北京地区冬夏季持续性雾-霾发生的环境气象条件对比分析

在北京地区,除冬季供暖期外盛夏也是雾-霾天气的高发季节,与我国南方不同.使用微波辐射仪、风廓线和常规气象探测资料、NCEP再分析资料以及大气成分观测结果,通过对比分析揭示了冬、夏季持续6 d的2个雾-霾过程形成和维持机制的异同.冬季雾-霾过程出现在高空西北气流、低层多短波活动的背景下,其形成和维持的主要机制是边界层内始终有逆温层、地面弱风场、底层湿度逐渐增大.逆温层昼高夜低、湿度昼小夜大是影响PM2.5质量浓度和能见度日变化的重要环境因子.在雾-霾天气持续期间地面弱风场能够维持主要源于冷空气势力弱、常不能影响到地面.此外,入夜后地面迅速辐射降温、边界层上层有暖平流以及空气过山后下沉增温在逆温层的形成中起了关键作用.然而,对于夏季持续性雾-霾天气,气溶胶区域输送、环境大气保持对流性稳定、空气的高饱和度是其发生的重要条件.在副热带高压长时间控制下对流层低层盛行偏南风,北京的PM2.5质量浓度随着偏南风风速增大升高.对流层底层系统性偏南风与北京附近的山谷风共同构成了从北京以南气溶胶累积地向北输送的机制.夏季雾-霾过程低层没有逆温,但是北京上空一直维持超过200 J·kG-1的对流抑制能量,它同样限制了污染物的垂直扩散.夏季自由对流高度也存在昼夜变化,其对PM2.5浓度和能见度的作用与逆温层高度升降相同.因此,冬、夏个例分别代表了2种不同类型的持续性雾-霾过程,导致差异的根本原因在于大气环流型.

Comparative Analysis on Meteorological Condition for Persistent Haze Cases in Summer and Winter in Beijing

Summer is another peak season for haze besides winter in Beijing area, which is different from that in South China. The data of microwave radiometer, profiler, sounding, AWS, NCEP (NCAR) and air pollution monitors were used in the analysis of two haze cases which occurred in winter and summer, respectively. Both cases lasted for 6 days. This research focused on the difference in the mechanism of the formation and persistence of haze cases in various seasons. In winter, north-westerly flow dominated Beijing at upper-levels and a few of shallow troughs passed by during persistent haze development. The main meteorological reasons for lower visibility in 6 days were: there was an inversion in the boundary layer all the time; wind was weak at surface and moisture went up gradually. The change of inversion height and humidity day and night led to the diurnal variation of PM_2.5 concentration and visibility. The surface wind speed kept lower because the weak cold air could not often hit the surface during the haze case. In addition, three factors played key roles in the inversion formation in boundary layer. One was that the rapid decrease in the surface temperature after sunset due to the radiation. At the same time, there was some warm advection at upper boundary layer. The third one attributed to the temperature increase after the air flowing over the mountains and down. However, in summer, regional transportation of aerosol, sustained convective stability and high air saturation were very important factors for the haze formation. Under the sub-tropic high control, the wind direction at lower troposphere was south. The PM_2.5 concentration went up when the speed of south wind increased. The south flow caused by both synoptic scale systems and mountain-valley breeze near Beijing transported the aerosol northward from higher polluted area. There was no inversion in the summer haze case. But, the convective inhibition was kept over 200 J·kG^-1. As the result, it was not favorable for the pollutant diffusion upward. Furthermore, the level of free convection also changed during the day and at the night, which had similar effect on the PM_2.5 concentration and visibility as the daily variation of inversion. In conclusion, these cases in winter and summer were categorized into two different kinds of persistent haze. The main reason leading to the difference was the synoptic pattern.