PNNL

Abstract

Surface active organic compounds, surfactants, can affect aerosol activation by two mechanisms: lowering surface tension and altering the bulk hygroscopicity of the particles. A simulation model has been developed to predict the activation of aerosol particles consisting of an internal uniform chemical mixture of organic surfactants and inorganic salts in a parcel of air rising adiabatically at constant speed. Equations reflecting water balance of the air parcel were used together with a modified form of Kohler theory to model droplet nucleation while considering surface effects. Numerical simulations of a NaCl-Sodium Dodecyl Sulfate SDS mixture indicate that displacement of NaCl by SDS results in an increase in aerosol particle critical supersaturation Sc and a decrease in the number activated. The increase in Sc is due to the fact that the reduction in hygroscopicity dominates the reduction in surface tension. In this paper we also extend a parametric representation of aerosol activation to the case of a mixture of inorganic salts and surface active organic compounds by modifying the Raoult term in Kohler theory assuming additive behavior and using an adjustment factor to account for surface effects. The close agreement between numerical and parametric results validates our modifications. Thus, the modified form of the parameterization can account for the influence of organic surfactants. However, further work is required to extend the parameterization to other organic surfactants.