基于车载雷达探测的一次华北冬季重污染天气成因研究

Study on a heavy pollution episode in winter in North China based on lidar detection

基于车载微脉冲气溶胶激光雷达、多普勒风廓线激光雷达和扭转拉曼廓线激光雷达的中山大学环境气象综合观测车,于2018年12月18日-22日在河北省望都县PM_(2.5)重污染期间开展定点观测.结合地面PM_(2.5)浓度和气象要素观测资料,对本次污染过程的成因展开分析.本次重污染过程日均PM_(2.5)浓度为163.2μg·m^(-3),PM_(2.5)浓度的日变化特征明显,表现为白天PM_(2.5)浓度降低,傍晚至次日早晨PM_(2.5)浓度升高.气溶胶激光雷达观测结果发现,污染期间,700 m高度以下存在明显的消光系数高值区;夜间存在明显的消光系数高值区分层现象,气溶胶消光系数高值区出现高度可达1700 m.本次PM_(2.5)重污染过程受静稳边界层气象条件和高空气溶胶输送、沉降共同影响.在污染时段内,大气边界层低层小风持续,近地面和大气低层逆温和同温层频发,静稳边界层条件不利于PM_(2.5)的输送和扩散;此外,夜间高空气溶胶伴随强西风带出现,随着动量通量的向下传输而沉降,使地面PM_(2.5)浓度升高.

A field campaign was conducted during a severe PM_(2.5)pollution episode in North China in December 18-22,2018.The Environment and Meteorology Mobile Observation Vehicle equipped with a micro pulse lidar,a Doppler wind profile lidar,and a Raman temperature profile lidar was used in the campaign,detecting aerosol extinction and polarization profiles,wind profiles and temperature profiles.Additionally,the formation mechanisms of the pollution episode was investigated with the aid of surface PM_(2.5)concentration and meteorological observations.The daily averaged PM_(2.5)concentration reached 163.2μg·m^(-3)during the episode,and it was found that hourly PM_(2.5)concentration always increased from sunset to the next day morning,while decreased during daytime.A high extinction layer always existed below 700 m during the episode.The vertical decoupling of high extinction layer was observed during the entire nighttime,simultaneously appearing at 1700 m and lower altitudes.Stagnant boundary layer condition and the upper transport and deposition of aerosol were responsible for this pollution episode.During the episode,weak wind maintained at the low-level atmospheric boundary layer,temperature inversion and isothermal layer frequently appeared,resulting in the disadvantage transport and dispersion of surface PM_(2.5).Besides,strong westerly higher than 1000 m accompanied aerosol transport,which deposited along with the downward momentum flux,leading to the increase of surface PM_(2.5)concentration.