长江中游地区PM2.5重污染过程的典型天气环流分型及区域传输影响

作为一个新的区域性霾污染中心，长江中游地区地理位置特殊，是我国中东部地区大气污染物区域传输的重要枢纽，天气环流对该区域不同传输和累积型PM_2.5重污染的形成机制还不甚了解.利用T-mode斜交旋转主成分分析法（PCT），对2015~2019年采暖季长江中游地区74 d PM_2.5重污染事件进行天气环流分型，得到：PCT1高压底部传输型（天数：41 d，占比：55.4%）、PCT2低压辐合累积型（天数：12 d，占比：16.2%）、PCT3高压静稳累积型（天数：11 d，占比：14.9%）和PCT4高压后部传输型（天数：10 d，占比：13.5%）这4种主要的大气环流类型.区域传输型污染（PCT1和PCT4）占比高达69%，是长江中游地区PM_2.5重污染发生的主导因素，突显了地域特殊性.其中，PCT1是最主要的环流型，冷锋南侵伴随强偏北风驱动上游地区污染物快速传输，使得PM_2.5浓度暴发式增长.境内传输通道城市襄阳、荆门和荆州PM_2.5传输过程具有12 h滞后特征，其PM_2.5影响源区主要分布在上游的河南中北部、山东西部和华北大部分地区.PCT4传输型受低层偏东风输送影响，污染上升速率也相对较快.PCT2和PCT3为静稳天气环流型，地面风速较小，低层水平辐合和下沉运动有利本地PM_2.5重污染累积，污染上升速率和持续时间都相对传输型更长.

Analysis of Typical Weather Circulation Patterns of Heavy PM2.5 Pollution and the Transport Pattern in the Yangtze River Middle Basin

The dominant transportation and accumulation patterns of heavy PM_2.5 pollution events over the Yangtze River middle basin were identified based on the obliquely rotated T-mode principal component analysis (PCT) method and the daily mean surface pressure. The heavy PM_2.5 pollution events over the Yangtze River middle basin during 2015-2019 were divided into four patterns, namely, PCT1:high-pressure bottom transport pattern (number of days:41 d, accounting for 55.4% of the total heavy PM_2.5 pollution days), PCT2:low-pressure convergence accumulation pattern (12 d, 16.2%), PCT3:high-pressure static stability accumulation pattern (11 d, 14.9%), and PCT4:high-pressure rear transport pattern (10 d, 13.5%). Regional transport patterns (PCT1 and PCT4) accounted for 69% of the total heavy PM_2.5 pollution days and were the major pattern of heavy PM_2.5 pollution in the Yangtze River middle basin. PCT1 occurred most frequently among the four patterns, accompanied with strong northerly winds, which could drive the rapid transportation of pollutants from the upstream areas and cause the explosive increase in PM_2.5 over the Yangtze River middle basin. The PM_2.5 pollution events in the transport corridor, including Xiangyang, Jingmen, and Jingzhou, exhibited a 12-hour lag feature. Most parts of northern China were the source of PM_2.5, especially in central and northern Henan and western Shandong. The PCT4 transport pattern was featured by the low-level easterly winds, and the pollution level rose quickly. The PCT2 and PCT3 were characterized by the low ground wind speed, associated with the low-level horizontal convergence and subsidence. Such synoptic conditions were favorable for the accumulation of local PM_2.5 pollution, and the pollution rise rate was slower, and the duration was longer than those of other patterns.