Microphysical properties of refractory black carbon during the Atmospheric Tomography (ATom) mission

Microphysical properties of refractory black carbon (rBC) in the atmosphere can dramatically change its contribution to direct, semi-direct and indirect radiative forcing. This microphysics is strongly related to the rBC's original source as well as aging during transport. Particles found over remote ocean waters, most of which have been transported over days or longer from land-based source regions, are highly likely to have become internally mixed (or 'coated') with non-refractory materials. These coatings have the potential to enhance rBC's solar absorption, leading to substantially larger direct radiative forcing. Estimates of this effect have mostly relied on theoretical models, as data throughout remote regions of the globe remain sparse.

The Atmospheric Tomography (ATom) mission, occurring from August 2016-May 2018, obtained in situ measurements of refractory black carbon (rBC) aerosol using a Single Particle Soot Photometer. The dataset ranges 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 rBC microphysical properties observed throughout the mission, with a focus on mass (volume equivalent diameter) size distributions and coating thicknesses (calculated via Mie theory). These results will help constrain model predictions and ultimately improve our understanding of rBC's effect on climate.