Spatial and Altitudinal Contrast in Aerosol Radiative Properties across the Indo-Gangetic Plain

Abstract

With a view to understanding the vertical distribution of aerosols and estimating the radiative impacts of elevated aerosols in the lower free troposphere, extensive profiling of the vertical variation of the optical properties, namely the extinction/ scattering and absorption coefficients (respectively $\sigma _{\mathrm{ext}}/ \sigma _{\mathrm{sca}}\mathrm {t}/ \sigma _{\mathrm{abs}})$ have been carried out from three base stations in the Indo-Gangetic Plain (IGP) using an instrumented aircraft, prior to onset of the Indian Summer Monsoon. These stations represented the semiarid western IGP (Jodhpur, JDR), the anthropogenically affected central IGP (Varanasi, VNS), and the industrialized coastal location in the eastern end of the IGP, close to the northern Bay of Bengal (Bhubaneswar, BBR). The vertical profiles of the optical properties differed significantly across these locations and this resulted in a regionally significant heating rate gradient. While the integrated (ground to 3 km altitude $) \sigma _{\mathrm{scat}}$ remained quite comparable across the IGP, the highest $\sigma _{\mathrm{abs}}$ and hence the lowest single scattering albedo (SSA) occurred in the central IGP (Varanasi). Size distribution, inferred from the spectral variation of the scattering coefficient, showed a gradual shift from coarse particle dominance in the western IGP to strong accumulation dominance in the eastern coast, with the central IGP coming in-between. Source speciation of aerosol, using spectral aerosol properties, revealed aerosol system in the west IGP is predominantly natural (dust and sea-salt) and that in the east IGP is highly anthropogenic type (industrial emissions, fossil fuel and biomass combustion).The central IGP exhibited a mixture of both.