无锡夏季颗粒污染物的垂直特征分析

The study of vertical distribution of particulate matter in summer,Wuxi City

针对无锡地区2013年8月13日至31日期间的晴朗、降雨、污染过程,利用地基遥感设备大气颗粒物监测激光雷达对气溶胶光学特性的时空演变特征进行监测.结果表明晴朗天气下污染物早晚分布较少、中午集聚较多的特点,并且午间污染物的扩散高度可以达到1.5km以上.晴朗天气下气流的水平、垂直运动增强有利于污染的扩散,这也是污染程度较轻和能见度超过20km的重要原因;降雨过程中,雨云层分布高度在500m以下,雨云中的水滴产生的消光系数大于2km-1,能见度不足10km,降雨过程中空气对流增强,风速增大,加上颗粒物的湿沉降作用减轻污染程度;污染时段,由于静稳天气,使得污染物扩散不利,颗粒物主要集中在近地面1km以内,颗粒物产生的消光系数较长时段超过0.5km-1.激光雷达的探测结果显示,8月30日外来污染物的输送和局地污染物的复合使得污染程度加重.通过后向轨迹模式分析发现,该污染团可能来自站点的西北方向.近地面PM2.5的质量浓度在晴天和降雨过程中均不超过50μg·m-3,但是在污染过程中,PM2.5峰值接近160μg·m-3.近地面碳黑浓度的变化趋势PM2.5一致,尤其在污染过程中,碳黑的质量浓度达到9μg·m-3.结合近地面观测与颗粒物激光雷达的地基遥感监测结果,能够对无锡地区颗粒物的垂直分布进行初步解析.

A continuous observation to study the vertical distribution of particulate matter has been conducted at Wuxi city during August 13^th to 31^st 2013.In this experiment,the ground-based particulate LiDAR（Light Detection and Ranging）system was used for the retrieval the aerosol optical properties（extinction coefficients,depolarization coefficients）varying along with height in clear days,rainy days and polluted days,e.g.haze pollution.The results showedthat particulate matter accumulated rather rare in the early morning and in the evening for clear days,while it was rich at noon.The top of particulate matter layer extended to 1.5km due to the strong convection of airflow in vertical and horizontal direction.In this case,the visibility range reached over 20 km.LiDAR detection also revealed that the rainy clouds covered at 500 moff ground level in rainy interval.The extinction coefficients exceeded 2km^-1 due to the large volume of water drops.We found that the visibility was no more than 10 km in raining duration.The fast airflow movement and strong wind combining with wet deposition of atmosphere particles may contribute for the less pollution in rainy days.In polluted episodes,the calm condition was not good for particulate matter dispersing in vertical and horizontal level;therefore,most of the aerosol was trapped in 1km from ground level.The maximum in the extinction coefficients profile of aerosol always exceed 0.5km^-1 from LiDAR detection.A polluted air mass transferred over-boundary made local air quality worse on August 30 th.Model simulation results from HYSPLIT estimated the possible origin of this air mass was from the Northwest of monitoring station.PM2.5concentration was below 50μg·m^-3 in clear days and rainy days,but the peak value of PM2.5in haze pollution was closed to 160μg·m^-3.A better consistence between PM2.5and black carbon（BC）has been shown in polluted days than the other two cases.The climax of BC was approaching 9μg·m^-3 in polluted period.The exact observations of PM2.5and BC perfectly supported the detection of LiDAR and together constructed a whole picture of particulate matter vertical distribution for Wuxi city.