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. Nitrate is an increasingly important component of fine particulate matter (PM2.5) in Chinese cities. The production of nitrate is not onlyrelated to the abundance of its precursor, but it is also supported by the atmospheric photochemical oxidants, raising a new challenge for the currentemission control actions in China. This paper uses comprehensive measurements and a regional meteorology–chemistry model with optimized mechanismsto establish the nonlinear responses between particulate nitrate and the emission controls of nitrogen oxides (NOx) in the megalopolisesof China. Nitrate is an essential component of PM2.5 in eastern China, accounting for 9.4 %–15.5 % and 11.5 %–32.1 % ofthe PM2.5 mass for the warm and cold seasons. The hypothetical NOx emission reduction scenarios(−10 % to −80 %) during summer–autumn result in almost linearly lower PM2.5 by −2.2 % in Beijing–Tianjin–Hebei(BTH) and −2.9 % in Yangtze River Delta (YRD) per 10 % reduction of NOx emissions, whereas they lead to a rather complicatedresponse of PM components in winter. Wintertime nitrate is found to increase by +4.1 % in BTH and +5.1 % in YRD per 10 % reduction ofNOx emissions, with nearly unchanged nitric acid (HNO3) and higher dinitrogen pentoxide (N2O5) intermediateproducts produced from the increased atmospheric oxidant levels. An inflexion point appears at 30 %–50 % NOx emissionreduction, and a further reduction in NOx emissions is predicted to cause −10.5 % reduction of nitrate for BTH and −7.7 %for YRD per 10 % reduction of NOx emissions. In addition, the 2012–2016 NOx control strategy actually leads tono changes or even increases of nitrate in some areas (8.8 % in BTH and 14.4 % in YRD) during winter. Our results also emphasize thatammonia (NH3) and volatile organic compounds (VOCs) are effective in controlling nitrate pollution, whereas decreasing thesulfur dioxide (SO2) and NOx emissions may have counterintuitive effects on nitrate aerosols. This paper helpsunderstand the nonlinear aerosol and photochemistry feedbacks and defines the effectiveness of proposed mitigations for the increasingly seriousnitrate pollution in China.

Nonlinear responses of particulate nitrate to NO&lt;sub&gt;x&lt;/sub&gt; emission controls in the megalopolises of China

Nonlinear responses of particulate nitrate to NO<sub>x</sub> emission controls in the megalopolises of China