Isotopic Examination of Variations in Sulfur(IV) Oxidation Pathways during the COVID Shutdown

Sulfate, SO42–, is an oxidation product of the atmospheric pollutant SO2 and an important component of atmospheric aerosols that impacts air quality, human health, and Earth’s radiative balance. The three stable isotope system of oxygen (16O, 17O, 18O) is an excellent tracer to distinguish SO42– formation pathways because SO2 oxidation by H2O2, and especially O3, results in a mass-independent 17O excess for the product SO42–, termed Δ17O(SO42–). Herein, we report on unusually high Δ17O(SO42–) values in excess of 2‰ observed in San Diego, California, between mid-March and mid-April 2020 during the COVID-19 emergency shutdown. To assess the relationship between Δ17O(SO42–) and reduced emissions from anthropogenic activities, we employed the Community Multiscale Air Quality (CMAQ) modeling system to simulate Δ17O(SO42–) under different emissions scenarios. This dual observational-modeling approach indicated that the elevated Δ17O(SO42–) values were primarily attributable to changes in local meteorology and aerosol chemistry rather than emission reductions resulting from the COVID shutdown. An evaluation of the environmental factors driving these local-scale changes revealed that aerosol acidity was the most important contributor. The implications of these findings for accurately forecasting Δ17O(SO42–) in chemical transport models are discussed alongside approaches to improve agreement between observed and modeled S(IV) oxidation chemistry. Overall, the large variations in Δ17O(SO42–) observed in this work sharpen our ability to attribute isotopic fractionations and compositions to specific atmospheric reaction mechanisms.