PNNL

Abstract

Aerosol non-sea-salt sulfate (NSS sulfate) forms by secondary reactions of emissions from marine phytoplankton and shipping in the atmosphere, with gas-phase as well as cloud and aerosol aqueous reactions controlling production. Twelve months of Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) measurements at Scripps Pier in La Jolla, California, showed the highest NSS sulfate mass concentrations occurred for the northwesterly back-trajectories over 64% of the year, with an average of 0.90 µg/m³ that contributed 76% of annual NSS sulfate concentration. Multi-Linear Regression (MLR) and a refractory black carbon tracer method attributed 76-80% of the sulfur dioxide (SO2) sources of submicron NSS sulfate to marine biogenic emissions and 20-24% to shipping emissions. MLR indicated that Downwelling Shortwave Radiation (DWS) driving photochemical oxidation accounted for 34% of annual sulfate production, Upwind Cloud Vertical Fraction (UCVF) controlling cloud-associated oxidation accounted for 29%, and relative humidity (RH) associated with aerosol-phase oxidation accounted for 36%. NSS sulfate was correlated moderately (r=0.3-0.5) to UCVF during April-June and August but to RH in October-January. These findings show the apportionment of SO2 emissions to biogenic and anthropogenic sources and provide observational constraints for the mechanisms for sulfate production from SO2 in the atmosphere.