Effect of Meteorological Factors on PM_2.5 in Late Summer and Early Autumn of Beijing

The characteristics of PM_2.5 concentration of Beijing and the relationship with meteorological factors are investigated, using the PM_2.5 data of Baolian, Shangdianzi stations, sounding data, wind profiling radar data and precipitation data during July, August and September from 2006 to 2008. The results show that the average PM_2.5 concentration of late summer and early autumn at urban station (Baolian) are significantly higher than that of rural station (Shangdianzi). The prevailing southerly air flow of July to September of Beijing can transport the pollutants that generated in Tianjin, Hebei and Shanxi to Beijing. Due to the mountains blocking in the north, the pollutants are very likely to accumulate and form particle pollution. So the wind direction at 1500 m level has significant influence on PM_2.5 concentration. The transport of pollutants by the southerly flow not only enhances the pollution level of urban, but also contributes to the fine particle pollution in regional background areas. The northerly flow is clean, and plays the important role of effective dilution and diffusion of PM_2.5 in Beijing area. The ground wind speed plays different roles in urban and rural areas. The southerly and northerly wind could diffuse PM_2.5 in urban area when the wind speed exceeds 2 m·s^-1 and 3.5 m·s^-1, respectively. In rural area, the northerly wind could dilute and diffuse PM_2.5 effectively, but the southerly wind has no effect of diffusion. The variation of PM_2.5 concentration has positive correlation with precipitation amount and PM_2.5 initial concentration. The precipitation amount and PM_2.5 initial concentration in the process of below-cloud scavenging in urban and rural areas are different. As the most critical factor in the below-cloud scavenging process, the capture coefficient depends on the raindrop size, particle size and the raindrop falling speed. Owing to high PM_2.5 concentrations in urban area, the capture efficiency of PM_2.5 per unit time is greater than that of rural area under the same precipitation condition. In rural area where PM_2.5 concentration is lower, the scavenging process depends on the increase of precipitation amount. The PM_2.5 concentration decreases with the increase of mixing layer when it exceeds 1000 m. Vertical diffusion of PM_2.5 is more effective when the mixing height breakthrough 1500 m.