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 (∼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.