Meteorological effects on the evolution of high ozone episodes in the greater seoul area

Abstract

Three high O-3 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 O-3 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 O-3 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 O-3 concentration to rise to a record high of 322 parts per billion by volume (ppbv). All three high O-3 episodes ended as a result of precipitation, and subsequent rises in O-3 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 O-3 concentrations, lasted only a few days.