Surface-active organic species are ubiquitous in atmospheric aerosol particles and can impact the particles’ cloud condensation nuclei (CCN) activity. Köhler theory describes the thermodynamic equilibrium of the cloud droplet formation process and has been extensively confirmed for inorganic aerosols, but the situation becomes more complicated when the particles are mixtures of inorganic and organic species. The goal for this study is to quantify the importance of organic films for cloud condensation nuclei activity for aerosol populations with realistic mixing states. Many studies that use κ-Köhler theory to calculate the CCN activity for organic/inorganic mixtures calculate the hygroscopicity of the mixtures by using a volume-weighted average of the component hygroscopicity. However, this ignores the decrease of surface tension for aerosol particles that consist of a hydrophobic organic-rich shell (organic film) coating a hygroscopic inorganic core. Recently, an effective surface tension model was developed to calculate the evolving surface tension values of organic/inorganic mixtures during the droplet growth process. In this study, this effective surface tension model was incorporated into the particle-resolved aerosol model PartMC-MOSAIC. PartMC-MOSAIC is a Lagrangian box model where PartMC simulates the dynamics of an aerosol with a stochastic Monte Carlo approach, and MOSAIC simulates the change of composition of each particle during the evolution. The particle-resolved modeling approach offers unique advantages over traditional modal or sectional modeling approaches, since it does not require any a priori assumptions about aerosol mixing state. This means that we can apply the effective surface tension model on a per-particle level to investigate its impact on CCN concentrations. In our study we use a large ensemble of PartMC-MOSAIC model simulations covering a wide range of aerosol mixing states that may be encountered in the ambient atmosphere. We will show the evaluation of the differences in CCN concentrations between including organic films and neglecting them. This comparison will help us to better quantify the contribution of organic films on cloud droplet formation and their influence on the Earth’s climate.

Quantifying the impact of organic films on cloud condensation nuclei activity using a particle-resolved model

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