Background

The atmosphere is a complex photochemical reactor which contains a gas-particles mixture of natural and anthropogenic compounds with diverse properties, producing adverse effects on human health, ecology, ozone depletion, and climate (Finlayson-Pitts and Pitts, 2000). Since the industrial revolution, a strong anthropogenic influence on the tropospheric composition has taken place.

Aromatic hydrocarbons (AHs) are an important class of volatile organic compounds (VOC) emitted into atmosphere. Benzene, toluene, ethyl benzene and the xylene isomers (BTEX) are the main aromatics present especially in urban air as a result of human activities (Calvert et al., 2002; Lin et al., 2011; Vega et al., 2011). AHs contribution to the photooxidant formation in Europe is situated to 30 - 40% (Calvert et al., 2002). AHs have an important contribution as precursors to secondary organic aerosol (SOA) formation (Derwent et al., 2010), the role of SOA in the environmental processes being well known (Kanakidou et al., 2005; Hallquist et al., 2009). SOA formation is significant both in terms of climate but also from a health perspective where SOA can effectively deliver toxins into the human blood stream.

Particular interest is represented by oxygenated AHs gas phase chemistry. If the BTEX are mainly the result of the human activities (traffic and industry), the oxygenated aromatics, e.g. cresols, catechols (1,2-dihydroxybenzenes), are considered important products of biomass burning and residential wood combustion (Hawthorne et al., 1989). Cresols and catechols are known to be also photooxidation products of BTEX gas phase degradation (Calvert et al., 2002).

The major removal process of AHs in the atmosphere will be the reaction with OH radicals, however, for phenol, cresols and catechols have been reported also fast reaction with NO3 radicals (Calvert et al., 2002; Olariu et al., 2004), therefore it is to be expected that reactions of NO3 radicals with hydroxyarenes, in general, will be fast and important not only during the night but also potentially during the day in areas with high NOx levels.

The degradation of BTEX with OH radicals received more attention and the mechanism is well established (Calvert et al., 2002; Hamilton et al., 2003; Zhao et al., 2005). The OH radical initiated oxidation of methylated AHs, e.g. toluene, consists of three main degradation routes:

•          The H-atom abstraction route which accounts for about ~8% of the overall reaction and in the case of toluene leads to the formation of benzaldehyde and benzylalcohol.

•          The addition route, where the hydroxycyclohexadienyl radical may lead to cresol formation by reaction with O2 or may add O2 and form a bicyclic peroxy radical (Glowacki et al., 2009). The subsequent chemistry of bicyclic peroxy radicals (Elrod, 2011; Birdsal and Elrod, 2011) is considered to be responsible for formation of ring opening products.

•          Dealkylation of aromatics is considered of less importance Aschmann et al., (2010).

The yield of phenol from the reaction of OH has been shown to be sensitive to the amount of NOx in the experimental system; under atmospheric NOx conditions the phenol yield will be between 50-60% (Berndt and Böge, 2006; Klotz et al., 2002; Volkamer et al., 2002). Our current understanding is limited to first steps of the oxidations of main non-oxygenated aromatic hydrocarbons with significant gaps in our understanding of other important AH.

Catechols (1,2-dihydroxybenzenes) are known important products of the further oxidation of phenol and the cresol isomers (Olariu et al., 2002; Bernd and Böge, 2003) and are also known products of biomass burning (Fine et al., 2001; Hays et al., 2005). The importance of the formation of catechol-type compounds from the photooxidation of other alkylated phenols still remains to be established.

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