Abstract: Spatial distribution and time scales of atmospheric diffusion over Beijing area are revealed by means of a random walk simulation model and practical meteorological data with a specified emission source from the city. Results show a southward transport pattern for wintertime while a northwest transport of pollutants in summer. The area is the least evidently influenced by the emission source in spring, while the largest in autumn.The time spent for instantly emitted material removing from the model domain varies from winter-spring to summer-autumn. The former was shorter one of less than 20 hours; the latter was longer one of approximately 30 hours. Distribution of occurrence probability for different removal times was not symmetry.Reducing slowly at the end of longer removal time, probability exists for pollutants remaining in this area a long time.

Key words: diffusion patterns, time scales, random walk simulation, Beijing area, historical state

摘要： 用随机游走大气扩散模式和1981年的历史气象观测资料，研究北京地区中尺度扩散的历史背景状态。由示踪物从北京城区连续排放条件的模拟结果，分析各季平均浓度的空间分布和示踪物在模式区域输送扩散的时间尺度。结果表明，冬季示踪物偏南、夏季偏西北的输送明显；春季扩散影响范围最小、秋季最大。示踪物从200km×200km模式区域输出的平均时间(去除时间)明显分为冬春季和夏秋季两组，前者较小，平均在20h以下，后者较大，平均约30h。不同去除时间出现的频率分布是非对称的，在长去除时间一侧，出现频率下降缓慢，显示污染物有在该区域内长时间滞留的可能。

关键词: 扩散模态, 时间尺度, 随机游走, 北京地区, 历史背景