Research On Characteristics Of Air Pollution And Typical Pollution Processes In The Beijing-Tianjin-Hebei Region

In view of the severe air pollution in the Beijing-Tianjin-Hebei（BTH）region at present,O₃ and PM_2.5were analyzed as the major pollutant in this region.The main contents include:based on the air quality data in the BTH region from 2014 to 2017,combined with the conventional meteorological observation data and NCEP/NCAR reanalysis data of the same period,the spatio-temporal characteristics of O₃ and PM_2.5were mainly analyzed and the relationship of meteorological elements and pollutant concentration were evaluated.In addition,we focused on the analysis of the pollution process and atmospheric circulation background of the high-pollution seasons of PM_2.5and O₃.On this basis,a comparative study was conducted on the meteorological conditions during the London smog event and the Los Angeles photochemical smog event at the same time to reveal the relationship between meteorological conditions and air pollution from a historical perspective.The mainly results show that:（1）PM_2.5 concentration in the BTH region decreased from 2014～2017,but it was much higher than the secondary standard limit.The 95th percentile and annual average of PM_2.5concentration of maximum multiples exceeding standard were 1.79 and 1.60,respectively.In2017,10 cities including Baoding and Shijiazhuang had heavy pollution for more than 20percent of the total number of exceeding days,indicating that ultra-high concentration of PM_2.5still remained the focus of prevention and control in the future.At the same time,O₃-8h increased,with an average annual increase of 4.50μg/m³.O₃ pollution situation in the whole region was aggravated,especially in Beijing and Baoding.The monthly O₃ concentration and the over-standard situation mainly presented single-peak variations in 2014～2015,with the maximum in May.However,the irregular double-peak variations were found in 2016～2017,showing one peak value during May to June and the other in September.（2）Winter and summer are the seasons with the highest concentrations and most over-standard days of PM_2.5 and O₃ in the BTH region.The correlation between O₃ and PM_2.5concentrations and meteorological factors indicated that the influence of meteorological factors on O₃and PM_2.5 had obvious seasonal differences.In spring,autumn and winter,the relative humidity and wind speed were the main factors affecting PM_2.5concentration,while temperature was the main factor in summer.But O₃concentration was affected by temperature in spring,summer,autumn,and relative humidity wind speed in winter.Besides,it was indicted that the thresholds for the formation of high PM_2.5 and O₃ concentrations in Beijing,Tianjin and Shijiazhuang were different in winter and summer.（3）Studies of the typical pollution processes in the BTH region and London in winter suggested that the circulation configurations were relatively similar.The geopotential heights at the upper level and near surface were anomalously high,the surface wind speed was relatively low,the local humidity was high,and there both existed vertical descending movements and anomalous temperature inversion layers.The difference of the meteorological conditions was that the London smog event was more affected by the temperature inversion,meanwhile,the warm high-pressure change at the mid–upper level was more obvious and its intensity was stronger,while the oceanic cold high-pressure at the low level was also strong.This kind of configuration was more conducive to the stable maintenance of temperature inversion.In addition,during the BTH pollution processes,there was an obvious humidification phenomenon in the atmosphere,but the humidity conditions did not change significantly and even a dehumidification phenomenon occurred during the pollution process in London,which was closely related to the different climatic conditions in the two places.（4）When the typical O₃ pollution processes occurred in the BTH region and Los Angeles in summer,the situation at both upper level and near surface were relatively similar.The geopotential heights at the upper level was anomalously high,in which Los Angeles was under the control of the strong high pressure system.The surface wind speed and relative humidity were relatively low,the air temperature was relatively high,and existed anomalous temperature inversion layers,which made the atmosphere surface layer more stable.In addition,O₃ was mainly generated by photochemical reactions and local accumulation in Los Angeles photochemical smog event;In the BTH region were affected by a variety of factors,photochemical reactions,horizontal transport and local accumulation,they together lead to continuous O₃ exceeding concentration.