Introduction

Aerosols perturb the Earth's radiation budget directly through scattering and absorption of sunlight,1 and indirectly through interaction with clouds.2 The change in the radiative fluxes at the top-of-the-atmosphere (TOA) and at the surface due to the aerosols (natural + anthropogenic) is termed as aerosol radiative forcing. One major uncertainty in aerosol direct radiative forcing (ADRF) arises from limited knowledge of the relative proportion of the anthropogenic and natural components and their mixing state.3'4

Fig. 1. The IGB in the northern India with the locations of major urban areas, as indicated by (1) Delhi, (2) Agra, (3) Kanpur, (4) Allahabad, (5) Varanasi, (6) Patna, and (7) Kolkata.

The Indo-Gangetic basin (IGB) in India (Fig. 1) is a region, where anthropogenic and natural aerosols have changing pattern in their loadings in different seasons.55-7 In terms of aerosol loading, despite being one of the most polluted regions in the world,8-12 the spatio-temporal distribution of ADRF in the IGB is poorly understood. Most of the previous studies are limited in spatial and temporal scale, e.g. Dey and Tripathi12 and Tripathi et al.9 have studied the aerosol and BC radiative effects in Kanpur (Fig. 1) during the winter months (December-February) based on the direct measurements of aerosol parameters during a land campaign conducted by Indian Space Research Organization in December 2004-January 2005.11,13 Aerosol DRF estimation have been carried out for Delhi,14 Hissar (semi-urban site in IGB),15 and Nainital (high altitude site in the Himalaya),16 but only for wintertime. Ramanathan and Ramana8 have also investigated the climatic impact of absorbing haze over the IGB during the dry months (October-May). During the pre-monsoon (March-May) dust loading season, ADRF estimation has been carried out only for Kanpur7 and Delhi17 regions. All these studies indicated a very high magnitude of surface cooling (i.e. solar dimming) and atmospheric absorption (heating effect) over the respective locations, which has adverse effect on the regional climate. But how much of it is the anthropogenic fraction is not known, neither its spatio-temporal variability. Here, we report the spatio-temporal variations of the anthropogenic contribution to the ADRF over the IGB using ground-based and satellite observations, and through a regional aerosol optical model.

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