MECHANISM OF SPRING HIGH OXIDANT EPISODE - A METEOROLOGICAL ANALYSIS IN AND AROUND THE HOKURIKU DISTRICT, JAPAN

In order to clarify the mechanism of spring surface level high oxidant episodes, both upper and surface level ozone observations were carried out from April to June 1983. The results were analyzed using aerological data and AMeDAS (Automated Meteorological Data Acquisition System) data. During the observation period, there were 11 days with high oxidant episodes which were defined as when the recorded ozone concentration exceeded 100 ppb at any of five stations. Meteorological conditions under which the surface episodes occurred were examined for three cases when upper level ozone distributions were observed, and the mechanism of the episode were investigated. In this paper, the analysis for one case is explained in detail. For this case: (i) the peak time of oxidant (ozone) concentration at each surface station was controlled by the transition pattern of photochemical oxidant under the local wind system and (ii) the stratospheric air with high concentrations of ozone intruded into the troposphere through the tropopause gap (tropopause folding) behind the cyclone and continued to descend gradually. In the lower troposphere, it descended further due to the influence of topography, and contributed to high surface oxidant episodes. The descent of stratospheric ozone was related to the position of both cyclones and anticyclones. The other two cases had similar upper meteorological conditions but different surface wind systems such as land and sea breeze. © 1990.