Progress of Research on Global Tropospheric Ozone Variation Characteristics during COVID-19 Pandemic

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 O₃. However, is this highlighted O₃ 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 O₃ 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 O₃, leading to an increase in O₃ in such highly polluted areas (10%–50%). However, O₃ 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 O₃ was still increasing in most cities in China with significantly decreased NO_x concentration, indicating that the effective way to control surface O₃ concentration in urban areas in China is controlling O₃ precursors based on the sensitive area of O₃ chemical generation. However, the drastic change in NO_x in each region could change the sensitive area of O₃ chemical generation, leading to a change in O₃ 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 O₃ in each region. In the future, the characteristics of O₃ in different regions and corresponding O₃ regulation strategies influenced by COVID-19 and global warming are also worthy of further study.