Role of black carbon in aerosol properties and radiative forcing over western India during premonsoon period

The present study addresses the role of black carbon (BC) in aerosol radiative forcing (ARF) over western India, where the Thar Desert produces large amount of dust aerosols during pre-monsoon season (Mar-May) and its mixing with BC makes the investigation a real challenge. Measurements of aerosol physical and optical parameters were carried out at three stations, Ahmedabad (urban area), Udaipur (semi-arid region) and Mt. Abu (a hill-top representing background conditions), to investigate the regional variation of ARF during April 2007. Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) measurements show presence of dust layer in the altitude region from 1 to 5 km over western India throughout the month leading to uniform distribution of dust. Ahmedabad has slightly higher AOD(500) (0.31) due to production of anthropogenic aerosols with BC concentration of 1.8 mu g.m(-3) at surface, followed by Udaipur (AOD(500) = 0.30 and BC = 0.9 mu g.m(-3)) and Mt. Abu (0.28 and 0.7 mu g.m(-3), respectively). The longwave ARF is found to be similar over all three stations whereas the shortwave ARF depends on type of location. The shortwave ARF at the top of atmosphere (TOA), surface, and within the atmosphere are found to be 1.7, -46 and 47.7W m(-2), respectively, at Ahmedabad, -1.5, -35 and 33.5W m(-2) at Udaipur and -1.5, -31 and 29.5W m(-2) at Mt. Abu. On the other hand, the heating rates in the lower atmosphere (up to 5 km) are 1.3, 1.0 and 0.4 K/day over Ahmedabad, Udaipur and Mt. Abu, respectively. Sensitivity analysis shows that a 40% enhancement of BC could increase the heating rate by up to 50% over western India. Higher aerosol-induced heating in the atmosphere during premonsoon may have a large impact on the regional climate dynamics and hydrological processes. (C) 2011 Elsevier B.V. All rights reserved.