Abstract
Abstract. The Chinese capital Beijing is one of the global megacities where the effects of rapid economic growth have led to complex air pollution problems that are not well understood. In this study, ambient particle number size distributions in Beijing between 2004 and 2006 are analysed as a function of regional meteorological transport. An essential result is that the particle size distribution in Beijing depends to large extent on the history of the synoptic scale air masses. A first approach based on manual back trajectory classification yielded differences in particulate matter mass concentration by a factor of two between four different air mass categories, including three main wind directions plus the case of stagnant air masses. A back trajectory cluster analysis refined these results, yielding a total of six trajectory clusters. Besides the large scale wind direction, the transportation speed of an air mass was found to play an essential role on the PM concentrations in Beijing. Slow-moving air masses were shown to be associated with an effective accumulation of surface-based anthropogenic emissions due to both, an increased residence time over densely populated land, and their higher degree of vertical stability. For the six back trajectory clusters, differences in PM1 mass concentrations by a factor of 3.5, in the mean air mass speed by a factor of 6, and in atmospheric visibility by a factor of 4 were found. The main conclusion is that the air quality in Beijing is not only degraded by anthropogenic aerosol sources from within the megacity, but also by sources across the entire Northwest China plain depending on the meteorological situation.
Highlights
Beijing with approximately 17 million inhabitants is one of the megacities in the world that are challenged by the imperative necessity of air quality management
Along with the rapid economic growth and increase in motor traffic during the last decade, the characteristics of air pollution in Beijing have changed from typical coal-combustion pollution to a complex mixture of domestic, industrial and traffic emissions combined with the secondary gas and aerosol formation processes in the regional atmosphere
The results shown here generally encompass the Beijing particle size distribution data between March 2004 and March 2006
Summary
Beijing with approximately 17 million inhabitants is one of the megacities in the world that are challenged by the imperative necessity of air quality management. The problem of particulate air pollution has become more and more serious during the last decades in terms of visibility degradation as well as an increase in particle-related adverse health symptoms Beside these local effects, changes in the regional climate became obvious, such as the decrease of light rain events (Qian et al, 2007b) and a decrease in sunshine duration in China (Kaiser and Qian, 2002). The first number-based environmental particle measurements in China were performed with poor size resolution and limited to diameters >100 nm (Li et al, 2002; Zhong et al, 2003) These authors found general relationships between aerosol number concentration and local meteorology as well as great changes in the characteristics of air pollution in Beijing during the last two decades. A major goal is to understand the circumstances causing long-lasting pollution episodes in Beijing and to separate local and regional pollution sources
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