Size-resolved mixing state of black carbon in the Canadian high Arctic and implications for simulated direct radiative effect

Andreas.Herber [ at ]


Transport of anthropogenic aerosol into the Arc- tic in the spring months has the potential to affect regional climate; however, modeling estimates of the aerosol direct radiative effect (DRE) are sensitive to uncertainties in the mixing state of black carbon (BC). A common approach in previous modeling studies is to assume an entirely exter- nal mixture (all primarily scattering species are in separate particles from BC) or internal mixture (all primarily scat- tering species are mixed in the same particles as BC). To provide constraints on the size-resolved mixing state of BC, we use airborne single-particle soot photometer (SP2) and ultrahigh-sensitivity aerosol spectrometer (UHSAS) mea- surements from the Alfred Wegener Institute (AWI) Polar 6 flights from the NETCARE/PAMARCMIP2015 campaign to estimate coating thickness as a function of refractory BC (rBC) core diameter and the fraction of particles contain- ing rBC in the springtime Canadian high Arctic. For rBC core diameters in the range of 140 to 220 nm, we find av- erage coating thicknesses of approximately 45 to 40 nm, re- spectively, resulting in ratios of total particle diameter to rBC core diameters ranging from 1.6 to 1.4. For total par- ticle diameters ranging from 175 to 730 nm, rBC-containing particle number fractions range from 16% to 3%, respec- tively. We combine the observed mixing-state constraints with simulated size-resolved aerosol mass and number dis- tributions 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 ranges from

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DOI 10.5194/acp-18-11345-2018

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Kodros, J. K. , Hanna, S. J. , Betram, A. K. , Leaitch, W. R. , Schulz, H. , Herber, A. , Zanatta, M. , Burkhart, J. , Willis, M. D. , Abbatt, J. P. and Pierce, J. R. (2018): Size-resolved mixing state of black carbon in the Canadian high Arctic and implications for simulated direct radiative effect , Atmos. Chem. Phys., 18 , pp. 11345-11361 . doi: 10.5194/acp-18-11345-2018

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