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

## TL;DR

This study explores how surfactants in atmospheric droplets self-assemble and affect droplet growth and activation under different humidity conditions.

## Contribution

The paper introduces a model combining water activity and Köhler theory to study surfactant self-assembly in atmospheric droplets.

## Key findings

- Surfactant self-assembly has minor effects at cloud droplet activation but major impacts during early droplet growth.
- Surfactant aggregates bind sodium ions, reducing their influence on droplet water activity.
- Variations in droplet size due to surfactants affect aerosol liquid water content predictions.

## Abstract

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.