extinction-to-backscatter) ratio (LR) estimation for that dust plume over all stations when a suitable aerosol scenario for lidar data retrieval is selected. Lidar-retrieved LR values of 65–70 sr were obtained during the principal dusty episodes. These similar LR values found in all the stations suggest that dust properties were kept unchanged in the course of its medium-range transport. In addition, the potential impact on surface of that Saharan dust intrusion over the Iberian Peninsula was evaluated by ground-level in situ measurements for particle deposition assessment together with backtrajectory analysis. However, no connection between those dust plumes and the particle sedimentation registered at ground level is found. Differences on particle deposition process observed in both Southern Iberian Peninsula sites are due to the particular dust transport pattern occurred in each station.
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Zhuang, B. L., T. J. Wang, J. Liu, Y. Ma, C. Q. Yin, S. Li, M. Xie, et al. "Absorption coefficient of urban aerosol in Nanjing, west Yangtze River Delta of China." Atmospheric Chemistry and Physics Discussions 15, no. 12 (June 16, 2015): 16175–213. http://dx.doi.org/10.5194/acpd-15-16175-2015.
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Abstract. Absorbing aerosols can significantly modulate shortwave solar radiation in the atmosphere, affecting regional and global climate. Aerosol absorption coefficient (AAC) is an indicator to assess the impact of absorbing aerosols on radiative forcing. In this study, the near-surface AAC and absorption angstrom exponent (AAE) in urban Nanjing, China, are characterized on the basis of measurements in 2012 and 2013 using the 7-channel Aethalometer (model AE-31, Magee Scientific, USA). The AAC is estimated with direct and indirect corrections, which show consistent temporal variations and magnitudes of AAC at 532 nm. The mean AAC at 532 nm is about 43.23 ± 28.13 M m−1 in urban Nanjing, which is much lower than that in Pearl River Delta and as the same as that in rural areas (Lin'an) in Yangtze River Delta. The AAC in urban Nanjing shows strong seasonality (diurnal variations), high in cold seasons (at rush hours) and low in summer (in afternoon). It also show synoptic and quasi-two-week cycles in response to weather systems. Its frequency distribution follows a typical lognormal pattern. The 532 nm-AAC ranging from 15 to 65 M m−1 dominates, accounting for more than 72% of the total data samples in the entire study period. Frequent high pollution episodes, such as those observed in June 2012 and in winter 2013, greatly enhanced AAC and altered its temporal variations and frequency distributions. These episodes are mostly due to local emissions and regional pollutions. Air masses from northern China to Nanjing can sometimes be highly polluted and lead to high AAC at the site. AAE at 660/470 nm from the Schmid correction (Schmid et al., 2006) is about 1.56, which might be more reasonable compared to that from the Weingartner correction (Weingartner et al., 2003). Low AAEs mainly appear in summer in response to the relative humidity (RH). AAC increases with increasing AAE at a fixed aerosol loading. The RH-AAC relationship is more complex. Overall, AAC peaks around RH values of 40% (1.3 < AAE < 1.6), 65% (AAE < 1.3 and AAE > 1.6), and 80% (1.3 < AAE < 1.6).
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Denjean, C., F. Cassola, A. Mazzino, S. Triquet, S. Chevaillier, N. Grand, T. Bourrianne, et al. "Size distribution and optical properties of mineral dust aerosols transported in the western Mediterranean." Atmospheric Chemistry and Physics Discussions 15, no. 15 (August 10, 2015): 21607–69. http://dx.doi.org/10.5194/acpd-15-21607-2015.
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Abstract. This study presents in situ aircraft measurements of Saharan mineral dust transported over the western Mediterranean basin in June–July 2013 during the ChArMEx/ADRIMED (the Chemistry-Aerosol Mediterranean Experiment/Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region) airborne campaign. Dust events differing in terms of source region (Algeria, Tunisia and Morocco), time of tranport (1–5 days) and height of transport were sampled. Mineral dust were transported above the marine boundary layer, which conversely was dominated by pollution and marine aerosols. The dust vertical structure was extremely variable and characterized by either a single layer or a more complex and stratified structure with layers originating from different source regions. Mixing of mineral dust with pollution particles was observed depending on the height of transport of the dust layers. Dust layers carried higher concentration of pollution particles at intermediate altitude (1–3 km) than at elevated altitude (> 3 km), resulting in scattering Angstrom exponent up to 2.2 within the intermediate altitude. However, the optical properties of the dust plumes remained practically unchanged with respect to values previously measured over source regions, regardless of the altitude. Moderate light absorption of the dust plumes was observed with values of aerosol single scattering albedo at 530 nm ranging from 0.90 to 1.00 ± 0.04. Concurrent calculations from the aerosol chemical composition revealed a negligible contribution of pollution particles to the absorption properties of the dust plumes that was due to a low contribution of refractory black carbon in regards to the fraction of dust and sulfate particles. This suggests that, even in the presence of moderate pollution, likely a persistent feature in the Mediterranean, the optical properties of the dust plumes could be assimilated to those of native dust in radiative transfer simulations, modeling studies and satellite retrievals over the Mediterranean. Measurements also showed that the coarse mode of mineral dust was conserved even after 5 days of transport in the Mediterranean, which contrasts with the gravitational depletion of large particles observed during the transport of dust plumes over the Atlantic. Simulations with the WRF mesoscale meteorological model highlighted a strong vertical turbulence within the dust layers that could prevent deposition of large particles during their atmospheric transport. This has important implications for the dust radiative effects due to surface dimming, atmospheric heating and cloud formation. The results presented here add to the observational dataset necessary for evaluating the role of mineral dust on the regional climate and rainfall patterns in the western Mediterranean basin.
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Byčenkienė, Steigvilė, Vidmantas Ulevicius, Vadimas Dudoitis, and Julija Pauraitė. "Identification and Characterization of Black Carbon Aerosol Sources in the East Baltic Region." Advances in Meteorology 2013 (2013): 1–11. http://dx.doi.org/10.1155/2013/380614.
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One-year continuous measurements of aerosol black carbon (BC) at the background site Preila (55°55′N, 21°00′E, 5 m a.s.l., Lithuania) were performed. Temporal and spatial evolution and transport of biomass burning (BB) and volcanic aerosols observed within this period were explained by the air mass backward trajectory analysis in conjunction with the fire detection data produced by the MODIS Rapid Response System and AERONET database. The surface measurements and analysis of the Angström exponent of the absorption coefficient done separately for shorter and longer wavelengths (i.e.,α=370–520 nm andα=660–950 nm) showed that high levels of aerosol BC were related to the transport of air masses rich in BB products from Ukraine caused by active grass burning. During the events the highest mean values of the Angström exponent of the absorption coefficientsα370–520andα590–950 nm were observed (2.4±0.1and1.5±0.1, resp.). The ash plume of the Grimsvötn eruption on May 21, 2011 offered an exceptional opportunity to characterize the volcanic aerosols. The largest ash plume (in terms of aerosol optical thickness) over Lithuania was observed at May 24/25, 2011. The highest values of the Angström exponent of the absorption coefficientsα370–520andα590–950 nm were reached (1.3±0.1and1.4±0.1, resp.).
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Cachorro, V. E., C. Toledano, M. Antón, A. Berjón, A. de Frutos, J. M. Vilaplana, A. Arola, and N. A. Krotkov. "Comparison of UV irradiances from Aura/Ozone Monitoring Instrument (OMI) with Brewer measurements at El Arenosillo (Spain) – Part 2: Analysis of site aerosol influence." Atmospheric Chemistry and Physics Discussions 10, no. 7 (July 1, 2010): 16385–423. http://dx.doi.org/10.5194/acpd-10-16385-2010.
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Abstract. Several validation studies have shown a notable overestimation of the clear sky ultraviolet (UV) irradiance at the Earth's surface derived from satellite sensors such as the Total Ozone Mapping Spectrometer (TOMS) and the Ozone Monitoring Instrument (OMI) with respect to ground-based UV data at many locations. Most of this positive bias is attributed to boundary layer aerosol absorption that is not accounted for in the TOMS/OMI operational UV algorithm. Therefore, the main objective of this study is to analyse the aerosol effect on the bias between OMI erythemal UV irradiance (UVER) and spectral UV (305 nm, 310 nm and 324 nm) surface irradiances and ground-based Brewer spectroradiometer measurements from October 2004 to December 2008 at El Arenosillo station, with meteorological conditions representative of the south-west of Spain. The effect of other factors as clouds, ozone and the solar elevation over this intercomparison were analysed in detail in a companion paper (Antón et al., 2010). In that paper the aerosol effects were analysed making only a rough evaluation based on aerosol optical depth (AOD) information at 440 nm wavelength (visible range) without applying any correction. Here, the correction of the OMI UV data is proposed based on a detailed study about the determination of absorbing aerosols provided by AERONET data. Because of the difficulty to have reliable data about absorbing aerosol properties at many sites, first we consider AOD, Angstrom exponent and also OMI-Aerosol Index for this determination, but finally single scattering albedo (SSA) from AERONET was used since it represents a much more precise information. An aerosol correction expression was applied to the OMI operational UV data using two approaches to estimate the UV absorption aerosol optical depth, AAOD. The first approach was based on an assumption of constant SSA value of 0.91. This approach reduces OMI UVER bias from 13.4 to 8.4%. Second approach uses daily AERONET SSA values reducing the bias only to 11.6%. Therefore we have obtained a 37% and 12% of improvement, respectively. Similar results were obtained for spectral irradiances at 305 nm, 310 nm and 324 nm, where for 324 nm the OMI bias is reduced from 10.5 to 6.98% for constant SSA and to 9.03% for variable SSA. Contrary to what was expected, the constant SSA approach has a greater bias reduction than variable SSA, but this is a reasonable result according to the discussion about the reliability of SSA values. Our results reflect the level of accuracy that may be reached at the present time in this type of comparison, which may be considered as satisfactory taking into account the remaining dependence on other factors. Nevertheless, improvements must be accomplished to determine reliable absorbing aerosol properties, which appear as a limiting factor for improving OMI retrievals.
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Tao, Ye, Ning Sun, Xudong Li, Zhuzi Zhao, Shuaishuai Ma, Hongying Huang, Zhaolian Ye, and Xinlei Ge. "Chemical and Optical Characteristics and Sources of PM2.5 Humic-Like Substances at Industrial and Suburban Sites in Changzhou, China." Atmosphere 12, no. 2 (February 19, 2021): 276. http://dx.doi.org/10.3390/atmos12020276.
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The chemical and optical properties and sources of atmospheric PM2.5 humic-like substances (HULIS) were investigated from October to December 2016 in both industrial and suburban areas in Changzhou, China, during polluted and fair days. The average PM2.5 concentration in the industrial region was 113.06 (±64.3) μg m−3, higher than 85.27 (±41.56) μg m−3 at the suburban site. The frequency of polluted days was significantly higher in the industrial region. In contrast, the chemical compositions of PM2.5 at the two sampling sites exhibited no statistically significant differences. Rapidly increased secondary inorganic ions (SNA = NH4+ + SO42− + NO3−) concentrations suggested secondary formation played an important role in haze formation. The daily mean concentration of humic-like substance (HULIS) was 1.8–1.9 times that of HULIS-C (the carbon content of HULIS). Our results showed that HULIS accounted for a considerable fraction of PM2.5 (industrial region: 6.3% vs. suburban region: 9.4%). There were no large differences in the mass ratios of HULIS-C/WSOC at the two sites (46% in the industrial region and 52% in the suburban region). On average, suburban HULIS-C constituted 35.1% of organic carbon (OC), higher than that (21.1%) in the industrial region. Based on different MAE (mass absorption efficiency) values under different pollution levels, we can infer that the optical properties of HULIS varied with PM levels. Moreover, our results showed no distinct difference in E2/E3 (the ratio of light absorbance at 250 nm to that at 365 nm) and AAE300–400 (Absorption Angstrom Exponent at 300–400 nm) for HULIS and WSOC. the MAE365 (MAE at 365 nm) value of HULIS-C was different under three PM2.5 levels (low: PM2.5 < 75 μg m−3, moderate: PM2.5 = 75–150 μg m−3, high: PM2.5 > 150 μg m−3), with the highest MAE365 value on polluted days in the industrial region. Strong correlations between HULIS-C and SNA revealed that HULIS might be contributed from secondary formation at both sites. In addition, good correlations between HULIS-C with K+ in the industrial region implied the importance of biomass burning to PM2.5-bound HULIS. Three common sources of HULIS-C (i.e., vehicle emissions, biomass burning, and secondary aerosols) were identified by positive matrix factorization (PMF) for both sites, but the contributions were different, with the largest contribution from biomass burning in the industrial region and secondary sources in the suburban region, respectively. The findings presented here are important in understanding PM2.5 HULIS chemistry and are valuable for future air pollution control measures.
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Pérez-Ramírez, D., H. Lyamani, F. J. Olmo, D. N. Whiteman, and L. Alados-Arboledas. "Day and night columnar aerosol properties at Granada (Spain) retrieved from sun-and star-photometry." Atmospheric Chemistry and Physics Discussions 12, no. 5 (May 9, 2012): 11941–78. http://dx.doi.org/10.5194/acpd-12-11941-2012.
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Abstract. This works present the first analysis of long-term day- and night-time columnar aerosol optical properties. To this end we have used a combination of sun and star photometer measurements at the city of Granada (37.16° N, 3.60° W, 680 m a.s.l.; South-East of Spain) from 2007 to 2010. For the whole study period, mean aerosol optical depth (AOD) at 440 nm (± standard deviation) is 0.18 ± 0.10 and 0.19 ± 0.11 for day- and night-time, respectively, while the mean Angström exponent (α) is 1.0 ± 0.4 and 0.9 ± 0.4 for day- and night-time. The ANOVA statistical tests reveal that there are no significant differences between the AOD and α obtained at day-time and those obtained at night-time. Additionally, the mean day-time values of AOD and α obtained during this period are within the values obtained in the surrounding AERONET stations. On the other hand, AOD presents an evident seasonal pattern characterized by large values in summer (mean values of 0.20 ± 0.10 both at day- and night-time) and low values in winter (mean values of 0.15 ± 0.09 at day-time and 0.17 ± 0.10 at night-time). The Angström exponent presents clear seasonal pattern with low values in summer (mean values of 0.8 ± 0.4 and 0.9 ± 0.4 at day- and night-time) and relatively large values in winter (mean values of 1.2 ± 0.4 and 1.0 ± 0.3 at day- and night-time). These seasonal patterns are explained by the differences in the meteorological conditions and by the differences in the strength of aerosol sources during day and night. To take more insight about the changes in aerosol particles between day and night, the spectral difference of the Angström exponent (δα) as function of α is also studied. This analysis reveals an increase in the fine mode radius and in the fine mode contribution to AOD during night-time, being more remarkable in summer seasons. These changes are explained by the changes in the local aerosol source emissions and meteorological conditions between day- and night-time, as well as aerosol aging processes.
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Mallet, M., O. Dubovik, P. Nabat, F. Dulac, R. Kahn, J. Sciare, D. Paronis, and J. F. Léon. "Absorption properties of Mediterranean aerosols obtained from multi-year ground-based remote sensing observations." Atmospheric Chemistry and Physics 13, no. 18 (September 16, 2013): 9195–210. http://dx.doi.org/10.5194/acp-13-9195-2013.
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Abstract. Aerosol absorption properties are of high importance to assess aerosol impact on regional climate. This study presents an analysis of aerosol absorption products obtained over the Mediterranean basin or land stations in the region from multi-year ground-based AERONET observations with a focus on the Absorbing Aerosol Optical Depth (AAOD), Single Scattering Albedo (SSA) and their spectral dependence. The AAOD and Absorption Angström Exponent (AAE) dataset is composed of daily averaged AERONET level 2 data from a total of 22 Mediterranean stations having long time series, mainly under the influence of urban-industrial aerosols and/or soil dust. This dataset covers the 17-yr period 1996–2012 with most data being from 2003–2011 (~89% of level-2 AAOD data). Since AERONET level-2 absorption products require a high aerosol load (AOD at 440 nm > 0.4), which is most often related to the presence of desert dust, we also consider level-1.5 SSA data, despite their higher uncertainty, and filter out data with an Angström exponent < 1.0 in order to study absorption by carbonaceous aerosols. The SSA dataset includes AERONET level-2 products. Sun-photometer observations show that values of AAOD at 440 nm vary between 0.024 ± 0.01 (resp. 0.040 ± 0.01) and 0.050 ± 0.01 (0.055 ± 0.01) for urban (dusty) sites. Analysis shows that the Mediterranean urban-industrial aerosols appear "moderately" absorbing with values of SSA close to ~0.94–0.95 ± 0.04 (at 440 nm) in most cases except over the large cities of Rome and Athens, where aerosol appears more absorbing (SSA ~0.89–0.90 ± 0.04). The aerosol Absorption Angström Exponent (AAE, estimated using 440 and 870 nm) is found to be larger than 1 for most sites over the Mediterranean, a manifestation of mineral dust (iron) and/or brown carbon producing the observed absorption. AERONET level-2 sun-photometer data indicate a possible East-West gradient, with higher values over the eastern basin (AAEEast = 1.39/AAEWest = 1.33). The North-South AAE gradient is more pronounced, especially over the western basin. Our additional analysis of AERONET level-1.5 data also shows that organic absorbing aerosols significantly affect some Mediterranean sites. These results indicate that current climate models treating organics as nonabsorbing over the Mediterranean certainly underestimate the warming effect due to carbonaceous aerosols.
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Regmi, Jeevan, Khem N. Poudyal, Amod Pokhrel, Madhu Gyawali, Anthony Barinelli, and Rudra Aryal. "Analysis of Aerosol Optical Depth and Angstrom Exponents over an AERONET site at Pokhara, Nepal." BIBECHANA 18, no. 1 (January 1, 2021): 118–27. http://dx.doi.org/10.3126/bibechana.v18i1.29448.
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The monthly variability of Aerosol Optical Depth at 0.50 μm (AOD0.50) and Ångström exponents (AE) based on spectral AODs over an Aerosol Robotic Network (AERONET) site Pokhara, are analyzed by using aerosol data of the year 2017. The AOD0.50 are characterized by low average values (0.21± 0.12) in monsoon, and highest values in pre-monsoon (0.67± 0.14) followed by winter (0.46± 0.28) and post-monsoon (0.33±.02) with an overall mean of 0.43 ± 0.02. The average AE obtained by using AODs at 0.44 μm and 0.87 μm is 1.20± 0.04 in pre-monsoon, 1.37± 0.05 in monsoon, 1.41±.01 in post-monsoon, and 1.37± 0.07 in winter with an annual average value of 1.35 ± 0.08. These overall variations of AE indicate that the majority of aerosol loading during the study period was mixture of fine and coarse mode aerosols and the influence of anthropogenic aerosols. The monthly average AOD suggests low aerosol loading in the months of the monsoon season (June to September) than other months of pre-monsoon season (March to May) and post-monsoon season (October and November).
BIBECHANA 18 (2021) 118-127
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Guerrero-Rascado, J. L., F. J. Olmo, I. Avilés-Rodríguez, F. Navas-Guzmán, D. Pérez-Ramírez, H. Lyamani, and L. Alados Arboledas. "Extreme Saharan dust event over the southern Iberian Peninsula in september 2007: active and passive remote sensing from surface and satellite." Atmospheric Chemistry and Physics Discussions 9, no. 4 (July 24, 2009): 15673–723. http://dx.doi.org/10.5194/acpd-9-15673-2009.
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Abstract. This study investigates aerosol optical properties during the extreme Saharan dust event detected from 3 to 7 September 2007 over Granada, southern Iberian Peninsula, with both active and passive remote sensing instrumentation from surface and satellite. The intensity of the event was visualized on the aerosol optical depth series obtained by the sun-photometer Cimel CE 318-4 operated at Granada in the framework of AERONET from August 2004 until December 2008 (level 2 data). A combination of large aerosol optical depth (0.86–1.50) at 500 nm, and reduced Angström exponent (0.1–0.25) in the range 440–870 nm, was detected on 6 September during daytime. This Saharan dust event also affected other Iberian Peninsula stations included in AERONET (El Arenosillo and Évora stations). During the most intense stage, on 6 September, maximum aerosol backscatter values were a factor of 8 higher than other maxima during this Saharan dust event. Values up to 1.5×10−2 km−1 sr−1 at 355 and 532 nm were detected in the layer with the greatest aerosol load between 3–4 km a.s.l., although aerosol particles were also detected up to 5.5 km a.s.l. In this stage of the event, dust particles at these altitudes showed a backscatter-related Angström exponent between −0.44 and 0.53 for the two spectral intervals considered. The results from different measurements (active/passive and ground-based/satellite) reveal the importance of performing multi-instrumental measurements to properly characterize the contribution of different aerosol types from different sources during extreme events.
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Guerrero-Rascado, J. L., F. J. Olmo, I. Avilés-Rodríguez, F. Navas-Guzmán, D. Pérez-Ramírez, H. Lyamani, and L. Alados Arboledas. "Extreme Saharan dust event over the southern Iberian Peninsula in september 2007: active and passive remote sensing from surface and satellite." Atmospheric Chemistry and Physics 9, no. 21 (November 6, 2009): 8453–69. http://dx.doi.org/10.5194/acp-9-8453-2009.
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Abstract. This study investigates aerosol optical properties during the extreme Saharan dust event detected from 3 to 7 September 2007 over Granada, southern Iberian Peninsula, with both active and passive remote sensing instrumentation from surface and satellite. The intensity of the event was visualized on the aerosol optical depth series obtained by the sun-photometer Cimel CE 318-4 operated at Granada in the framework of AERONET from August 2004 until December 2008 (level 2 data). A combination of large aerosol optical depth (0.86–1.50) at 500 nm, and reduced Angström exponent (0.1–0.25) in the range 440–870 nm, was detected on 6 September during daytime. This Saharan dust event also affected other Iberian Peninsula stations included in AERONET (El Arenosillo and Évora stations), and it was monitored by MODIS instrument on board Aqua satellite. Vertically resolved measurements were performed by a ground-based Raman Lidar and by CALIPSO satellite. During the most intense stage, on 6 September, maximum aerosol backscatter values were a factor of 8 higher than other maxima during this Saharan dust event. Values up to 1.5×10−2 km−1 sr−1 at 355 and 532 nm were detected in the layer with the greatest aerosol load between 3–4 km a.s.l., although aerosol particles were also detected up to 5.5 km a.s.l. In this stage of the event, dust particles at these altitudes showed a backscatter-related Angström exponent between –0.44 and 0.53 for the two spectral intervals considered. The results from different measurements (active/passive and ground-based/satellite) reveal the importance of performing multi-instrumental measurements to properly characterize the contribution of different aerosol types from different sources during extreme events. The atmospheric stabilization effect of the aerosol particles has been characterized by computing the solar heating rates using SBDART code.
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Brogniez, C., V. Buchard, and F. Auriol. "Validation of UV-visible aerosol optical thickness retrieved from spectroradiometer measurements." Atmospheric Chemistry and Physics Discussions 8, no. 1 (February 26, 2008): 3895–919. http://dx.doi.org/10.5194/acpd-8-3895-2008.
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Abstract. Global and diffuse UV-visible solar irradiances are routinely measured since 2003 with a spectroradiometer operated by the Laboratoire d'Optique Atmosphérique (LOA) located in Villeneuve d'Ascq, France. The analysis of the direct irradiance derived by cloudless conditions enables retrieving the aerosol optical thickness (AOT) spectrum in the 330–450 nm range. The site hosts also sunphotometers from the AERONET/PHOTONS network performing routinely measurements of the AOT at several wavelengths. On one hand, comparisons between the spectroradiometer and the sunphotometer AOT at 440 nm as well as, when available, at 340 and 380 nm, show good agreement. On the other hand, the AOT's spectral variations have been compared using the Angström exponents derived from AOT data at 340 and 440 nm for both instruments. The comparisons show that this parameter is difficult to retrieve accurately due to the small wavelength range and due to the weak AOT values. Thus, AOT derived at wavelengths outside the spectroradiometer range by means of an extrapolation using the Angström parameter would be of poor value, whereas, spectroradiometer's spectral AOT could be used for direct validation of other AOT, such as those provided by satellite instruments.
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Mallet, M., O. Dubovik, P. Nabat, F. Dulac, R. Kahn, J. Sciare, D. Paronis, and J. F. Léon. "Absorption properties of Mediterranean aerosols obtained from multi-year ground-based and satellite remote sensing observations." Atmospheric Chemistry and Physics Discussions 13, no. 4 (April 8, 2013): 9267–317. http://dx.doi.org/10.5194/acpd-13-9267-2013.
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Abstract. Aerosol absorption properties are of high importance to assess aerosol impact on regional climate. This study presents an analysis of aerosol absorption products obtained over the Mediterranean Basin or land stations in the region from multi-year ground-based AERONET and satellite observations with a focus on the Absorbing Aerosol Optical Depth (AAOD), Single Scattering Albedo (SSA) and their spectral dependence. The AAOD and Absorption Angström Exponent (AAE) data set is composed of daily averaged AERONET level 2 data from a~total of 22 Mediterranean stations having long time series, mainly under the influence of urban-industrial aerosols and/or soil dust. This data set covers the 17 yr period 1996–2012 with most data being from 2003–2011 (~89% of level-2 AAOD data). Since AERONET level-2 absorption products require a high aerosol load (AOD at 440 nm > 0.4), which is most often related to the presence of desert dust, we also consider level-1.5 SSA data, despite their higher uncertainty, and filter out data with an Angström exponent <1.0 in order to study absorption by carbonaceous aerosols. The SSA data set includes both AERONET level-2 and satellite level-3 products. Satellite-derived SSA data considered are monthly level 3 products mapped at the regional scale for the spring and summer seasons that exhibit the largest aerosol loads. The satellite SSA dataset includes the following products: (i) Multi-angle Imaging SpectroRadiometer (MISR) over 2000–2011, (ii) Ozone Monitoring Instrument (OMI) near-UV algorithm over 2004–2010, and (iii) MODerate resolution Imaging Spectroradiometer (MODIS) Deep-Blue algorithm over 2005–2011, derived only over land in dusty conditions. Sun-photometer observations show that values of AAOD at 440 nm vary between 0.024 ± 0.01 (resp. 0.040 ± 0.01) and 0.050 ± 0.01 (0.055 ± 0.01) for urban (dusty) sites. Analysis shows that the Mediterranean urban-industrial aerosols appear "moderately" absorbing with values of SSA close to ~0.94–0.95 ± 0.04 (at 440 nm) in most cases except over the large cities of Rome and Athens, where aerosol appears more absorbing (SSA ~0.89–0.90 ± 0.04). The aerosol Absorption Angström Exponent (AAE, estimated using 440 and 870 nm) is found to be larger than 1 for most sites over the Mediterranean, a manifestation of mineral dust (iron) and/or brown carbon producing the observed absorption. AERONET level-2 sun-photometer data indicate the existence of a moderate East–West gradient, with higher values over the eastern basin (AAEEast. = 1.39/AAEWest. = 1.33) due to the influence of desert dust. The North–South AAE gradient is more pronounced, especially over the western basin. Our additional analysis of AERONET level-1.5 data also shows that organic absorbing aerosols significantly affect some Mediterranean sites. These results indicate that current climate models treating organics as nonabsorbing over the Mediterranean certainly underestimate the warming effect due to carbonaceous aerosols. A~comparative analysis of the regional SSA variability has been attempted using satellite data. OMI and MODIS data show an absorbing zone (SSA ~0.90 at 470–500 nm) over Northeastern Africa that does not appear in the MISR retrievals. In contrast, MISR seems able to observe the East–West SSA gradient during summer, as also detected by AERONET. Also, the analysis of SSA provided by satellites indicates that the aerosol over the Mediterranean Sea appears less absorbing during spring (MAM) than summer (JJA).
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Brogniez, C., V. Buchard, and F. Auriol. "Validation of UV-visible aerosol optical thickness retrieved from spectroradiometer measurements." Atmospheric Chemistry and Physics 8, no. 16 (August 12, 2008): 4655–63. http://dx.doi.org/10.5194/acp-8-4655-2008.
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Abstract. Global and diffuse UV-visible solar irradiances are routinely measured since 2003 with a spectroradiometer operated by the Laboratoire d'Optique Atmosphérique (LOA) located in Villeneuve d'Ascq, France. The analysis of the direct irradiance derived by cloudless conditions enables retrieving the aerosol optical thickness (AOT) spectrum in the 330–450 nm range. The site hosts also sunphotometers from the AERONET/PHOTONS network performing routinely measurements of the AOT at several wavelengths. On one hand, comparisons between the spectroradiometer and the sunphotometer AOT at 440 nm as well as, when available, at 340 and 380 nm, show good agreement: in 2003–2005 at 440 nm the correlation coefficient, the slope and the intercept of the regression line are [0.97, 0.95, 0.025], and in 2006 at 440, 380 and 340 nm they are [0.97, 1.00, −0.013], [0.97, 0.98, −0.007], and [0.98, 0.98, −0.002] respectively. On the other hand, the AOT's spectral variations have been compared using the Angström exponents derived from AOT data at 340 and 440 nm for both instruments. The comparisons show that this parameter is difficult to retrieve accurately due to the small wavelength range and due to the weak AOT values. Thus, AOT derived at wavelengths outside the spectroradiometer range by means of an extrapolation using the Angström parameter would have large uncertainties, whereas spectroradiometer's spectral AOT could be used for direct validation of other AOT, such as those provided by satellite instruments.
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Che, H., X. Zhang, X. Xia, P. Goloub, B. Holben, H. Zhao, Y. Wang, et al. "Ground-based aerosol climatology of China: aerosol optical depths from the China Aerosol Remote Sensing Network (CARSNET) 2002–2013." Atmospheric Chemistry and Physics Discussions 15, no. 8 (April 29, 2015): 12715–76. http://dx.doi.org/10.5194/acpd-15-12715-2015.
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Abstract. Long-term measurements of aerosol optical depths (AOD) and Angstrom exponents (Alpha) made for CARSNET were compiled into a climatology of aerosol optical properties for China. Quality-assured monthly mean AODs are presented for 50 sites representing remote, rural, and urban areas. AODs were 0.14, 0.34, 0.42, 0.54, and 0.74 at remote stations, rural/desert regions, the Loess Plateau, central and eastern China, and urban sites, respectively, and the corresponding Alpha values were 0.97, 0.55, 0.82, 1.19, and 1.05. AODs increased from north to south, with low values (< 0.20) over the Tibetan Plateau and northwestern China and high AODs (> 0.60) in central and eastern China where industrial emissions and anthropogenic activities were likely sources. AODs were 0.20–0.40 in semi-arid and arid regions and some background areas in north and northeast China. Alphas were > 1.20 over the southern reaches of the Yangtze River and at clean sites in northeastern China. In the northwestern deserts and industrial parts of northeast China, Alphas were lower (< 0.80) compared with central and eastern regions. Dust events in spring, hygroscopic particle growth during summer, and biomass burning contribute the high AODs, especially in northern and eastern China. The AODs show decreasing trends from 2006 to 2009 but increased ~ 0.03 yr−1 from 2009 to 2013.
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Che, Huizheng, Bing Qi, Hujia Zhao, Xiangao Xia, Thomas F. Eck, Philippe Goloub, Oleg Dubovik, et al. "Aerosol optical properties and direct radiative forcing based on measurements from the China Aerosol Remote Sensing Network (CARSNET) in eastern China." Atmospheric Chemistry and Physics 18, no. 1 (January 15, 2018): 405–25. http://dx.doi.org/10.5194/acp-18-405-2018.
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Abstract. Aerosol pollution in eastern China is an unfortunate consequence of the region's rapid economic and industrial growth. Here, sun photometer measurements from seven sites in the Yangtze River Delta (YRD) from 2011 to 2015 were used to characterize the climatology of aerosol microphysical and optical properties, calculate direct aerosol radiative forcing (DARF) and classify the aerosols based on size and absorption. Bimodal size distributions were found throughout the year, but larger volumes and effective radii of fine-mode particles occurred in June and September due to hygroscopic growth and/or cloud processing. Increases in the fine-mode particles in June and September caused AOD440 nm > 1.00 at most sites, and annual mean AOD440 nm values of 0.71–0.76 were found at the urban sites and 0.68 at the rural site. Unlike northern China, the AOD440 nm was lower in July and August (∼ 0.40–0.60) than in January and February (0.71–0.89) due to particle dispersion associated with subtropical anticyclones in summer. Low volumes and large bandwidths of both fine-mode and coarse-mode aerosol size distributions occurred in July and August because of biomass burning. Single-scattering albedos at 440 nm (SSA440 nm) from 0.91 to 0.94 indicated particles with relatively strong to moderate absorption. Strongly absorbing particles from biomass burning with a significant SSA wavelength dependence were found in July and August at most sites, while coarse particles in March to May were mineral dust. Absorbing aerosols were distributed more or less homogeneously throughout the region with absorption aerosol optical depths at 440 nm ∼ 0.04–0.06, but inter-site differences in the absorption Angström exponent indicate a degree of spatial heterogeneity in particle composition. The annual mean DARF was −93 ± 44 to −79 ± 39 W m−2 at the Earth's surface and ∼ −40 W m−2 at the top of the atmosphere (for the solar zenith angle range of 50 to 80∘) under cloud-free conditions. The fine mode composed a major contribution of the absorbing particles in the classification scheme based on SSA, fine-mode fraction and extinction Angström exponent. This study contributes to our understanding of aerosols and regional climate/air quality, and the results will be useful for validating satellite retrievals and for improving climate models and remote sensing algorithms.
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Katsev, I. L., A. S. Prikhach, E. P. Zege, J. O. Grudo, and A. A. Kokhanovsky. "Speeding up the AOT retrieval procedure using RTT analytical solutions: FAR code." Atmospheric Measurement Techniques Discussions 3, no. 2 (April 12, 2010): 1645–705. http://dx.doi.org/10.5194/amtd-3-1645-2010.
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Abstract. We present here the aerosol retrieval technique that uses radiative transfer computations in the process of retrieval rather than look-up tables (LUT). This approach provides operational satellite data processing due to the use of the accurate and extremely fast radiative transfer code RAY previously developed by authors along with approximate analytical solutions of the radiative transfer theory. The aerosol optical thickness (AOT) and Angström exponent are optimized in the iteration process using the least-squares technique with fast computations of the derivatives of radiative characteristics in respect to retrieved values. The developed technique can be adapted for processing data of various satellite instruments (including any spectral multi-angle polarization-sensitive sensors). Beside, two important problems that determine the accuracy of the AOT retrieval are considered. The first one is the effect of the preliminary choice of the aerosol model, particularly for retrieval from satellite instrument providing only spectral data (MERIS, MODIS). The second problem is the influence of clouds in adjacent pixels. As for our knowledge, this problem has not been given required attention up to now and it should be properly accounted in the AOT retrieval algorithms.
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Groß, Silke, Josef Gasteiger, Volker Freudenthaler, Thomas Müller, Daniel Sauer, Carlos Toledano, and Albert Ansmann. "Saharan dust contribution to the Caribbean summertime boundary layer – a lidar study during SALTRACE." Atmospheric Chemistry and Physics 16, no. 18 (September 16, 2016): 11535–46. http://dx.doi.org/10.5194/acp-16-11535-2016.
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Abstract. Dual-wavelength lidar measurements with the small lidar system POLIS of the Ludwig-Maximilians-Universität München were performed during the SALTRACE experiment at Barbados in June and July 2013. Based on high-accuracy measurements of the linear depolarization ratio down to about 200 m above ground level, the dust volume fraction and the dust mass concentration within the convective marine boundary layer can be derived. Additional information from radiosonde launches at the ground-based measurement site provide independent information on the convective marine boundary layer height and the meteorological situation within the convective marine boundary layer. We investigate the lidar-derived optical properties, the lidar ratio and the particle linear depolarization ratio at 355 and 532 nm and find mean values of 0.04 (SD 0.03) and 0.05 (SD 0.04) at 355 and 532 nm, respectively, for the particle linear depolarization ratio, and (26 ± 5) sr for the lidar ratio at 355 and 532 nm. For the concentration of dust in the convective marine boundary layer we find that most values were between 20 and 50 µgm−3. On most days the dust contribution to total aerosol volume was about 30–40 %. Comparing the dust contribution to the column-integrated sun-photometer measurements we see a correlation between high dust contribution, high total aerosol optical depth and a low Angström exponent, and of low dust contribution with low total aerosol optical depth.
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Papayannis, A., R. E. Mamouri, V. Amiridis, S. Kazadzis, C. Pérez, G. Tsaknakis, P. Kokkalis, and J. M. Baldasano. "Systematic lidar observations of Saharan dust layers over Athens, Greece in the frame of EARLINET project (2004–2006)." Annales Geophysicae 27, no. 9 (September 29, 2009): 3611–20. http://dx.doi.org/10.5194/angeo-27-3611-2009.
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Abstract. In this paper we present a statistical analysis on the geometrical and optical properties of Saharan dust layers observed over Athens, Greece, in a three-year period from 1 January 2004 up to 31 December 2006. The observations of the vertical aerosol profile were performed by the multi-wavelength (355-532-1064-387-607 nm) Raman lidar system of the National Technical University of Athens (NTUA) operated in the city of Athens (37°98' N, 23°77' E), Greece, in the frame of the European Aerosol Research Lidar Network (EARLINET-ASOS) project. The number of dust events was greatest in late spring, summer, and early autumn periods. This was evident also by aerosol observations during dust outbreaks obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS). In our lidar measurements, multiple aerosol dust layers of variable thickness (680–4800 m) were observed. The center of mass of these layers was located in altitudes between 1600 and 5800 m. However, the mean thickness of the dust layer typically stayed around 2700 m and the corresponding mean center of mass was of the order of 2900 m. The top of the dust layer ranged from 2000 to 8000 m, with a mean value of the order of 4700 m. MODIS observations during dust outbreaks showed that the AOD values at 550 nm ranged between 0.3–0.6, while the corresponding Angström exponent (AE) values were of the order of 0.5–0.65, indicating the presence of rather large particles.
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de Oliveira, Aline M., Cristina T. Souza, Nara P. M. de Oliveira, Aline K. S. Melo, Fabio J. S. Lopes, Eduardo Landulfo, Hendrik Elbern, and Judith J. Hoelzemann. "Analysis of Atmospheric Aerosol Optical Properties in the Northeast Brazilian Atmosphere with Remote Sensing Data from MODIS and CALIOP/CALIPSO Satellites, AERONET Photometers and a Ground-Based Lidar." Atmosphere 10, no. 10 (October 2, 2019): 594. http://dx.doi.org/10.3390/atmos10100594.
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A 12-year analysis, from 2005 to 2016, of atmospheric aerosol optical properties focusing for the first time on Northeast Brazil (NEB) was performed based on four different remote sensing datasets: the Moderate Resolution Imaging Spectroradiometer (MODIS), the Aerosol Robotic Network (AERONET), the Cloud-Aerosol LIDAR with Orthogonal Polarization (CALIOP) and a ground-based Lidar from Natal. We evaluated and identified distinct aerosol types, considering Aerosol Optical Depth (AOD) and Angström Exponent (AE). All analyses show that over the NEB, a low aerosol scenario prevails, while there are two distinct seasons of more elevated AOD that occur every year, from August to October and January to March. According to MODIS, AOD values ranges from 0.04 to 0.52 over the region with a mean of 0.20 and occasionally isolated outliers of up to 1.21. Aerosol types were identified as sea spray, biomass burning, and dust aerosols mostly transported from tropical Africa. Three case studies on days with elevated AOD were performed. All cases identified the same aerosol types and modeled HYSPLIT backward trajectories confirmed their source-dependent origins. This analysis is motivated by the implementation of an atmospheric chemistry model with an advanced data assimilation system that will use the observational database over NEB with the model to overcome high uncertainties in the model results induced by still unvalidated emission inventories.
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Gerasopoulos, E., M. O. Andreae, C. S. Zerefos, T. W. Andreae, D. Balis, P. Formenti, P. Merlet, V. Amiridis, and C. Papastefanou. "Climatological aspects of aerosol optical properties in Northern Greece." Atmospheric Chemistry and Physics Discussions 3, no. 2 (April 15, 2003): 2059–99. http://dx.doi.org/10.5194/acpd-3-2059-2003.
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Abstract. Measurements of aerosol optical properties (aerosol optical depth, scattering and backscattering coefficients) have been conducted at two ground-based sites in Northern Greece, Ouranoupolis (40° 23' N, 23° 57 E, 170 m a.s.l.) and Thessaloniki (40° 38' N, 22° 57 E, 80 m a.s.l.), between 1999 and 2002. Their frequency distributions have revealed the presence of individual modes of high and low values, indicating the influence from different sources. At both sites, the mean aerosol optical depth at 500 nm was 0.23. Values increase considerably during summer when they remain persistently between 0.3 and 0.5, going up to 0.7–0.8 during specific cases. The mean value of 65±40 Mm−1 of the particle scattering coefficient at 550 nm reflects the impact of continental pollution in the regional boundary layer. Trajectory analysis has shown that higher values of aerosol optical depth and the scattering coefficient are found in the east sector (former Soviet Union countries, eastern Balkan countries), whereas cleaner conditions are found for the NW direction. The influence of Sahara dust events is clearly reflected in the Angström exponents. About 45–60% of the observed diurnal variation of the optical properties was attributed to the growth of aerosols with humidity, while the rest of the variability is in phase with the evolution of the sea-breeze cell. Local pollution is estimated to contribute 35±10% to the average aerosol optical depth at the Thessaloniki site during summer. Finally, the aerosol scale height was found to be related to the height of the boundary layer with values between 0.5–1 km during winter and up to 2.5–3 km during summer.
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Kumar, R., M. C. Barth, G. G. Pfister, M. Naja, and G. P. Brasseur. "WRF-Chem simulations of a typical pre-monsoon dust storm in northern India: influences on aerosol optical properties and radiation budget." Atmospheric Chemistry and Physics Discussions 13, no. 8 (August 23, 2013): 21837–81. http://dx.doi.org/10.5194/acpd-13-21837-2013.
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Abstract. The impact of a typical pre-monsoon season (April–June) dust storm event on the regional aerosol optical properties and radiation budget in northern India is analyzed. The dust storm event lasted from 17 to 22 April 2010 and the WRF-Chem model estimated total dust emissions of 7.5 Tg over the model domain. Both in situ (AERONET) and satellite observations show significant increase (>50%) in local to regional scale aerosol optical depth (AOD) and decrease (>70%) in the Angström exponent (α) during this period. Amongst the AERONET sites in this region, Kanpur was influenced the most where the AOD reached up to 2.1 and the α decreased to −0.09 during the dust storm period. The WRF-Chem model reproduced the spatial and temporal distributions of dust plumes and aerosol optical properties but generally underestimated the AOD. The average MODIS and WRF-Chem AOD (550 nm) values in high dust laden region are estimated as 0.80 ± 0.30 and 0.68 ± 0.28, respectively. Model results show that dust particles cool the surface and the top of the atmosphere, and warm the atmosphere. The regionally averaged radiative perturbation due to dust aerosols is estimated as −2.0 ± 3.0 W m−2 at the top of the atmosphere, 2.3 ± 1.8 W m−2 in the atmosphere and −4.4 ± 3.1 W m−2 at the surface. The impact of these radiative perturbations on the surface energy budget is estimated to be small on a regional scale but significant locally.
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Horvath, Helmuth, Lucas Alados Arboledas, and Francisco José Olmo Reyes. "Angular scattering of the Sahara dust aerosol." Atmospheric Chemistry and Physics 18, no. 23 (December 13, 2018): 17735–44. http://dx.doi.org/10.5194/acp-18-17735-2018.
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Abstract. Soil erosion aerosols can be transported considerable distances, the Sahara being one of the major sources in the world. In June 2016 the volume scattering function of the atmospheric aerosol was determined in the Sierra Nevada, Spain, at an altitude of 2500 m. Measurements were performed with a polar nephelometer permitting measurements between scattering angles of 5 to 175∘. The values at the missing angles could be estimated to a high accuracy, using the shape of the scattering function adjacent to the missing angles, and thus a complete volume scattering function was available. During the measuring period intrusions of long-range transported Sahara aerosol happened several times. The classification of the aerosol was done by back trajectories and by the Angström exponent of the wavelength-dependent scattering coefficient, which was determined by a three-wavelength Integrating Nephelometer. The phase function of the Sahara aerosol had a stronger forward scattering, and less backscattering compared to the non-Sahara aerosol, which is in agreement with other findings for irregular particles. The asymmetry parameter of the phase function is the best characteristic to distinguish Sahara aerosol from non-Sahara aerosol. In this study the asymmetry parameter for the Sahara aerosol was larger than 0.65, whereas the non-Sahara aerosol had an asymmetry parameter below 0.6. A comparison with measurements performed with long-range transported Gobi desert aerosols observed in Kyoto, Japan, showed very similar results.