乌鲁木齐重污染日PM_(2.5)不同增长型的污染特征及气象条件分析

Pollution Characteristics and Meteorological Conditions of Different Growth Types of PM_(2.5) on Heavy Pollution Days in Urumqi

基于2015—2021年近7年乌鲁木齐冬季逐小时地面常规观测资料和空气质量数据,并结合ERA5再分析资料对重污染日PM_(2.5)不同增长类型的污染特征、环流形势以及气象条件进行综合分析。结果表明:近7年乌鲁木齐冬季PM_(2.5)重污染及以上级别的比例由41.2%降至8.6%,PM_(2.5)重污染天数由63 d降至13 d,超过70%重污染日PM_(2.5)浓度增长分布在60μg·m^(-3)以内。依据PM_(2.5)增长类型判别方法,近7年乌鲁木齐冬季重污染日以缓慢型增长为主。对比分析爆发型增长和缓慢型增长的天气背景形势表明,两种增长类型在欧亚范围内500 hPa高空形势上均主要受西北或偏西气流影响,爆发型增长的高压脊势力较强,乌鲁木齐处于高压中心后部且气压梯度显著;而缓慢型增长的高压脊较为平直,乌鲁木齐位于高压后部的均压场控制下,气压梯度相对较弱。对比两种类型边界层内逆温厚度和强度发现,爆发型增长在925~700 hPa的逆温层平均厚度明显大于缓慢型增长,前者逆温强度达到1.8℃·(100 m)^(-1),明显高于缓慢型增长的逆温强度1.2℃·(100 m)^(-1),造成两种PM_(2.5)不同类型增长与边界层内的逆温垂直特征分布结构存在密切联系。

In this paper,the pollution characteristics,circulation situation and meteorological conditions of different growth types of PM_(2.5)on heavy pollution days were analyzed in winter in Urumqi during 2015-2021 based on hourly ground-based conventional observations,air quality data and combined with ERA5 reanalysis data.The results show that,the proportion of heavy PM_(2.5)pollution decreases from 41.2%to 8.6%,and the number of heavy PM_(2.5)pollution days decreases from 63 days to 13 days in winter in Urumqi during 2015-2021.Besides,more than 70%of heavy pollution days with PM_(2.5)concentration growth distribute within 60μg/m^(3).According to the identification method of different growth types,PM_(2.5)heavy pollution in Urumqi are mostly slow growth type in winter during 2015-2021.By comparing and analyzing explosive growth type and slow growth type of circulation situations,both types are mainly influenced by northwesterly or westerly air currents in the 500 hPa high altitude over the Eurasian.The high pressure ridge of the explosive growth type is stronger,and Urumqi is located behind the high pressure centre and has a larger pressure gradient.While the high pressure ridge of the slow growth type is flatter,and Urumqi is under the control of the mean pressure field behind the high pressure,and the pressure gradient is relatively small.By comparing the thickness and intensity of the inverse temperature in the boundary layer of the two types,it is found that the thickness of the average inverse temperature layer of the explosive growth type is significantly larger than that of the slow growth type between 925-700 hPa,and the intensity of the inverse temperature of the former reaches 1.8℃·(100 m)^(-1),which is significantly higher than that of the slow growth type(1.2℃·(100 m)^(-1)),thus it can be seen that the main factor causing the two types is due to the vertical characteristic distribution structure of the inverse temperature in the boundary layer.