Vertical Distribution and Monthly Variations of Biomass Burning Aerosols as Observed by the Micropulse Lidar during LASIC

Biomass burning (BB) in southern Africa injects vast amounts of radiatively effective and chemically active aerosols into the atmosphere. Global models indicate that some of the largest aerosol radiative forcings occur over the southeast Atlantic downwind of these BB activities. However, the sign and the magnitude of the BB aerosol forcing are still highly uncertain associated with aerosol vertical distributions.

The DOE Atmospheric Radiation Measurement (ARM) Program Mobile Facility deployment for the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign took place on Ascension Island (8°S and 14.5°W), about 1600 kilometers from the west coast of Africa, from June 2016 to October 2017. It provides an excellent dataset to improve the current understandings of BB aerosol vertical distributions, temporal evolution and radiative impact.

In particular, the micropulse lidar (MPL), deployed on Ascension Island during LASIC, provides continuous observations of the vertical distribution of clouds and aerosol. The raw MPL measurements consist of profiles of backscattered signal from atmospheric particles. The normalized relative backscatter (NRB) signal is calculated by applying the lidar corrections (overlap, afterpulse, deadtime and range correction) to the raw signal. Retrieval periods are identified when at least 5 minutes of cloud-free data is available. Approximately 300 such periods are identified for the LASIC MPL dataset. The NRB signal is then inverted with the Klett’s (1981) method to acquire an average extinction profile during the selected retrieval period.

In this study, we have retrieved aerosol extinction profiles from the complete MPL dataset during the LASIC campaign. Monthly variations of the retrieved extinction profiles 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. Preliminary results 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 to higher altitudes in August. 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. We also compare the MPL retrieved extinction profiles with the CALIPSO satellite retrievals and Energy Exascale Earth System Model (E3SM) simulations.