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<span classCombining double low line"inline-formula">2.5</span>) 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, <span classCombining double low line"inline-formula">"17O</span>(<span classCombining double low line"inline-formula"><math xmlnsCombining double low line"http://www.w3.org/1998/Math/MathML" idCombining double low line"M3" displayCombining double low line"inline" overflowCombining double low line"scroll" dspmathCombining double low line"mathml"><mrow classCombining double low line"chem"><msubsup><mi mathvariantCombining double low line"normal">SO</mi><mn mathvariantCombining double low line"normal">4</mn><mrow><mn mathvariantCombining double low line"normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svgCombining double low line"http://www.w3.org/2000/svg" widthCombining double low line"29pt" heightCombining double low line"17pt" classCombining double low line"svg-formula" dspmathCombining double low line"mathimg" md5hashCombining double low line"6060a0eb6022af681aa55d19b3180df9"><svg:image xmlns:xlinkCombining double low line"http://www.w3.org/1999/xlink" xlink:hrefCombining double low line"acp-19-6107-2019-ie00001.svg" widthCombining double low line"29pt" heightCombining double low line"17pt" srcCombining double low line"acp-19-6107-2019-ie00001.png"/></svg:svg></span></span>), 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&thinsp;% of sulfate production in clean and polluted conditions, partially reducing the modeled low bias in sulfate concentrations. Model sensitivity studies in comparison with the <span classCombining double low line"inline-formula">"17O</span>(<span classCombining double low line"inline-formula"><math xmlnsCombining double low line"http://www.w3.org/1998/Math/MathML" idCombining double low line"M5" displayCombining double low line"inline" overflowCombining double low line"scroll" dspmathCombining double low line"mathml"><mrow classCombining double low line"chem"><msubsup><mi mathvariantCombining double low line"normal">SO</mi><mn mathvariantCombining double low line"normal">4</mn><mrow><mn mathvariantCombining double low line"normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svgCombining double low line"http://www.w3.org/2000/svg" widthCombining double low line"29pt" heightCombining double low line"17pt" classCombining double low line"svg-formula" dspmathCombining double low line"mathimg" md5hashCombining double low line"28cd4f8c12cf9ef751545712573a522a"><svg:image xmlns:xlinkCombining double low line"http://www.w3.org/1999/xlink" xlink:hrefCombining double low line"acp-19-6107-2019-ie00002.svg" widthCombining double low line"29pt" heightCombining double low line"17pt" srcCombining double low line"acp-19-6107-2019-ie00002.png"/></svg:svg></span></span>) observations suggest that heterogeneous sulfate formation is dominated by transition metal ion-catalyzed oxidation of <span classCombining double low line"inline-formula".