es9b07248_si_001.pdf (1.36 MB)
Persistent Heavy Winter Nitrate Pollution Driven by Increased Photochemical Oxidants in Northern China
journal contribution
posted on 2020-03-13, 19:34 authored by Xiao Fu, Tao Wang, Jian Gao, Peng Wang, Yiming Liu, Shuxiao Wang, Bin Zhao, Likun XueNitrate
is an increasingly important component of fine particulate
matter (PM2.5) 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 NOX to nitrate
over the NCP. Increasing O3 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 O3 and OH (by ∼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.