Seasonal Variation of Refractory Black Carbon Loadings over the Remote Atlantic Observed During the Atmospheric Tomography (ATom) Mission

The recently completed Atmospheric Tomography (ATom) mission obtained in situ measurements of refractory black carbon (rBC) aerosol ranging from near the surface to the upper troposphere / lower stratosphere over the Atlantic and Pacific basins with near pole-to-pole coverage ( 80N-80S latitude) in each of the four seasons. Here, we present an initial analysis of an extensive seasonally-resolved observation of rBC mass mixing ratio and size distribution over the remote Atlantic. Low-to-mid tropospheric rBC loadings above the Atlantic range over several orders of magnitude, with the highest concentrations due to African biomass burning outflow. The AeroCom model ensemble actually under-predicts loadings from this major source; a result contrary to what has been observed anywhere over the remote Pacific (eg. HIPPO). In fact, the disagreement in measurement and model is to the extent such that estimates of global direct radiative forcing over remote ocean waters may be under-, rather than over-predicted. These results will help constrain model predictions of black carbon loadings in remote regions, and ultimately, improve our understanding of its effect on climate change.