Journal
ENVIRONMENTAL SCIENCE & TECHNOLOGY
Volume 54, Issue 7, Pages 3881-3889Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.est.9b07248
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Funding
- National Natural Science Foundation of China [91544213, 91544226, 21625701]
- Hong Kong Research Grants Council [A-PolyU502/16, T24-504/17-N]
- Beijing Municipal Science and Technology Project [Z191100009119001]
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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 O-3 and OH productions are sufficiently high in winter to facilitate fast gas-phase and heterogeneous conversion of NOx to nitrate over the NCP. Increasing O-3 and OH productions from higher precursor levels and fast ROx cycling accelerate the nitrate generation during heavy pollution. We find that the 31.8% reduction of NOx 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 O-3 and OH (by similar to 30%), which has subsequently increased the conversion efficiency of NOx to HNO3 (by 38.7%). Future control strategies for the winter haze should also aim to lower photochemical oxidants, via larger and synchronized NOx and VOCs emissions reduction, to overcome the effects of nonlinear photochemistry and aerosol chemical feedback.
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