Spatio-temporal Change And Cause Analysis Of NO₂ Column Density In Beijing-tianjin-hebei Region Based On Multi-source Remote Sensing Data

With the accelerated urbanization of Beijing-Tianjin-Hebei region,it is urgent to solve air pollution problems caused by economic development and to accurately obtain spatial and temporal distribution of NO₂density,in order to provide scientific basis for collaborative management and joint prevention and control in Beijing-Tianjin-Hebei region.The shortcomings of NO₂ground monitoring stations are uneven distribution,sparse stations over ocean,forests and polar regions,and inability to conduct all-round three-dimensional monitoring.This study used the advantages of remote sensing data with wide coverage,easy access and dynamic monitoring,to validate remote sensing data in combination with ground data.Using OMI remote sensing data of Aura satellite,spatial and temporal changes of NO₂column density in Beijing,-Tianjin-Hebei region from 2005 to 2020were analyzed,and changes of NO₂column density during the epidemic were analyzed based on the current highest resolution TROPOMI remote sensing data of Sentinel-5P satellite.The reasons of NO₂column density change were analyzed from six aspects of natural factors（topography,wind speed,wind direction and land cover types）and anthropogenic factors（motor vehicle ownership,industrial structure and factory agglomeration）.HYSPLIT model was applied to analyze the clustering trajectories of typical cities（Beijing,Shijiazhuang,Tangshan and Chengde）at different pollution levels to understand pollution transmission channels Concentration-weighted Trajectory（CWT）method was used to analyze the distribution and contribution values of potential source areas of NO₂external pollution at city scale and seasonal scale.The main conclusions are as follows:（1）The inter-annual spatial distribution of NO₂column density in Beijing,Tianjin and Hebei from 2005 to 2020 shows a clear spatial heterogeneity within the region.There are obvious differences between the north and south,with low concentrations in the Taihang Mountains in the west and northern Yanshan area,and high concentrations in the southeastern plains.From 2005 to 2007,the area of high density continued to expand,with higher density expanding over a large area and showing a trend of continuous distribution;from 2008 to2014,the overall high value area showed a fluctuating trend of expansion,forming the"Shijiazhuang-Xingtai-Handan"and"Beijing-Tianjin-Tangshan"areas.From 2008 to 2014,the overall high value area showed a fluctuating trend of expansion,forming two major high density areas:"Shijiazhuang-Xingtai-Handan"and"Beijing-Tianjin-Tangshan";from 2015 to2020,the high value area disappeared and the higher density area shrank to a large extent.The seasonal spatial distribution shows obvious seasonal differences and cyclical fluctuations,showing that summer<spring<autumn<winter,and the distribution of high density areas in winter is the largest and shows a continuous trend.（2）The temporal change of NO₂column density in Beijing-Tianjin-Hebei region from2005 to 2020 was analyzed,.The monthly average value of NO₂column density showed yearly"U"shaped cyclical fluctuation with obvious seasonality,and there were peaks and valleys in each cycle.The highest NO₂column density occurred in January 2013 at42×10¹⁵molec/cm²and the lowest value occurred in July 2020 with a NO₂column density of3.56×10¹⁵molec/cm².The seasonal variation of NO₂column density values was winter>autumn>spring>summer.The NO₂column density fluctuated greatly in winter and autumn,spring and summer fluctuated are stable。The maximum NO₂value was 13.40×10¹⁵molec/cm²in 2013 and the lowest NO₂value was 8.75×10¹⁵molec/cm²in 2017.（3）NO₂column density in Beijing-Tianjin-Hebei region during the COVID-19 epidemic in Shijiazhuang in 2021 decreased by 2.40×10¹⁵molec/cm²compared with the same period in2019.At city scale,NO₂column density in Shijiazhuang decreased the most with decrease rate of 56.97%,followed by Xingtai city,but NO₂column density in Chengde and Zhangjiakou cities increased by 35.40%and 49.55%respectively compared with the same period in 2019.In terms of spatial variation,there was no regional distribution of high NO₂density in Beijing-Tianji-Hebei region during the epidemic period,93%reduction in higher density areas compared to 2019。with 93%fewer higher concentration areas than in 2021.The COVID-19 epidemic had greater impact on NO₂density in Shijiazhuang and Xingtai cities.（4）The reasons of serious pollution of NO₂density in Beijing-Tianjin-Hebei region was analyzed mainly from both natural and anthropogenic aspects.Among natural factors,topography of Yanshan and Taihang mountains leaded to the accumulation of NO₂pollutants.Wind speed and direction had significant effect on NO₂column density in Beijing-Tianjin-Hebei region;precipitation was significantly correlated negatively with NO₂column density,the more precipitation,the lower NO₂column density.In terms of land use types,the highest NO₂column density was on urban land,and the order of NO₂column density was urban land>agricultural land>water>unused land>forest land>grassland.Among socio-economic factors,motor vehicle ownership,industrial structure and factory agglomeration had greater influence on NO₂density.（5）The backward trajectory clustering using Traj Stat module in Meteinfo showed that the longer and faster of air masses trajectory,the stronger the transport and dilution effect on pollutants.At interannual scale,the pollution pathways of typical cities（Beijing,Shijiazhuang,Tangshan and Chengde）at different pollution levels in Beijing-Tianjin-Hebei region were mainly southeast short-range transport and northwest long-range transport.The northwest pathways started from Russia,passed through Outer and Inner Mongolia and reached the typical cities,while the southeast pathways varies from region to region.At seasonal scale,the northwest airflow trajectory was absolutely dominant in winter,and the northwest trajectory lengths of Beijing and Shijiazhuang cities were larger than those of Tangshan and Chengde cities,the trajectories of Shijiazhuang city were more aggregated.（6）The analysis of potential source areas of NO₂density using CWT method showed that the distribution of potential source areas of NO₂density in Beijing,Shijiazhuang,Tangshan and Chengde cities had obvious variability at interannual scale,and the contribution of potential source areas of NO₂density showed increase trend from surrounding areas to the center.At seasonal scale,the areas with high contribution values of NO₂density potential source areas in Beijing,Shijiazhuang and Tangshan cities in winter showed strip-like distribution,the WCWT value was 80～100μg/m³and pollution source areas was concentrated.