Abstract
Abstract. The formation of organic acids during photooxidation of 1,3,5-trimethylbenzene (TMB) in the presence of NOx was investigated with an online ion chromatography (IC) instrument coupled to a mass spectrometer (MS) at the Paul Scherrer Institute's smog chamber. Gas and aerosol phase were both sampled. Molecular formulas were attributed to 12 compounds with the help of high-resolution MS data from filter extracts (two compounds in the gas phase only, two in the aerosol phase only and eight in both). Seven of those species could be identified: formic acid, acetic acid, glycolic acid, butanoic acid, pyruvic acid, lactic acid and methylmaleic acid. While the organic acid fraction present in the aerosol phase does not strongly depend on the precursor concentration (6 to 20%), the presence of SO2 reduces this amount to less than 3% for both high and low precursor concentration scenarios. A large amount of acetic acid was injected during one experiment after aerosol formation, but no increase of acetic acid particle concentration could be observed. This indicates that the unexpected presence of volatile organic acids in the particle phase might not be due to partitioning effects, but to reactive uptake or to sampling artefact.
Highlights
Aromatic compounds are ring-containing volatile organic compounds (VOCs) emitted into the atmosphere by fuel combustion and evaporation, where they are oxidised by hydroxyl radicals (OH) or nitrate radicals (NO3)
Ozone photolysis triggers the formation of OH radicals, which oxidise TMB
Note that NO3 do not play a role in TMB photooxidation under these experimental conditions; NO3 is rapidly photolysed to NO or NO2
Summary
Aromatic compounds are ring-containing volatile organic compounds (VOCs) emitted into the atmosphere by fuel combustion and evaporation, where they are oxidised by hydroxyl radicals (OH) or nitrate radicals (NO3). They play a major role in urban areas and can represent 13–44 % of the total hydrocarbon mass in the atmosphere (Calvert, 2002; Dommen et al, 2003; Molina et al, 2007; Vega et al, 2011). The organic fraction of the atmospheric aerosol, to which SOA contributes, is a complex mixture of many different compounds (Kanakidou et al, 2005; Williams et al, 2007). Only 10 % of this water-soluble organic fraction of the aerosol can typically be chemically identified (Saxena and Hildemann, 1996)
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