Surface-tension lowering of aerosols by organics in urban atmospheres: implication to cloud condensation nuclei prediction

Surface-active organics lower the aerosol surface tension (σ_s/a), leading to enhanced cloud condensation nuclei (CCN) activity and potentially exerting impacts on the climate. Quantification of σ_s/a is mainly limited to laboratory or modeling work for particles with selected sizes and known chemical compositions. Inferred values from ambient aerosol populations are deficient. In this study, we propose a new method to derive σ_s/a by combining field measurements made at an urban site in northern China with the κ-Köhler theory. The results present new evidence that organics remarkably lower the surface tension of aerosols in a polluted atmosphere. Particles sized around 40 nm have an averaged σ_s/a of 53.8 mN m^-1, while particles sized up to 100 nm show σ_s/a values approaching that of pure water. The dependence curve of σ_s/a with the organic mass resembles the behavior of dicarboxylic acids, suggesting their critical role in reducing the surface tension. The study further reveals that neglecting the σ_s/a lowering effect would result in lowered ultrafine CCN (diameter < 100 nm) concentrations by 6.8% to 42.1% at a typical range of supersaturations in clouds, demonstrating the significant impact of surface tension on the CCN concentrations of urban aerosols.