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

Abstract

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-theart 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 NO X to nitrate over the NCP. Increasing O 3 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 O 3 and OH (by ∼30%), which has subsequently increased the conversion efficiency of NO X to HNO 3 (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.

Atmospheric Ammonia in Beijing during the COVID-19 Outbreak: Concentrations, Sources, and Implications

“…This was likely due to the enhanced oxidation of SO 2 and NO x , contributed by the increased photochemical oxidants, e.g., O 3 ( Figure S2 ) and also OH, which increase the efficiencies of formation of SO 4 2– and NO 3 – . 34 , 35 In addition, meteorological conditions strongly regulate near-surface air pollutant concentrations. 36 In this study, lower boundary layer heights ( Figure S4 ), higher temperatures, and comparable RHs ( Figure S5 ) were observed during the sampling period in 2020 compared to 2017.…”

Drivers for the poor air quality conditions in North China Plain during the COVID-19 outbreak

“…They determined that the NO x emission reduction might cause an increase of aerosol in the VOC s -limited zone due to the enhanced atmospheric oxidation capacity. Recent evidence from field observations ( Fu et al, 2020 ) also supported that the persistent heavy aerosol pollution in China over recent years was driven by the increased photochemical oxidants. Besides, the other causes for rapid haze increase include the stable boundary layer, weak ventilation and high humidity in winter that favored the accumulation and secondary formation of aerosol species ( Wang et al, 2015 ; Yang et al, 2015 ; Zhang et al, 2015 ).…”

“…The persistent particulate pollution in China is typically associated with emissions ( Geng et al, 2017 ; Wang et al, 2013 ), enhanced secondary production ( Fu et al, 2020 ; Huang et al, 2020 ) and unfavorable meteorology ( Ji et al, 2014 ; Jiang et al, 2015 ; Li et al, 2019 ; Yang et al, 2015 ; Zhang et al, 2015 ). Regional emission controls are effective in reducing the emissions and hence improving the air quality in several specific events, e.g., Beijing Olympic Games ( Wang et al, 2010 ) and Asia-Pacific Economic Cooperation Conference ( Wang et al, 2016 ).…”

Is reducing new particle formation a plausible solution to mitigate particulate air pollution in Beijing and other Chinese megacities?

“…As a result, SO2 concentrations in the surface air have decreased considerably during the recent years 40 . Though sulfate aerosol mass decreased, the observed reduction in the PM2.5 concentration was smaller 40 , partly due to compensating effects by more efficient nitrate formation 41 . Other contributors to PM2.5 (SOA, NH4 + and NO3 -) will likely be targeted in future emission reductions.…”