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 in NOx emissions. This intriguing NOx-nitrate relationship may originate from non-linear nitrate-formation chemistry, but it is unclear which feedback mechanisms dominate in NCP. In this study, we re-interpret the wintertime observations of O-17 excess of nitrate ( increment O-17(NO3-)) in Beijing using the GEOS-Chem (GC) chemical transport model to estimate the importance of various nitrate-production pathways and how their contributions change with the intensity of haze events. We also analyze the relationships between other metrics of NOy chemistry and [PM2.5] in observations and model simulations. We find that the model on average has a negative bias of -0.9 parts per thousand and -36% for increment O-17(NO3-) and [O-x,O-major] (equivalent to [O-3] + [NO2] + [p-NO3-]), respectively, while overestimating the nitrogen oxidation ratio ([NO3-]/([NO3-] + [NO2])) by +0.12 in intense haze. The discrepancies become larger in more intense haze. We attribute the model biases to an overestimate of NO2-uptake on aerosols and an underestimate in wintertime O-3 concentrations. Our findings highlight a need to address uncertainties related to heterogeneous chemistry of NO2 in air-quality models. The combined assessment of observations and model results suggest that N2O5 uptake in aerosols and clouds is the dominant nitrate-production pathway in wintertime Beijing, but its rate is limited by ozone under high-NOx-high-PM2.5 conditions. Nitrate production rates may continue to increase as long as [O-3] increases despite reduction in [NOx], creating a negative feedback that reduces the effectiveness of air pollution mitigation. Plain Language Summary Nitrate, a major component of particles in urban air, has been identified as an important driver for recent trends in wintertime haze in the North China Plain. While it has long been known that many chemical reactions can convert gas-phase nitrogen oxides into particulate nitrate in the atmosphere, the contribution from different reactions in intense haze remains elusive. Recently, analysis of oxygen stable isotopes (O-16, O-17, and O-18) in nitrate has become a promising tool for understanding its chemical origins. In this study, we re-examine the isotopic observations of nitrate in wintertime Beijing and compare them with predictions made by an air-quality model. Our analysis of observations suggests that the model likely overestimates nitrate production via the reactions between nitrogen dioxide gas (NO2) and particles during intense haze events. After removing this nitrate formation pathway in the model, we demonstrate that nitrate production during intense haze events in Beijing is strongly modulated by ozone, a secondary pollutant whose formation is dependent on nitrogen oxides and volatile organic compounds (VOCs). Policies that result in a reduction of ozone concentrations, possibly through reductions in VOC emissions, will also reduce the formation of nitrate during wintertime haze events.

Automated summary

Studies show persistent particulate-nitrate pollution in the North China Plain despite rapid reduction in NOx emissions. Analysis suggests N2O5 uptake in aerosols and clouds as the dominant nitrate-production pathway in wintertime Beijing, limited by ozone under high-NOx-high-PM2.5 conditions. Policies reducing ozone concentrations may effectively decrease nitrate formation during wintertime haze events.