Influence of meteorological conditions on summer ozone levels in the central Kanto area of Japan

Abstract

We investigated the influence of meteorological factors that affect ozone in summer using both measurement analysis and numerical simulation. The results show that there is a close relationship between changes in meteorological conditions and variations in ozone concentrations over the central Kanto area. In summer, up to 84% of long-term variations in peak ozone concentrations may be accounted for by changes in the seasonally averaged daily maximum temperatures and seasonally averaged wind speeds. The ozone episodes in the Kanto region are dominated by three major patterns, of which Patterns I and II are regular summertime pressure patterns with a 26% and 16% frequency of occurrence, respectively. A detailed process analysis of ozone formation under urban heat island (UHI) at two areas in the Kanto region – urban and rural area – indicates that ozone formation is mainly controlled by chemistry, dry deposition, vertical transport, and horizontal transport processes. The groundlevel ozone concentrations are enhanced mainly by the vertical mixing of ozone-rich air from aloft, whereas dry deposition process mainly depletes ozone. Horizontal transport and chemistry processes play opposite roles in the net change of ozone concentration between the two areas. The results of numerical simulations also indicate that the sea breeze has significant effects on the ozone accumulation and distribution in the Kanto area. The high ozone was first observed in urban area and then was transported to the rural area by sea breeze. At rural area, the highest ozone concentrations were found in late afternoon, about two hours later in comparison with the urban area.