A method for aerosol profiling in the lower troposphere with coupled scatter and meteorological rawindsondes and first data from the tropical Atlantic off Sahara

Abstract

We developed a simple prototype balloonborne aerosol scattering profiler by coupling a small nephelometer with a meteorological sonde and we performed one sounding over Sal Island, Cape Verde, in February 1996 during a turbid episode of dust transport with an aerosol optical thickness (AOT) larger than 0.6 at 870 nm . We also monitored space and time variations in AOT with both Meteosat and a Sun photometer. This allows us to document for the first time the vertical distribution of aerosols during the typical winter situation of low-altitude transport of African dust in the trade winds of the tropical North Atlantic. We derive the vertical profile of aerosol extinction from side-scattering nephelometer measurements by applying corrections for aerosol optical properties. We find that most of the aerosol extinction occurs between about 600 and 1400 m in altitude in a layer of dry air above the atmospheric marine boundary layer (MBL). Our results suggest that about 90% of the dust is transported above the MBL. They confirm previous findings of well-structured African dust layers observed at north mid-latitudes with lidar systems.