Online gas and aerosol measurement of water soluble carboxylic acids in Zurich

[ 1] We discuss the diurnal and seasonal variability of low molecular weight organic acids in Zurich city on the basis of online quasi-continuous measurement in the gas and aerosol phase using a wet effluent diffusion denuder/aerosol collector(WEDD/AC) coupled to ion chromatography. The measurements were performed during August - September 2002 and March 2003. Acetic acid exhibited the highest concentration in the gas phase during all the measurement periods, followed by formic acid. Oxalic acid was predominantly found in the aerosol phase and often below the detection limit in the gas phase. In addition, filter samples were analyzed using ion chromatography - mass spectrometry (IC-MS) to provide more information on organic acids in the aerosol phase. From the offline IC-MS measurements, 20 monocarboxylic, dicarboxylic, and tricarboxylic acids were determined. In addition, more than 20 different masses were detected with the MS; however, identification of the organic acids was not possible. The sum of the carboxylic acids contributed on average 2% to the water soluble organic carbon (WSOC). The fraction of dicarboxylic acids to the WSOC was higher in summer compared to winter suggesting that dicarboxylic acids are mainly a result of photochemical reactions in summer whereas in winter they mainly result from primary sources.