Aerosol optical properties and the spectral behavior of the aerosol optical depth (AOD) over Kanpur, an urban-industrial city in the Ganga basin, have been presented for the first time. Measurements by the ground-based Aerosol Robotic Network (AERONET) during January 2001 to December 2003 show pronounced seasonal influence, with maximum dust loading during the premonsoon season (April–May). The distribution of AOD is found to be large with a mean value of 0.6 at 500 nm wavelength. The frequency distribution of the Ångström parameter α reveals two modes (α < 1 dominant dusty condition and α > 1 urban aerosols). Diurnal variations of AOD, water vapor content (WVC), and Ångström parameter show strong seasonal influence. Maximum variation of AOD is found during the monsoon season (presence of mixed types of aerosols), maximum variation of WVC is observed during the winter season (frequent changes in humidity and air pressure), and α shows maximum variations during the premonsoon (dust dominating the atmospheric optical conditions) season. The aerosol volume size distributions show two distinct modes, fine (geometric mean radii of 0.17 μm and standard deviation of 0.03) and coarse (geometric mean radii of 3.37 μm and standard deviation of 0.1), but during May–August (period of dust loading), a third mode (around 1–2 μm) appears because of hygroscopic growth of finer aerosols. The single-scattering albedo (SSA) is found to increase with wavelength in the presence of dust and shows a reverse trend in dust-free conditions. Refractive indices show the presence of dust as the main component during the premonsoon season and dominance of anthropogenic urban-industrial aerosols during the winter season, when the optical state of the atmosphere is more absorbing. The aerosol parameters show distinct interannual variations, with increasing aerosol burden over the region. Aerosol loading over Kanpur is found to be controlled by the regional climatology.
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