An increasing trend in ground-level ozone ( ) concentrations has recently been recognized in Japan, although concentrations of ozone precursors, nitrogen oxides (NOx), volatile organic compounds (VOCs) and nonmethane hydrocarbons (NMHCs) have decreased. In this paper, the relationship between meteorological factors (temperature and wind speed) and ground-level ozone concentrations in the summer over the central Kanto area of Japan was examined using both statistical analyses and numerical models. The Fifth-Generation NCAR/Penn State Mesoscale Model (MM5) and the Community Multiscale Air Quality (CMAQ) model were employed in this study. It was found that there is a close relationship between meteorological conditions and ground-level ozone concentrations over the central Kanto area. In summer, up to 84% of the long-term variation in peak ozone concentrations may be accounted for by changes in the seasonally averaged daily maximum temperature and seasonally averaged wind speed, while about 70% of the recent short-term variation in peak ozone depends on the daily maximum temperature and the daily averaged wind speed. The results of numerical simulations also indicate that urban heat island (UHI) phenomena can play an important role in the formation of high ozone concentrations in this area. 1. Introduction A high concentration of ground level ozone has been recognized as a harmful pollutant for decades because it is the primary ingredient in photochemical smog and has detrimental effects on human health and the environment [1]. In the Kanto region, the most highly developed, urbanized, and industrialized part of Japan (see Figure 1), high ozone episodes frequently occur during the summer season. According to the Tokyo Metropolitan Government Environmental White Paper from 2006 (http://www.kankyo.metro.tokyo.jp/), the concentrations of most air pollutants are decreasing in the Tokyo Metropolitan area due to the application of exhaust control regulations to factories and industrial complexes and the introduction of regulations to control diesel emissions from automobiles. However, the concentration of photochemical oxidants has not been lowered to the Environmental Quality Standards in Japan (a one-hour value (i.e., averaged over one hour) of 60 parts per billion (ppb) or less), and the number of days on which high concentrations of ozone (one-hour ozone concentrations in excess of 120？ppb) are recorded has been increasing. Figure 2 shows the annually averaged values of ozone, NOx, and NMHCs from 1990 to 2005 in the Tokyo area. It is evident that both NOx
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