Quantifying Aerosol Surfactant Effects on Cloud Droplet Activation

Description

Köhler theory has become an important ingredient to modeling studies aimed at assessing the impact of aerosols on clouds and climate. In the basic theory, surface tension is assumed to be that of pure water, neglecting effects from surface active and/or partially soluble compounds found typically among the organic constituents of cloud-condensation nuclei (CCN). More recently there have been a number of studies focused on bringing surfactants and surface-bulk partitioning into the Köhler framework and much progress has been made, yet a systematic and seemless integration has yet to be achieved. Molecular-based approaches towards quantifying the influence of surface-active species on CCN efficiency are also in need of development. This paper introduces novel approaches towards each of these goals: Gibbsian thermodynamics is shown to seamlessly integrate surface effects into the Köhler equations, while quantitative methods from multicomponent nucleation theory, developed for condensation nuclei (CN) activation at the nanoscale, are successfully scaled to the sub-micron sizes of CCN and the larger sizes of activated solution drops. Sensitivities of cloud droplet activation to seed molecular composition and solute partitioning between surface and bulk are obtained using nucleation theorems and stoichiometric analysis. Regimes where surfactant dominates solute in lowering the barrier to CCN activation, and vice-versa, are identified and discussed.