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

Abstract

Abstract. Nitrate is an increasingly important component of fine particulate matter (PM2.5) in Chinese cities. The production of nitrate is not only related to the abundance of its precursor, but also supported by atmospheric photochemical oxidants. The control of nitrogen oxides (NOx) emissions may thereby lead to nonlinear changes of nitrate concentrations, raising a new challenge to 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 NOx emission controls in the megalopolises of China. Nitrate is an essential component of PM2.5 in eastern China, accounting for 9.4–15.5 % and 11.5–32.1 % of the PM2.5 mass for the warm and cold seasons. The hypothetical NOx emission reduction scenarios (−10 %~−80 %) during summer-autumn result in almost linearly lower PM2.5 by −2.65 % in Beijing-Tianjin-Hebei (BTH) and −2.79 % in Yangtze River Delta (YRD) per 10 % cut of NOx emissions, whereas they increase the oxidant levels and lead to a rather complicated response of PM components in winter. Wintertime nitrate is found to increase by 4.28 % in BTH and 4.60 % in YRD, with higher dinitrogen pentoxide (N2O5) intermediate products produced from increased ozone introduced by lower NOx emissions. An inflexion point appears at 40–50 % NOx emission reduction, and a further cut in NOx emission is predicted to cause −8.74 % reduction of nitrate for BTH and −10.59 % for YRD per 10 % cut of NOx emissions. In addition, the 2012–2016 NOx control strategy actually leads to no change or even increase of nitrate in some areas (8.82 % in BTH and 14.41 % in YRD) during winter. 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.