不对称街谷内PM_(2.5)浓度垂直分布特征及成因

Vertical distribution characteristics and origin of PM_(2. 5) concentration in asymmetrical street canyon

当前细颗粒物PM2.5已成为城市环境的主要污染物,研究城市不对称街谷内PM2.5浓度的垂直分布特征,对居民日常生活与健康出行有现实意义。实验选取2013年3个不同阶段对高度在1-35 m范围的街谷进行PM2.5浓度监测,同时引用街谷内流场模型与浓度场模型,对PM2.5浓度垂直分布特征及成因进行探究。结果表明,不对称街谷受大气对流、风速、风向影响,街谷内细颗粒物存在不均匀分布特点,在较高侧随着壁面高度的增加PM2.5浓度大体呈＂S＂型曲线变化。同时在同一阶段监测的4天中街谷内PM2.5浓度分布特征大体一致,而阶段之间差异明显;街谷内PM2.5浓度垂直分布的最高浓度差出现在阶段1,高达75μg/m^3,阶段2与阶段3浓度差相对减弱,仅在20-30μg/m^3之间。通过阶段2与阶段3对比可知,北京冬季供暖燃煤对大气细颗粒物的贡献较大,导致颗粒物浓度偏高;而非采暖期气温回升,大气对流作用较强,有助于大气颗粒物扩散,因而街谷内PM2.5污染程度相对较低。

Fine particulate matter PM2. 5has currently become the main pollutant in urban environments.The purpose of this work was to study the vertical distribution of fine particulate matter in an asymmetric street canyon,which holds significant merit for daily life activities and efficient transportation. In order to perform this analysis,three phases were selected for different height levels（ from 1 to 35 m） in the street canyon in 2013. At the time of collection of PM2. 5samples,the flow model and the concentration field model were integrated. The results showed that the asymmetric street canyon was significantly influenced by factors such as atmospheric convection,wind speed,and wind direction. The particulate matter showed uneven distribution characteristics in street canyon; with an increase in the wall heights,PM2. 5concentrations showed an ‘S＇shaped curve. While the data spanning four days showed the distribution of PM2. 5concentrations to be similar,the differences between the phases were obvious. Our results also showed that the highest concentration of PM2. 5in poor street valley occurred in phase 1,with the differences in concentration reaching up to 75 μg / m^3. The differences in the PM2. 5concentrations between phases 2 and 3 were relatively smaller,being from 20 to 30 μg / m^3. A comparison of the results from phases 2 and 3 showed that coal,which is used for heating during winter in Beijing,was the main contributor of atmospheric particles. During the non-heating period,however,the PM2. 5concentrations were found to be low owing to high temperatures and strong air convection in street canyon.