持续鞍型场导致的西安市PM2.5重污染过程分析

为了解鞍型场对西安市PM_2.5重污染过程的影响.以西安市2016年2月6—14日重污染过程ρ（PM_2.5）及气象要素的小时变化为研究对象，综合分析了此次重污染过程特征、天气型以及气象要素变化.结果表明:①西安市此次重污染过程可分为污染上升阶段（6—7日）、污染维持阶段（8—11日）及污染减轻阶段（12—14日），3个阶段分别处于均压场、鞍型场、高压前部等天气型的影响下.②此次鞍型场发生时，天气持续静稳，气压梯度力小，且西安市处于气流的辐合地带，导致污染物的形成和积累，ρ（PM_2.5）最高值达198 μg/m3.③在鞍型场的控制下，西安市日均气温维持在偏高的水平（最高达7.2℃），相对湿度呈上升的趋势，最高达86.5%；而风速和能见度则波动下降，平均风速和能见度最低值分别为0.8 m/s和0.5 km.高温、高湿、小风的气象条件有利于污染物的吸湿增长从而导致PM_2.5重污染.④受鞍型场的影响，西安市边界层高度较低，最低时只有55 m，且逆温层较厚，强度较大，最大值达3.8℃/（100 m），极低的边界层高度和较厚的逆温层削弱了污染物的垂直扩散能力，污染物被抑制在近地面，形成较严重的污染.研究显示，鞍型场天气型导致的均压场、暖湿、静风、低边界层及强逆温层是此次西安市PM_2.5重污染过程的重要原因. 

Analysis of Heavy PM2.5Pollution Episode in Xi'an City Caused by Stable Saddle Pattern of Pressure Field

In this paper, the hourly variation of ρ(PM_2.5) and meteorological elements in Xi'an City from February 6th to 14th, 2016 is taken as the research objects, and the characteristics of the heavy pollution episode, weather patterns, and meteorological elements are analyzed in order to learn the influence of saddle pattern of pressure field on the heavy PM_2.5 pollution episode. The results show that: (1) The heavy pollution episode can be divided into three stages. During the development stage of heavy pollution from February 6th to 7th, Xi'an City was controlled by a uniformly distributed pressure field. During the maintenance stage of heavy pollution from February 8th to 11th, Xi'an City was under the influence of saddle pattern of pressure field. During the dissipation stage of heavy pollution from February 12th to 14th, Xi'an City was at the front of a high-pressure area. (2) The steady and stable weather and low pressure gradient were conducive to the formation and persistence of heavy PM_2.5 pollution episode under the impact of the saddle pattern of pressure field. The maximum ρ(PM_2.5) was 198 μg/m3. (3) The saddle pattern of pressure field led to higher than normal temperature and higher relative humidity in Xi'an City. The highest temperature and highest relative humidity were 7.2 ℃ and 86.5%, respectively. While the minimum wind speed and minimum visibility were only 0.8 m/s and 0.5 km, respectively, and showed a fluctuating downward trend. The high temperature, high relative humidity and breeze promoted aerosol hygroscopic growth and aggravated the heavy PM_2.5 pollution during this period. (4) Under the control of the stable saddle pattern of pressure field, the boundary layer was lower (55 m) than normal and the inversion layer was stronger (3.8 ℃/(100 m)) than normal. The extremely low boundary layer and thicker inversion layer weakened the diffusion ability of atmosphere in the vertical direction, resulting in a heavy PM_2.5 pollution episode. In conclusion, the uniform distribution of the pressure field, warm and moist, light wind and strong inversion layer caused by the saddle pattern of pressure field were the main causes of the heavy PM_2.5 pollution episode in Xi'an City.