Vertical and Horizontal Gradients in Aerosol Black Carbon and Its Mass Fraction to Composite Aerosols over the East Coast of Peninsular India from Aircraft Measurements

During the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) experiment of ISRO-GBP, altitude profiles of mass concentrations of aerosol black carbon ( ) and total (composite) aerosols ( ) in the lower troposphere were made onboard an aircraft from an urban location, Chennai (13.04？ , 80.17？ ). The profiling was carried out up to 3？km (AGL) in eight levels to obtain higher resolution in altitude. Besides, to explore the horizontal gradient in the vertical profiles, measurements were made at two levels [500？m (within ABL) and 1500？m (above ABL)] from ~10？ to 16 and ~80？ to 84 . The profiles showed a significant vertical extent of aerosols over coastal and offshore regions around Chennai with BC concentrations (~2？ g ) and its contribution to composite aerosols remaining at the same level (between 8 to 10% for ) as at the surface. Even though the values are not unusually high as far as an urban location is concerned, but their constancy throughout the vertical column will have important implications to climate impact of aerosols. 1. Introduction Direct radiative forcing due to black carbon (BC) aerosols crucially depends on the vertical profile of BC. Elevated BC layer over scattering aerosol/cloud layer will enhance the atmospheric forcing and can even reverse the “white house effect” [1]. Tripathi et al. [2] have reported that the difference in the short-wave, clear sky forcing between the steadily decreasing and increasing BC aloft is as much as a factor of 1.3. Lubin et al. [3] have shown that this difference can be as much as a factor of two in the case of long wave. Haywood and Ramaswamy [4] have reported from GCM simulation that the direct radiative forcing of a BC aerosol layer increases approximately by a factor of 5, as the layer is moved between the surface and 20？km. Based on model simulation and observation during INDOEX, Ackerman et al. [5] reported that enhanced layer of BC aerosols reduces the cloud cover by BC-induced atmospheric heating and hence offsets the aerosol-induced radiative cooling at the top of the atmosphere on a regional scale. Thus information on the altitude variation of BC and its mass fraction to total composite aerosols is very important in estimating its radiative forcing. Even though LIDAR can give information on the vertical distribution of scattering aerosol, it cannot give any information on the altitude distribution of absorbing aerosols. Thus, in situ measurement of absorbing aerosol such as BC from aircraft is very important. Such measurements are very limited worldwide, especially over India