Abstract
Abstract. Measurements of the vertical profiles of the optical properties (namely the extinction coefficient and scattering and absorption coefficients respectively σext ∕ σscat ∕ σabs) of aerosols have been made across the Indo-Gangetic Plain (IGP) using an instrumented aircraft operated from three base stations – Jodhpur (JDR), representing the semi-arid western IGP; Varanasi (VNS), the central IGP characterized by significant anthropogenic activities; and the industrialized coastal location in the eastern end of the IGP (Bhubaneswar, BBR) – just prior to the onset of the Indian summer monsoon. The vertical profiles depicted region-specific absorption characteristics, while the scattering characteristics remained fairly uniform across the region, leading to a west–east gradient in the vertical structure of single-scattering albedo (SSA). Integrated from near the ground to 3 km, the highest absorption coefficient and hence the lowest 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, arising from a change in the aerosol type from a predominantly natural (dust and sea salt) type in the western IGP to a highly anthropogenic type (industrial emissions, fossil fuel and biomass combustion) in the eastern IGP, with the central IGP exhibiting a mixture of both. Aerosol-induced short-wave radiative forcing, estimated using altitude-resolved SSA information, revealed significant atmospheric warming in the central IGP, while a top-of-atmosphere cooling is seen, in general, in the IGP. Atmospheric heating rate profiles, estimated using altitude-resolved SSA and column-averaged SSA, revealed considerable underestimation in the latter case, emphasizing the importance and necessity of having altitude-resolved SSA information as against a single value for the entire column.
Highlights
Ground-based, as well as space-borne observations, have established that the Indo-Gangetic Plain (IGP) remains one of the aerosol hotspots in the world, depicting persistently high aerosol loading (Babu et al, 2013; Gautam et al, 2010; Dey and Di Girolamo, 2010), especially during the dry winter and premonsoon seasons
While the altitude variation is very weak at JDR and moderate at BBR, it is rather strong at the central IGP (VNS)
Combining our results with those reported by Babu et al (2016) and assuming the interannual variations to be less significant, in Fig. 6 we present the temporal evolution of aerosol absorption over the IGP from winter to spring to just prior to the onset of the Indian summer monsoon (ISM)
Summary
Ground-based, as well as space-borne observations, have established that the Indo-Gangetic Plain (IGP) (the vast stretch of apparently contiguous plain land along the east–west with an area of approximately 7 million km bounded between the Iranian Plateau to the west, the Bay of Bengal to the east, the Himalayas to the north and Chota Nagpur Plateau and Aravalli ranges to the south) remains one of the aerosol hotspots in the world, depicting persistently high aerosol loading (Babu et al, 2013; Gautam et al, 2010; Dey and Di Girolamo, 2010), especially during the dry winter and premonsoon seasons. Most of the impact assessments of aerosols over this region have used optical properties of aerosols, especially the most important parameter, the single-scattering albedo (SSA), derived either indirectly (Ramachandran et al, 2006) or from surface measurements (Ram et al, 2016), while information on the vertical structure of the optical properties (scattering, absorption, SSA) has been very sparse. This information is very important to accurately estimate the vertical structure of atmospheric heating rate resulting from absorption by aerosols. These results are examined in light of available information and the implications are discussed
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