title: [Comparative analysis on meteorological condition for persistent haze cases in summer and winter in Beijing].
abstract: 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 PM2.5 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 PM2.5 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 x 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 PM2.5 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.
authors.forename: Xiao-Nong
authors.initials: XN
authors.lastname: Liao
authors.forename: Xiao-Ling
authors.initials: XL
authors.lastname: Zhang
authors.forename: Ying-Chun
authors.initials: YC
authors.lastname: Wang
authors.forename: Wei-Dong
authors.initials: WD
authors.lastname: Liu
authors.forename: Jia
authors.initials: J
authors.lastname: Du
authors.forename: Ling-Hui
authors.initials: LH
authors.lastname: Zhao
mesh_terms: D000336:Aerosols / Q000032:analysis; D000393:Air Pollutants / Q000032:analysis*; D002681:China; D002947:Cities; D004784:Environmental Monitoring; D006813:Humidity; D052638:Particulate Matter / Q000032:analysis; D012621:Seasons; D013696:Temperature; D014919:Wind
chemical_list: D000336:Aerosols; D000393:Air Pollutants; D052638:Particulate Matter