Trends in concentration of ground-level ozone and meteorological conditions during high ozone episodes in the Kao-Ping airshed, Taiwan

This work analyzes the variations in daily maximum 1-hr ozone (O-3) concentrations and the long-term trends in annual means of hourly ambient concentrations of O-3, nitrogen oxides (nitrous oxide + nitrogen dioxide), and nonmethane hydrocarbons in the three administrative regions of Kao-Ping airshed in southern Taiwan over a recent 8-yr period. The annual or monthly means of all maxima, most 95th percentiles, and some 90th percentiles of the daily maximum 1-hr O-3 concentrations exceed the daily limit of 120 parts per billion by volume in all three regions, namely, Kao-hsiung City, Kso-hsiung County, and P'ing-tung County. The monthly means of daily maximum 1-hr O-3 concentrations exhibit distinct seasonal variations, with a bimodal form with the maxima in autumn and late winter to the middle of spring and a minimum in summer. The long-term variations in the annual means of hourly O-3 concentrations in the three regions exhibit increasing trends. These increases in O-3 are associated with the decline in ambient concentrations of nitrogen oxides and, nonmethane hydrocarbons. High O-3 episodes occur most often in autumn and most rarely in summer. The seasonal mean mixing heights in descending order follow the order. of spring, summer, autumn, and winter. Meteorological parameters in autumn and winter indicate that the ground-level O-3 tends to accumulate and trigger a high O-3 episode on a warm day with sufficient sunlight and low wind in a high-pressure system, consistent with the low mixing heights in these two seasons.