The emergence of urban ozone episodes in autumn and air temperature rise in Hong Kong

In recent years urban photochemical episodes accompanied by unusually high ozone concentrations have been observed in the urban area of Hong Kong every autumn season except 2003. These episodes always occurred in summer time since 1990, but starting in 2000 additional autumn episodes could be observed. During those episodes the ozone concentrations exceeded the Hong Kong Air Quality Objective and reached an hourly maximum of 332 µg m−3 in September 2005. The maximum ozone concentrations of both summer and autumn episodes seem to be on an upward trend. It is the objective of this paper to examine a possible role of the air temperature factor in the emergence of ozone episodes in autumn. Significant correlations could be found between the maximum hourly ozone concentrations and the maximum and mean air temperatures during autumn months whereas the association between ozone and particulates is weaker. Autumn air temperature data from the Hong Kong Observatory headquarters and two ozone monitoring sites had been compared for the period from 1989 to 2007. This analysis takes into account the ozone trend and the unfavorable conditions posed by decreasing solar radiation and increasing cloud cover. Mean temperatures for October and November at the Hong Kong Observatory headquarters as well as two ozone monitoring sites (Sha Tin and Tai Po) show a slightly upward trend for the period under investigation. The September values, however, show a slight negative slope. The overall upward trend for air temperatures in autumn occurs at a time when the mean air temperatures in Hong Kong have been on the rise for over 120 years from 1885 to 2007, amidst the global warming trend. It is believed that the air temperature increase may have contributed to the emergence of urban ozone episodes in autumn in Hong Kong in 2000. Related to this influence of air temperature on ozone are high background pollutant concentrations in Hong Kong, as indicated by the minimum PM10 concentrations and the serious NO2 pollution dominating the Pearl River Delta region.