Aromatic compounds, troposphere

Tropospheric Chemistry of Aromatic Compounds Emitted from Anthropogenic Sources... 

The primary tropospheric oxidants are OH, O3, and NO3, with "OH and O3 reactions with hydrocarbons dominating primarily during daytime hours, and NO3 reactions dominating at night. Rate constants for the reactions of many different aromatic compounds with each of the aforementioned oxidants have been determined through laboratory experiments [16]. The rate constant data as well as atmospheric lifetimes for the reactions of toluene, m-xylene, p-xylene, m-ethyl-toluene, and 1,2,4-trimethylbenzene appear in Table 14.1. Only these particular aromatic compoimds will be discussed in this review paper, since much of the computational chemistry efforts have focused on these compounds. When considering typical atmospheric concentrations of the major atmospheric oxidants, OH, O3, and NO3 of 1.5 x 10, 7 x 10, and 4.8 x 10, molecules cm , respectively [17], combined with the rate constants, it is clear that the major atmospheric loss process for these selected aromatic compounds is reaction with the hydroxyl... 

Although many advances have been made in understanding the tropospheric reactions of anthropogenic aromatic compounds, additional work is clearly needed. Specific areas of foci for future closely coordinated computational and laboratory-based studies are in the areas of ... 

Aromatic, olefinic, and acetylenic hydrocarbons, but especially saturated hydrocarbons belong to persistent pollutants difficult to eliminate from the troposphere. The only exceptions are some aromatic compounds that undergo direct photodegradation in result of solar irradiation. Alkanes are undoubtedly much less reactive than other organic compounds including unsaturated hydrocarbons, surely because they are more completely saturated and their activation involves cleavage of the relatively strong C—H bond (s = 415 kJ). 

PiUing, M. J. et al. (2003). Effects of the oXidation of Aromatic Compounds in the Troposphere (EXACT, EVK2-CT-1999-0005 3), Final Report., http //www.chem.leeds.ac.uk/exact... 

FIGURE 6.16 Aromatic compounds of interest in tropospheric chemistry. 

The results of LACTOZ have provided an extended kinetic data base for the following classes of reactions reactions of OH with VOCs, reactions of NO3 with VOCs and peroxy radicals, reactions of O3 with alkenes, reactions of peroxy radicals (self reactions, reaction with HO2, other RO2, NO, NO2), reactions of alkoxy radicals (reactions with O2, decomposition, isomerisation), thermal decomposition of peroxynitrates. Photolysis parameters (absorption cross-section, quantum yields) have been refined or obtained for the first time for species which photolyse in the troposphere. Significantly new mechanistic information has also been obtained for the oxidation of aromatic compounds and biogenic compounds (especially isoprene). These different data allow the rates of the processes involved to be modelled, especially the ozone production from the oxidation of hydrocarbons. The data from LACTOZ are summarised in the tables given in this report and have been used in evaluations of chemical data for atmospheric chemistry conducted by international evaluation groups of NASA and lUPAC. 

Three unsaturated 1,4-dicarbonyl compounds, (butenedial, 4-oxo-2 pentenal and 3-hexene-2,5-dione), formed in the OH initiated oxidation of aromatic compounds, were studied in both UV and VIS regions (Becker). The results indicate that both reaction with OH radicals and photolysis are major atmospheric sinks for these species. In contrast, doubly unsaturated 1,6-dicarbonyl compounds (e.g. 2,4-hexadienedial) were found to photolyse extremely rapidly in the UV range at 254 nm, but negligibly in the visible range. These results may suggest that the latter compounds are not photolysed in the troposphere (Becker). 

Aromatic hydrocarbons (benzene, toluene, xylene isomers) are a major class of organic compounds associated with the urban environment and model calculations appear to indicate that they might contribute more than 30 % of the photo-oxidant formation in urban areas. However, the mechanisms currently used in models of tropospheric chemistry to describe the photo-oxidation of aromatic compounds are highly speculative. Studies within LACTOZ have involved both direct and smog chamber investigations and have provided data on many different aspects of the mechanisms for oxidation of aromatic compounds related to tropospheric ozone and oxidant formation. Most of this work was carried out by four of the LACTOZ groups (Becker, Devolder, Kerr, Zetzsch). 

The project has resulted in a considerable extension of the kinetic data base for the reactions of OH radicals with the VOC classes of aliphatic ethers and of aromatic compounds. Such data are essential in the determination of tropospheric lifetimes of the VOC. 

The temperature dependence of the OH-radical reaction of some aromatic compounds under simulated tropospheric conditions,... 

The results presented in this study allow for the elucidation of the reaction mechanism and kinetics of the major products of alkylated aromatic compounds in the troposphere. The results show that the peroxy radical chemistry achieves the conversion of dimethylphenol to unsaturated dicarbonyls and aldehydes, thereby aiding oxidation and combustion processes that either releases large amount of energy or form reactive free radicals. 

Aryl halides tend to be chemically unreactive and include persistent environmental pollutants such as dichloro-diphenyl-trichloroethane (DDT), polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and polybrominated diphenyl ethers (PBDEs). Many studies of the photochemistry of halogenated aromatic compounds have been stimulated by environmental concerns, the goal often being to understand whether photolysis is an important sink for these compounds in natural waters - or in the atmosphere.The photochemistry of aryl halides causes problems in this context because many aryl halides have minimal absorption in the region of the tropospheric solar spectrum (>295 nm), and experiments at environmentally irrelevant wavelengths such as 254 nm are... 

Biacetyl is a major product of the atmospheric oxidation of many aromatic compounds, e.g., toluene, o-xylene, 1,2,3- and 1,2,4-trimethylbenzene. Its photochemistry is of interest to atmospheric scientists since it is a likely source of free radicals within the troposphere. The absorption of biacetyl extends well into the visible region of the sunlight see figure IX-F-14. The analysis of the absorption bands of biacetyl have been studied and rationalized theoretically (e.g., see Brand and Mau, 1974) Huang et al., 2005). The photochemistry of biacetyl in the absence of oxygen has been the subject of numerous studies since the early 1940s (e.g., see Henriques and Noyes, 1940 Roof and Blacet, 1941 Anderson and RoUefson, 1941 Blacet and Bell, 1953 Bell and Blacet, 1954 Sheats and Noyes, 1955a, b Ausloos and Steacie, 1955 Okabe and Noyes, 1957 Heicklen, 1959 Noyes et al., 1962 Parmenter and Poland, 1969 Caro et al., 1969 Abuin et al., 1971 Horowitz and Calvert, 1972 Sidebottom et al.. 

The photodecomposition of the various oxidation products of the alkanes, alkenes, and the aromatic hydrocarbons play important roles in the chemistry of the urban, mral, and remote atmospheres. These processes provide radical and other reachve products that help drive the chemistry that leads to ozone generation and other important chemistty in the troposphere. In this chapter, we have reviewed the evidence for the nature of the primary processes that occur in the aldehydes, ketones, alkyl nitrites, nittoalkanes, alkyl nitrates, peroxyacyl nitrates, alkyl peroxides, and some representative, ttopospheric, sunlight-absorbing aromatic compounds. Where sufficient data exist, estimates have been made of the rate of the photolytic processes that occur in these molecules by calculation of the photolysis frequencies ory-values. These rate coefficients allow estimation of the photochemical lifetimes of the various compounds in the atmosphere as well as the rates at which various reactive products are formed through photolysis. 

Semadeni, M., D.W. Stocker, and J.A. Kerr (1995), The temperature dependence of the OH radical reactions with some aromatic compounds under simulated tropospheric conditions, Int. J. Chem. Kinetics, 27, 287-304. 

In addition to reactions with HO, tropospheric organic compounds may be oxidized by ozone (via ozonation of non-aromatic carbon/carbon double bonds, Atkinson 1990) and in some cases by reaction with nitrate radical, described below. Table I gives representative trace-gas removal rates for these three processes. In spite of these competing reactions, HO largely serves as... 

The transformation of arenes in the troposphere has been discussed in detail (Arey 1998). Their destruction can be mediated by reaction with hydroxyl radicals, and from naphthalene a wide range of compounds is produced, including 1- and 2-naphthols, 2-formylcinnamaldehyde, phthalic anhydride, and with less certainty 1,4-naphthoquinone and 2,3-epoxynaphthoquinone. Both 1- and 2-nitronaphthalene were formed through the intervention of NO2 (Bunce et al. 1997). Attention has also been directed to the composition of secondary organic aerosols from the photooxidation of monocyclic aromatic hydrocarbons in the presence of NO (Eorstner et al. 1997) the main products from a range of alkylated aromatics were 2,5-furandione and the 3-methyl and 3-ethyl congeners. 

That is, it abstracts from saturated hydrocarbons and aldehydes and adds to unsaturated hydrocarbons. As discussed in Chapter 6, the reactions with aromatic hydrocarbons are generally too slow to be important in the troposphere the exceptions are particular compounds such as the cresols where the reaction is rapid. 

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