Increase in daytime ozone exposure due to nighttime accumulation in a typical city in eastern China during 2014–2020

Tropospheric ozone (O3) pollution, as one of the major environmental problems, poses a significant threat to human health and ecosystems. In the past, the study of O3 risk has only focused on the daytime, neglecting to analyze the role of nighttime concentrations, which may have led to underestimation of O3 exposure hazards. In this study, we investigated the long-term variation of O3 during April to September from 2014 to 2020 in Rizhao, an industrial city on the east coast of China. We also calculated its exposure effects on human health and vegetation with variety of metrics especially both daytime and nighttime indexes. The results showed that various indexes of O3 in Rizhao were on the rise during 2014–2020, and the growth rate of O3 at nighttime was greater than that during the daytime. This increase in concentration at nighttime have weakened the benefits of the reduction in O3 formation from the precursor emissions control, which contributed more than 94% to the risk of O3 exposure to human health. Short-term field observations showed that O3 in Rizhao is still sensitive to volatile organic compounds (VOCs). This suggests that emission reduction measures for Rizhao should consider the emissions ratio of NOx and VOCs instead of NOx only, as well as take into account the emissions of precursors at nighttime to reduce the nighttime O3 level. Such an approach has the potential to reduce O3 pollution and its harm to human health and vegetation effectively.