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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 11 Sep 2019

Submitted as: research article | 11 Sep 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Vertical profiles of light absorption and scattering associated with black-carbon particle fractions in the springtime Arctic above 79° N

W. Richard Leaitch1, John K. Kodros2, Megan D. Willis3,a, Sarah Hanna4, Hannes Schulz5, Elisabeth Andrews6,8, Heiko Bozem7, Julia Burkart3,b, Peter Hoor7, Felicia Kolonjari1, John A. Ogren8, Sangeeta Sharma1, Meng Si4, Knut von Salzen9, Allan K. Bertram4, Andreas Herber5, Jonathan P. D. Abbatt3, and Jeffrey R. Pierce2 W. Richard Leaitch et al.
  • 1Environment and Climate Change Canada, Toronto, ON, Canada
  • 2Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA
  • 3Department of Chemistry, University of Toronto, Toronto, ON, Canada
  • 4Department of Chemistry, University of British Columbia, Vancouver, BC, Canada
  • 5Alfred Wegener Institute, Helmholtz Center for POLAR and Marine Research, Bremerhaven, Germany
  • 6University of Colorado, Boulder, CO, USA
  • 7Institute of Atmospheric Physics, Johannes Gutenberg-University, Mainz, Germany
  • 8National Oceanic and Atmospheric Administration (NOAA), Boulder, CO, USA
  • 9Environment and Climate Change Canada, Victoria, BC, Canada
  • anow at: Lawrence Berkeley National Laboratory, Chemical Sciences Division, Berkeley, CA, USA
  • bnow at: Aerosol Physics and Environmental Physics, University of Vienna, Austria

Abstract. Despite the potential importance of black carbon (BC) to radiative forcing of the Arctic atmosphere, vertically-resolved measurements of the particle light scattering coefficient (Bsp) and light absorption coefficient (Bap) in the springtime Arctic atmosphere are infrequent, especially measurements at latitudes at or above 80oN. Here, relationships among vertically-distributed aerosol optical properties Bap, Bsp, and single scattering albedo or SSA), particle microphysics and particle chemistry are examined for a region of the Canadian archipelago between 79.9oN and 83.4oN from near the surface to 500 hPa. Airborne data collected during April, 2015, are combined with ground-based observations from the observatory at Alert, Nunavut and simulations from the GEOS-Chem-TOMAS model (Kodros et al., 2018) to increase our knowledge of the effects of BC on light absorption in the Arctic troposphere. The results are constrained for Bsp less than 15 Mm-1, which represent 98% of the observed Bsp, because the single scattering albedo (SSA) has a tendency to be lower at lower Bsp, resulting in a larger relative contribution to Arctic warming. At 18.4 m2 g-1, the average BC mass absorption coefficient (MAC) from the combined airborne and Alert observations is substantially higher than the two averaged modelled MAC values (9.5 m2 g-1 and 7.0 m2 g-1) for two different internal mixing assumptions, the latter of which is based on previous observations. The higher observed MAC value may be explained by an underestimation of BC and possible differences in BC microphysics and morphologies between the observations and model. We present Bap and SSA based on the assumption that Bap is overestimated in the observations in addition to the assumption that the higher MAC is explained. Median values of the measured Bap, rBC and organic component of particles all increase by a factor of 1.8±0.1 going from near-surface to 750 hPa, and values higher than the surface persist to 600 hPa. Modelled BC, organics, and Bap agree with the near-surface measurements, but do not reproduce the higher values observed between 900 hPa and 600 hPa. The differences between modelled and observed optical properties follow the same trend as the differences between the modelled and observed concentrations of the carbonaceous components (black and organic). Some discrepancies in the model may be due to the use of a relatively low imaginary refractive index of BC as well as by the ejection of biomass burning particles only into the boundary layer at sources. For the assumption of the higher observed MAC value, the SSA range between 0.88 and 0.94, which is significantly lower than other recent estimates for the Arctic, in part reflecting the constraint of Bsp <15 Mm-1. The large uncertainties in measuring optical properties and BC as well as the large differences between measured and modelled values, here and in the literature, argue for improved measurements of BC and light absorption by BC as well as more vertical profiles of aerosol chemistry, microphysics, and other optical properties in the Arctic.

W. Richard Leaitch et al.
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W. Richard Leaitch et al.
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Short summary
Black carbon is a factor in the warming of the Arctic atmosphere due to its ability to absorb light, but the uncertainty is high and few observations have been made in the high Arctic above 80 north. We combine airborne and ground-based observations in the springtime Arctic, at and above 80 north, with simulations from a global model to show that light absorption by black carbon may be much larger than modelled. However, the uncertainty remains high.
Black carbon is a factor in the warming of the Arctic atmosphere due to its ability to absorb...