Isotopic Evidence Reveals the Nonlinear Mitigation Efficacy of Coal-derived Sulfate: Enhanced Role of TMI-Catalyzed Oxidation

Abstract Despite stringent SO2 emission reductions, particulate sulfate (SO42–) concentrations often exhibit a nonlinear response. To reconcile this discrepancy, this study utilized synchronous sulfur isotope (δ34S) observations of precursor SO2 and product sulfate in Nanjing across a decade (winter 2015 vs 2024). Sulfate reductions are observed to lag behind SO2, declining together with elevated δ34S values in ambient sulfate (5.7 ± 1.1‰ vs 6.2 ± 1.3‰) and significantly in SO2 (1.4 ± 1.2‰ vs 6.5 ± 1.6‰), which indicates a shift in emission structure and sulfate formation chemistry. Unexpectedly, while SO2 source apportionment demonstrates a significant decline in contribution of coal combustion to regional SO2 emissions, coal combustion remains the predominant contributor to secondary sulfate, maintaining a stable relative share. This discrepancy implies enhanced conversion efficiency for coal-emitted SO2. We attribute this to the increased contribution of co-emitted transition metal ion (TMI)-catalyzed oxidation (rising from 35.6% to 59.0%), with kinetic calculations underscoring the dominance of the Mn-surface catalysis pathway. Ultimately, these findings suggest that intensified TMI-catalyzed oxidation sustains high sulfate production from coal-emitted SO2 even under low-SO2 conditions, providing a mechanistic explanation for the nonlinear response. These results also highlight the necessity for synergistic co-control strategies within coal-dependent industries.