Morphology Rather Than Surface Tension Determines CCN Activity of Submicron Inorganic Salt/Organic Oxidized Sulfur Mixed Particles

Organic oxidized sulfur (OOS) compounds, particularly surface-active ones like organosulfates and organosulfonates, constitute a significant atmospheric sulfur reservoir and play a crucial role in cloud formation. Due to limited measurements, the physical characteristics of OOS aerosols under high relative humidity (RH) and subsequent cloud activation potential remain unclear. In this study, we measured phase state, surface tension and morphology of mixtures containing inorganic salts (sodium chloride (NaCl) and ammonium salfate (AS)) and OOS under high RH (>99.5%), and investigated their cloud condensation nuclei (CCN) activity. We found that the κ CCN of the studied OOS ranged from 0.15 to 0.37, showing a negative relationship with molar volume. κ CCN and chemistry-derived κ Chem agreed well for mixtures containing OOS with short carbon chain length (≤4), which could be explained by the ideal solution because of the liquid phase and slight surface tension reduction compared to that of water. However, κ Chem overpredicted κ CCN by 35% and 17% on average for NaCl–OOS and AS–OOS with long carbon chain length (seven and eight), respectively, which was the opposite of results from inorganic–dicarboxylic acid mixtures we previously reported. Although surface tension notably decreased by 25%–44%, inorganic–OOS mixtures exhibited core-shell morphology and the OOS shell suppressed water uptake, whereas inorganic–dicarboxylic acid particles exhibited homogeneous state thus not inhibiting water uptake. Based on the results, we emphasize the pivotal role of surfactant in determining aerosol morphology and thereby CCN activations. Key Points Inorganic–OOS mixtures exhibited liquid phase at high relative humidity and surface tension was affected by organic carbon chain length Slightly reduced surface tension in short carbon chain systems could explain the closure between predicted and measured cloud condensation nuclei (CCN) activity CCN ability of the mixtures with long carbon chain length was mainly affected by initial morphology rather than surface tension Plain Language Summary Ubiquitous surface-active organic oxidized sulfur (OOS) compounds significantly influence the aerosol physical properties and subsequent cloud droplet activation. In this study, phase state, surface tension, morphology, and cloud condensation nuclei (CCN) activity of submicron particles containing inorganic salt and OOS were investigated. For mixtures containing OOS with short organic carbon chain length (≤4), the liquid phase and slight reduction in surface tension compared to that of water under nearly saturated conditions indicate ideal solution, leading to good closure between predicted and measured CCN activity. Inorganic-OOS mixtures exhibited core-shell morphology and water uptake suppression by the outer OOS shell contributed to the overprediction of CCN ability, whereas inorganic-dicarboxylic acid particles were homogeneous and water uptake was not inhibited. This study sheds light on the significance of OOS compounds in influencing submicron aerosol morphology and subsequent cloud formation.