Elucidating ozone and PM_2.5 pollution in the Fenwei Plain reveals the co-benefits of controlling precursor gas emissions in winter haze

Abstract

The Fenwei Plain, home to 50 million people in central China, is one of the most polluted regions in China. In 2018, the Fenwei Plain was designated as one of the three key regions for the “Blue Sky Protection Campaign”, along with the Beijing–Tianjin–Hebei (BTH) and Yangtze River Delta (YRD) regions. However, compared to BTH and YRD, our understanding of the current status of air pollution in the Fenwei Plain is limited partly due to a lack of detailed analysis of the transformation from precursor gases to secondary products including secondary organic aerosol (SOA) and ozone. Through the analysis of 7 years (2015–2021) of surface monitoring of the air pollutants in Xi'an, the largest city in the Fenwei Plain, we show that roughly two-thirds of the days exceeded either the PM2.5 or the O3 level-1 air quality standard, highlighting the severity of air pollution. Moreover, an increase in O3 pollution in the winter haze was also revealed, due to the constantly elevated reactive oxygenated volatile organic compounds (OVOCs), in particular formaldehyde, with an ozone formation potential of over 50 µg m−3, in combination with the reduced NO2. The abrupt decrease of NO2, as observed during the lockdown in 2020, provided real-world evidence of the control measures, targeting only NOx (70 % decrease on average), and were insufficient to reduce ozone pollution because reactive OVOCs remained constantly high in a volatile organic compound (VOC)-limited regime. Model simulation results showed that with NO2 reduction from 20 %–70 %, the self-reaction rate between peroxy radicals, a pathway for SOA formation, was intensified by up to 75 %, while the self-reaction rate was only reduced with a further reduction of VOCs of > 50 %. Therefore, a synergic reduction in PM2.5 and O3 pollution can only be achieved through a more aggressive reduction of their precursor gases. This study elucidates the status of ozone and PM2.5 pollution in one of the most polluted regions in China, revealing a general trend of increasing secondary pollution, i.e., ozone and SOA in winter haze. Controlling precursor gas emissions is anticipated to curb both ozone and SOA formation, which will benefit not just the Fenwei Plain but also other regions in China.