Characteristics of vertical profiles and sources of PM2.5, PM10 and carbonaceous species in Beijing

摘要

In August 2003 during the anticipated month of the 2008 Beijing Summer Olympic Games, we simultaneously collected PM10 and PM2.5 samples at 8, 100, 200 and 325 m heights up a meteorological tower and in an urban and a suburban site in Beijing. The samples were analysed for organic carbon (OC) and elemental carbon (EC) contents. Particulate matter (PM) and carbonaceous species pollution in the Beijing region were serious and widespread with 86% of PM2.5 samples exceeding the daily National Ambient Air Quality Standard of the USA (65 μg m-3) and the overall daily average PM10 concentrations of the three surface sites exceeding the Class II National Air Quality Standard of China (150 μg m-3). The maximum daily PM2.5 and PM10 concentrations reached 178.7 and 368.1 μg m-3, respectively, while those of OC and EC reached 22.2 and 9.1 μg m-3 in PM2.5 and 30.0 and 13.0 μg m-3 in PM10, respectively. PM, especially PM 2.5, OC and EC showed complex vertical distributions and distinct layered structures up the meteorological tower with elevated levels extending to the 100, 200 and 300 m heights. Meteorological evidence suggested that there exist fine atmospheric layers over urban Beijing. These layers were featured by strong temperature inversions close to the surface (50 m) and more stable conditions aloft. They enhanced the accumulation of pollutants and probably caused the complex vertical distributions of PM and carbonaceous species over urban Beijing. The built-up of PM was accompanied by transport of industrial emissions from the southwest direction of the city. Emissions from road traffic and construction activities as well as secondary organic carbon (SOC) are important sources of PM. High OC/EC ratios (range of 1.8-5.1 for PM 2.5 and 2.0-4.3 for PM10) were found, especially in the higher levels of the meteorological tower suggesting there were substantial productions of SOC in summer Beijing. SOC is estimated to account for at least 33.8% and 28.1% of OC in PM2.5 and PM10, respectively, with higher percentages at the higher levels of the tower.