Meteorological Conditions and Impact Factors of a Heavy Air Pollution Process at Xi'an in December 2013

As one of the most famous historical and cultural cities, Xi'an is located at the central part of the Guanzhong Basin, its northern and southern areas are the Loess Plateau and the Qinling Mountains, respectively. Increasing trend of air pollution weather disturbs Xi'an in recent years. Based on high resolution observations from air quality monitor, laser radar and automatic weather stations, meteorological conditions and impact factors of heavy air pollution process from 18 Dec to 25 Dec in 2013 are analyzed. Results show that during heavy air pollution process, the circulation at 500 hPa is meridional circulation pattern including a trough and a ridge in high latitude area. Shaanxi Province is located at the front of warm ridge with weak wind, and south to the Mongolia cold high on ground. When air quality improves, the front and wind at 500 hPa increase, and ground cold front move southeast quickly. Laser radar monitoring indicates the aerosol conglomerations below 0.5 km are uplift and the boundary layer height increases significantly. Heavy air pollution process leads to severe haze and its daily average visibility is less than 1.5 km. The boundary layer height is less than 0.7 km and its peak appears at about 1500 BT. Temperature and humidity conditions are conducive to strengthen pollution, significantly different from common periods. An inversion of temperature exists at 2~3.2 km or 0.7~1.5 km height and relative vertical humidity distribution is wet-dry-wet below 3.5 km. Severe haze is mainly wet haze and its daily average duration in suburban is 5 hours longer than urban. Concentration of fine particle PM_2.5 is much higher than that of coarse particulate defined by PM₁₀ minus PM_2.5 during heavy air pollution process. The former has obvious increasing trend and the latter doesn't change obviously with time. Concentrations of PM₁₀ and PM_2.5 reach peaks at 1300 BT and 2200 BT, and maintain low values from 0500 BT to 1000 BT every day. Pollutant concentration rises fast from 1000 BT to 1300 BT. Unlike typical process with a downward trend after noontime, there is an average concentration peak of pollutants in Xi'an during the heavy air pollution process. This phenomenon is probably caused by factors including very low boundary layer height, the Guanzhong Basin terrain and diurnal variation of local ground wind.