The Developing, Maintaining, and Diminishing Process of Summertime Pollution in Urban Area of Beijing

All the summer air pollution cases in Beijing during the period of 2006 to 2013 are identified based on daily PM10 concentration data collected at stations in urban area of Beijing. A pollution case is determined if daily mean maximum PM10 concentration satisfies the criterion PM10 concentration >150 μg/m³. The corresponding atmospheric circulation patterns are then analyzed. It is found that the PM10 concentration in Beijing is significantly correlated with southerly winds in the surrounding regions of Beijing. When PM10 concentration is high in Beijing, strong southerly wind anomalies tend to appear to the south of Beijing with the maximum southerly wind anomalies along the eastern flank of the Taihang Mountain. Since pollution emissions are large in some regions to the south of Beijing, southerly winds apparently are favorable for pollution transport from Hebei and Shandong to Beijing, resulting in air pollution in Beijing. During periods of air pollution, positive geopotential height anomalies occur in the middle and upper troposphere above Beijing and Inner Mongolia; in the lower troposphere, positive pressure anomalies occur in the east and negative anomalies occur in the west. Beijing is located in between the positive and negative pressure anomalies, where the surface pressure gradient anomalies lead to southerly wind anomalies in the regions surrounding Beijing. Furthermore, the positive geopotential height anomalies in the upper-level associated with air pollution in Beijing are quasi-stationary, indicating that summertime air pollution in Beijing is possibly associated with quasi-stationary circulation anomalies. Large positive geopotential height anomalies in the upper troposphere over the Mongolia often establish four days before the pollution occurrence. Such anomalies gradually extend southward and strengthen, and eventually control Beijing and Inner Mongolia on the pollution day. As the anomalous circulation pattern weakens, air pollution diminishes correspondingly. However, positive geopotential height anomalies still control Beijing and Inner Mongolia four days after the pollution disappears, suggesting that summer air pollution in Beijing is highly correlated with upper-level quasi-stationary circulation anomalies.