A Novel Algorithm Applied to Common Thermal-Optical Transmission Data for Determining Mass Absorption Cross Sections of Atmospheric Black Carbon: Applications to the Indian Outflow

High wintertime concentrations of black carbon aerosols (BCA) over South Asia and the Northern Indian Ocean are thought to have a large impact on the regional climate. Direct absorption of sunlight by BCAs causes heating of the atmosphere and cooling at the surface. To quantify such effects it is important to characterize a number of different properties of the aerosols. Here we present a novel application of the thermal-optical (OCEC) instrument in which the laser beam is used to obtain optical information about the aerosols. In particular, the novel algorithm accounts for non-carbon contributions to the light extinction. Combining these light extinction coefficients with the simultaneously constrained Elemental Carbon (EC) concentrations, the Mass Absorption Cross Section (MAC) is computed. Samples were collected during a continuous 14-month campaign Dec 2008 - Mar 2009 at Sinaghad in Western India and on Hanimaadhoo, the Northernmost Island in the Maldives. This data set suggests that the MAC of the BCAs are variable, sometimes by a factor of 3 compared to the mean. This observation adds to the complexity of calculating the radiative forcing for BCAs, reinforcing previous observations that parameters such as aerosol mixing state and sources need to be taken into account.