Impact of Bulk-Phase Self-Assembly on Growth and Activation of Aqueous Surfactant Aerosol

Surface active compounds (surfactants) have been found in atmospheric aerosols from many environments. In microscopic aqueous droplets, their presence may cause a variety of effects that are challenging to model comprehensively. In this work, we investigate the solute effects of atmospheric surfactant self-assembly to form micelles and small clusters in aqueous droplets. Several water activity models are employed in combination with Köhler theory to represent self-assembly phenomena within the droplet bulk while also accounting for surfactant bulk-surface partitioning. Our results show that surfactant self-assembly has only minor effects at the critical point of cloud droplet activation; however, very significant effects are observed during earlier stages of droplet growth at subsaturated ambient humidities. A major driver is the binding of free sodium counterions to the surfactant aggregates, strongly limiting the effect of dissolved sodium ions on droplet water activity. Variations in droplet equilibrium size at a given subsaturated humidity due to the presence of surfactant aggregates lead to different estimates of droplet water uptake and aerosol liquid water content. As a result, predictions of atmospheric processes involving surfactant aerosol could be highly sensitive to how surfactant self-assembly and related phenomena are taken into account.