Nitrate is an increasingly important component of fine particulate matter (PM_2.5) in Chinese cities. The production of nitrate is not only related to the abundance of its precursor, but it is also supported by the atmospheric photochemical oxidants, raising a new challenge for the current emission control actions in China. This paper uses comprehensive measurements and a regional meteorology–chemistry model with optimized mechanisms to establish the nonlinear responses between particulate nitrate and the emission controls of nitrogen oxides (NO_x) in the megalopolises of China. Nitrate is an essential component of PM_2.5 in eastern China, accounting for 9.4 %–15.5 % and 11.5 %–32.1 % of the PM_2.5 mass for the warm and cold seasons. The hypothetical NO_x emission reduction scenarios (−10 % to −80 %) during summer–autumn result in almost linearly lower PM_2.5 by −2.2 % in Beijing–Tianjin–Hebei (BTH) and −2.9 % in Yangtze River Delta (YRD) per 10 % reduction of NO_x emissions, whereas they lead to a rather complicated response of PM components in winter. Wintertime nitrate is found to increase by +4.1 % in BTH and +5.1 % in YRD per 10 % reduction of NO_x emissions, with nearly unchanged nitric acid (HNO₃) and higher dinitrogen pentoxide (N₂O₅) intermediate products produced from the increased atmospheric oxidant levels. An inflexion point appears at 30 %–50 % NO_x emission reduction, and a further reduction in NO_x emissions is predicted to cause −10.5 % reduction of nitrate for BTH and −7.7 % for YRD per 10 % reduction of NO_x emissions. In addition, the 2012–2016 NO_x control strategy actually leads to no changes or even increases of nitrate in some areas (8.8 % in BTH and 14.4 % in YRD) during winter. Our results also emphasize that ammonia (NH₃) and volatile organic compounds (VOCs) are effective in controlling nitrate pollution, whereas decreasing the sulfur dioxide (SO₂) and NO_x emissions may have counterintuitive effects on nitrate aerosols. This paper helps understand the nonlinear aerosol and photochemistry feedbacks and defines the effectiveness of proposed mitigations for the increasingly serious nitrate pollution in China.

Severe haze pollution frequently occurred in China during winter. Mechanisms for the formation and evolution of high PM_2.5 (particulate matter with aerodynamic diameter of 2.5 μm or less) episodes, however, remain poorly understood. We characterize two extreme haze episodes in the Yangtze River Delta region of China from 1 to 9 December (Episode I) and 19 to 24 December (Episode II) in 2016 using comprehensive measurements and model analyses. The aqueous sulfur dioxide (SO₂) oxidation catalyzed by mineral ions and the heterogeneous uptakes of SO₂, sulfuric acid (H₂SO₄), nitrogen dioxide (NO₂), nitrogen trioxide (NO₃), nitrogen pentoxide (N₂O₅), and nitric acid (HNO₃) on mineral aerosols are included in the model to better represent the formation of sulfate‐nitrate‐ammonium. The optimized mechanisms substantially improve the simulations of PM_2.5 composition, particularly for sulfate and nitrate. The two episodes show different synoptic conditions and evolution stages, with gradual PM_2.5 increase under stagnant weather conditions in Episode I (Stage I: Slow Increase Stage, Stage II: Rapid Formation Stage, and Stage III: Dissipation Stage) and with explosive PM_2.5 increase mostly associated with cross‐border transport from North China in Episode II (Stage I′: Clean Stage, Stage II′: Fast Transport Stage, and Stage III′: Clear Stage). The concentrations of sulfate‐nitrate‐ammonium increased evidently and became the key components of PM_2.5 during haze episodes. The heterogeneous conversion from SO₂ to sulfate on mineral aerosols is the main reason for sulfate increase, accounting for more than 50% of sulfate production. This study provides a better understanding of the causes for winter haze in China.

Ozone (O₃) is of great importance to air quality in the Yangtze River Delta (YRD) region, China. Focusing on surface O₃ in the YRD, its spatiotemporal characteristics, meteorological driving factors, and health impacts are investigated based on surface O₃ observations in this study. We find that from 2015 to 2019, most cities failed to meet the national standards and O₃ pollution mainly occurred in warm seasons (April‐September). Surface O₃ in the YRD increased in the recent years, so did tropospheric column O₃. Spatially, surface shows a significant positive autocorrelation over the YRD except for 2019, and high O₃ values mainly gather in the central of the YRD. The first two empirical orthogonal functions (EOFs) of O₃ accounted for 37.4% and 20.0% of the total variance in O₃, respectively, with their spatial patterns characterized by the same phase and west and east contrast, respectively. The EOF1 is related to the change in radiation and the EOF2 is attributed to regional transport by prevailing westerly wind. Downward UV radiation, temperature and u wind are crucial factors. The premature mortality caused by O₃ for respiratory disease in the YRD is estimated to be 5,889 cases per year. Furthermore, we find that premature mortality is more sensitive to O₃ concentration than population, suggesting that controlling peak O₃ concentration can bring great health benefits.

Agricultural fires could affect tropospheric photochemistry by emitting trace gases, interacting with solar radiation or providing reactive surfaces for heterogeneous reactions. This study examines the effects of a typical agricultural fire event (8–13 June 2012) on ozone photochemistry over the Yangtze River Delta region, East China. The online coupled Weather Research and Forecasting‐Chemistry model is used, with the inclusion of a detailed agricultural fire emission inventory and seven heterogeneous reactions on soot surfaces. Comparisons with satellite and ground observational data indicate that the model is capable of reproducing the transport and evolution of this crop fire event. The precursor emissions from agricultural fires play a major role in modifying ozone photochemistry, with a maximal increase in ozone mixing ratio reaching 20 ppb (40%) near the fire zones in northern Anhui. The radiation feedback generates a small increase of surface NO₂ levels by 2% and a decrease in surface O₃ levels by 1% in the smoke plume. The effects due to heterogeneous uptake on soot are rather small, on average, changing the average O₃, NO₂, ˙OH, and HO₂˙ concentrations by +0.8%, −0.5%, −0.7%, and +0.8%, respectively. This study highlights the complicated interactions between agricultural fire emissions and photochemistry.