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

<div>Tropospheric ozone (O₃) pollution, as one of the major environmental problems, poses a significant threat to</div> <div>human health and ecosystems. In the past, the study of O₃ risk has only focused on the daytime, neglecting to</div> <div>analyze the role of nighttime concentrations, which may have led to underestimation of O₃ exposure hazards. In</div> <div>this study, we investigated the long-term variation of O₃ during April to September from 2014 to 2020 in Rizhao,</div> <div>an industrial city on the east coast of China. We also calculated its exposure effects on human health and</div> <div>vegetation with variety of metrics especially both daytime and nighttime indexes. The results showed that</div> <div>various indexes of O₃ in Rizhao were on the rise during 2014&#8211;2020, and the growth rate of O₃ at nighttime was</div> <div>greater than that during the daytime. This increase in concentration at nighttime have weakened the benefits of</div> <div>the reduction in O₃ formation from the precursor emissions control, which contributed more than 94% to the risk</div> <div>of O₃ exposure to human health. Short-term field observations showed that O₃ in Rizhao is still sensitive to</div> <div>volatile organic compounds (VOCs). This suggests that emission reduction measures for Rizhao should consider</div> <div>the emissions ratio of NO<em>x </em>and VOCs instead of NO<em>x </em>only, as well as take into account the emissions of precursors</div> <div>at nighttime to reduce the nighttime O₃ level. Such an approach has the potential to reduce O₃ pollution and its</div> <div>harm to human health and vegetation effectively.</div>