The Variation Characteristics Of PM_2.5 Concentration In Beijing And Impact Of Meteorological Conditions

Fine particulate matter PM_2.5 has already been the primary pollutants in Beijing. Thus, the basis of controlling the air pollution is to have a comprehensive understanding of its variation characteristics and impact mechanism. The temporal evolvement of PM_2.5 concentration and the variation characteristics of moderate or above pollution in urban and rural environments of Beijing in recent 10 years have been investigated by use of hourly data from BAOLIAN atmospheric composition monitoring station（representing urban area） from 2005 to 2014 and SHANGDIANZI BAP-station（representing rural area） from 2006 to 2014. Meanwhile, difference between urban and suburban stations have been analyzed by combining data of SHUNYI station and CHANGPING station of 2014. Furthermore, the number of moderate and abo ve pollution days and diurnal variation of PM 2.5 concentration in different grades have been studied based on the division standards of PM_2.5 pollution. As meteorological condition is a critical factor influencing the variation of PM_2.5 concentration, so radiosonde data, ground-based observation data, NCEP/NCAR reanalysis data, wind profiler data, meteorological satellite data and solor radiation data are used to discuss potential impact mechanism.PM_2.5 concentration in Beijing shows a decreasing trend generally during the 10-year observation period, but it becomes worse in recent 2 years. The results show that the PM_2.5 pollution is the most severe during 2005 to 2007. Since 2008, the annual mean mass concentration and the days of mo derate or above pollution have been reduced. However, mean concentration of PM_2.5 from 2013 to 2014 is higher than that from 2008 to 2014 in the urban area and the number of serious pollution day is increasing continuously during these two years. Furthermo re, the annual mean concentration of rural areas is also increasing and the number of severe pollution day increases dramatically to 14 in 2014 which has been the maximum since 2006.The seasonal distribution of PM 2.5 concentration in the urban area and rural area are quite different, but there is a convergence in 2014. In the urban area, the PM_2.5 concentration of autumn and winter is higher than that of spring and summer. On the contrary, the concentration of spring and summer is higher in the rural aera. This indicates that influencing factors between urban and rural areas are different. Moderate and severe pollution days appear in each season in Beijing, but serious pollution days in the urban generally only appeare in heating season, and there is only one serious pollution day（May 2007） in the rural area since 2006. In recent years, the pollution days tend to appear in autumn and winter but decrease in spring and summer which indicates that the seasonal gape becomes larger in the urban area. At the same time, the seasonal distribution of PM_2.5 concentration in rural areas is likely same as that in urban area. These results indicated that PM 2.5 pollution has become more regional in Beijing area.The diurnal variations of PM_2.5 concentration not only have differences between urban and rural areas, but also differ with different seasons. Furthermore, it also varies with deterioration of PM 2.5 pollution. In the urban area, diurnal variations in PM_2.5 concentration displays a bimodal pattern, with peaks correspond to the moring and evening rush-hour. In the rural aera, the PM 2.5 concentration shows a unimodal pattern with concentration increases continuously during the daytime. In the urban area, t he morning peak is significant in the summer but weakened in the winter, and PM_2.5 concentration rises to daily maximum during the evening in winter. In the rural area, PM_2.5 concentration rises sharply from afternoon to the evening in the summer, but keep ed a very low level in the daytime in winter. Furthermore, PM_2.5 concentration increases after morning rush hour till afternoon in severe and serious pollution days which is different from mean pattern and that in moderate pollution days in the urban area.The enlarged seasonal differences of PM_2.5 pollution have relationships to the decrease of spring sand-dust days and the variation of western pacific subtropical high in summer. The correlation coefficient of PM 2.5 concentration and sand-dust days in spring is 0.8 in recent 10 years. Statistically, the number of sand-dust days in spring in Beijing maintains a lower level within 3 days after 2009. The reduction of sand-dust day may be one of the main factors influencing the decrease of PM 2.5 concentration. The decrease of sand-dust days is affected by the adjustment of circulation pattens, surface wind speed and precipitation of dust sources. In addition, pollution days in high summer usually happen when subtropical high stability controls mid-latitudes and the temperature and humidity are high. Research shows that the intensity of subtropical high over Beijing is w eaker and the time of this control is shorter, and this may be one of the reason of the decrease of PM_2.5 concentration in the summer.The seasonal differences and differences between different pollution levels of PM_2.5 daily variation have relationships with meteorological conditions and solor radiation. The mass concentration in the moring rush-hour reaches its top in the summer in urban areas which is some related with high relative humidity as wel l as great water vapor pressure, and also the high solor radiation. But the smaller variety of mass concentration in the late afternoon attributes to the thicker mixing layer, higher wind speed and more showers. By contraries, it goes up greatly in the late afternoon in the winter. One of the main reason is that the top of mixing layer decreases sharply during 14:00 p.m to 17:00 p.m. Moreover, PM_2.5 concentration increase after morning rush hour till afternoon in some severe and serious polluted days. The main mechanism attribute to the aerosol from aerial source around being usually brought by south wind to Beijing.The diffusion condition was worse than climatological normal and that may be the main reason for the aggravation of PM 2.5 pollution in 2014. In February and October 2014, the thickness of atmospheric mixed layer is 150 m lower than 10-year average. Besides, the surface wind speed is 0.5m/s lower and the relative humidiy is 10% higher than climatologic in the urban area. In the rural area, the occurrence frequency of southwes t wind increases significantly which help the transportation of pollutants from urban to rual areas.Based on the long-term monitoring data of PM_2.5 concentration, this research reveals the temporal variations of PM_2.5 concentration and fine characteristics on different levels of pollution and its differences between urban and rural areas. Furthermore, this paper also reveals effects of synoptic-scale systems such as subtropical high, the vertical distribution of wind in boundary layer, surface meteorological elements, solor radiation on seasonal and diurnal variations of PM_2.5 concentration, and also discusses the effect of high stack sources around on the aggravation of pollution in Beijing. The research results not only improve the cognition of fine particle pollution in the city of Beijing, but also would be helpful to make fine emission control policy when the atmosphere is under poor diffusion conditions.