AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Size-resolved Mixing State of Black Carbon in the Arctic and Implications for Simulated Direct Radiative Forcing
JACK KODROS, Sarah Hanna, Allan Bertram, W. Richard Leaitch, Hannes Schulz, Andreas Herber, Marco Zanatta, Julia Burkart, Megan Willis, Jonathan Abbatt, Jeffrey R. Pierce, Colorado State University, Fort Collins, USA
Abstract Number: 225 Working Group: Regional and Global Air Quality and Climate Modeling
Abstract Transport of anthropogenic aerosol emissions into the Arctic in the spring months has the potential to affect regional climate; however, modeling estimates of the aerosol direct radiative effect (DRE) are strongly sensitive to uncertainties in the mixing state of black carbon (BC). A common approach in previous modeling studies is to assume an entirely internal mixture (all primarily scattering species are mixed in the same particles as BC) or external mixture (all primarily scattering species are in separate particles from BC). In order to provide constraints on the size-resolved mixing state of BC, we use airborne Ultra-High Sensitivity Aerosol Spectrometer (UHSAS) and Single Particle Soot Photometer (SP2) measurements from the POLAR6 flights from the NETCARE/PAMARCMIP2015 campaign to estimate coating thickness as a function of BC core diameter as well as the fraction of particles containing BC in the springtime Canadian high Arctic. For BC core diameters in the range of 200 to 500 nm, we find average coating thicknesses of approximately 30 to 50 nm and BC-containing particle number fractions ranging from 3 to 20%. We combine the observed mixing-state constraints with simulated size-resolved aerosol mass and number distributions from GEOS-Chem-TOMAS to estimate the DRE with observed bounds on mixing state as opposed to assuming an entirely external or internal mixture. We find that the pan-arctic average springtime DRE is 0.12 W m-2 less negative when constraining the coating thickness of the mixed particles compared to an entirely external mixture but only 0.04 W m-2 less negative when constraining the BC-containing particle fraction compared to an entirely external mixture. The difference between these methods is due to an underestimation of BC mass fraction in the springtime arctic by a factor of 3 in GEOS-Chem-TOMAS compared to POLAR6 observations.