Ozone trends in Hong Kong were assessed from the ozone data recorded at three urban stations and one station in the new development area. Three parameters were selected as the robust trend indicators in the study: monthly mean, monthly averaged daily 1-h maximum, and number of annual hours >120μg m~-3. As the ozone data displayed obvious seasonal variation, using deseasonalized monthly average parameters to estimate the ozone trends could smooth out the influence of seasonal fluctuations. The detection of an increase of more than 10/100 per year in ozone concentration at one station was particulary alarming. Effects of meteorological factors, control programmes of ozone precursors, and long-range transport on the ambient ozone level were examined in the study.
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