Heterogeneous Nitrate Production Mechanisms in Intense Haze Events in the North China Plain

Studies of wintertime air quality in the North China Plain (NCP) show that particulate-nitrate pollution persists despite rapid reduction NOx emissions. This intriguing NOx-nitrate relationship may originate from non-linear nitrate-formation chemistry, but it is unclear which feedback mechanisms dominate NCP. In this study, we re-interpret observations 17O excess nitrate (∆17O(NO3−)) Beijing using GEOS-Chem (GC) chemical transport model to estimate importance various nitrate-production pathways and how their contributions change with intensity haze events. We also analyze relationships between other metrics NOy chemistry [PM2.5] simulations. find on average has a negative bias −0.9‰ −36% for ∆17O(NO3−) [Ox,major] (≡ [O3] + [NO2] [p-NO3−]), respectively, while overestimating nitrogen oxidation ratio ([NO3−]/([NO3−] [NO2])) by +0.12 intense haze. The discrepancies become larger more attribute biases an overestimate NO2-uptake aerosols underestimate O3 concentrations. Our findings highlight need address uncertainties related heterogeneous NO2 air-quality models. combined assessment results suggest N2O5 uptake clouds dominant pathway Beijing, its rate limited ozone under high-NOx-high-PM2.5 conditions. Nitrate production rates continue increase as long increases [NOx], creating reduces effectiveness mitigation.