Satellite images reveal that during the COVID-19 shutdown in California there was a reduction of local emissions of SO2, allowing for observation of atmospheric processes with minimal local anthropogenic contribution. Anthropogenic sulfur dioxide (SO2) emissions lead to the formation of atmospheric aerosol sulfate (SO42−), directly influencing air quality, climate, and radiative forcing. The production of aerosol sulfate in the atmosphere is a natural process that occurs via the chemical oxidation of SO2 from OH radicals in the homogenous gas phase or H2O2/O3 in the aerosol aqueous phase, predominantly ozone. Most atmospheric processes obey a mass-dependent isotopic fractionation effect—as δ17O and δ18O vary in proportion relative to their reduced mass—but reaction of SO2 with H2O2, and especially O3, introduces a mass independent 17O excess in the product SO42−. This mass independent anomaly is denoted by Δ17O, where Δ17O = δ17O − 0.52 × δ18O, and makes Δ17O values obtained from sulfate an ideal tracer to study the major oxidative pathways of sulfate formation in the stratosphere and free troposphere (FT) due to isotopic constraints of the oxidative species. 35S is a cosmogenic radioisotope that is generated through cosmic ray spallation in the stratosphere and has been shown to be a reliable tracer to identify stratospheric intrusions and stratosphere-troposphere exchange (STE) events within the planetary boundary layer (PBL) due to rapid oxidation to SO2. Therefore, analysis of 35S in conjunction with stable oxygen isotopic measurements have the potential to study variations in the oxidative formation pathways of sulfate in the atmosphere. In this study, atmospheric samples were collected at a coastal site in southern California and in situ measurements of 35S using low-level liquid scintillation counting and stable triple oxygen isotope analysis using stable isotope ratio mass spectrometry were employed to observe any variations in the various oxidative pathways of SO2 during the COVID-19 shutdown.

Investigation of atmospheric sulfate aerosol formation pathways using cosmogenic 35S and stable triple oxygen isotopes during the COVID-19 shutdown in California

ACS Spring Meeting 2021

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