Evaluation of Moderate-Resolution Imaging Spectroradiometer (MODIS) Collection 004 (C004) aerosol retrievals at Kanpur, Indo-Gangetic Basin

[1] We have compared the spectral aerosol optical depth (AOD, τλ) and aerosol fine mode fraction (AFMF) of Collection 004 (C004) derived from Moderate-Resolution Imaging Spectroradiometer (MODIS) on board National Aeronautics and Space Administration's (NASA) Terra and Aqua platforms with that obtained from Aerosol Robotic Network (AERONET) at Kanpur (26.45°N, 80.35°E), India for the period 2001–2005. The spatially-averaged (0.5° × 0.5° centered at AERONET sunphotometer) MODIS Level-2 aerosol parameters (10 km at nadir) were compared with the temporally averaged AERONET-measured AOD (within ± 30 minutes of MODIS overpass). We found that MODIS systematically overestimated AOD during the pre-monsoon season (March to June, known to be influenced by dust aerosols). The errors in AOD at 0.66 μm were correlated with the apparent reflectance at 2.1 μm (ρ*_2.1) which MODIS C004 uses to estimate the surface reflectance in the visible channels (ρ_0.47 = ρ*_2.1/4, ρ_0.66 = ρ*_2.1/2). The large errors in AOD (Δτ_0.66 > 0.3) are found to be associated with the higher values of ρ*_2.1 (0.18 to 0.25), where the uncertainty in the ratios of reflectance is large (Δρ_0.66 ± 0.04, Δρ_0.47 ± 0.02). This could have resulted in lower surface reflectance, higher aerosol path radiance and thus lead to overestimation in AOD. While MODIS-derived AFMF has binary distribution (1 or 0) with too low (AFMF < 0.2) during dust-loading period, and ∼1 for the rest of the retrievals, AERONET showed range of values (0.4 to 0.9). The errors in τ_0.66 were also high in the scattering angle range 110°–140°, where the optical effects of nonspherical dust particles are different from that of spherical particles.