Evaluation Of MODIS C006 Aerosol Products Over The Beijing-Tianjin-Hebei Region And The Exploration Of Retrieval Algorithm

Some important improvements of algorithms are included in the MODerate resolution Imaging Spectroradiometer?MODIS?Collection 6?C006?aerosol retrievals in comparison with the previous Collection 5.1?C051?.A finer scale Aerosol Optical Depth?AOD?,a nominal 3 km product,is also introduced in the C006.The MODIS C051 retrievals have been validated regionally and globally.However,few regional validations of C006 have been conducted.It is worth noting that the MODIS C006 AOD retrievals should be validated completely before they will be applied to identifying the regional air pollution.On basis of the necessity,the following researches have been carried out.A scale of temporal-spatial matching in this region has been proposed.This study evaluates performance of the MODIS-Aqua C051 and C006 AOD retrievals and the 3km retrievals over the Beijing-Tianjin-Hebei region in China from 2006 to 2015 using the AERONET?AErosol RObotic NETwork?data.The distribution and variation of AOD over Beijing-Tianjin-Hebei based on MODIS-Aqua C006 10 km and 3 km data set was analyzed.A vector radiative transfer model?6SV?was used to simulate the Top of Atmosphere?TOA?reflectance and polarized reflectance in accordance with improvement of algorithm in wind speed from C051 to C006.The main conclusions are as follows:?1?For the DT?Dark Target?algorithm products,both DT C051 and DT C006AOD values are overestimated over the two sites,which are caused by the underestimation of surface reflectance and aerosol single albedo.For the DB?Deep blue?algorithm products,enhanced DB algorithm?C006?AOD retrievals perform better than those of DB C051,as a better correlation is observed between the DB C006 retrievals and AERONET.The spatial distribution of the AOD values is high over the southeastern region?⁰.7?and low over the northwestern region?⁰.3?.Although the seasonal geographical distribution exhibits similar spatial patterns as the yearly mean distribution,the AOD values are higher in spring and summer and relatively low in autumn and winter.?2?The 3 km AOD retrievals are also overestimated over this region.The finer resolution AOD products slightly outperform the 10 km products over vegetated surface?Xianghe and Xinglong sites?.Conversely,the 3 km products perform worse than the 10 km,especially in the bright urban surface.The two products perform worse during spring and summer.Due to the retention of some bright pixels in retrieval window which should be removed in 10 km,the 3 km retrievals introduce some“noise”.?3?In the direction of specular reflection,the TOA reflectance decreases with the increase of wind speed.The TOA reflectance increases with the increase of wind speed when deviates the reflection direction.The changes?⁰.002?of TOA reflectance caused by chlorophyll a concentration cannot be ignored and the polarization reflectance is not sensitive to chlorophyll a concentration(?R_p<10^-4).