Remarkable spring increase overwhelmed hard-earned autumn decrease in ozone pollution from 2005 to 2017 at a suburban site in Hong Kong, South China

Ozone (O₃) pollution has been a persistent problem in Hong Kong, particularly in autumn when severe O₃ pollution events are often observed. In this study, linear regression analyses of long-term O₃ data in suburban Hong Kong revealed that the variation of autumn O₃ obviously leveled off during 2005–2017, mainly due to the significant decrease of autumn O₃ in 2013–2017 (period II), despite the increase in 2005–2012 (period I). In addition, the rise of O₃ in summer and winter also ceased since 2013. In contrary, O₃ continuously increased throughout the spring of 2005–2017, especially in period II. Consequently, an incessant increase of overall O₃ was observed during 2005–2017. A statistical model combining Kolmogorov-Zurbenko filter with multiple linear regressions, and a photochemical box model incorporating CB05 mechanism were applied to probe the causes of the above trends. In general, O₃ production was controlled by VOC-limited regime throughout 13 years. The meteorological variability and regional transport facilitated the O₃ growth in period Ι. In contrast, the unchanged O₃ level in period II was attributable to the negative impact of meteorological variability and reduction of regional transport effect on O₃ formation and accumulation, as well as the negligible change in locally-produced O₃. In autumn of period II, the inhibitory meteorological variability, reduced regional transport, and alleviated local production were the driving force for the hard-earned decrease of O₃. However, the remarkable rise of spring O₃ was caused by the reduction of NO_x, especially in the spring of period II. The findings of the long-term and seasonal variations of O₃ pollution in Hong Kong are helpful for future O₃ mitigation.