Abstract. The formation of organic acids during photooxidation of 1,3,5-trimethylbenzene (TMB) in the presence of NO x 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 SO 2 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.

Abstract. The formation of organic acids during photooxidation of 1,3,5-trimethylbenzene (TMB) was investigated with an online ion chromatography (IC) instrument coupled to a mass spectrometer (MS) at the Paul Scherrer Institute (PSI) smog chamber. Gas and aerosol phase were both sampled. Molecular formulae were attributed to twelve 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 unambiguously (each of them present in gas and aerosol phase): formic acid, acetic acid, glycolic acid, butyric acid, pyruvic acid, lactic acid and methylmaleic acid. The influence of the precursor concentration (TMB: 1200 and 600 ppbv) and of the presence of 2 ppbv of sulphur dioxide (SO2) on aerosol and gas phase products were further investigated. While the organic acid fraction present in the aerosol phase does not strongly depend on the precursor concentration (6 to 14%), the presence of SO2 reduces this amount to less than 3% for both high and low precursor concentration scenarios. The addition of acetic acid during the experiments indicated that the presence of small acids in the particle phase might not be due to partitioning effects.