Analysis of Visible/SWIR surface reflectance ratios for aerosol retrievals from satellite in Mexico City urban area

摘要

The surface reflectance ratio between the visible (VIS) and shortwaveinfrared (SWIR) radiation is an important quantity for the retrieval of theaerosol optical depth (τ_a) from the MODIS sensor data. Based onempirically determined VIS/SWIR ratios, MODIS τ_a retrieval usesthe surface reflectance in the SWIR band (2.1 µm), where theinteraction between solar radiation and the aerosol layer is small, topredict the visible reflectances in the blue (0.47 µm) and red (0.66 µm) bands. Therefore, accurate knowledge of the VIS/SWIR ratio isessential for achieving accurate retrieval of aerosol optical depth fromMODIS. We analyzed the surface reflectance over some distinct surface coversin and around the Mexico City metropolitan area (MCMA) using MODIS radiancesat 0.66 µm and 2.1 µm. The analysis was performed at 1.5 km×1.5 km spatial resolution. Also, ground-based AERONET sun-photometer dataacquired in Mexico City from 2002 to 2005 were analyzed for aerosol depth and other aerosol optical properties. In addition, a network ofhand-held sun-photometers deployed in Mexico City, as part of the MCMA-2006Study during the MILAGRO Campaign, provided an unprecedented measurement ofτ_a in 5 different sites well distributed in the city. We foundthat the average RED/SWIR ratio representative of the urbanized sitesanalyzed is 0.73±0.06 for scattering angles <140° and goes up to0.77±0.06 for higher ones. The average ratio for non-urban sites wassignificantly lower (approximately 0.55). In fact, this ratio stronglydepends on differences in urbanization levels (i.e. relative urban tovegetation proportions and types of surface materials). The aerosol opticaldepth retrieved from MODIS radiances at a spatial resolution of 1.5 km×1.5 kmand averaged within 10×10 km boxes were compared with collocated 1-hτ_a averaged from sun-photometer measurements. The use of the newRED/SWIR ratio of 0.73 in the MODIS retrieval over Mexico City led to asignificant improvement in the agreement between the MODIS andsun-photometer AOD results; with the slope, offset, and the correlationcoefficient of the linear regression changing from (τ_aMODIS=0.91τ_{a sun-photometer}+0.33, R2=0.66) to (τ_aMODIS=0.96 τ_{a sun-photometer}−0.006, R2=0.87).Indeed, an underestimation of this ratio in urban areas lead to asignificant overestimation of the AOD retrieved from satellite. Therefore,we strongly encourage similar analyses in other urban areas to enhance thedevelopment of a parameterization of the surface ratios accounting for urbanheterogeneities.