Strong Deviations from Thermodynamically Expected Phase Partitioning of Low-Molecular-Weight Organic Acids during One Year of Rural Measurements

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An upgraded monitor for aerosols and gases in ambient air (MARGA) was applied for one year at the Central European TROPOS research site Melpitz investigating the gas- and particle-phase partitioning of formic, acetic, propionic, butyric, glycolic, pyruvic, oxalic, malonic, succinic, malic, and methanesulfonic acid (MSA). Gas- and PM 10 particle-phase mean concentrations were 12–445 and 7–31 ng m –3 for monocarboxylic acids (MCAs), between 0.6–8 and 4–31 ng m –3 for dicarboxylic acids (DCAs), and 2 and 31 ng m –3 for MSA, respectively. Assuming full dissolution in nonideal aerosol solutions, empirical noneffective Henry’s law constants ( H emp ) were calculated and compared with literature values ( H lit ). Mean H emp were 4.5 × 10 9 –2.2 × 10 10 mol L –1 atm –1 for MCAs, 3.6 × 10 10 –7.5 × 10 11 mol L –1 atm –1 for DCAs, and 7.5 × 10 7 mol L –1 atm –1 for MSA and, thus, factors of 5.1 × 10 3 –9.1 × 10 5 and 2.5–20.3 higher than their corresponding H lit for MCAs and DCAs, respectively, and 9.0 × 10 –5 lower than H lit,MSA . Data analyses and thermodynamic calculations implicate that the formation of chemical association complexes and organic salts inhibits the partitioning of organic acids toward the gas phase and, thus, at least partly explains higher H emp values for both MCAs and summertime DCAs. Low H emp,MSA are unexpected because of the high MSA solubility and are reported here for the first time. Overall, processes responsible for the observed stronger partitioning of carboxylic acids toward the particle phase need to be accounted for in complex multiphase chemistry models affecting the contribution of organic acids to secondary organic aerosol mass, their chemical processing, and lifetime.