Molecular Characterization of Nascent and Aged Sea Spray Aerosol.

The chemical aging of sea spray aerosol (SSA) was examined in the Sea Spray Chemistry and Particle Evolution (SeaSCAPE) experiment in which nascent SSA particles were generated from seawater by breaking waves in a glass wave channel. Particles and gases in the air in the wave channel headspace were aged in an oxidative flow reactor. Nascent SSA (before reaction) and aged SSA (after reaction) particles were chemically analyzed for inorganic ions, organic carbon (OC), and select organic species, including organosulfates, fatty acids, and alkyl amines. Nascent SSA mass primarily consisted of inorganic ions associated with sea salt. On average, OC accounted for 52% of particle mass <0.25 μm and 0.2% of mass in both supermicron and submicron particles, with an increase in OC relative to Na⁺ with decreasing particle size. The aging process increased the sulfate, phosphate, nitrate, ammonium, and OC concentrations relative to sodium. The largest increases in the sulfate and OC to Na⁺ ratios (by factors of 7 and 5) in aged SSA were observed in particles with diameters <0.25 μm. Organosulfates, which accounted for approximately 1% of SSA OC mass in PM_1.0, were enhanced in aged SSA and indicated the formation of low-volatility secondary organic aerosol products associated with aging biological molecules such as unsaturated fatty acids, isoprene, and monoterpenes. For example, isoprene-derived organosulfates (e.g., 2-methyltetrol sulfate, C₅H₁₁SO₇ ^-; m/z 215.0225) increased by a factor of 40 in samples of aged SSA and marine volatile organic compounds. Among the strongest organosulfate signals in nascent and aged SSA were alkyl organosulfates, which are known to be anthropogenic surfactants in coastal waters. Homologous series of saturated and unsaturated fatty acids, fatty acid derivatives, and alkyl amines were also identified in nascent and aged SSA, with some species enhanced by aging (i.e., diethylamine) and others not. Together, these bulk and molecular analyses provide insight into molecular modifications that occur upon the chemical aging of nascent SSA.