The COVID-19 shutdown in 2020 resulted in reductions of sulfur dioxide emissions for Southern California. Sulfur dioxide (SO2) is an important atmospheric pollutant that leads to formation of atmospheric aerosol sulfate (SO42-), directly influencing air quality, climate, and radiative forcing. Formation of atmospheric SO42- can occur via the gas phase reaction of SO2 with OH radical or aerosol aqueous phase oxidation of SO2 by H2O2/O3. Triple oxygen (δ16O, δ17O, δ18O) stable isotope measurements are an excellent tracer to discern the contributions of these various pathways because oxidation of SO2 by H2O2, and especially O3, results in a mass-independent 17O excess (Δ17O) for the product SO42-. In this study, triple oxygen isotope analysis was performed on atmospheric samples were collected at a coastal site in southern California to assess variations in the oxidative pathways of SO2 during the COVID-19 shutdown. From March-May 2020, the decreases in local SO2 emissions led to reductions in aerosol SO42- concentrations. The SO42- concentration decreases were accompanied by an increase in Δ17O, indicating that natural sulfate sources have a higher proportion of oxidation by O3. These findings will improve estimates of anthropogenic versus natural SO42- sources in Southern California as well as modelling of SO42- atmospheric chemistry and transport.

Triple oxygen stable isotope insights into sulfate sources and production during the COVID-19 shutdown in Southern California

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