Persistent Heavy Winter Nitrate Pollution Driven by Increased Photochemical Oxidants in Northern China

Nitrate is an increasingly important component of fine particulate matter (PM_2.5) during winter in northern China. Past emission control has been ineffective in reducing winter nitrate. Here, we use extensive observations and a model with state-of-the-art nitrogen chemistry to identify the key factors that control the nitrate formation in the heavily polluted North China Plain (NCP). In contrast to the previous view of weak winter photochemistry, we show that the O₃ and OH productions are sufficiently high in winter to facilitate fast gas-phase and heterogeneous conversion of NO_X to nitrate over the NCP. Increasing O₃ and OH productions from higher precursor levels and fast RO_X cycling accelerate the nitrate generation during heavy pollution. We find that the 31.8% reduction of NO_X emissions from 2010 to 2017 in the NCP lowers surface nitrate by only 0.2% and even increases nitrate in some polluted areas. This is mainly due to the increase of O₃ and OH (by ∼30%), which has subsequently increased the conversion efficiency of NO_X to HNO₃ (by 38.7%). Future control strategies for the winter haze should also aim to lower photochemical oxidants, via larger and synchronized NO_X and VOCs emissions reduction, to overcome the effects of nonlinear photochemistry and aerosol chemical feedback.