Determining the dependence of relative humidity and oxidative aging on the hygroscopicity and phase state of sea spray aerosol

Phase state (i.e., liquid, semi-solid, solid) affects the diffusion rate of trace gases into aerosol, thus influencing the reactivity, lifetime, and composition of atmospheric aerosols. While sea spray aerosol (SSA) is the largest contributor to aerosol mass globally, its phase state, morphology, and reactivity in the presence of trace reactive gases, is poorly characterized. In this work, as part of the Sea Spray Chemistry and Particle Evolution (SeaSCAPE) experiment in summer 2019 at the NSF-Center for Aerosol Impacts on Chemistry of the Environment (NSF-CAICE), we investigate the humidity-dependent phase states and molecular composition of unoxidized and oxidized nascent SSA (nSSA) generated in a laboratory mesocosm with breaking waves. Furthermore, the phase and composition of secondary marine aerosol (SMA) produced from the oxidation of volatile compounds emitted without breaking waves is examined after sending the particles and gases through a potential aerosol mass oxidative flow reactor (PAM-OFR). We utilize an electrical low-pressure impactor and a scanning electrical mobility spectrometer to directly infer transient variations in phase state based on particle bounce fractions (BF) and analyze the phase, morphology, and chemical composition of individual size-fractionated particles using atomic force microscopy infrared-spectroscopy and high-resolution mass spectrometry. Preliminary results indicate significant differences in phase state, concurrent with hygroscopicity of nSSA, SMA, and mixed nSSA/SMA as a function of oxidative aging, relative humidity, and phytoplankton bloom age. All modes exhibit decreasing bounce with increasing relative humidity. With increasing OH exposure, SMA particles, when dried, exhibit less bounce. Preliminary results also indicate that BF varies with the average molar mass of the dry organic components in the particles. Further analysis of SSA hygroscopicity and water-soluble ion/organic carbon content will enable the first quantitative estimates of nSSA and SMA glass transition temperatures/viscosity as a function of particle size and chemical age, which can be used to guide multiphase chemistry in aerosol transport models.