Systematic bias in MODIS dust aerosol retrieval at Kanpur (AERONET) Indo-Gangetic Basin

We have compared the spectral aerosol optical depth (AOD) and aerosol fine mode fraction (AFMF) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) with those of Aerosol Robotic Network (AERONET) at Kanpur (26.45N, 80.35E), northern India for the pre-monsoon season (March to June, 2001-2005). We found that MODIS systematically overestimates AOD during pre-monsoon season (known to be influenced by dust transport from north-west of India). The errors in AOD were correlated with the MODIS top-of-atmosphere apparent surface reflectance in 2.1 μm channel (ρ*2.1). MODIS aerosol algorithm uses (ρ*2.1) to derive the surface reflectance in visible channels (ρ0.47, ρ0.66) using an empirical mid IR-visible correlation (ρ0.47= ρ2.1/4, ρ0.66 ρ2.1/2). The large uncertainty in estimating surface reflectance in visible channels (Δρ0.66±0.04, Δρ0.47±0.02) at higher values of ρ*2.1 (ρ*2.1>0.18) leads to higher aerosol contribution in the total reflected radiance at top-of-atmosphere to compensate for the reduced surface reflectance in visible channels and thus leads to overestimation of AOD. This was also reflected in the very low values of AFMF during pre-monsoon whose accuracy depends on the aerosol path radiance in 0.47 and 0.66 μm channels and aerosol models. The errors in AOD were also high in the scattering angle range 110°-140°, where the effect of dust non-spherity on its optical properties is significant. The direct measurements of spectral surface reflectance are required over the Indo-Gangetic basin in order to validate the mid IR-visible relationship. MODIS aerosol models should also be modified to incorporate the effect of non-spherity of dust aerosols.