2015年冬季京津冀两次重污染天气过程气象成因

为了分析京津冀地区2015年11月27日~12月1日和12月19日~25日这2次重污染过程，从环流形势、大气稳定度条件、动力条件、水汽条件、近地层风场输送等几个方面对重污染天气的形成机制展开分析，结果表明：这2次重污染天气过程均属于静稳型，津京冀各地重度以上污染时长均超过50%.在大范围静稳形势存在时，过程一期间边界层内的垂直扩散条件较过程二偏弱，过程一期间地面辐合线位置偏北且维持不动，过程二期间辐合线位置偏南且略微南北摆动，导致了2次过程重污染区域和污染增长速率的不同.对北京而言，过程一前期降雪融化提供了有利水汽条件，弱偏南风有利于污染物和水汽的输送，混合层高度持续异常偏低（京津冀平均混合层高度339m）、过程期间伴随弱下沉运动（0~2Pa/s）、多层逆温（且厚度大）造成日变化不明显，地面辐合线在北京中部维持等多重因素，使得污染浓度极高，北京地区PM_2.5峰值浓度达593mg/m³.过程二前期采取了减排措施，能见度和PM_2.5日变化大、污染发展较过程一前期平缓；后期不利气象条件叠加污染排放，导致了PM_2.5爆发式增长，其中邢台PM_2.5峰值浓度达70mg/m³，增长率超过7.2mg/（m³·h）.

Analysis of meteorological causes of two heavily polluted weather processes in Beijing-Tianjin-Hebei Region in winter of 2015

In order to analyze the two heavy pollution processes in Beijing-Tianjin-Hebei area from November 27 to December 1 (process 1) and December 19 to 25(process 2), 2015, the formation mechanism of heavy pollution weather was analyzed from circulation situation, atmospheric stability condition, dynamic condition, water vapor condition and near-surface wind field transportation. These two heavy polluted weather processes belonged to stationary type, and the duration of heavy polluted weather in Beijing, Tianjin and Hebei was more than 50%. In the presence of large-scale static and stable situation, the vertical diffusion condition in the boundary layer during process 1 was weaker than that of process 2. During the first period of the process, the position of the surface convergence line was northward and remains unchanged, while the position of the convergence line was south and slightly north-south oscillation during the second process, which resulted in the difference of heavy pollution area and pollution growth rate between two processes. For Beijing, snowfall melting provided favorable water vapor conditions in the early stage of process 1. Weak southerly wind was beneficial to the transport of pollutants and water vapor. The height of mixing layer remained abnormally low (the average height of mixing layer in Beijing, Tianjin and Hebei was 339m), accompanied by weak subsidence movement (0~2Pa/s) and multi-layer inversion (and thickness) during the process, which caused no obvious diurnal variation. Simultaneously, the surface convergence line maintained in central Beijing. Due to these multiple factors, Beijing was seriously polluted in process 1 with peak concentration of PM_2.5 reached to 593mg/m³. In the early stage of process 2, emission reduction measures were adopted, visibility and PM_2.5 changed greatly, and pollution development was more gentle than that of process 1. In the later stage of process 2, adverse meteorological conditions combined with pollution emissions resulted in the explosive growth of PM_2.5, with the peak concentration of PM_2.5 in Xingtai reaching 700mg/m³ and the growth rate exceeding 7.2mg/(m³·h).