Aerosol Vertical Structure as Assessed by a Micropulse Lidar upon Ascension Island during the LASIC Campaign

The vertical structure of the absorbing aerosols is a critical first step to characterizing possible aerosol-cloud interactions. The DOE Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign on Ascension Island (8S, 14W) in the remote Atlantic deployed a micropulse lidar, providing the only consistent indicator of aerosol vertical structure during the 17-month-long campaign. Two aircraft campaigns coincided at times with LASIC. Here we provide an assessment of the lidar-derived aerosol extinction and depolarization ratio vertical structure. The extinctions are derived using a molecular scattering profile fitting implementing local sounding pressure measurements to find the aerosol top and column optical depth. These are constrained by AERONET-derived aerosol optical depth. The lidar normalized backscatter signal is inverted to extinction using the Fernald (1984) Algorithm. Highly-attenuated profiles are excluded because they distort the perceived aerosol vertical structure. Findings to date indicate that aerosol during the biomass-burning season (June-October), when present above the cloud layer, is typically resting on it, most evident through the depolarization ratio. The depolarization ratios of more desiccated aerosol is typically higher, indicating more aspherical particles. The most remarkable finding is that aerosol is often present above the cloud layer between December to April, peaking in February, when the large-scale circulation is not encouraging the westward transport of smoke off of continental southern Africa. Elevated depolarization ratios suggest the aerosol is more likely to be dust. The monthly evolution in the altitudes of the aerosol layer tops will be assessed.