边界层动力过程对成都冬季细颗粒物污染事件的影响

Influence of Dynamic Process of Boundary Layer on Fine Particulate Matter Pollution Events in Chengdu

利用地面常规观测气象资料、风廓线雷达等资料,从环流形势、边界层输送扩散条件等方面对比分析了成都地区2017年12月至2018年1月两次冬季典型重污染生消过程的异同,结果表明:(1)两次过程均为静稳型天气背景,高层盛行纬向环流,近地面为均压场、弱气压场,存在多层逆温,夜间相对湿度高,风速小,风向多变,近地面主导风向为西北风、西风,风向对污染的区域输送效果明显;(2)在污染累积阶段,边界层内存在平均风速小于2m/s、风向多变的小风层,清除阶段的小风层特征则不显著;(3)大气折射率结构常数C_(n)^(2)与垂直速度可以作为垂直扩散条件的判据:污染累积阶段,C_(n)^(2)大值区的高度一般为500~1500m,清除阶段,C_(n)^(2)大值区的高度显著抬升,且垂直速度强下沉区与C_(n)^(2)大值区基本重合;(4)局地回流指数(R_(F))与通风系数对污染过程生消的指示性非常显著。污染累积阶段,1000m高度以下的R_(F)小于0.6,近地层的R_(F)小于0.2,通风系数日平均值仅为1455m^(2)/s;清除阶段的R_(F)大于0.7,通风系数通常大于3000m^(2)/s;此外,较大的通风系数也可以起到传输上游污染物的作用。

In order to compare the similarities and differences of the two typical heavy pollution processes in Chengdu from December 2017to January 2018,the generation and elimination are analyzed from the circulation situation,boundary layer transport and diffusion conditions by using the data of observation meteorological and wind profiler radar.The results show that:(1)Both processes were formed by stagnant weather.The zonal circulation prevailed in the upper layer,and the lower layer was an equal or weak pressure field with multiple layers of temperature inversion.The relative humidity was high at night,the wind speed was low,and the direction was changeable.Near the ground,the dominant wind direction was northwest,and west wind and wind direction affected the regional transport of air pollutants.(2)In the pollution accumulation stage,there was a small wind layer with an average wind speed of less tha_(n)^(2)m·s-1 and variable wind direction in the boundary layer.In the cleaning stage,this feature was not apparent.(3)The refractive index structure constant C_(n)^(2)and vertical velocity can be used as the criteria for vertical diffusion conditions.In the pollution accumulation stage,the height of the high-value area of C_(n)^(2)was generally 500to 1500m,while it was above 3000min the removal stage.Meanwhile,the location and durations of the large value area of C_(n)^(2)and the large value area of vertical velocity were basically the same in the removal stage.(4)The recirculation index(RF)and the ventilation index(VI)were very indicative of the pollution process.In the pollution accumulation stage,the RF below 1000mwas less than0.6,the near-surface RF was less than 0.2,and the daily average VI was only 1455 m^(2)·s^(-1).In the removal stage,the RF was above 0.7,and the VI exceeds 3000m^(2)·s^(-1).In addition,VI with a large value also played a role in transmitting upstream pollutants.