Atmospheric Environment 40 (2006) 6504–6515 Spatial variations in aerosol characteristics and regional radiative forcing over India: Measurements and modeling of 2004 road campaign experiment A. Jayaraman à , H. Gadhavi, D. Ganguly, A. Misra, S. Ramachandran, T.A. Rajesh Space and Atmospheric Sciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India Received 5 August 2005; received in revised form 4 January 2006; accepted 4 January 2006 Abstract Results from a variety of complementary aerosol measurements made as part of the Indian Space Research Organization-Geosphere Biosphere Program (ISRO-GBP) road campaign experiment conducted during Feb 2004 in the central Indian region are reported. An instrumented vehicle fitted with the Micro Pulse Lidar for aerosol vertical profile measurements, nephelometer for scattering and athelometer for absorption coefficients measurements, Microtops II sun- photometer for column aerosol optical depth, Quartz Crystal Microbalance for aerosol mass concentration and Grimm spectrometer for aerosol number concentration was used in the campaign. The synergy of results from these complementary measurements is reflected in the computed aerosol radiative forcing for the region. We show that the large spatial and temporal variations found in the aerosol characteristics over the northwest Indian region are caused mainly by the naturally produced dust particles. The variation in their concentration modulates the overall atmospheric radiative forcing over this region. We also show that apart from the large north to south gradient in aerosol forcing caused due to transport of aerosols from the continent to the Indian Ocean region and reported during INDOEX, there is also a positive gradient in aerosol forcing from west to east, from the Arabian Sea to the Bay of Bengal region. A considerable portion of particles produced from the Indian main land is flushed to the Bay of Bengal region before being transported to the Indian Ocean, causing this west to east gradient. r 2006 Elsevier Ltd. All rights reserved. Keywords: Aerosol vertical profiles; Aerosol scattering and absorption coefficients; Aerosol radiative forcing over central India 1. Introduction Aerosols exert large radiative imbalance over regional scales both by scattering and absorption of radiation and also by modifying cloud properties such as cloud cover and cloud albedo (IPCC, 2001). The aerosol radiative forcing could be an order of magnitude larger than compared to well-mixed greenhouse gases over polluted regions, such as that reported for the Indo-Asian haze (Ramanathan et al., 2001). Similar ‘hot spots’ exist around the globe, for example the Saharan dust plume that spreads over most of the sub-tropical Atlantic, plumes from Indonesia, Brazil and southern Africa which consist mostly of biomass burning products etc. The challenging job is to estimate quantitatively ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.01.034 à Corresponding author. Tel.: +91 79 26314658; fax: +91 79 26301502. E-mail address: jraman@prl.res.in (A. Jayaraman).