Simulation of Biomass Burning Aerosol Transport Over the South African-Atlantic Region

Biomass burning is a major source of trace gases and aerosol in the atmosphere. Although aerosol from industrial activity and biofuels intermingle with clouds in many regions, the unique situation in the South Atlantic merits special scrutiny. The large-scale transport of aerosol plumes, originating from seasonal burning of agricultural residue in the southwestern African Savannah region, allows for a non-interacting aerosol layer to reside over vast stretches of marine stratus clouds in the Atlantic Ocean. Accurate simulations of the existence, location and properties of these aerosols determine the direct, indirect and semi-direct radiative forcing due to this seasonal biomass-burning aerosol. Vertical profiles of aerosol occurrence and extinction coefficient from the GOCART (Goddard Chemistry Aerosol Radiation and Transport) model were compared with measurements from CALIOP on board the CALIPSO satellite over the region of interest. There was reasonable agreement in the source region, but model aerosol properties after long-range transport did not bear much resemblance to observations. The cause for this underestimation by the model in both frequency and optical thickness of aerosol above cloud occurrence was investigated. Aerosol wet and dry deposition and GOCART chemistry were examined by comparing aerosol fields along the plume trajectory with that of carbon monoxide, a tracer gas emitted during biomass burning along with the aerosols. Sensitivity tests using two different emission inventories (GFED and QFED) for the simulations were performed to find the influence of source strength. Model dynamical fields responsible for horizontal and vertical transport of aerosols were also compared to those from previous modeling and observational studies over the same region. The results of these comparisons determine the relative roles of emission inventory, dynamics and chemistry in accurate simulation of the location and optical thickness of absorbing aerosol over clouds in the South Atlantic.