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 NOx, 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.
Keywords: Local formation; Long-term trends; Meteorological variability impact; Ozone pollution; Regional transport.
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