Correlation between air temperature and surface ozone in their extreme ranges in the greater Tokyo region

Abstract High-level ozone (O3) events observed around major urban regions in the middle latitudes are correlated with high temperatures (T-O3 correlation). Therefore, the effects of global warming on the future O3 levels are a matter of concern. The T-O3 correlation is caused by various physicochemical and meteorological processes, the importance of which can differ by region. This statistical analysis focused on the correlation in the extremely high ranges, because the lower ranges would only act as noise in elucidating the conditions at which high temperatures and high levels of O3 occur. This methodology was applied to the greater Tokyo region after 2001, where severe O3 events frequently occurred when the sea breeze system developed in summer. To select sample days for the analysis, this study set up twofold filtering: (1) a large threshold for midday sunshine duration and (2) a typical variation pattern to roughly judge sea breeze days, mostly essential weather pattern for high-level O3 events in the region. The most notable result was a decrease in O3 corresponding to the reduction in non-methane hydrocarbons (NMHC) from Period I (2001–2007) to Period III (2017–2019). As the NMHC rank reduced, the linear regression line for the T-O3 correlation shifted downward, but its slope (ppb/°C) remained around 10, except that temporary spikes in O3 levels and temperatures occurred at moderate NMHC levels. From an urban meteorological perspective, the wind speed in the mature stage of the sea breeze is the major factor behind the T-O3 correlation