Vertical aerosol profiles from Raman polarization lidar observations during the dry season AMMA field campaign

Abstract

Lidar measurements of the vertical distribution of optical particle properties were performed in January 2006 in the frame of the dry season field experiment of the African Monsoon Multidisciplinary Analyses (AMMA). The measurements were conducted at a remote site in West Africa in the Sahel region just south of the Saharan desert to investigate the influence of particles on the West African monsoon. The portable Raman polarization lidar system POLIS of the University of Munich was operated each forenoon and during selected nights. These measurements are the first Raman and depolarization lidar measurements taken in the Sahel. The particle depolarization ratio and the extinction coefficient profiles allow the characterization of the particles and the distinction between Saharan dust particles and biomass burning aerosol (BBA). The origin of this aerosol was investigated by trajectory analyses. The profiles show a varying dust layer in the lowermost troposphere during the whole period. Above the dust layer a broad layer of BBA reaching up to an altitude of 5 km was frequently observed. The extinction coefficient in the dust layer was between 0.2 and 0.4 km−1 at 355 nm, and the particle depolarization was around 25%. In the BBA layer the extinction coefficient was lower with values between 0.1 and 0.2 km−1, and the particle depolarization ratio was below 10%. Lidar ratio values around 55 sr were found for the dust layer and around 75 sr for the BBA layer. From previous observations, lidar ratios between 40 and 80 sr were expected for BBA.