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 the ir approximation through vertically constant properties. The data sets foll owed from combined six wavelength aerosol lidar and Sun photometer measurem ents made on March 25, 1999, in the framework of the Indian Ocean Experimen t (INDOEX). On this day a multilayered pollution plume, which was character ized by highly absorbing particles, extended to 3.5 km above ground. The ae rosol optical thickness was as high as 0.57 at 530 nm. The uncertainty of t he 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-ave raged values if Sun photometer measurements are used alone. The radiative f orcing 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 det ermined 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.