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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-111
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Apr 2018

Research article | 13 Apr 2018

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

Modelling black carbon absorption of solar radiation: combining external and internal mixing assumptions

Gabriele Curci1,2, Ummugulsum Alyuz3, Rocio Barò4, Roberto Bianconi5, Johannes Bieser6, Jesper H. Christensen7, Augustin Colette8, Aidan Farrow9, Xavier Francis9, Pedro Jiménez-Guerrero4, Ulas Im7, Peng Liu10, Astrid Manders11, Laura Palacios-Peña4, Marje Prank12,13, Luca Pozzoli3,13, Ranjeet Sokhi9, Efisio Solazzo14, Paolo Tuccella1,2, Alper Unal3, Marta G. Vivanco15, Christian Hogrefe16, and Stefano Galmarini14 Gabriele Curci et al.
  • 1Department of Physical and Chemical Sciences, University of L’Aquila, L’Aquila, Italy
  • 2Center of Excellence in Telesening of Environment and Model Prediction of Severe Events (CETEMPS), University of L’Aquila, L’Aquila (AQ), Italy
  • 3Eurasia Institute of Earth Sciences, Istanbul Technical University, 34469 Istanbul, Turkey
  • 4Department of Physics, University of Murcia, Murcia, 30003, Spain
  • 5Enviroware s.r.l., Concorezzo (MB), 20863, Italy
  • 6Helmholtz-Zentrum Geesthacht, Zentrum für Material- und Küstenforschung GmbH, Geesthacht, 21502, Germany
  • 7Atmospheric Modelling Secton (ATMO), Department of Environmental Science, Aarhus University, Frederiksborgvej 399, 4000 Roskilde, Denmark
  • 8Atmospheric Modelling and Environmental Mapping Unit, INERIS, BP2, Verneuil-en-Halatte, 60550, France
  • 9Centre for Atmospheric and Instrumentation Research (CAIR), University of Hertfordshire College Lane, Hatfield, AL10 9AB, UK
  • 10NRC Research Associate at Computational Exposure Division, National Exposure Research Laboratory, U.S. Environmental Protection Agency (EPA), Research Triangle Park, NC 27711, USA
  • 11TNO, PO Box 80015, 3508 TA Utrecht, the Netherlands
  • 12Finnish Meteorological Institute, Atmospheric Composition Research Unit, Helsinki, 00560, Finland
  • 13Cornell University, Department of Earth and Atmospheric Sciences, Ithaca, 14853, NY, USA
  • 14Joint Research Centre (JRC), European Commission, Ispra (VA), 21027, Italy
  • 15CIEMAT, Madrid, 28040, Spain
  • 16Computational Exposure Division, National Exposure Research Laboratory, U.S. Environmental Protection Agency (EPA), Research Triangle Park, NC 27711, USA

Abstract. An accurate simulation of the absorption properties is key for assessing the radiative effects of aerosol on meteorology and climate. The representation of how chemical species are mixed inside the particles (the mixing state) is one of the major uncertainty factors in the assessment of these effects. Here we compare aerosol optical properties simulations over Europe and North America, coordinated in the framework of the third phase of the Air Quality Model Evaluation International Initiative (AQMEII), to one year of AERONET sunphotometer retrievals, in an attempt to identify a mixing state representation that better reproduces the observed single scattering albedo and its spectral variation. We use a single post-processing tool (FlexAOD) to derive aerosol optical properties from simulated aerosol speciation profiles, and focus on the absorption enhancement of black carbon when it is internally mixed with more scattering material, discarding from the analysis scenes dominated by dust.

We found that the single scattering albedo at 440nm (ω0,440) is on average overestimated (underestimated) by 3–5% when external (core-shell internal) mixing of particles is assumed, a bias comparable in magnitude with the typical variability of the quantity. The (unphysical) homogeneous internal mixing assumption underestimates ω0,440 by ~14%. The combination of external and core-shell configurations (partial internal mixing), parameterized using a simplified function of air mass aging, reduces the ω0,440 bias to −1/−3%. The black carbon absorption enhancement (Eabs) in core-shell with respect to the externally mixed state is in the range 1.8–2.5, which is above the currently most accepted upper limit of ~1.5. The partial internal mixing reduces Eabs to values more consistent with this limit. However, the spectral dependence of the absorption is not well reproduced, and the absorption Angostrom exponent AAE440675 is overestimated by 70–120%. Further testing against more comprehensive campaign data, including a full characterization of the aerosol profile in terms of chemical speciation, mixing state, and related optical properties, would help in putting a better constraint on these calculations.

Gabriele Curci et al.
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Status: final response (author comments only)
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Atmospheric carbonaceous aerosols are able to absorb solar radiation and they continue to contribute some of the largest uncertainties in projected climate change. One important detail is how the chemical species are arranged inside each particle, i.e. the knowledge of their mixing state. We use an ensemble of regional model simulations to test different mixing state assumptions and found that a combination of internal and external mixing may better reproduce sunphotometer observations.
Atmospheric carbonaceous aerosols are able to absorb solar radiation and they continue to...
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