| Abstract
| - The role of water-soluble organic compounds on thehygroscopic properties of atmospheric aerosols hasrecently been the subject of many studies. In particular,low molecular weight dicarboxylic acids and somemultifunctional organic acids have been found or areexpected to exist in atmospheric aerosols in urban, semiurban,rural, and remote sites. Unlike for their inorganiccounterparts, the hygroscopic properties of organic acidshave not been well characterized. In this study, thehygroscopic properties of selected water-soluble dicarboxylicacids (oxalic acid, malonic acid, succinic acid, andglutaric acid) and multifunctional acids (citric acid, dl-malic acid, and l-(+)-tartaric acid) were studied using singledroplets levitated in an electrodynamic balance at 25 °C.The water activities of bulk samples of dilute solutions werealso measured. Solute evaporation was observed in thedicarboxylic acids but not in the multifunctional acids. Oxalicacid, succinic acid, and glutaric acid droplets crystallizeupon evaporation of water, but, except for glutaricacid droplets, do not deliquesce even at 90% relativehumidity (RH). Mass transfer limitation of the deliquescenceprocess was observed in glutaric acid. Neither crystallizationnor deliquescence was observed in malonic acid, citricacid, dl-malic acid, or l-(+)-tartaric acid. Malonic acid andthese three hydroxy-carboxylic acids absorb water evenat RH much lower than their respective deliquescence RH.The growth factor (Gf), defined as the ratio of the particlediameter at RH = 10% to that at RH = 90%, of oxalicacid and succinic acid was close to unity, indicating nohygroscopicity in this range. The remaining acids (malonicacid, glutaric acid, citric acid, malic acid, and tartaricacid) showed roughly similar hygroscopicity of a Gf of 1.30−1.53, which is similar to that of “more hygroscopic”aerosols in field measurements reported in the literature.A generalized equation for these four acids, Gf = (1−aw)-0.163,was developed to represent the hygroscopicity of theseacids. Water activity predictions from calculations using theUNIFAC model were found to agree with the measuredwater activity data to within 40% for most of the acids butthe deviations were as large as about 100% for malicacid and tartaric acid. We modified the functional groupinteraction parameters of the COOH−H2O, OH−H2O, and OH−COOH pairs by fitting the UNIFAC model with the measureddata. The modified UNIFAC model improves the agreementof predictions and measurements to within 38% for allthe acids studied.
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