Comparison of the radiative impact of aerosols derived from vertically resolved (lidar) and vertically integrated (Sun photometer) measurements : Example of an Indian aerosol plume

Radiative forcing calculations both at the top of the atmosphere (TOA) and at the surface were done for vertically varying particle properties and their approximation through vertically constant properties. The data sets followed from combined six wavelength aerosol lidar and Sun photometer measurements made on March 25, 1999, in the framework of the Indian Ocean Experiment (INDOEX). On this day a multilayered pollution plume, which was characterized by highly absorbing particles, extended to 3.5 km above ground. The aerosol optical thickness was as high as 0.57 at 530 nm. The uncertainty of the radiative forcing can be as large as 30% owing to the uncertainty of the refractive index and size distribution, which are only given as column-averaged values if Sun photometer measurements are used alone. The radiative forcing calculated for an ocean albedo of 0.05 varies between -5 and -12 W m(-2) at TOA and -55 and -81 W m(-2) at the surface, whereas the forcing determined by height dependent aerosol properties is about -10 W m(-2) at TOA and -60 W m(-2) at the surface. The forcing efficiency is in the range -11 and -24 W m(-2) per unit optical thickness at TOA and -101 and -154 W m(-2) per unit optical thickness at the surface.