Reconciling Modeled and Observed Δ17O(NO3−) in Beijing Winter Haze With Heterogeneous Chlorine Chemistry

Abstract

AbstractThe fine particulate matter (PM2.5) in Chinese megacities has declined significantly after the implementation of strict mitigation strategies since 2013. However, the concentration of wintertime nitrate in PM2.5 (p‐NO3‐) has only changed slightly and became the dominant inorganic component despite considerable precursor emission reductions. Discerning chemical mechanisms leading to nitrate growth during haze events is critical to implementing effective pollution mitigation policies. The oxygen isotope anomaly of nitrate (Δ17O(NO3−)) is a powerful means to distinguish nitrate formation mechanisms. Nevertheless, the observed high Δ17O(NO3−) values were significantly underestimated by chemical transport models during extreme haze events (PM2.5 > 225 μg m−3) in Beijing, indicating an incomplete understanding of nitrate chemistry. To reconcile this model‐observation discrepancy, we compiled reported Δ17O(NO3−) data in Beijing haze along with relevant observational parameters (e.g., hydroxyl (OH) reactivity, peroxyl radical concentrations), then tested assumptions on Δ17O of key precursors (e.g., OH and nitrogen dioxide (NO2)), recalculated Δ17O(NO3−) and compared them with observations. Our results indicate that considering heterogeneous dinitrogen pentoxide reactions on chlorine‐containing aerosols (N2O5 + Cl− chemistry) with a nitryl chloride (ClNO2) yield of ∼0.75 can explain the observed high Δ17O(NO3−) during extreme haze events. According to the Δ17O(NO3−) data, on average this heterogeneous N2O5 + Cl− chemistry can explain ∼60% of nighttime nitrate production and make daytime and nocturnal pathways equally important in winter Beijing haze (PM2.5 > 75 μg m−3). Our results highlight the critical role of reactive chlorine chemistry in air pollution/chemistry in inland cities.