Research On The Temporal And Spatial Distribution Characteristics Of NO₂ Gas In Beijing Area Based On MAX-DOAS

As the capital of our country,Beijing has an absolute core position in politics,economy and culture.Nowadays,a series of air pollution problems brought about by the rapid development of urbanization and industrialization are particularly evident in Beijing and its surrounding areas.Nitrogen dioxide（NO₂）,as an important atmospheric trace gas,not only plays an important role in the physical and chemical operation mechanism of the atmospheric environment,but is also directly related to air quality,which affects human health at all times.Therefore,it is necessary to carry out long-term continuous effective monitoring of atmospheric NO₂concentration in Beijing area and adopt relevant control methods,and it is also of great value in the subsequent air pollution prevention and control work.Ground-based multi-axis differential absorption spectroscopy（MAX-DOAS）technology is a typical optical remote sensing technology emerging in recent years.It can retrieve and analyze trace gas column concentration and vertical distribution information,with high time resolution,high sensitivity,and wide measurement range.And the advantages of not being restricted by the construction of the platform have been widely adopted in recent years.Based on MAX-DOAS technology,this thesis conducted a monitoring experiment on atmospheric trace gases in Beijing.With NO₂as the research object,combined with atmospheric radiation transmission model,profile inversion algorithm and backward trajectory model to analyze its temporal change characteristics and vertical distribution Laws and potential sources of pollution.The main conclusions are as follows:1.The vertical column concentration（VCD）of tropospheric NO₂is inverted by MAX-DOAS technology,and the characteristics of the change of NO₂concentration over time are analyzed accordingly.The results show that the tropospheric NO₂VCD in Beijing area changes significantly with the seasons,presenting a trend of highest in winter and lowest in summer.The average value of winter season reaches2.94×10¹⁶molec·cm^-2,which is 1.6 times that of summer.The daily average change in different seasons is generally There was a significant difference in the afternoon,with a maximum difference of 2.17×10¹⁶molec·cm^-2.There is a certain regularity in the daily concentration changes during a week,and the average concentration on Sunday is about 17%lower than other times,and a certain degree of weekend effect appears.2.Based on MAX-DOAS observations,the VCD was inverted using two methods:geometric approximation and SCIATRAN radiation transmission model.The two methods respectively calculate NO₂VCD in summer and winter.Because VCD_GEodoes not consider factors such as atmospheric temperature,aerosol conditions,and surface albedo,it will be overestimated or underestimated compared to VCD_Mod.The correlation coefficient R between the results of VCD_Modand VCD_Geoin summer was 0.78,and in winter was 0.91.The daily and monthly average correlation coefficients for the whole year reached 0.91 and 0.84,respectively,showing good correlation.The results show that the geometric approximation method is a reliable and effective method for calculating tropospheric VCD,and it is relatively more accurate when calculating winter VCD.3.By comparing the VCD results of ground-based MAX-DOAS observations with the ground-based national control official park monitoring station（116.339°E,39.929°N）point measurement results,the correlation coefficient R of the annual average daily value of the two numerical concentrations is 0.74.The monthly mean correlation coefficient R is 0.91,and the comparison shows that the overall trend of the two is very consistent.Studies have shown that the ground-based MAX-DOAS technology is an effective method for analyzing changes in NO₂pollution with seasons and time,with high accuracy and reliability.4.Combined with the profile inversion algorithm HEIPRO,the vertical structure of NO₂is analyzed.The analysis results of the correlation between the bottom concentration of the profile and the concentration measured by the national control point show that the correlation R between the two can reach 0.76,and the inverted profile results have a high degree of reliability.The highly layered information of the profile shows that the bottom layer of the profile usually has a high NO₂concentration,while the upper level of the profile shows a low concentration.The ratios of the average concentration of the top and middle layers of NO₂profile to the bottom layer in summer are 17.59%and 37.06%,respectively,and in winter are 26.85%and 50.70%,and the concentrations of each layer in winter are generally higher than those in summer.This may be related to the lowering of the boundary layer in winter and the large accumulation of pollutant gases in the entire boundary layer,especially near the ground.5.Combined with the model HYSPLIT,backward trajectory clustering,potential source contribution function（PSCF）and concentration weighted trajectory（CWT）analysis can be carried out.Based on this,the potential source area of NO₂ in the Beijing area is explored and found that under the influence of the Beijing monsoon climate,the local winter The air mass trajectory mainly comes from the long-distance transmission in the northwest,while the main air mass in summer comes from the southeast periphery.Regardless of whether it is winter or summer,NO₂pollution in Beijing is mainly affected by local and neighboring provinces,coastal ports and maritime shipping emissions,and a certain degree of external transportation under the influence of monsoon.