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

Abstract

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.

Insights into the role of dicarboxylic acid on CCN activity: implications for surface tension and phase state effects

“…Solubility and molar volume of dicarboxylic acids were essential factors influencing their hygroscopicity (Kumar et al, 2003;Han et al, 2022). In this study, we considered two regimes: highly soluble organic components (with water solubility over 100 g L −1 ) and slightly soluble organic components (with water solubility between 10-100 g L −1 ), which was consistent with previous study (Kuwata et al, 2013).…”

“…In Fig. 2b, CCN values of sparely soluble components (AA, PA, SA and OA) showed an increased trend with solubility, as organic matter with the higher water solubility would dissolve more and have a higher molar concentration, resulting in reduction in water activity and higher hygroscopicity (Luo et al, 2020;Han et al, 2022).…”

Constraining the Effect of Surfactants on the Hygroscopic Growth of Model Sea Spray Aerosol Particles

“…The pure glucose particles have GFs larger than 1.0 across the RH range studied, which increase to almost 1.4 at 90% RH and match previous HTDMA studies performed on glucose particles , (Table ). The hygroscopic behavior of sugars has been linked to water solubility and oxidation state, and previous measurements have demonstrated that glucose particles are somewhat hygroscopic, , with κ values of 0.14–0.20 (Table ). The κ for glucose calculated in this study falls within that range (κ = 0.193 ± 0.012 at 90% RH; Table ).…”

“…The hygroscopic growth of organic compounds in mixed aerosols has also been linked to the particle mixing state and organic coating thickness. , In the presence of salt, sugars can form an organic shell around the salt core, and the composition of the organic coating will determine the water uptake. , Previous studies have shown that in core–shell particle morphologies, organic shells may suppress the uptake of water by hindering water diffusion to the inorganic core, but this does not occur for all core–shell particles. , The effect of the organic compound on the salt particle water uptake is dependent on the organic composition and specifically linked to the solubility of the organic compound, which may vary in an ionic environment − , such as an aqueous salt particle.…”

“…Lower hygroscopic growth of complex sugar particles compared to simple sugar particles has been observed previously across a wider RH range and was linked to the higher molecular weights of complex sugars. 93,99 An increase in molecular weight and the number of glucose units has been tied to an increase in sugar solubility, 100,101 which highlights solubility as a determining factor in the hygroscopic growth of sugars.…”