Surface ozone changes during the COVID-19 outbreak in China: An insight into the pollution characteristics and formation regimes of ozone in the cold season

The countrywide lockdown in China during the COVID-19 pandemic provided a natural experiment to study the characteristics of surface ozone (O-3). Based on statistical analysis of air quality across China before and during the lockdown, the tempo-spatial variations and site-specific formation regimes of wintertime O-3 were analyzed. The results showed that the O(3 )pollution with concentrations higher than air quality standards could occur widely in winter, which had been aggravated by the emission reduction during the lockdown. On the national scale of China, with the significant decrease (54.03%) in NO2 level from pre-lockdown to COVID-19 lockdown, the maximum daily 8-h average concentration of O-3 (MDA8h O-3) increased by 39.43% from 49.05 to 64.22 mu g/m(3). This increase was comprehensively contributed by attenuated NOx suppression and favorable meteorological changes on O-3 formation during the lockdown. As to the pollution states of different monitoring stations, surface O-3 responded oppositely to the consistent decreased NO2 across China. The O-3 levels were found to increase in the northern and central regions, but decrease in the southern region, where the changes in both meteorology (e.g. temperature drops) and precursors (reduced emissions) during the lockdown had diminished local O-3 production. The spatial differences in NOx levels generally dictate the site-specific O-3 formation regimes in winter, with NOx-titration/VOCs-sensitive regimes being dominant in northern and central China, while VOCs-sensitive/transition regimes being dominant in southern China. These findings highlight the influence of NOx saturation levels on winter O-3 formation and the necessity of VOCs emission reductions on O-3 pollution controls.