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Surfactants are surface-active organic compounds that can influence cloud condensation nuclei (CCN) activity by forming a film at the gas-liquid interface, reducing surface tension and affecting water uptake. However, many implementations of κ-Köhler theory simplify CCN activation by assuming constant surface tension, often that of pure water, which may not reflect the behavior of realistic aerosol mixtures. This study investigates the impact of surfactants on CCN activation by incorporating an effective surface tension (EST) model into the particle-resolved aerosol model PartMC-MOSAIC and its regional extension WRF-PartMC. We simulate CCN concentrations in both idealized polluted urban plume scenarios and a regional case study over California. In the urban plume scenario, neglecting surfactants led to underestimations of CCN concentrations by approximately 60%. In the regional simulations at 0.3% supersaturation, CCN concentrations were underestimated by up to 81% at the surface (with a domain-average relative difference of 21%) and up to 57% at 1 km altitude (average 19%), especially near urban emission sources. The strongest effects were observed for particles in the 40–70 nm diameter range with high organic content. Our results provide process-level evidence within a regional-scale context that surfactants can significantly alter activation behavior, indicating that neglecting these effects may lead to systematic biases in modeled aerosol-cloud interactions.Copyright © 2026 American Association for Aerosol Research
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