Nonlinear response of nitrate to NO(x) reduction in China during the COVID-19 pandemic

In recent years, nitrate plays an increasingly important role in haze pollution and strict emission control seems ineffective in reducing nitrate pollution in China. In this study, observations of gaseous and particulate pollutants during the COVID-19 lockdown, as well as numerical modelling were integrated to explore the underlying causes of the nonlinear response of nitrate mitigation to nitric oxides (NO(x)) reduction. We found that, due to less NO(x) titration effect and the transition of ozone (O(3)) formation regime caused by NO(x) emissions reduction, a significant increase of O(3) (by ∼ 69%) was observed during the lockdown period, leading to higher atmospheric oxidizing capacity and facilitating the conversion from NO(x) to oxidation products like nitric acid (HNO(3)). It is proven by the fact that 26–61% reduction of NO(x) emissions only lowered surface HNO(3) by 2–3% in Hebi and Nanjing, eastern China. In addition, ammonia concentration in Hebi and Nanjing increased by 10% and 40% during the lockdown, respectively. Model results suggested that the increasing ammonia can promote the gas-particle partition and thus enhance the nitrate formation by up to 20%. The enhanced atmospheric oxidizing capacity together with increasing ammonia availability jointly promotes the nitrate formation, thereby partly offsetting the drop of NO(x). This work sheds more lights on the side effects of a sharp NO(x) reduction and highlights the importance of a coordinated control strategy.