Enhanced atmospheric oxidation capacity and associated ozone increases during COVID-19 lockdown in the Yangtze River Delta

Highlights: • Enhanced atmospheric oxidation capacity is observed in the Yangtze River Delta during COVID-19 lockdown. • NO_x reduction is the reason for increases of oxidants (OH, HO₂, and NO₃). • O₃ is increased in the central Yangtze River Delta, corresponding to atmospheric oxidation capacity enhancement. Aggressive air pollution control in China since 2013 has achieved sharp decreases in fine particulate matter (PM_2.5), along with increased ozone (O₃) concentrations. Due to the pandemic of coronavirus disease 2019 (COVID-19), China imposed nationwide restriction, leading to large reductions in economic activities and associated emissions. In particular, large decreases were found in nitrogen oxides (NO_x) emissions (>50%) from transportation. However, O₃ increased in the Yangtze River Delta (YRD), which cannot be fully explained by changes in NO_x and volatile organic compound (VOCs) emissions. In this study, the Community Multi-scale Air Quality model was used to investigate O₃ increase in the YRD. Our results show a significant increase of atmospheric oxidation capacity (AOC) indicated by enhanced oxidants levels (up to +25%) especially in southern Jiangsu, Shanghai and northern Zhejiang, inducing the elevated O₃ during lockdown. Moreover, net P(HO_x) of 0.4 to 1.6 ppb h⁻¹ during lockdown (Case 2) was larger than the case without lockdown (Case 1), mainly resulting in the enhanced AOC and higher O₃ production rate (+12%). This comprehensive analysis improves our understanding on AOC and associated O₃ formation, which helps to design effective strategies to control O₃.