Meteorological Effects on the Evolution of High Ozone Episodes in the Greater Seoul Area

ABSTRACT

Three high O₃ episodes—7 days in 1992 (July 3–July 9), 9 days in 1994 (July 21–July 29), and another 3 days in 1994 (August 22–August 24)—were selected on the basis of morning (7:00 a.m.–10:00 a.m.) average wind direction and speed and daily maximum O₃ concentrations in the greater Seoul, Korea, of 1990–1997. To better understand their characteristics and life cycles, surface data from the Seoul Weather Station (SWS) and surface and 850-hPa wind field data covering northeast Asia around the Korean Peninsula were used for the analysis. In the July 1992 episode, westerly winds were most frequent as a result of the influence of a high-pressure system west of the Korean Peninsula behind a trough. In contrast, in the July 1994 episode, easterly winds were most frequent as a result of the effect of a typhoon moving north from the south of Japan. Despite different prevailing wind directions, the peak O₃ concentrations for each episode occurred when a sea/land breeze developed in association with weak synoptic forcing. The August 1994 episode, which was selected as being representative of calm conditions, was another typical example in which a well-developed sea/land breeze helped cause the peak O₃ concentration to rise to a record high of 322 parts per billion by volume (ppbv). All three high O₃ episodes ended as a result of precipitation, and subsequent rises in O₃ concentrations were also suppressed by a series of precipitation events afterwards. In particular, two heavy rainfalls were the main reason why the August 1994 episode, which had the highest and second-highest O₃ concentrations, lasted only a few days.