Comparison of MODIS and AERONET derived aerosol optical depth over the Ganga Basin, India

Abstract. The Moderate Resolution Imaging Spectroradiometer (MODIS) onboard EOS Terra measures global aerosol optical depth and optical properties since 2000. MODIS aerosol products are freely available and are being used for numerous studies. In this paper, we present a comparison of aerosol optical depth (AOD) retrieved from MODIS with Aerosol Robotic Network (AERONET) data for the year 2004 over Kanpur, an industrial city lying in the Ganga Basin in the northern part of India. AOD retrieved from MODIS (τ_aMODIS) at 0.55µm wavelength has been compared with the AERONET derived AOD (τ_aAERONET), within an optimum space-time window. Although the correlation between τ_aMODIS and τ_aAERONET during the post-monsoon and winter seasons (R²~0.71) is almost equal to that during the pre-monsoon and monsoon seasons (R²~0.72), MODIS is found to overestimate AOD during the pre-monsoon and monsoon period (characterized by severe dust loading) and underestimate during the post-monsoon and winter seasons. The absolute difference between τ_aMODIS and τ_aAERONET is found to be low (0.12±0.11) during the non-dust loading season and much higher (0.4±0.2) during dust-loading seasons. The absolute error in τ_aMODIS is found to be about ~25% of the absolute values of τ_aMODIS. Our comparison shows the importance of modifying the existing MODIS algorithm during the dust-loading seasons, especially in the Ganga Basin in northern part of India.