Assessing Radiative Impacts of African Smoke Aerosols Over the Southeastern Atlantic Ocean

Biomass burning smoke aerosols are efficient at attenuating incoming solar radiation. The Layered Atlantic Smoke Interactions with Clouds campaign was conducted from June 2016 to October 2017. The U. S. Department of Energy mobile Atmospheric Radiation Measurement site located on Ascension Island (AMF-ASI) identified several instances of smoke plume intrusions. Increases in surface and column measurements of aerosol loading were directly related to increases in fine mode fraction, number concentrations of aerosols (Na), and cloud condensation nuclei (NCCN). During periods of weak lower tropospheric stability, smoke particles were more likely to be advected downward either by boundary layer turbulence or cloud top entrainment under non-overcast sky conditions. Backward trajectory analysis illustrated that smoke aerosols reaching the AMF-ASI site were fine mode, less aged, strongly absorbing, and had shorter boundary layer trajectories while longer boundary layer trajectories denoted mixtures of weakly absorbing smoke and coarse mode marine aerosols. The most polluted smoke cases of August 2016 and 2017 revealed a notable contrast in radiative forcing per unit aerosol optical depth or radiative forcing efficiency (Delta Feff) at the top of the atmosphere (TOA) and near-surface (BOA). The weakly (strongly) absorbing 2016 cases exhibited weaker (stronger) Delta Feff at the TOA and BOA suggesting a warming (cooling) effect within the boundary layer. The 2017 cases featured the strongest Delta Feff suggesting more of a cooling effect at the TOA and BOA due to mixing of fresh smoke with marine aerosols during transport. Fires are often observed in Central Africa throughout the year due to agricultural and natural processes. As a result of changes in pressure patterns along with wind direction and strength, the smoke aerosols can transport to regions thousands of kilometers away from the fires. The aerosols can aid in cloud development while at the same time, have variable impacts on the amount of solar radiation reaching the surface and returning to space which can cause either warming or cooling along their transport pathway. In the Southern Hemisphere tropics, there are few observations to document aerosol radiative forcing impacts on global climate. This study uses short-term and long-term measurements of aerosol physical properties in conjunction with meteorological analysis to diagnose the impact of smoke aerosols on incoming solar radiation and cloud development during transport. African biomass burning smoke plumes feature highly variable top of the atmosphere and BOA radiative forcing efficiencies when mixed with marine aerosols Smoke aerosols with stronger radiative forcing efficiencies present a cooling effect which can stabilize the marine boundary layer Longer back trajectories suggest aging of smoke aerosols and mixing with marine aerosols which facilitate cloud development