武清地区冬季一次重污染过程垂直分布特征

The Vertical Distribution of Air Pollutants in a Typical Winter Haze Episode in Wuqing Area

为研究京津冀地区重污染过程大气污染物的垂直分布特征,于2016年12月13日重污染前(11:49-12:18)和12月18日重污染期间(11:00-11:16)在北京市、天津市、河北省交界处的武清地区利用系留气球开展1 000 m以下的大气观测,探究污染物的垂直分布特征及对流边界层、覆盖逆温层和混合层等要素对重污染形成的影响.结果表明:(1)在重污染前,大气层结不稳定,ρ(PM2.5)、ρ(NOx)与ρ(O3)随高度变化不明显,存在明显的垂直对流运动,有利于大气污染物的扩散;PM2.5/PM10[ρ(PM2.5)/ρ(PM10)]在800 m以下为0.60~0.80,在800~1 000 m以上大于0.90.(2)重污染期间,近地面大气层分为对流边界层(距地面0~150 m)、覆盖逆温层(150~370 m)、混合层(370~500 m)和自由大气(500 m以上)4个层次.(3)NOx主要在对流边界层内聚积;高空O3在向近地面扩散时受强混合层阻挡,在混合层出现一个小峰值;PM2.5不仅在近地面聚积,而且在覆盖逆温层内聚积,ρ(PM2.5)在覆盖逆温层内呈双峰(峰值分别出现在150和370 m)分布,其粒径集中在0.5~1.0μm,属于积聚态气溶胶.研究显示,在不利扩散条件下,汽车排放、村镇居民供暖排放的污染物聚积及二次颗粒物的生成是重污染形成的重要因素.

To study the vertical distributions of air pollutants during heavy pollution in the Beijing-Tianjin-Hebei Region,tethered balloons were used over the Wuqing Area,which was located at the junction part of Beijing-Tianjin-Hebei Region.The observation campaign was conducted under 1000 m during the pre-heavy-pollution period(11:49-12:18,December 13th,2016)and the heavy-pollution period(11:00-11:16,December 18th,2016).The purpose of this study was to obtain the vertical distributions of air pollutants and explore the impacts of boundary layer,air temperature inversion and blending layer on the formation of haze pollution episode.The results showed that:(1)During the pre-heavy-pollution period,the atmospheric stratification was unstable and the vertical distribution ofρ(PM2.5),ρ(NOx)andρ(O3)was uniform,which indicated obvious vertical convection and favorable diffusion condition.The PM2.5/PM10(ρ(PM2.5)/ρ(PM10))values were 0.60-0.80 at an altitude below 800 m while they were larger 0.90 at an altitude of 800-1000 m.(2)During the heavy-pollution period,the atmosphere near the ground was divided into four layers:convective boundary layer(0-150 m above the ground),thermal inversion layer(150-370 m),mixing layer(370-500 m)and free atmosphere(above 500 m).(3)NOx tended to accumulate below the boundary layer.The downward O3from the high-altitude was hindered by the strong mixing layer,and formed a small O3peak at 500 m.PM2.5,which mainly consisted of accumulative mode aerosols with the size of 0.5-1.0μm,accumulated not only near the ground but also in the inversion layer with a bimodal distribution(peaks at 150 and 370 m).Under unfavorable diffusion conditions,the accumulation of vehicle exhausts,village residents heating emissions and the secondary particles were the key factors for the formation of heavy pollution during this period.