Strong deviations from thermodynamically expected phase partitioning of organic acids during one year of rural field measurements

Organic acids are ubiquitous compounds in the troposphere and can affect human health, the climate, air quality, and the linked ecosystems. Depending on their solubility and volatility, they can partition in both gas phase and in the particle phase. In the particle phase, organic acids partly represent about 10% of the water-soluble organic matter. However, their partitioning between different phases is not fully understood yet. Therefore, an upgraded monitor for aerosols and gases in ambient air (MARGA) was applied for one year at the Central European TROPOS research site Melpitz to study the gas- and particle-phase partitioning of formic, acetic, propionic, butyric, glycolic, pyruvic, oxalic, malonic, succinic, malic, and methanesulfonic acid (MSA). Measured gas- and PM₁₀ 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). Calculated mean H_emp were 4.5 × 10⁹−2.2 × 10¹⁰ mol L⁻¹ atm⁻¹ for MCAs, 3.6 × 10¹⁰−7.5 × 10¹¹ mol L⁻¹ atm⁻¹ for DCAs, and 7.5 × 10⁷ mol L⁻¹ atm⁻¹ for MSA and, thus, factors of 5.1 × 10³−9.1 × 10⁵ 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 also unexpected because of the high MSA solubility and are reported for the first time in this study. Overall, the results of the present study implicate that processes responsible for the observed stronger partitioning of carboxylic acids toward the particle phase need to be further investigated and accounted for in complex multiphase chemistry models as they affect the contribution of organic acids to secondary organic aerosol mass, their chemical processing, and lifetime.