相对湿度和PM2.5浓度对乌鲁木齐市冬季能见度的影响

基于2016~2017年冬季乌鲁木齐市城区PM_2.5和气象要素观测数据，采用线性和非线性回归、变量分类分析等统计方法，研究了大气能见度与相对湿度（RH）、PM_2.5浓度的定量关系.结果表明：乌鲁木齐市冬季能见度日变化呈单峰形分布，中午13：00前后和夜晚20：00前后能见度分别达到最高和最低.相对湿度增加、PM_2.5污染加重都会造成冬季大气能见度明显降低，但低能见度天气的主要影响因素是PM_2.5污染.在RH < 90%时PM_2.5累积及其吸湿增长对能见度变化起控制作用，特别是70%￡RH < 90%时，PM_2.5浓度对大气能见度影响最大.RH³90%时相对湿度成为决定因素.在PM_2.5污染逐渐加重的过程中，相对湿度对能见度的影响在减弱.能见度增加与PM_2.5浓度降低之间存在非线性响应.在PM_2.5污染由严重减轻至中度污染级别过程中，能见度改善并不明显.只有把PM_2.5浓度控制在115μg/m³以下（轻度污染或优良级别），PM_2.5浓度降低，能见度才开始出现显著提高.但冬季要达到较高能见度水平（8km），PM_2.5浓度需要继续严控至39μg/m³以下.本文对乌鲁木齐市冬季大气污染治理具有重要指导意义.

Impacts of relative humidity and PM2.5 concentration on atmospheric visibility during winter in Urumqi Urban Area

In this study the visibility, PM_2.5 concentration and meteorological data from December in 2016 to February in 2017 were analyzed by using statistical methods to present variations of visibility in winter in Urumqi. The quantitative relation among the visibility, relative humidity (RH) and PM_2.5 concentration was explored and the thresholds of PM_2.5 concentration corresponding to different visibility grades were proposed.The results indicated that the average diurnal variations of visibility showed a single peak in winter, coincident with these of PM_2.5 concentration and RH. The highest and lowest values of visibility were observed respectively at around 13:00 BJT (Beijing time) and 20:00 BJT. High RH and severe PM_2.5 pollution could both reduce visibility significantly in winter, but low visibility in winter was mainly caused by PM_2.5 pollution. The variations of visibility at RH < 90% were primarily controlled by the accumulation of PM_2.5 and its hygroscopic growth. Especially for 70%￡RH < 90%, PM_2.5 concentration was the most important influencing factor for visibility variations. When RH³90%, the visibility degradation was mainly contributed to the influences of RH. The influences of RH on visibility were weakened gradually as PM_2.5 concentration increased in the transitions from good air quality to severely polluted condition. The visibility was significantly negatively correlated with PM_2.5 concentration, and the increasing of visibility responded nonlinearly to the descending of PM_2.5 concentration. We found that visibility did not increase obviously when PM_2.5 pollution level transformed from severely polluted into moderately polluted. The visibility would increase quickly when PM_2.5 concentrations dropped below 115μg/m³ (i.e. excellent, good air quality, slightly polluted condition). However, the PM_2.5 concentration corresponding to the visibility of 8km (relatively high visibility in winter) needed to be further limited under 39μg/m³. This finding has important implications for air pollution prevention and control in winter in Urumqi.