Academic literature on the topic 'Photolyse UV-C'

Abstract Photomorphogenic regulators COP1 (Constitutive Photomorphogenic 1) and HY5 (Elongated Hypocotyl 5) play a key role in plant development by guiding the transition from dark to light growth. In Arabidopsis they are also implicated in the transcriptional control of photolyase genes. Here we characterize the transcript abundance of COP1 and HY5 gene homologues in barley in relation to light-grown conditions and UV-damage response. Etiolated and green 6-day-old seedlings were UV-C irradiated and exposed to light or kept in darkness. The abundance of barley COP1 and HY5 transcripts was assessed by real-time RT-PCR. In etiolated leaves we found several-fold lower levels of COP1 transcripts which reached the levels of the green ones after 1 h of light exposure. Barley HY5 transcripts were very low in the dark-grown seedlings and after 1 h of illumination they increased drastically to levels significantly exceeding those measured in the green leaves. Both genes were upregulated by light in the irradiated plants as well, but to a lesser extent compared with their controls, probably due to the presence of non-repaired DNA damage in the etiolated leaves soon after irradiation. The enhanced transcription of barley COP1 under light is unexpected in view of the well-known function of COP1 as a negative regulator of plant photomorphogenesis but conforms to the positive role reported for AtCOP1 in UV-B signalling. HY5 is recognized as a stimulator of light-inducible genes and our data support such a role for the barley HY5 homologue as well. Our study shows that, in barley seedlings, the regulation of COP1 and HY5 gene expression is achieved through light-positive transcriptional modulation, suggesting that both genes contribute to the de-etiolation phase in barley. According to our knowledge, this is the first quantitation of the COP1 and HY5 mRNAs in barley that also regards the UV-damage response of this crop.

Abstract As with alkanes, reaction with OH is the dominant loss mechanism for haloalkanes. However, unlike the case for alkanes, photolysis can be an important loss mechanism for brominated and iodinated alkanes in the troposphere, chlorinated alkanes in the stratosphere, and fluorinated alkanes in the mesosphere. UV spectra and photolysis lifetimes for haloalkanes are discussed in chapter VII. In this chapter we first review the available kinetic data for reactions of OH radicals with haloalkanes (section VI-A). Reactions with Cl, O(3P), NO3, and O3 are then considered in section VI-B. Rules for the empirical estimation of rate coefficients of OH radicals with the haloalkanes are presented in section VI-C. The atmospheric chemistry of haloalkylperoxy and haloalkoxy radicals is described in sections VI-D and VI-E, respectively. Finally, the major products of the OH-initiated oxidation of haloalkanes are discussed in section VI-F.

Organic acids, particularly formic and acetic acid, are ubiquitous components of the troposphere (Chebbi and Carlier, 1996); see table I-D-1. However, the atmospheric budget of these species is at present poorly constrained, and global models often underestimate their abundance (von Kuhlmann et al., 2003). The presence of organic acids in the atmosphere can be attributed to two distinct mechanisms: direct emission from anthropogenic and natural sources; and in situ production via gas-phase or condensed-phase chemistry. Direct emissions result from biomass burning (e.g., Christian et al., 2007), from motor vehicle use (Kawamura et al., 2000) and other anthropogenic activities (see chapter I), and from biogenic sources (e.g., Seco et al., 2007). Production in the gas phase can occur via the reactions of acylperoxy radicals with HO2: . . . CH3C(O)O2 + HO2 → CH3C(O)OOH + O2 . . . . . . CH3C(O)O2 + HO2 → CH3C(O)O + OH + O2 . . . . . . CH3C(O)O2 + HO2 → CH3C(O)OH + O3 . . . or via the ozonolysis of unsaturated species (Orzechowska and Paulson, 2005a, b). Additional in situ acid production (particularly with multi-functional species and diacids) likely occurs in the condensed phase as well, via the oxidation of carbonyl and other oxygen-containing and multi-functional organics (e.g., Ervens et al., 2004). In general, the organic acid moiety, —C(O)OH, is rather unreactive in the gas phase. This is in large part due to the strength of the O—H bond, ∼460 kJ mole−1 versus 400–420 kJ mole−1 for typical C—H bonds (Sander et al., 2006). The organic acid moiety also acts to inhibit somewhat the reactivity of neighboring sites (Kwok and Atkinson, 1995), further decreasing the reactivity of small saturated acids. UV spectra for unsubstituted acids are located at relatively short wavelengths, [e.g., λmax< 210 nm for acetic acid, Orlando and Tyndall (2003); see figure IX-A-1], so tropospheric photolysis is of negligible importance. Thus, the gas-phase lifetime for small saturated organic acids (e.g., formic and acetic acid) can be quite long, about 1 month.

Esters are emitted directly into the atmosphere from both natural and anthropogenic sources and are produced during the atmospheric oxidation of ethers. Methyl acetate and ethyl acetate have found widespread use as solvents. Vegetable oils and animal fats are esters. Transesterification of vegetable oils and animal fats with methanol gives fatty acid methyl esters (FAMEs) which are used in biodiesel. Many esters have pleasant odors and are present in essential oils, fruits, and pheromones, and are often added to fragrances and consumer products to provide a pleasant odor. Table VII-A-1 provides a list of common esters and their odors. It is surprising to note that despite their ubiquitous nature, volatility, and fragrance, it is only very recently that quantitative measurements of esters in ambient air have been reported (Niedojadlo et al., 2007; Legreid et al., 2007). The atmospheric oxidation of saturated esters is largely initiated by OH radical attack. Reaction with O3 and NO3 radicals contributes to the atmospheric oxidation of unsaturated esters. As discussed in chapter IX, UV absorption by esters is only important for wavelengths below approximately 240 nm and, hence, photolysis is not a significant tropospheric loss mechanism. When compared to the ethers from which they can be derived, the esters are substantially less reactive towards OH radicals. The ester functionality —C(O)O— in R1C(O)OR2 deactivates the alkyl groups to which it is attached with the deactivation being most pronounced for the R1 group attached to the carbonyl group. The atmospheric oxidation mechanisms of the esters are reviewed in the present chapter. The reaction of OH with methyl formate has been studied by Wallington et al. (1988b) and Le Calvé et al. (1997a) over the temperature range 233–372 K. Data are summarized in table VII-B-1 and are plotted in figure VII-B-1. The room temperature determination of k(OH + CH3OCHO) by Wallington et al. is in agreement with that by Le Calvé et al. (1997) within the experimental uncertainties. Significant curvature is evident in the Arrhenius plot in figure VII-B-1.

A thin film ZnO nanostructured catalyst exhibited a significantly greater superiority for the photodegradation of 2, 4, 6-TCP in water over photolysis via irradiation with UV of 254 nm wavelength. This ZnO photocatalyst was prepared via Zn metal evaporation and deposition on a glass sheet followed by calcination ature from 350 to 500 °C and the calcination time from 1 to 2h shows via SEM photography a decrease of ZnO nanoparticales sizes sheet followed by calcination (oxidation). Increasing the calcination temperature from 350 to 500 °C and the calcination time from 1 to 2h shows via SEM photo