Heterogeneous sulfate aerosol formation mechanisms during wintertime Chinese haze events: air quality model assessment using observations of sulfate oxygen isotopes in Beijing

Air quality models have not been able to reproduce the magnitude of the observed concentrations of fine particulate matter (PM_2.5) during wintertime Chinese haze events. The discrepancy has been at least partly attributed to low biases in modeled sulfate production rates, due to the lack of heterogeneous sulfate production on aerosols in the models. In this study, we explicitly implement four heterogeneous sulfate formation mechanisms into a regional chemical transport model, in addition to gas-phase and in-cloud sulfate production. We compare the model results with observations of sulfate concentrations and oxygen isotopes, Δ¹⁷O(${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ ), in the winter of 2014–2015, the latter of which is highly sensitive to the relative importance of different sulfate production mechanisms. Model results suggest that heterogeneous sulfate production on aerosols accounts for about 20 % of sulfate production in clean and polluted conditions, partially reducing the modeled low bias in sulfate concentrations. Model sensitivity studies in comparison with the Δ¹⁷O(${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ ) observations suggest that heterogeneous sulfate formation is dominated by transition metal ion-catalyzed oxidation of SO₂.