青岛冬季PM2.5持续重污染天气的大气边界层特征

鲜有出现空气质量问题的北方沿海城市青岛近年来也频频出现重污染天气. 2014年1月青岛市总计出现7 d重污染天气，其中1月15-18日是持续4 d的PM_2.5重污染，其余的则分别出现在1月6日、11日和30日.为了获得气象条件对持续重污染天气发展、维持和消除的影响机制，利用激光雷达、大气稳定度仪探测数据以及地面、高空气象观测和空气质量监测数据，重点分析了1月15-18日持续重污染期间青岛市大气边界层气象要素的时间和空间特征.结果表明，2014年1月影响青岛冷空气势力弱、青岛近地面低于3 m/s的风速不利于污染物扩散，66%以上的相对湿度有利于污染物浓度增大.在污染源稳定的背景下，气象要素的差异性导致了污染物浓度时空分布的差异.在持续的弱偏北风下污染物浓度居高不下；在偏南风影响下，污染物浓度趋于下降.边界层内存在高层干冷弱北风和低层暖湿弱南风的风切变、稳定层结、低层相对湿度为70%的高湿大气以及交替出现的近地面南北风是此次重污染持续的主要原因.大气边界层高度变化对污染物浓度具有6 h左右的延迟影响；而低边界层高度、大稳定度因子，低云的存在和较高的污染物浓度之间具有较好的一致性变化趋势.当近地面温度升高、相对湿度减小以及增大的偏南风和存在弱不稳定层结时，有利于提高青岛局地大气扩散能力. 

Characteristics of Planetary Boundary Layer for Persistent PM2.5 Heavy Pollution in Winter in Qingdao City

Qingdao, a northern coastal city previously with little air quality issue, has recently been invaded by severe air pollution. There were seven heavy air pollution days in January 2014 in Qingdao City. During this month, a severe PM_2.5 pollution event lasted four days from 15^th to 18^th January, and the other three single day event occurred on the day of 6^th, 11^th and 30^th, respectively. In order to understand the modulations of meteorological conditions on the development, persistence and removal of the long-lasting heavy pollution event, the spatial-temporal meteorological characteristics in the planetary boundary layer (PBL) during the period of 15^th to 18^th January were investigated by using Lidar, atmospheric stability monitoring as well as surface and upper-air meteorological and environmental observations. The results show that the weak cold air affected Qingdao and near surface wind speed less than 3 m/s were not conducive to diffusion of pollutant in January in Qingdao. On the other hand, relative humidity more than 66% was in favor of the increasing of pollutant concentration. Under stable emission conditions, different meteorological conditions may lead to the spatial-temporal differences of air pollutant. Under the situation of lasting weak northerly wind, the concentration of pollutants was higher and tended to be lower on the situation of southerly wind. The 4-day event was mainly caused by the following characteristics in the PBL: wind shear with cold-weak northern wind in the higher level and warm-humid southern wind in the lower level, stable vertical structure, high relative humidity with 70% at low level and weak near-surface southerly and northerly wind alternation. The effect of the PBL height variations on the persistence of high pollutant seems to have a lag of 6 hours. However, good temporal agreement exists among lower PBL height, larger stability factor and low clouds. We found that with the increase of near-surface temperature, decrease of relative humidity, enhanced southerly wind as well as the weak unstable layer existed in the PBL were helpful for the improvement of atmospheric diffusion ability in Qingdao.