UC San Diego

Aerosol non-sea-salt sulfate (NSS sulfate) forms by secondary reactions of emissions from marine phytoplankton and shipping in the atmosphere, with both cloud and aerosol water contributing to enhanced production. For 12 months of measurements during the Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) at Scripps Pier in La Jolla, California, the highest NSS sulfate mass concentration and fraction occurred for the northwesterly trajectories that followed the Southern California Coastline for 64% of the year, with an average NSS sulfate concentration of 0.72 µg/m³ per trajectory that contributed 74% of annual sulfate. Multi-Linear Regression (MLR) and an rBC-based tracer method were used to attribute ~85% (71-98%) of submicron NSS sulfate to marine biogenic emissions and ~15% (2-29%) to regional shipping emissions. The mechanism of NSS sulfate formation was investigated by comparing correlations of NSS sulfate mass concentration associated with Upwind Cloud Vertical Fraction (UCVF), local cloud liquid water content (LWC), and relative humidity (RH). UCVF showed weak to moderate correlations (0.3-0.6) with NSS sulfate in the cloudy months of April, May, June, and August, even though local LWC showed no significant correlations. The correlation with UCVF but not LWC indicates that UCVF is a better indicator of the extent of aqueous reactions than local LWC during EPCAPE. Moderate correlations (0.4-0.5) between RH and NSS sulfate were observed in October-January when RH had greater variability. MLR analysis indicated UCVF and RH together accounted for 27% of annual NSS sulfate, with RH associated with 73% of this contribution. These findings provide important constraints for accurately modeling coastal aerosol NSS sulfate in climate predictions and air quality simulations.