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

Abstract 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.