北京一次重污染过程的天气成因及来源分析

采用天气学分析和GRAPES-CUACE气溶胶伴随模式相结合的方式，探讨了北京市2016年2月29日~3月6日一次PM_2.5重污染过程的大气环流特征、污染形成和消散原因，并利用伴随模式追踪了造成此次重污染过程的关键排放源区及敏感排放时段.结果表明：此次重污染过程北京市PM_2.5浓度存在明显日变化，在3月4日20：00达到污染峰值，观测数据显示海淀站PM_2.5浓度达到506.4μg/m³.形成此次重污染过程的主要天气学原因是北京站地面处于低压中心，且无冷空气影响，风速较弱，逆温较强，大气层结稳定，混合层高度较低，500hPa西风急流较弱，污染物水平和垂直扩散条件差，大气污染物易堆积；此次过程中，500hPa短波槽过境、边界层偏南风急流和冷空气不完全渗透导致了本次严重污染PM_2.5浓度的短暂下降.伴随模式模拟结果表明，此次污染过程目标时刻的污染浓度受到来自河北东北部和南部、天津、山西东部、以及山东西北部污染物的共同影响，目标时刻PM_2.5峰值浓度对北京本地源响应最为迅速，山西响应速度最慢；北京、天津、河北及山西排放源对目标时刻前72h内的累积贡献比例分别为31.1%、11.7%、52.6%和4.7%.北京本地排放源占总累积贡献的1/3左右，河北排放源累积贡献占一半以上，天津和山西分别占1/10和1/20，河北源贡献占主导地位，天津和山西贡献较小；目标时刻前3h内，北京本地源贡献占主导地位，贡献比例为49.3%，目标时刻前4~50h内，河北源贡献占主导地位，贡献比例为48.6%，目标时刻前50~80h，山西源贡献占主导地位，贡献比例在50%以上.

The analysis of weather causes and sources of a heavy pollution process in Beijing

Synoptic analyses associated with the aerosol adjoint module of the atmospheric chemical modeling system GRAPES-CUACE are used to investigate characteristics of the atmospheric circulation, the formation and dissipation of air pollutants during a heavy PM_2.5 pollution episode from February 29 to March 6, 2016 in Beijing. The adjoint module is also applied to track the key source areas and sensitive emission period. Analyses reveal that the PM_2.5 concentration in Beijing exhibits remarkable daily variations and reach its peak at 20:00 BJT on March 4, and the observed PM_2.5 concentration attain 506.4μg/m³ at Haidian station. Beijing is controlled by the low pressure during the episode, with less influence of cold air, weak/calm winds, strong inversion of temperature, stable atmospheric stratification, low planetary boundary layer (PBL), facilitating the accumulation of air pollutants. The occurrence of a short-term PM_2.5 decrease is primarily caused by the 500hPa short-wave trough transit and the southerly jet in the PBL. Model results show that the PM_2.5 concentration at the target time of the pollution process in Beijing is affected jointly by the transport from the northeastern and southern regions of Hebei, Tianjin, and parts of Shanxi and Shandong. The peak PM_2.5 concentration at the target time in Beijing responses most quickly to the local emission source and most slowly to the Shanxi emissions. The cumulative contribution of emissions from Beijing, Tianjin, Hebei, and Shanxi to the PM_2.5 concentration in Beijing at the target time during the first 72hours is 31.1%, 11.7%, 52.6%, and 4.7%, respectively. Within 3 hours before the target time, the local emission dominates the PM_2.5 concentration in Beijing, with a contribution of 49.3%, but emissions from Hebei and Shanxi are dominant within 4h to 50h and within 50h to 80h before the target time, with contributions of 48.6% and over 50%, respectively.