Aerosols Optical Properties Profile Characterization over Sao Vincente, Cabo Verde, during the Askos Campaign

Abstract

Studying the vertical distribution of aerosol particles is essential for understanding their influence on the climate system. One way to evaluate this aspect of aerosols is to discriminate their source and optical properties in the atmosphere column. The main objective of this study was to characterize and vertically analyze aerosol's optical properties profile under distinct aerosol mixture states over São Vicente Island, Cabo Verde, during the ASKOS campaign that took part in July and September of 2021. Thus, the Elastic Extinction (ElEx) method has been applied, which allows aerosol backscattering profile separations by type and estimation of total extinction coefficient. The EIEx application requires particles' complete backscattering profile and vertically resolved particles' linear depolarization ratio at the same wavelength. These values were taken from the eVe lidar system, linear depolarization ratio and lidar ratio values of pure aerosol types that were taken from the literature. The total extinction coefficient profile is then estimated and compared with eVe Raman retrievals and with the results of a PollyXT Raman lidar, also operated in São Vicente during the ASKOS experiment. Based on this method, three study cases have been chosen for detailed discussion. The first and third cases consist of pure Saharan dust in the free troposphere, while in the Marine Boundary Layer (MBL), pure marine and volcanic sulfate were recorded, respectively. Regarding the second study case, a mixed scenario of dust and marine aerosols were found across the MBL and free troposphere. Comparison between the EIEx estimated optical properties profiles (extinction coefficient, lidar ratio) and the eVe and PollyXT lidar retrievals revealed a good general agreement. Nevertheless, differences can be observed in regions where the signal to noise ratio is low, such as close to the top of the aerosol layer and also inside the MBL. These differences in the MBL could be attributed to the presence of pollution particles or the limited ability of the lidars to correctly represent the aerosol optical properties in the near range due to the overlap problem.