Long-term (2007-2013) analysis of aerosol optical properties over four locations in the Indo-Gangetic plains.

The emphasis of the present work lies on the examination of the distribution and spectral behavior of the optical properties of atmospheric aerosols in the Indo-Gangetic plains (IGP). Measurements were performed using an AErosol RObotic NETwork (AERONET) Sun photometer at four sites (Karachi, Lahore, Jaipur, and Kanpur) with different aerosol environments during the period 2007-2013. The aerosol optical depth (AOD) and Ångström exponent (α) were measured, and the results revealed a high AOD with a low α value over Karachi and Jaipur in July, while a high AOD with a high α value was reported over Lahore and Kanpur during October and December. The pattern of the aerosol volume size distribution (VSD) was similar across all four sites, with a prominent peak in coarse mode at a radius of 4.0-5.0 μm, and in fine mode at a radius of 0.1-4.0 μm, for all seasons. On the other hand, during the winter months, the fine-mode peaks were comparable to the coarse mode, which was not the case during the other seasons. The single scattering albedo (SSA) was found to be strongly wavelength-dependent during all seasons and for all sites, with the exception of Kanpur, where the SSA decreases with increasing wavelength during winter and post-monsoon. It was found that the phase function of the atmospheric aerosol was high at a small angle and stable around a scattering angle of 90°-180° at all sites and during all seasons. Spectral variation of the asymmetry parameter (ASY) revealed a decreasing trend with increasing wavelength, and this decreasing trend was more pronounced during the summer, winter, and post-monsoon as compared to pre-monsoon. Furthermore, extensive measurements suggest that both real (RRI) and imaginary (IRI) parts of the refractive index (RI) show contrasting spectral behavior during all seasons. Finally, the analysis of the National Oceanic and Atmospheric Administration hybrid single particle Lagrangian integrated trajectory model back trajectory revealed that the seasonal variation in aerosol types was influenced by a contribution of air masses from multiple source locations.