The trend for ground-level ozone concentrations to increase has recently been recognized in Japan, although concentrations of ozone precursors, nitrogen oxides and volatile organic compounds or non-methane hydrocarbons have decreased. One of the causes is thought to be an increase in long-range transported ozone and its precursors from East Asia, particularly China. However, ground-level ozone levels in the vicinity of major cities have also been rising during summer, despite the relatively clean air mass mainly transported from the Pacific Ocean during this season. It is well documented that along with global warming, urban warming has been recognized as one of the environmental problems resulting from rapid urbanization. The changes in local meteorological conditions associated with those events may significantly contribute to the variation in ground-level ozone concentrations. Ozone is a secondary pollutant, produced by a series of complicated chemical reactions, and overall concentration is influenced by different physical processes such as advection, diffusion, deposition processes, and so on. It is therefore difficult to understand the inner relationships among these processes and to identify which process is in control solely by observation analysis, due to the limited number of measurements available. Numerical simulation establishes a link between emissions and ambient concentrations based on the assumed physical and chemical processes, thus providing a powerful tool to analyze the detailed processes of ozone formation. In this thesis, three studies of ozone for Kanto area of Japan were undertaken to add to the body of knowledge concerning the mechanism of ozone formation and the possible factors for increasing ozone levels during summer in the Kanto area.

The first study is "Influence of meteorological and initial/boundary conditions on the ozone levels by MM5/CMAQ analysis". In this study, the MM5/CMAQ model is applied to simulate ozone levels in the Kanto area of Japan and the influence of meteorological conditions on atmospheric pollution is investigated over the following two summer periods; (1) a period with mild weather, and (2) a period which is associated with a hot and dry weather pattern. Additionally, the influence of initial and boundary condition on ozone simulations is also investigated. The simulation results were compared with observation data, and are found to accurately replicate most of the important observed characteristics. It is found that the simulated O3 concentrations are strongly influenced by meteorological conditions in the Kanto area. High temperatures with weak winds on hot and dry day lead to a significant increase in O3 concentrations, especially in the afternoon. The initial and boundary conditions also have a significant effect on simulated O3 concentration However, there are few observation data. The importance of external influence from transport outside the area study is very uncertain, it is, therefore, necessary to have more measured data and studies to evaluate the influence of initial and boundary conditions and develop a database for air quality models.

The second study is "Process analysis on ozone formation under different weather conditions using the MM5/CMAQ model". This study aims to assess the contributions of individual physical and chemical atmospheric processes on ozone formation under different weather conditions during a typical summer month, August 2005, using the MM5/CMAQ model. For this purpose, we have developed a simple method to clarify weather patterns and look at its relationship with high ozone levels. The role of processes on ozone formation is then quantitatively analyzed using the Integrated Process Rate analysis tool available in the CMAQ model. The results show that there is a close relationship between prevailing weather patterns and ozone concentrations in Kanto area. The process analysis at two selected sites in the Kanto area indicates that ozone formation is mainly controlled by advection, vertical diffusion, dry deposition, and chemical processes. The ozone concentrations in the surface layer are mainly enhanced by the vertical diffusion of ozone-rich air from aloft, whereas dry deposition and chemical processes mainly deplete ozone concentrations. By investigating the effect of each process under different weather conditions, it is found that the contributions of processes differ depending on prevailing weather patterns. Of the 3 major patterns that are identified, meteorological conditions for Pattern I are less favorable for photochemical and accumulation processes resulting in lower ozone concentrations. By contrast, under conditions of higher stagnation such as Patterns II and III, there is the significant increase in accumulated ozone and its precursors, and- as a result - increased ozone chemical production. On the basis of this study, it can be suggested that the incidence of high ozone levels in the central Kanto area is strongly affected by meteorological and photochemical conditions. The significant decrease in ozone removal due to chemical and advection processes under conditions of higher stagnation is the most important reason for the enhanced levels of ozone concentrations in this area.

The last study is "Analysis of relationship between changes in meteorological conditions and the variation in summer ozone levels". This study investigate the relationship between meteorological factors (e.g. temperature, wind speed) and ground-level ozone concentrations in summer over the central Kanto area of Japan using both statistical analysis and numerical models. It was found that there is a close relationship between changes in meteorological conditions and the variation in ozone concentrations over the central Kanto area. In summer, up to 84.1% of the long-term variation in peak ozone may be accounted for by changes in the seasonally averaged daily maximum temperature, and seasonally averaged wind speed, while about 70.3% of the short-term variation in peak ozone depends on the daily maximum temperature and daily averaged wind speed. The results suggest changes in meteorological conditions may be one cause leading to the rising ozone concentrations in this area. The results also indicated that significantly high ozone concentrations appear on days associated with urban heat islands. High temperatures and calm conditions under urban heat island (UHI) can cause high ozone levels in this area. UHI and its interaction with the sea breeze strongly affect ozone concentrations. Although the temperature difference between land and sea helps to develop a summer sea breeze, the sea breeze cannot pass through the city due to the persistent UHI. Therefore, dispersion of ozone is limited and the high ozone concentrations can be observed.

本論文は「日本の関東地方における気象条件が夏季のオゾン濃度へ与える影響に関する数値解析」と題して、関東地方において気象条件が夏季のオゾン濃度の増加とオゾン形成のメカニズムへ与える影響について統計分析と数値シミュレーションを用いて明らかにしている。

本論文の構成は以下の通りである。

第1章では、近年の日本の光化学大気汚染問題、既往研究の全体的な説明及び目的について説明している。

第2章では、地表付近や対流圏におけるオゾンの化学反応、有機化合物、窒素酸化物、気象影響のような関連する要素に関する文献調査を行っている。

第3章では、気象モデル(MM5)と化学輸送モデル(CMAQ)について簡潔に説明している。この章では、シミュレーションのための地域の選択、気象モデルへ初期値を与えるための気象データの入力、大気質モデルに必要な排出データの算出、気象モデルと大気質モデルの連成などについて説明している。

第4章では、MM5/CMAQモデルは関東地方のオゾン濃度のシミュレーションへ適用される。大気汚染における初期条件と境界条件の影響を、(1) 温暖な天候、(2) 暑熱な天候の夏季の2期間で調査した。また、関東地方へ適用されたMM5/CMAQモデルの性能を評価した。さらに、オゾン濃度のシミュレーションへの気象条件の影響も考慮されている。シミュレーション結果は実測データと比較し、大部分の観測された特性を正確に再現できることを示している。初期条件と境界条件はシミュレーションのオゾン濃度にかなり影響を与えることも示している。

第5章では、関東地方における夏の気象要素(気温、風速)と地表付近のオゾン濃度の関係を調査している。まず、実測に基づいた気象条件の変化がオゾン濃度の変動へ与える影響を評価し、これらの関係は連成したMM5/CMAQモデルのシミュレーションに考慮されている。関東地表の中央部における気象条件の変化とオゾン濃度の変動は親密な関係あることを示している。また、関東地方のオゾン形成はいくつかの気象要素と現象の影響を受けており、風速場と都市のヒートアイランド現象が高オゾン濃度となる重要な役割を果たしている知見を得た。

第6章では、典型的な夏の8月における異なった気象状況でのオゾン形成における大気の化学的プロセスや物理的プロセスの影響についてMM5/CMAQモデルを用いて分析している。天候のパターンを明確にするために簡単な方法を開発し、高オゾン濃度との関係を見出している。そして、オゾン形成のプロセスは、CMAQモデルの分析ツールを用いて定量的に分析する。また、関東地方の一般的な天候パターンとオゾン濃度は密接な関係があることを示している。さらに、関東地方の二つの地域の過程分析を行い、オゾン形成を移流、鉛直拡散、乾性沈着、化学反応の過程で主に抑制できることを示している。

最後に第7章では、本論文の結論をまとめ、今後の課題を示している。

以上を総括するに、本論文は数値シミュレーションによる局所オゾン濃度解析手法を全体的に明確に説明しており、オゾン濃度への気象条件の影響について役立つ知見を得ている。都市化は地域や地方の気象状況を変化させ、その結果、環境問題を起こしている。この研究では、大気汚染濃度と気象要素や現象が、特に関東地方の都市温暖化に密接な関係があることを示唆している。また、気象条件の変化はこの地域のオゾン濃度上昇に影響を与える可能性がある。研究成果は関東地方の光化学オキシダント濃度を抑制する対策の評価に利用できると思われる。

よって、本論文は博士(工学)の学位請求論文として合格と認められる。