Evolution of Sea Spray Aerosol Particle Phase State Across a Phytoplankton Bloom

Abstract

Sea spray aerosol (SSA) represents the largest flux of aerosol mass annually, impacting Earth’s climate and the hydrological cycle by acting as cloud condensation and ice nuclei. The phase state (viscosity, η) of aerosol particles is important for aerosol impacts on climate and air quality, but the viscosity of SSA and chemical and biological drivers have not been evaluated. We measured variability in the relative humidity (RH)-dependent phase states of SSA (with a peak number-weighted aerodynamic diameter of ∼0.2 μm) based on particle bounce factors (BFs) and calculated glass transition temperatures and η of the organic components in SSA (ηorg) from online molecular composition measurements during an induced phytoplankton bloom. The results demonstrate that the SSA BFs and ηorg vary throughout the bloom, with the bulk of the SSA number concentration as liquidlike below the deliquescence and efflorescence RH of sea salt prior to phytoplankton growth on the basis of a measured BF of ≤0.2 and an estimated ηorg of <102 Pa·s. In contrast, the organic components in SSA exhibit viscous semisolid-like phase states over the same RH range during the peak of phytoplankton growth (BF∼0.8 and estimated ηorg∼103 Pa·s). Including the η of the aqueous inorganic salt, under the assumption that it is internally and homogeneously mixed with the organic components, significantly reduces the estimated η of the SSA mixture. The differences in SSA phase state across the bloom appear to be partially driven by specific biological processes in the seawater that alter the average molecular properties of the organic compounds in SSA (e.g., molar mass and hygroscopicity) depending upon the stage of the phytoplankton bloom. These findings have implications for aerosol processing, particularly in the biologically active coastal marine environment, including potential influences on ice nucleation and gas–particle partitioning.