CCN predictions on particles containing mixed organic and elemental carbon using Köhler theory

Anthropogenic atmospheric aerosols play a large role in climate change by acting as cloud condensation nuclei (CCN) and influencing cloud reflectivity. The ability of anthropogenic aerosols to contribute to cloud droplet activation depends on particle size and composition. Up to 90% of fine particle mass (PM2.5) is composed of carbonaceous (elemental and organic carbon) compounds, which play a significant yet unknown role in cloud droplet activation. While elemental carbon (EC) has limited solubility and mainly contributes to particle size, the organic carbon (OC) could add a substantial amount of soluble material to the particle depending on the nature of the organic species present. Here we present size-resolved aerosol measurements of single-particle chemistry in an urban environment that was heavily influenced by carbonaceous particle types (OC and EC particles accounted for 43.5% and 39.1% of the total submicron particle count, respectively). Köhler calculations were used to determine the CCN activity of different particle diameters of internally mixed OC and EC particles. Our results demonstrate how the soluble fraction (%OC) and size of observed mixed OC and ECcontaining particles affect the ability to act as CCN and how sensitivity parameters such as density, solubility, and molecular weight impact CCN. Compared to pure 600 nm EC-containing particles, adding 40% OC (using oxalic acid parameters) to the particles decreased the critical supersaturation from 0.28% to 0.021% and increased the fraction of CCN from 0.264 to 0.815. This means that organic compounds could contribute significantly to cloud droplet number concentration and potentially to the Twomey effect in highly aged urban environments.