| The Beijing-Tianjin-Hebei region has always been one of the regions with the most serious air pollution in China.The pollution situation is serious and lasts for a long time,it is a key area for the government to fight against the blue sky.It is also a hot spot for many scholars to study air pollution.Aerosol Optical Depth(AOD)is defined as the integral of the extinction coefficient of the medium in the vertical direction.It is an important factor for measuring aerosol and can be used to characterize atmospheric turbidity or total aerosol content.It can effectively reflect the degree of regional pollution.Based on this,this study used the 2010-2016 MODIS 3km AOD data to analyze the temporal and spatial variation characteristics of AOD and its influencing mechanism in the Beijing-Tianjin-Hebei region,which provides a reference for air pollution control.The temporal and spatial characteristics of AOD in the Beijing-Tianjin-Hebei region in the past 7 years were analyzed at first.The results showed that:(1)The monthly change trend of the year showed an obvious "W"-type distribution.The high-value month of AOD were January,February,June,July and December,The low-value month of AOD were April,May,September,and October,the highest was January and the lowest was September.Moreover,the AOD values were more obvious in different seasons.AOD was the highest in winter,and it had been stable and high In the past 7 years,followed by summer,showing a decreasing trend,the AOD value in spring was low and it had a decreasing trend,and the AOD value in autumn was the lowest,and it had a significantly increased trends.The average annual value of AOD in the Beijing-Tianjin-Hebei region from 2010 to 2016 had not changed much,and it basically fluctuated around 0.82.(2)The spatial distribution characteristics of AOD in Beijing-Tianjin-Hebei region from 2010 to 2016 indicated that the space was generally high in the south and low in the north,and the high-value areas were concentrated in three regions,namely Beijing,Tianjin,and southern Hebei.The low value area of AOD was located in the mountainous area in the northwest of the study area.The spatial distribution of AOD in different years had certain differences.(3)The spatial evolution trend of AOD indicates that the air quality in most areas of Beijing-Tianjin-Hebei region remained at the original level in 2010-2016.About one-third of the study area had a slight improvement.A few areas were showing an increase.The air quality of the Beij ing-Baoding-Shij iazhuang-Xingtai-Handan line had deteriorated significantly;the air quality of Chengde-Zhangjiakou,Tangshan-Qinhuangdao,the eastern part of Cangzhou-Hengshui and the Taihang Mountains had improved.The evolution trends of different cities were quite different.(4)Analysis of the center of gravity transfer model indicates that the focus of the hot spot was located in the western border area of Cangzhou City.The center of cold spot was located at the junction of the northern part of Beijing and Zhangjiakou City.The center of the insignificant area was located at the junction of Beijing,Langfang and Baoding.In the past seven years,the overall center of gravity pattern in the Beijing-Tianjin-Hebei region has changed little,and the hot-spot and cold-spot migrations had generally moved southward and then moved northward,and the insignificant migration and hot-spot trajectories were opposite.The specific environment in each year caused the direction and speed of gravity movement in different years to be different.On this basis,combined with the precipitation,wind speed,relative humidity data in the meteorological site,GDP,secondary industry GDP data,and population density data in the statistical yearbook and MOD 13 NDVI data,the impact factors of AOD were analyzed from multiple perspectives.The influence factors of the temporal and spatial changes of AOD in the Beijing-Tianjin-Hebei region were firstly revealed by correlation analysis from the regional and pixel scales to reveal the strength of different AOD forces.Then the geo-detector tool was used to analyze the trend of the contribution rate of impact factors in different regions and their dominant influence factors.The results of correlation analysis and geo-detector analysis showed that the population density,the secondary industry and other human factors were dominant and the natural factors were second,the interaction between the two was greater than the single factor in the Beijing-Tianjin-Hebei region.Beijing was mainly affected by NDVI and population density.It was necessary to expand the vegetation cover and effectively exert its positive effects.At the same time,it should also control the discharge of human activities.Tianjin was mainly affected by wind speed,population density and secondary industry.It should focus on prevention and control of construction,soil dust,industrial emissions and vehicle emissions.Hebei province was mainly dominated by human factors,except for Qinhuangdao,the remaining 10 cities must adjust their industrial structure and reduce industrial emissions.Shijiazhuang,Zhangjiakou,and Cangzhou should optimize energy structure and control emissions from human production and living pollution. |
