Chemical Composition of an Ultrafine Sea Spray Aerosol during the Sea Spray Chemistry and Particle Evolution Experiment

ABSTRACT: Sea spray is a significant global aerosol source with impacts on marine cloud formation and climate. The physical properties and atmospheric fate of the sea spray aerosol (SSA) depend on its chemical composition, but the current understanding of the sources and composition of the marine aerosol or SSA remains limited particularly for the smallest aerosol. The composition of ultrafine (<100 nm diameter) SSA particles controls the critical diameter for activation to cloud droplets. This study presents online measurements of sea salt and organic mass fractions in an ultrafine SSA measured during the Sea Spray Chemistry and Particle Evolution experiment conducted in summer 2019 at the Scripps Institution of Oceanography. Primary SSA particles were generated in a wave flume mesocosm study with coastal seawater obtained from the Scripps Pier in San Diego, CA. Ultrafine particle composition measurements were performed using the thermal desorption chemical ionization mass spectrometer (TDCIMS). Trends in inorganic and organic fractions show dependence on the biological activity of the ocean water, where heterotrophic bacterium concentrations were correlated with organic mass fractions of the ultrafine SSA. At low phytoplankton concentrations, ultrafine sea spray particles were mainly composed of inorganic salts. Characteristic positive ion fragments indicate influence from polysaccharides and fatty acids likely of bacterial origin in the smallest sizes. In contrast, polysaccharide and fatty acid species were below detection levels in TDCIMS measurements of the larger SSA (similar to 100-200 nm). Comparisons with the submicron aerosol composition measured by an aerosol mass spectrometer (AMS) showed high correlation between AMS and general TDCIMS organic fractions but anticorrelation between measured, individual TDCIMS organics. These results suggest biological drivers for inorganic and organic aerosol compositions and a strong size dependence on the organic composition of nascent sea spray, consistent with previous findings.