Investigating the Bulk-Surface Partitioning and Surface Tension of Structurally Different Organosulfates Aerosol through Measurements and Simple Thermodynamic Model
HOI TANG LAW, Masayoshi Nakamura, Masao Gen, Man Nin Chan, The Chinese University of Hong Kong
Abstract Number: 263
Working Group: Aerosols, Clouds and Climate
Abstract
Organosulfur compounds, such as organosulfates (OSs), sulfones, sulfonates, and sulfoxides, exist in ambient aerosols across rural and heavily polluted areas. OSs are key anionic surfactants in ambient aerosols, with concentrations reaching up to 30% of organic aerosol mass. For example, the properties of sodium dodecyl sulfate (SDS) have been extensively studied to simulate marine aerosols. Other OSs derived from isoprene, limonene, methacrolein, methyl vinyl ketone, and α-pinene are also surface-active, reducing surface tension. According to Köhler theory, aerosol surface tension significantly influences the fraction of aerosols activated as cloud condensation nuclei (CCN). However, accurately estimating this fraction and predicting cloud formation are challenging due to surfactants in aerosols. To improve this accuracy, we systematically investigated the surface activities of OSs and aerosol surface tension in relation to OS chemical structures.
In this study, we conducted surface tension measurements of single-component OS aerosols under varying relative humidity (RH) using the single-particle levitation method in an RH-controlled electrodynamic balance (EDB) chamber coupled with quasi-elastic light scattering (QELS). From these measurements, we extracted the surface tension of aerosols with varying solute mass fractions under different RH levels to simulate the surface saturation point. We determined the extent of surface-bulk partitioning of OSs by fitting the results to the Langmuir isotherm adsorption model, calculating the ratio of OS adsorption and desorption rate constants. The decrease in surface tension plateaued at high OS concentrations, likely due to surfactant surface saturation. Moreover, the measurements align with previous studies, and the model accurately reproduces the decrease in surface tension with increasing OS concentration.