Substantial nitrogen oxides emission reduction from China due to COVID-19 and its impact on surface ozone and aerosol pollution.

A top-down approach was employed to estimate the influence of lockdown measures implemented during the COVID-19 pandemic on NO emissions and subsequent influence on surface PM and ozone in China. The nation-wide NO emission reduction of 53.4% due to the lockdown in 2020 quarter one in China may represent the current upper limit of China's NO emission control. During the Chinese New Year Holiday (P2), NO emission intensity in China declined by 44.7% compared to the preceding 3 weeks (P1). NO emission intensity increased by 20.3% during the 4 weeks after P2 (P3), despite the unchanged NO column. It recovered to 2019 level at the end of March (P4). The East China (22°N - 42°N, 102°E - 122°E) received greater influence from COVID-19. Overall NO emission from East China for 2020 first quarter is 40.5% lower than 2019, and in P4 it is still 22.9% below the same period in 2019. The 40.5% decrease of NO emission in 2020 first quarter in East China lead to 36.5% increase of surface O and 12.5% decrease of surface PM. The elevated O promotes the secondary aerosol formation through heterogeneous pathways. We recommend that the complicated interaction between PM and O should be considered in the emission control strategy making process in the future.