Resolving systematic errors in sulfate source apportionment: a field-validated kinetic isotope fractionation framework

Abstract

Abstract. Sulfates are critical constituents of atmospheric fine particulate matter (PM2.5) that significantly influence air quality and climate dynamics. While stable isotope fractionation analysis is a powerful tool for tracing atmospheric sulfate formation mechanisms, conventional isotopic models rely on idealized complete SO2 oxidation scenarios. This oversimplification introduces systematic errors and neglects vital kinetic isotope effects generated during incomplete SO2 processing. To address this knowledge gap, we established a field-validated analytical framework combining seasonal PM2.5 observations in Nanjing, China, with Bayesian isotope mixing and process-specific Rayleigh fractionation modeling. Our kinetic fractionation-corrected approach accounts for actual atmospheric oxidation processes, revealing that transition-metal ion (TMI)-catalyzed and NO2-mediated pathways dominate secondary sulfate production. Conversely, comparative analysis demonstrates that traditional complete-oxidation models disproportionately diminish TMI pathway contributions. Furthermore, implementing kinetic fractionation corrections successfully resolves systematic biases in source apportionment. We demonstrate that traditional models misrepresent source contributions, overestimating coal combustion by 10 % and underestimating traffic emissions by 8 % during summer photochemical episodes. These findings establish a refined isotopic tracing framework that resolves long-standing calculation discrepancies. Ultimately, this framework delivers essential constraints for atmospheric sulfur cycle modeling and underscores the necessity for multi-pollutant regulation strategies targeting vehicular emissions and co-emitted transition meals.