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Volume 18, issue 19
Atmos. Chem. Phys., 18, 14037-14057, 2018
https://doi.org/10.5194/acp-18-14037-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Atmos. Chem. Phys., 18, 14037-14057, 2018
https://doi.org/10.5194/acp-18-14037-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 04 Oct 2018

Research article | 04 Oct 2018

Effects of mixing state on optical and radiative properties of black carbon in the European Arctic

Marco Zanatta1,2,a, Paolo Laj1,3,4, Martin Gysel2, Urs Baltensperger2, Stergios Vratolis5, Konstantinos Eleftheriadis5, Yutaka Kondo6, Philippe Dubuisson7, Victor Winiarek7, Stelios Kazadzis8, Peter Tunved9, and Hans-Werner Jacobi1 Marco Zanatta et al.
  • 1Univ. Grenoble Alpes, CNRS, IRD, Grenoble INP, Institute for Geosciences and Environmental Research (IGE), 38000 Grenoble, France
  • 2Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
  • 3Division of Atmospheric Science, Department of Physics, University of Helsinki, 00014, Helsinki, Finland
  • 4Institute of Atmospheric Sciences and Climate of the National Research Council of Italy, Bologna, Italy
  • 5ERL, Demokritos National Center of Scientific Research, Institute of Nuclear Technology and Radiation Protection, Attiki, Greece
  • 6National Institute of Polar Research, 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan
  • 7Laboratoire d'Optique Atmosphérique, Université Lille, 59 655 Lille, France
  • 8Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center (PMOD/WRC), 7260 Davos, Switzerland
  • 9Department of Applied Environmental Science, Stockholm University, Stockholm, Sweden
  • anow at: Alfred Wegener Institute (AWI), Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany

Abstract. Atmospheric aging promotes internal mixing of black carbon (BC), leading to an enhancement of light absorption and radiative forcing. The relationship between BC mixing state and consequent absorption enhancement was never estimated for BC found in the Arctic region. In the present work, we aim to quantify the absorption enhancement and its impact on radiative forcing as a function of microphysical properties and mixing state of BC observed in situ at the Zeppelin Arctic station (78°N) in the spring of 2012 during the CLIMSLIP (Climate impacts of short-lived pollutants in the polar region) project.

Single-particle soot photometer (SP2) measurements showed a mean mass concentration of refractory black carbon (rBC) of 39ngm−3, while the rBC mass size distribution was of lognormal shape, peaking at an rBC mass-equivalent diameter (DrBC) of around 240nm. On average, the number fraction of particles containing a BC core with DrBC>80nm was less than 5% in the size range (overall optical particle diameter) from 150 to 500nm. The BC cores were internally mixed with other particulate matter. The median coating thickness of BC cores with 220nm<DrBC< 260nm was 52nm, resulting in a core–shell diameter ratio of 1.4, assuming a coated sphere morphology. Combining the aerosol absorption coefficient observed with an Aethalometer and the rBC mass concentration from the SP2, a mass absorption cross section (MAC) of 9.8m2g−1 was inferred at a wavelength of 550 nm. Consistent with direct observation, a similar MAC value (8.4m2g−1 at 550nm) was obtained indirectly by using Mie theory and assuming a coated-sphere morphology with the BC mixing state constrained from the SP2 measurements. According to these calculations, the lensing effect is estimated to cause a 54% enhancement of the MAC compared to that of bare BC particles with equal BC core size distribution. Finally, the ARTDECO radiative transfer model was used to estimate the sensitivity of the radiative balance to changes in light absorption by BC as a result of a varying degree of internal mixing at constant total BC mass. The clear-sky noontime aerosol radiative forcing over a surface with an assumed wavelength-dependent albedo of 0.76–0.89 decreased, when ignoring the absorption enhancement, by −0.12Wm−2 compared to the base case scenario, which was constrained with mean observed aerosol properties for the Zeppelin site in Arctic spring. The exact magnitude of this forcing difference scales with environmental conditions such as the aerosol optical depth, solar zenith angle and surface albedo. Nevertheless, our investigation suggests that the absorption enhancement due to internal mixing of BC, which is a systematic effect, should be considered for quantifying the aerosol radiative forcing in the Arctic region.

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The research community aims to quantify the actual contribution of soot particles to the recent Arctic warming. We discovered that mixing of soot with other components might enhance its light absorption power by 50 %. The neglection of such amplification might lead to the underestimation of radiative forcing by 0.12 W m−2. Thus a better understanding of the optical properties of soot is a crucial step for an accurate quantification of the radiative impact of soot in the Arctic atmosphere.
The research community aims to quantify the actual contribution of soot particles to the recent...
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