The role of jet and film drops in controlling the mixing state of submicron sea spray aerosol particles
Significance Submicrometer sea spray aerosol (SSA) particles play a critical role in determining cloud properties in marine environments. Typically, it is assumed that submicrometer SSA particles mainly originate from bubble cap films bursting at the sea surface. However, we show that in addition to this formation pathway, a substantial fraction of submicrometer SSA particles are also produced from jet drops. Using an experimental approach that relies on differences in the electrical mobility of jet and film drops, we show that jet drops contribute up to 43% of total submicrometer SSA number concentrations. As shown herein, these two different production pathways result in an externally mixed submicrometer SSA population with two distinct chemical compositions with significantly different ice nucleating activities. The oceans represent a significant global source of atmospheric aerosols. Sea spray aerosol (SSA) particles comprise sea salts and organic species in varying proportions. In addition to size, the overall composition of SSA particles determines how effectively they can form cloud droplets and ice crystals. Thus, understanding the factors controlling SSA composition is critical to predicting aerosol impacts on clouds and climate. It is often assumed that submicrometer SSAs are mainly formed by film drops produced from bursting bubble cap films, which become enriched with hydrophobic organic species contained within the sea surface microlayer. In contrast, jet drops formed from the base of bursting bubbles are postulated to mainly produce larger supermicrometer particles from bulk seawater, which comprises largely salts and water soluble organic species. However, here we demonstrate that jet drops produce up to 43% of total submicrometer SSA number concentrations, and that the fraction of SSA produced by jet drops can be modulated by marine biological activity. We show that the chemical composition, organic volume fraction, and ice nucleating ability of submicrometer particles from jet drops differ from those formed from film drops. Thus, the chemical composition of a substantial fraction of submicrometer particles will not be controlled by the composition of the sea surface microlayer, a major assumption in previous studies. This finding has significant ramifications for understanding the factors controlling the mixing state of submicrometer SSA particles and must be taken into consideration when predicting SSA impacts on clouds and climate.
Proceedings of the National Academy of Sciences