UC San Diego

The direct and indirect interactions of aerosols with solar and terrestrial radiation represent the most significant uncertainty in our understanding of Earth’s climate. Marine aerosols, comprised of freshly emitted or nascent sea spray aerosols (SSA), aged SSA, and secondary marine aerosol (SMA), significantly impact the global radiative budget, cloud and fog formation, and visibility and air quality in coastal marine environments. Biological processes in seawater impact marine aerosol chemical and physical properties. An important but understudied property of marine aerosol is its phase state or viscosity. Viscosity significantly influences gas-to-particle partitioning, liquid and ice cloud nucleation rates, and heterogeneous reaction kinetics, which can affect aerosol chemical composition, wet deposition rates, size, and thus aerosol impacts on climate and air quality. This work investigates the influence of biological activity, particle size, and oxidative aging processes on marine aerosol's molecular composition and phase states. Empirical observations of particle phase state are made through online particle bounce/rebound measurements and offline with atomic force microscopy. Findings indicate nascent SSA particles exhibit increasingly viscous or solid-like characteristics with greater seawater phytoplankton activity and decreasing particle size. SSA aged by OH radical is more viscous than nascent SSA. Increasing organic carbon content and chemical species with higher molecular weights were enriched in SSA during elevated phytoplankton growth in seawater and smaller SSA. We attribute the higher molecular weight and more viscous organic components in SSA to the production of colloidal or gel-like SSA formed through the complexation of polyanionic organic components with divalent cations, specifically Ca2+. In polluted air with high concentrations of marine volatile organic compounds and oxidants equivalent to a week of chemical aging in the atmosphere, SSA becomes more viscous, attributed to the functionalization of particulate organic carbon. Further aging up to the equivalent of weeks in the atmosphere leads to less viscous phase states, attributed to the volatilization and, thus, decreasing molecular weight of the SSA organic carbon. These results have the potential for a better understanding of processes such as ice nucleation and airborne transport of chemical pollutants and toxins in SSA in coastal environments, which are strongly influenced by viscosity.