Abstract:

Potential source area (footprint) related to air pollution of Beijing is investigated. Long-term modeling of meteorological fields is carried out by WRF model, from 2000 to 2014. A backward dispersion footprint model is used to derive hourly footprint using these meteorological data. Long-term mean source area is obtained as well as its seasonal variation. Heavy air pollution episodes in winter and autumn are selected from air pollution index (API) data in the period from 2000 to 2012. Relationship between air pollution and its potential source area is discussed. Results show that daily mean footprint varies both by its pattern and direction, indicating strong temporal variation of the source areas for Beijing. Long-term mean source area for Beijing is shaped like a triangle, and the southwest branch is the strongest, the other two are in the east and north. Beijing locates in the middle to norther position of the triangle. Seasonal variation of the mean source area show enhancement in the southwest and south in summer (July) and autumn (October). Local wind frequency may mislead assessment of source area, in comparison to the footprint derived by the backward dispersion model. By handling all dispersion processes, such as accumulation of air borne materials, the footprint model provides reasonable information of source area. Mean footprint of all heavy air pollution episodes reveals that a wide arc zone in front of the mountains is the most significant source area to air pollution of Beijing. This zone starts approximately from Shijiazhuang in the southwest to Beijing, and then turning to Tangshan in the east.

Key words: source area, footprint, long-term mean, heavy air pollution, Beijing

摘要：

探讨影响北京市区的大气污染潜在源区(印痕)。采用WRF气象模式, 对北京地区的气象场进行15年(2000—2014年)长期模拟分析。利用印痕模式, 计算15年的逐时印痕分布, 统计分析污染物源区的多年平均特性和季节变化。根据空气污染指数API, 筛选13年(2000—2012年)秋冬季的实际重污染事例, 统计分析其与污染物源区的关系。结果表明: 1) 逐日平均印痕的形态和分布变化极大, 说明影响北京的污染物源区是随时间而动态变化的; 2) 多年平均的污染物源区大致呈三角形分布, 偏西南的一角最强, 另外一角偏东, 一角大致偏北, 北京处于三角形中心以北的位置; 3) 源区多年的平均形态和分布随季节变化, 夏秋季(7月和10月)偏南和西南方向的源区范围扩大; 4) 根据局地风向频率来判断污染物来源方向是不可靠的, 印痕模式包含污染物累积等过程和机制, 可以获得合理的污染物源区; 5) 实际重污染过程与其平均印痕的关系显示, 西南方向从石家庄到北京, 再往东到唐山这一宽阔山前弧形地带是影响北京大气的最重要污染物源区。

关键词: 污染物源区, 印痕, 多年平均, 重污染, 北京