Custom Aerosol Type Based On 6S Model In Beijing Area Inversion Of Aerosol Optical Depth

In recent years,urban air pollution has become increasingly serious,and the heavy haze weather frequently appears in some cities,which affects the living standards and health of the people.Aerosol content is an important index to measure the air quality of the city,which is significant to study the temporal and spatial distribution of air pollution.Especially,the inhalable particulate matter smaller than2.5?m(PM_2.5)has become a research issue,because it has greatest damage to human respiratory system.Aerosol optical depth?AOD?is a crucial physical parameter which is used to describe the content and optical properties of atmospheric aerosol,and it can measure the pollution degree of the city.In this paper,taking the Beijing area as an example,we inverted the AOD by using the MODIS data and the ground-based observation data from ARRONET.Next,combined with the PM_2.5 measured data of12 ground-based air quality monitoring station in Beijing,a regression analysis was established with the AOD obtained from the inversion of a custom model,and the response relationship between AOD and PM_2.5 was studied by height and humidity correction.Specific research contents and conclusions are as follows:?1?According to the climatic conditions and regional characteristics of the Beijing area,we predefined the volume percentage ranges of aerosol components?dust,water,ocean,soot?.By combining MODIS data with ground-based solar photometer observation data,this study used Monte Carlo method and 6S radiation transmission model to obtain a custom aerosol type during the summer half year in the Beijing area.The results showed that the water-soluble particles accounted for the largest proportion with 60%,followed by the proportion of dust particles is 39%,and the proportion of soot particles is 1%.?2?In order to test the applicability of the dark target method,we simulated the sensitivity of the red,blue and mid-infrared channels of MODIS by using 6S model.Results showed that the sensitivity of apparent reflectance to AOD in red,blue and mid-infrared bands is different under different aerosol types.Specifically,the apparent reflectance of the custom type and the continental type are both sensitive to the surface reflectance and AOD in the red and blue bands,while the apparent reflectance in the mid-infrared band is not sensitive to AOD.Contrary to the former two types,urban type is not suitable for inversion of AOD in the Beijing area.?3?The AOD of summer half year in the Beijing area was inversed by using the custom aerosol type and built-in continental and urban aerosol types provided by 6S model.Comparing the inversion results with observed values,the results showed that the relative error of custom aerosol type is minimal and the effective range of absolute error is within±0.05±20%?,which indicated that its inversion value is closest to the observed value.Therefore,the inversion method of the custom aerosol type proposed in this paper is more suitable for the Beijing area than another two built-in types in 6S model,and it has advantage of high inversion accuracy,reliable algorithm and quite appliance value.?4?Based on the AOD inversion images obtained from the custom aerosol type,this paper analyzed the temporal and spatial distribution characteristics of AOD in the Beijing area.The results showed that the distribution of AOD presents a semi-enclosed structure,and the AOD in the central and southern urban areas is significantly higher than the north.From the point of topography,Beijing is located in a semi-enclosed terrain on the east side of Taihang mountain and the south side of Yanshan mountain.Affected by the downdraft of the leeward slope,the wind field is converged in the front of the mountain,leading the pollutants to accumulate in front of the mountain,which is not conducive to the spread of pollutants.Moreover,the central downtown of Beijing is situated in the south central administrative region,in which has dense population,high degree of industrialization and dense transportation networks,therefore,the local accumulation of man-made pollutants is also the main reason for high values in the south.?5?The response relationship between AOD and PM_2.5 obtained by inversion of custom model was studied,and it was found that there was a linear correlation between AOD and PM_2.5 data,and the correlation coefficient was 0.64.Then,using the boundary layer height and relative humidity respectively to correct the AOD.It is found that the contribution of the humidity factor to the correlation is higher than the height factor,mainly due to the high proportion of water-soluble fine particles in the air.Finally,two correction factors were introduced into the correlation analysis at the same time,and it was found that the two-factor correction significantly improved the correlation,and the correlation coefficient is 0.81,which is 0.16 higher than before the correction.Select the case analysis to correct the accuracy of the model before and after correction,and the average absolute error of the inversion after height and humidity correction is reduced from 18.4?g/8)³to 8.7?g/8)³.It is proved that remote sensing data can improve the accuracy of quantitative estimation of PM_2.5 through humidity and height correction,which is beneficial to study the distribution characteristics of PM_2.5 in space region and provides an effective supplementary means for ground monitoring.