Abstract:
Restriction measures against coronavirus disease 2019 (COVID-19) caused atmospheric trace species to change, especially in relation to air pollution. This severe pollutant emission reduction phenomenon during the pandemic led to intensive studies on its behavior. Most studies evidence a decrease in all pollutants except for O
3. However, is this highlighted O
3 trend a global trend? This study summarized the research results in the past two years and explored the characteristics, mechanisms, and potential environmental effects of tropospheric O
3 and its precursors during the COVID-19 pandemic. During lockdown periods, global anthropogenic NO
x emissions decreased by at least 15%; especially, those in high-anthropogenic areas decreased by 18%–25%. In some highly polluted areas volatile organic compound (VOC)-sensitive areas, NO
x emissions on the ground decreased by more than 50%. NO
x reduction led to the weakened titration effect of NO on O
3, leading to an increase in O
3 in such highly polluted areas (10%–50%). However, O
3 in remote areas and free troposphere (NO
x-sensitive areas) decreased, attributed to NO
x reduction and regional transmission effect. During the strict control period of the pandemic, surface O
3 was still increasing in most cities in China with significantly decreased NO
x concentration, indicating that the effective way to control surface O
3 concentration in urban areas in China is controlling O
3 precursors based on the sensitive area of O
3 chemical generation. However, the drastic change in NO
x in each region could change the sensitive area of O
3 chemical generation, leading to a change in O
3 production efficiency. However, due to the lack of VOC emission measurement and their atmospheric concentration, there are still great uncertainties in the trend and main controlling factors of O
3 in each region. In the future, the characteristics of O
3 in different regions and corresponding O
3 regulation strategies influenced by COVID-19 and global warming are also worthy of further study.