Hygroscopicity of organic compounds as a function of organic functionality, water solubility, molecular weight, and oxidation level

Abstract

Aerosol hygroscopicity strongly influences the number size distribution, phase state, optical properties, and multiphase chemistry of aerosol particles. Due to the large number of organic species in atmospheric aerosols, the determination of the hygroscopicity of ambient aerosols remains challenging. In this study, we measured the hygroscopic properties of 23 organics, including carboxylic acids, amino acids, sugars, and alcohols, using a hygroscopicity tandem differential mobility analyzer (HTDMA). Earlier studies have characterized the hygroscopicity either for a limited number of organic compounds using similar techniques or for particles at sizes beyond the microscale range or even bulk samples using other methodologies. Here, we validate these studies and extend the data by measuring the hygroscopicity of a broader suite of organics for particles with sizes under the submicrometer range that are more atmospherically relevant. Moreover, we systematically evaluate the roles of that related physicochemical properties play in organic hygroscopicity. We show that the hygroscopicity of organics varies widely with functional groups and organics with the same carbon number but that more functional groups show higher hygroscopicity. However, some isomers that are very similar in molecular structure show quite different hygroscopicity, demonstrating that other physicochemical properties, such as water solubility, may contribute to their hygroscopicity as well. If the organics are fully dissolved in water (solubility >7×10-1 g mL−1), we found that their hygroscopicity is mainly controlled by their molecular weight. For the organics that are not fully dissolved in water (slightly soluble: 5×10-4 g mL−1 < solubility < 7×10-1 g mL−1), we observed that some of them show no obvious water uptake, which is probably due to the fact that they may not deliquesce under our studied conditions up to 90 % relative humidity (RH). The other type of slightly soluble organic material is moderately hygroscopic, and the larger its solubility is, the higher its hygroscopicity will be. Moreover, the hygroscopicity of organics generally increased with O:C ratios, although this relationship is not linear.