Vertical Distribution and Monthly Variability of Biomass Burning Aerosols as observed by the Micropulse Lidar during LASIC

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Description

Biomass burning (BB) in southern Africa injects vast amounts of aerosols into the atmosphere. However, the vertical extent of the BB aerosol layers transported far from the sources are poorly represented in the global climate models (Das et al. 2017). The DOE Atmospheric Radiation Measurement Program Mobile Facility deployment for the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign took place on Ascension Island, about 1600 kilometers from the west coast of Africa, from June 2016 to October 2017. This unique field campaign location provides an excellent dataset to improve our current understandings of aerosol vertical distribution and their radiative impact throughout the BB season. The micropulse lidar (MPL) observations during LASIC are of particular interest to this work as it provides continuous measurements of the backscattered signal from atmospheric particles. The normalized relative backscatter (NRB) signal is calculated by applying the lidar corrections, and inverted with the Klett’s (1981) method to acquire average extinction profiles. We present retrieved aerosol extinction profiles from the complete MPL dataset during the LASIC campaign. Monthly variations of the retrieved extinctions are analyzed to assess the evolution of the pre-BB and BB season aerosol vertical structure as well as the thickness of the elevated BB layers in the downwind. We show that the smoke layer over Ascension is present mostly above boundary layer clouds between 1.5 to 3 km at the beginning of the burning season in July and deepens extending up to 4 km in September. Occurrences of the BB layers coincide with the peak black carbon concentrations (>1000 ng/m3) observed at the surface, suggesting that these aerosols are strongly absorbing, as back trajectories indicate that they originate from same continental BB regions. The mean July to October profiles between years 2016 and 2017 are compared to quantify the inter-annual variability of the BB aerosols demonstrating consistent vertical structure and depths between the two BB seasons. Inter-comparisons of extinction profiles retrieved via Klett method and Fernald-based inversion algorithm developed at the University of Miami are summarized. We also present comparisons of the MPL retrieved extinctions with the in-situ aircraft measurements during the Cloud Aerosol Radiation Interactions and Forcing field campaign that took place during the months of August and September in 2017.