2016年12月京津冀一次重污染天气过程分析

Analysis of A Heavily Polluted Event in Beijing-Tianjin-Hebei Region in December, 2016

2016年12月16—21日,京津冀地区出现了一次覆盖面积广、污染程度重、污染持续时间长、雾和霾天气相伴发生的重污染过程,北京于16日发布空气重污染红色预警。文章利用MODIS(Moderate Resolution Imaging Spectroradiometer)反演的气溶胶光学厚度(Aerosol optical depth,AOD),OMI(Ozone Monitoring Instrument)反演的气溶胶指数(Aerosol Index,AI),AERONET(AErosol RObotic NETwork,AERONET)观测的AOD、ERA-Interim风场和FNL边界层高度、及实况观测数据对此次重污染过程的发生、发展过程及可能原因进行分析,探讨污染物的本地排放和区域输送在不同阶段的贡献率,结论如下,此次污染过程的影响范围从河北中南部逐渐向北延伸加重,平原大部分地区能见度持续低于5 km,污染物浓度连续5 d(18—22日)超过150μg·m^(-3)。17日开始,AOD与AI在河北中南部及北京南部地区较高,均超过2.0,伴随风场和辐合线在北京南部和河北中南部南北摆动。16—19日,污染物的外地输送及固定源排放对北京和天津地区污染的发生发展和污染物的累积贡献较大,而本地污染物的排放和移动源的贡献比重自19日起逐渐增加,可能与京津冀地区采取联动减排有关。随着污染物质量浓度的增加,不同波长的气溶胶AOD均呈增加趋势,细粒子对AOD的贡献率也随之增加,有雾凇和弱冷空气出现的时段除外。稳定的气象条件、较高频率的静风和小风(尤其南风)、持续较低的边界层高度则是污染过程长时间维持的重要气象条件。同时,较高的相对湿度与颗粒物浓度是造成低能见度的重要原因,且经过线性与非线性拟合分析发现,同样的相对湿度条件下,PM_(2.5)对能见度的影响高于PM_(10)。

A heavily polluted event occurred in Beijing-Tianjin-Hebei （BTH） Region in December 16-21, 2016. The dense haze clouds covering the BTH Region during the severely polluted period were mixed with fogs, leading to the visibility below 5 km for almost one week. In this study, we investigated the formation process of the extreme air pollution event in BTH Region in December 2016, using satellite retrievals of Aerosol Optical Depths （AOD） and Aerosol Index （AI）, ground measurements of Aerosol Robotic Network （AERONET） AOD, wind data from ERA-interim, planetary boundary layer （PBL） heights from NCEP FNL reanalysis and other meteorological and aerosol observations. The haze clouds initially exhibited large spatial variations with peak aerosol concentrations in the north part of the BTH Region. In December 18—21, high AOD and AI values were observed in the south part of Beijing and the south-central Hebei province, which were driven by repeated alternation of airflows from different directions from the surface layer to 700 hPa, indicating accumulated aerosols and prevalent absorbing aerosols in these areas. Before December 19, the contributions of regional transports and residential emissions to aerosol concentrations in Beijing were higher than those of local emissions, while the contributions of local accumulated pollution and transportation sources increased after December 19, resulting from the regional emission reductions. Results also revealed that the AOD values and contributions of fine particles to total aerosols increased with enhanced aerosol mass concentrations, except for the events with weak cold air or rime. Stable synoptic conditions, low PBL heights, strong thermal inversion and weak south wind played important roles in the formation and persistence of the severe haze event. High relative humidity and aerosol concentrations were the main causes of low visibility. Using the method of fitting analysis, we found that the impacts of PM2.5 on visibility were higher than those of PM10 with the same relative humidity.