Risk tradeoffs between nitrogen dioxide and ozone pollution during the COVID-19 lockdowns in the Greater Bay area of China

Photochemical regime for ozone (O₃) formation is complicated in the sense that reducing emission of nitrogen oxides (NO_x) may increase O₃ concentration. The lockdown due to COVID-19 pandemic affords a unique opportunity to use real observations to explore the O₃ formation regime and the effectiveness of NO_x emission control strategies. In this study, observations from ground networks during the lockdowns were used to assess spatial disparity of the Ratio of Ozone Formation (ROF) for nitrogen dioxide (NO₂) reduction in the Greater Bay Area (GBA) of China. The health risk model from Air Quality Health Index (AQHI) system in Hong Kong was adopted to evaluate the risk tradeoffs between NO₂ and O₃. Results show that the levels of O₃ increase and NO₂ reduction were comparable due to high ROF values in urban areas of central GBA. The ozone reactivity to NO₂ reduction gradually declined outwards from central GBA. Despite the O₃ increases, the NO_x emission controls reduced the Integrated Health Risk (IHR) of NO₂ and O₃ in most regions of the GBA. When risk coefficients from the AQHI in Canada or the global review were adopted in the risk analyses, the results are extremely encouraging because the controls of NO_x emission reduced the IHR of NO₂ and O₃ almost everywhere in the GBA. Our results underscore the importance of using a risk-based method to assess the effectiveness of emission control measures and the overall health benefit from NO_x emission controls in the GBA. © 2022 Turkish National Committee for Air Pollution Research and Control.