[A Case Study on the Rapid Cleaned Away of PM2.5 Pollution in Beijing Related with BL Jet and Its Mechanism].

The concentration of PM2.5 decreased very rapidly from 18:00 to 23:00 on 17th Mar. 2015 in Beijing area. No cold air bringing strong north wind influenced Beijing. The reason leading to the clean away of PM2.5 was discussed. The results showed that a boundary layer jet played a key role. The ventilation in the boundary layer went up with the enhancement of southwesterly wind speed, which was favorable to the dilution of pollution. Besides, the development of jet also caused the increase of vertical wind shear. As a result, the turbulence in the boundary layer became more obvious and the mixing layer height rose. Furthermore, the geostrophic vorticity at the top of mixing layer was positive at 20:00 on 17th Mar. It means that the direction of Ekman-Pumping was upward. So, the pollution near the surface was brought to upper levels and transported downstream by the jet. The development of boundary layer jet attributed to inertial oscillation and atmospheric baroclinicity.

[Diurnal Variation of PM2.5 Mass Concentration in Beijing and Influence of Meteorological Factors Based on Long Term Date].

Diurnal variation of PM2.5 mass concentration is analyzed based on data of BAOLIAN (representing urban area) and SAHNDIANZI BAP-Station (representing rural area) from 2005 to 2014. Furthermore, the influence of meteorological factors was also discussed. The results showed that 10-year-average diurnal variation of PM2.5 mass concentration in urban area had a two-peak pattern being coincident with rush-hour. However, it became clear only after 2007. The monthly (including seasonal) variation presented the change from one-peak pattern to two-peak pattern with the max mass concentration appearing in the morning or late afternoon during rush hour. The mass concentration in the morning rush hour reached its maximum between May and August which was to some degree related with weak wind and high relative humidity as well as great water vapor pressure (indicating the absolute water content in the air). But the smaller variety of mass concentration in the late afternoon attributed to the thicker mixing layer, higher wind speed and more showers. By contraries, it went up greatly after 4 pm in Nov., Dec., Jan. and Feb.. One of the reasons was that the height of mixing layer top decreased sharply. Besides, in some severely and seriously polluted days, the PM2.5 mass concentration increased after morning rush hour till afternoon which was different from the mean pattern and that in moderately polluted day. The main mechanism attributed to the aerosol from aerial source around brought by south wind to Beijing. The more severe the pollution was, the greater the daily concentration fluctuated. The range of PM2.5 diurnal variation was determined by the max wind speed and daily change of relative humidity in a day. Besides, it also extended the diurnal variation of concentration when the south wind speed reached 4-6 m·s-1 in the afternoon. In rural area, the mean diurnal variation of PM2.5 mass concentration showed a one-peak pattern. And the time of concentration reaching its maximum was ahead of that of urban area. Moreover, the values in the day time during May and July were higher than that in winter. These results would be helpful to make policy for finer emission control when the atmosphere is in lower diffusivity situation.

[Scavenging Effect of Rime and East Wind on PM2.5 Under Air Heavy Pollution in Beijing].

In order to study the scavenging effect of rime and east wind on the concentrations of PM2.5 in Beijing during the air heavy pollution, PM2.5 concentration data of some mountains and plains stations was used, together with meteorological data, wind profile data, aerosol extinction coefficient radar data collected during December 19 to 27, 2015, to analyze the clearance mechanisms of rime and east wind on PM2.5.The results showed that ①the removal of PM2.5 by rime was a different mechanism from that by the north wind and rainy weather, in the cooling conditions, the droplets touched the branches, wire for solid condensation, leading to formation of rime, and the concentration of PM2.5 decreased;②the east wind is a special kind of wind in Beijing area, when easterly wind decreased with height, a strong upward motion could be formed, and the PM2.5 was uplifted from the surface layer to the top, while with larger westerly wind in top, it was removed into downstream and cleaned; when east wind increased with height, a weak downward movement was easy to form, and when this sinking motion could not reach the ground, the surface layer of PM2.5 capacity became small, which was beneficial to increase the concentration of PM2.5;③the PM2.5scavenging ability of east wind depended on two points, one was the strength and development of upward movement of the height formed by the east wind, and the second was the initial height of uplifted motion formed by east wind, the lower the origin of the ascending motion height, the more obvious the removal of PM2.5;④after occurrence of east wind, along with the ascending motion to m·s-1 magnitude, the mixed layer height increased to 1200-1800 meters, and PM2.5 was uplifted to top layer and cleaned.

[Meteorological mechanism for the formation of a serious pollution case in Beijing in the background of northerly flow at upper levels].

There is a kind of serious air pollution case occurring in the situation of northerly flow at the levels above 850 hPa in Beijing area. Meteorological data, NCEP reanalysis data and PM2.5 concentration survey were used to analyze the roles of meteorological mechanism in this kind of air pollution case. The results showed that the stable stratification did not exist all the time during the gradual increase of PM2.5 concentration. The favorable meteorological conditions in PM2.5 accumulation came from the layered structure of vertical speed and convergence in vertical direction. From the surface to the middle level of troposphere, the vertical velocity showed an updraft-downdraft-updraft distribution. And the divergence displayed a convergence-divergence-convergence structure. The convergence at lower levels led to the gathering of the polluted air from the surrounding and the ascending current brought it upwards. However, the polluted air could not go further up due to the prolonged downdraft layer above. As the result, PM2.5 near the surface was concentrated gradually. The vertical speed in layer structure was attributed to the high level northerly flow without invading into the boundary layer. In such case, the wind was still in southerly direction or kept calm at the surface. The weak cold air or no cold air at all was the main reason that the northerly flow could not hit the surface. The developed downdraft layer was relative to the convergence above which was resulted from the wind speed pulses in the northerly flow at upper levels. In conclusion, the air dynamical effect played a key role in the pollution type of northerly flow at upper levels. Therefore, it will be beneficial to enhance our capability in the analysis and forecasting of this kind of pollution case by focusing on 24 hours temperature variation at the middle and lower troposphere as well as the vertical distribution of vertical velocity and divergence.

[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 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.