Microphysical and Radiative Evolution of Aerosol Plumes Over the Tropical North Atlantic Ocean

Over the tropical North Atlantic Ocean in summer, plumes of aerosol extend from Saharan Africa to the Caribbean. The microphysical and radiative evolution of such plumes is studied using a Lagrangian column model and measurements obtained near the west coast of Africa (during ACE-2) and the Caribbean Sea (during PELTI). Mass and scattering in the plumes can be separated into two layers that overlay one another over much of the Atlantic Ocean. Mineral dust dominates in the lower free troposphere and sea-salt aerosol dominates in the boundary layer. Carbonaceous, sulfate and nitrate (CSN) aerosols are a minor component of mass but contribute significantly to total column optical depth. Combined, CSN aerosols and sea-salt contribute to more than half of total aerosol clear-sky shortwave forcing associated with such plumes. Satellite and model data suggest the reduction of plume forcing between the African coastline and the Caribbean is less than \( ~ \)20%. The reduction is due principally to settling of large dust particles and atmospheric subsidence; however the reduction of forcing remains small because a) boundary layer trade winds provide a steady source of sea-salt, b) dust particles are initially elevated 2.5 to 5.5 km from the surface and therefore have long settling distances before removal, and c) small CSN and dust particles in the free troposphere have high specific extinction and lack significant removal processes. Measurements and climatology suggest that the CSN aerosols in the free troposphere are anthropogenic pollution from Europe.