Resolving Systematic Errors in Sulfate Source Apportionment: A Field-Validated Kinetic Isotope Fractionation Framework

Abstract. Sulfates represent a critical constituent of atmospheric fine particulate matter (PM_2.5), significantly influencing air quality and climate dynamics. Precise quantification of atmospheric sulfate formation mechanisms and emission sources through stable isotope fractionation analysis represents a critical advancement in particulate matter pollution control. Conventional isotopic models relying on idealized complete SO₂ oxidation scenarios, while providing preliminary source apportionment estimates, exhibit systematic errors in reaction pathway quantification. Our field-validated approach incorporating actual atmospheric oxidation processes demonstrates that transition-metal ions (TMI)-catalyzed and NO₂-mediated pathways dominate sulfate production, with coal combustion (overestimate by 10.8 % in summer) and traffic emissions (underestimated by 8.2 % in summer) constituting primary sources. Comparative analysis reveals that traditional complete-oxidation models disproportionately diminish TMI pathway contributions, highlighting the necessity of kinetic fractionation corrections. These findings establish an improved isotopic tracing framework that resolves longstanding calculation discrepancies, delivering essential constraints for atmospheric sulfur cycle modeling and emission regulation strategies.