Studies on air pollution from automobile exhaust in tokyo : analysis of urban influence and estimates of air pollution potential
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概要
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In the present study, various urban influences on air pollution due to automobile exhaust gas (CO) have been examined in Tokyo. On the basis of these analyses, the climatic air pollution potential for automobile exhaust gas was calculated by the statistical model. First, the nature of urban effects was analyzed mainly in the case of the chosen highly polluted days. In this analysis, special care is taken for the hourly variation from evening to midnight, frequently struck by high concentration. Eventually, it is found that the stability difference, caused by differential cooling, plays an important role for the regional CI (the hourly concentration divided by the corresponding annual mean) distribution except for the western mountainous area affected by the early inflow of less polluted mountain wind. It is also shown quantitatively that the role of the heat island, a major cause of the differential cooling, is important in bringing about regional differences in the CI distribution. Secondly, the growth of the surface inversion layer from evening to midnight, deeply related to the CO concentration distribution, was examined by using a simple numerical model. Through these simulations, sensitivity tests were conducted to clarify the degree of various factors influencing inversion growth. The result shows that roughness increase doesn't affect the inversion growth in the case of a weak surface wind (u_s=1m/s) but tends to weaken it in the case of a moderate surface wind (u_s=3m/s), and screen-height cooling amount is almost linearly related to the inversion intensity; this is regardless of the cooling rates in the case of u_s=1m/s. The inflow of a stable stratified land breeze into the urban area contributes to intensifying the surface level inversion in a weak wind (u_s=1.5m/s), but the inversion strength is almost unchanged in a moderate wind (u_s=3m/s). The main cause of this seems to be due to the difference in vertical diffusion coefficients. Thirdly, to examine the hourly difference of the surface level vertical diffusion coefficient (Kz), Kz was calculated at the Tokyo tower (urban center) and the TVK tower (non-urban area). Then, a simple numerical simulation for concentration was performed on highly polluted days. From these analysis, it is clarified that the urban Kz substantially exceeds the non-urban Kz especially in December night, and that the hourly Kz variation plays an important role in forming a regionally different pattern of the hourly concentration variation from early evening to midnight in the case of the high concentration dates. Finally, on the basis of the obtained results, the method to predict the vertical potential temperature gradient (∂θ/∂z) from the routine observation was constructed statistically in the near-surface layer during the nighttime. By combining the estimated∂θ/∂z, tower measurements of ∂θ/∂z, and wind speed, the defined climatic air pollution potential for automobile exhaust gas was calculated all over the stations in Tokyo.
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