Physicochemical Mixing State of Sea Spray Aerosols: Morphologies Exhibit Size Dependence

Abstract

Sea spray aerosols (SSA) play an important role in radiative forcing by directly scattering solar radiation and indirectly by acting as cloud condensation or ice nuclei. These climate-relevant aerosol processes strongly depend on the mixing state and morphology of the individual aerosol. In this work, the distribution of different morphologies within nascent SSA as a function of particle size is investigated. SSA generated from wave breaking of natural seawater were collected for offline analysis using complementary scanning electron microscopy (SEM), thermal optical analysis, and atomic force microscopy-based photothermal infrared (AFM-PTIR) spectroscopy methods. SEM data revealed six key morphologies of SSA—prism-like, core–shell, rounded, rod-inclusion, aggregate, and rod. Of these, prism-like, core–shell, and rounded morphologies accounted for more than 99% of the entire SSA population and exhibited size-dependent trends. Thermal optical analysis data revealed a significant increase in the organic mass fraction of SSA with decreasing particle size. Concurrently, the SEM data showed a significant decrease in the relative abundance of prism-like morphology and corresponding increase of rounded and core–shell morphologies. AFM-PTIR spectroscopy showed that the SSA prism-like morphology is largely inorganic in nature, whereas the shell of the core–shell and rounded morphologies are predominantly organic. The study demonstrates that, instead of a single “representative” SSA morphology, the physicochemical mixing state of SSA is dynamic with respect to particle size. This should be taken into account to accurately predict the magnitude of the radiative forcing of SSA.