Isotopic Constraints on SO2 Oxidation Rates and Their Potential Relationship with Sulfate Formation Pathways in the Planetary Boundary Layer

Natural and anthropogenic emissions of sulfur-bearing species significantly alter the sulfur and energy budgets of the Earth’s atmosphere. Simulations of the atmospheric sulfur cycle, sulfate radiative forcing, and predictions of their future changes require a precise understanding of the SO₂ oxidation rates that control the formation of secondary sulfate aerosols. Given the unique single source of radiosulfur (cosmogenic ³⁵S radionuclide), combined measurements of atmospheric radiosulfur in both sulfur dioxide (³⁵SO₂) and sulfate (³⁵SO₄^2–) have been employed to constrain sulfur oxidation rates in the atmosphere. This approach employed box model calculations, incorporating several key assumed parameters, including sulfur deposition rates. However, previous calculations did not fully consider uncertainties in parametrizations, necessitating a re-examination of the estimated values. In this study, we applied a new approach to revisit existing combined measurements of ³⁵SO₂ and ³⁵SO₄^2– at coastal and inland sites. We estimated the temporospatial variability in SO₂ oxidation rates by incorporating a comprehensive consideration of parametrization uncertainties. We adopted deposition data from nine models of the Atmospheric Chemistry and Climate Model Intercomparison Project. Uncertainties in deposition data and other key parameters, such as cosmogenic ³⁵S production rates and ³⁵SO₂/³⁵SO₄^2– ratios in the free troposphere, were evaluated by using a Monte Carlo approach. Our new analysis reveals higher SO₂ oxidation rates than previously estimated, consistent with recent multiphase kinetics studies. Additionally, the potential relationship between changes in SO₂ oxidation rates and sulfate formation pathways was elucidated by comparing these results to sulfate oxygen-17 anomalies. Our approach and findings offer a stringent assessment of how various sulfate formation pathways contribute to the overall SO₂ oxidation rate in the planetary boundary layer and are therefore useful for evaluating the impacts of the atmospheric sulfur cycle on environmental health, public health, and climate.