Insights into the biogenic contribution to total sulphate in aerosol and precipitation in the Fraser Valley afforded by isotopes of sulphur and oxygen

Abstract

Contributions to atmospheric sulphate aerosol and precipitation in the Fraser Valley and at Saturna Island, Canada, from oxidation of biogenic dimethyl sulphide were determined using an isotopic mass balance approach. The S‐isotope composition of aerosol sulphate, and sulphur dioxide from a variety of industrial sources in the region were investigated and were found to have δ34S values ranging between −1.6 and +9‰. Isotopic analysis suggests a U.S. oil refinery and/or H2S from tidal flats on the coast contributes to the aerosol sulphate load in both summer and winter. On average, well‐mixed air in the region had δ34S values for SO2 near +1‰. Assuming this represents the average isotope composition of multiple sulphate sources from urban pollution, refinery emissions, and H2S, the DMS contributions to sulphate aerosol were calculated. On average, biogenic DMS‐sulphate made up 30% of the non‐sea salt sulphate in precipitation at Saturna Island in the Strait of Georgia. In contrast, it comprised only 13% of the aerosol sulphate load in the Fraser Valley. Proportionately more DMS aerosol sulphate is observed at Powell River and on Vancouver Island than in the vicinity of the city of Vancouver. Estimated concentrations are highest in the north, reaching 1.4 μgS/m3. These results are consistent with the isotope composition of oxygen in sulphate which show that at least half the aerosol sulphate in the Fraser Valley is formed from oxidation of SO2 at ambient temperatures.