大中型城市交通废气与光化学污染的数值模拟

The Numerical Simulation of the Emission from Traffic Exhaust-gas and Photochemical Pollutants in Urban Area

运用一个三维边界层光化学模式模拟了南京市主要由交通排放引起的多种光化学污染物的时空分布及相互联系 .模拟表明 :南京市冬季虽然没有形成光化学烟雾 ,但污染物浓度相当可观 ;夏季 ,不少地方臭氧浓度出现较高值 ,超过国家二级标准 ,并且其它光化学污染物的浓度亦较高 。

As we know, air pollution in cities is a serious environmental problem. It is the result of a complex interaction between natural and anthropogenic environmental conditions. With the improvement of protecting the environment, people pay more and more attention to it. So, what are the sources of air pollution in cities? Besides the industry, power plants, trade and domestic fuel, investigation indicates that motor traffic is major source of air pollution in urban area. Believe it or not, the total number of vehicles in our country will be 44 million in 2010. The proportion of nitrogen oxides (NO x) from the emission of motor traffic in total NO x has already been 39%～79.3%, such as in Beiing, Shanghai and Guangzhuo city. In Jiangsu province, the proportion of NO x in the total pollutants increases obviously after 1997, and the type of air pollution in some cities such as Nanjing, Suzhou, Wuxi etc. is changing from the type of coal to the mixed type (coal and motor traffic tail gas). Because the pollutants emitted from motor traffic are the main sourse of photochemical pollution, the simulation of them appears very important. Here, a three-dimensional photochemical pollution model with high spatial resolution for urban boundary layer is used to simulate the form and distribution of photochemical pollutants, which are mainly caused by urban traffic gas emission. The model is mainly composed of two parts-the meteorology and the photochemistry. In meteorology part we use a nonhydrostatic three-dimensional planet boundary layer (PBL) model and take a E-ε closure, which has strong stability and high resolution. So we put its result into the photochemistry part. Resorting to the uniting of the high resolution and precision photochemistry part and the PBL model, we can extract evident results. It can successfully reflect the diurnal variation in concentrations of chemically reactive species such as NO x, O 3 etc. Comparison with the observation data shows that the simulation result of O 3 is in good accordance with the observation. Other results show that total level of photochemical pollutants of Nanjing in winter is high, although photochemical fog hasn't been formed there. In summer, there are many places in Nanjing where high concentrations of photochemical pollutants are found. Those concentrations are much higher than the standards of the Nation. Some of them are 2～3 times the standard. On the other sider, in Nanjing, the pollution of PAN is more serious than ozone (O 3) That is to say, it is more likely to from the photochemical fog in Nanjing in summer. Because of the different wind directions in different seasons in Nanjing. there are different distributions of photochemical pollutants in the whole city, as we can see at the different height. Therefore, besides the control of such 'Classic' air pollutants as sulphur dioxide (SO 2), carbon monoxide (CO) etc., we should pay more attention to the pollutants resulting from the photochemical pollution, such as NO x, PAN and other oxides.