北京市冬季典型重污染时段PM2.5污染来源模式解析

为了探究近年来北京市PM_2.5污染区域来源规律和重污染累积过程中PM_2.5的生成途径,利用第三代三维空气质量模型CAMx的颗粒物源示踪(PSAT)和过程分析(PA)技术,模拟计算了北京市2013年和2014两次冬季典型重污染时段PM_2.5的源-受体关系和物理、化学过程对PM_2.5的生成贡献. 结果表明:在区域来源贡献中,随着空气污染等级由优升至严重污染,外地PM_2.5贡献率从42.9%升至67.4%,本地贡献率由57.1%降至32.6%,其中外地二次PM_2.5贡献率从20.2%升至39.8%,为北京市重污染时段的主要贡献因子；在外地贡献中,廊坊市、山东省、天津市、唐山市的贡献率较大,分别为3.2%~4.7%、3.8%~7.5%、3.6%~5.8%、2.2%~3.2%. PA分析结果表明:在不利气象条件(持续性的逆温层结)下,南边界的输送在重污染过程中起到了重要作用,对ρ(PM_2.5)增长的贡献速率可达10 μg/(m3·h). 此外,本地化学转化在重污染时段对ρ(PM_2.5)爆发性增长的贡献率也可以达到40.0%,其中特殊天气条件下二次PM_2.5生成贡献的显著增加是造成ρ(PM_2.5)出现峰值的主要原因. 研究显示,随着污染程度的加重,北京市受区域性污染的影响逐渐加大；在重污染过程中,不利气象条件下的本地化学转化与水平输送对近地层ρ(PM_2.5)峰值的出现与维持发挥了重要作用. 

Model Analytic Research of Typical Heavy PM2.5 Pollution Periods in Winter in Beijing

Particulate source apportionment technology (PSAT) and process analysis (PA) of CAMx were used for understanding the regional source contributions and formation processes of PM_2.5 in Beijing during heavy polluted periods in recent years. With the increasing of the air quality index from Ⅰ to Ⅵ, the modeling results showed that the regional contribution to PM_2.5 increased from 42.9% to 67.4%, while the contribution of local emissions decreased from 57.1% to 32.6%. The secondary portion of PM_2.5 became the major contributing factor during heavy PM pollution events, with the contribution ranging from 20.2% to 39.8%. Among the source regions in Beijing, Langfang, Tangshan in Hebei Province, Shandong Province and Tianjin City accounted for large fractions, supplying 3.2%-4.7%, 3.8%-7.5%, 3.6%-5.8% and 2.2%-3.2%. PA modeling results for two heavily polluted events were analyzed by PA process tool. The results indicated that the ratio of local chemical transformation contributed up to about 40.0% of incremental PM_2.5 concentration during the heavy pollution event. Transportation from the southern bounder of Beijing also played an important role, contributing 10 μg/m3 incremental concentration of PM_2.5 in one hour. Chemistry conversion during the night under extremely adverse meteorological conditions was the most important reason for the appearance of the peak concentration of PM_2.5, accounting for about 40.0% of the PM_2.5 rapid increment. This study concluded that the heavier the pollution in Beijing, the larger the influence of regional pollution. The local chemical conversion and horizontal transmission of PM_2.5 play important roles in the appearance of the peak concentration of PM_2.5 during heavy pollution.