Spectral- and size-resolved mass absorption efficiency of mineral dust aerosols in the shortwave: a simulation chamber study
Lorenzo Caponi1,2, Paola Formenti1, Dario Massabó2, Claudia Di Biagio1, Mathieu Cazaunau1, Edouard Pangui1, Servanne Chevaillier1, Gautier Landrot3, Meinrat O. Andreae4,11, Konrad Kandler5, Stuart Piketh6, Touraya Saeed7, Dave Seibert8, Earl Williams9, Yves Balkanski10, Paolo Prati2, and Jean-François Doussin11Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR 7583, CNRS, Université Paris-Est-Créteil et Université Paris Diderot, Institut Pierre Simon Laplace, Créteil, France 2University of Genoa, Department of Physics & INFN, Genoa, Italy 3Synchrotron SOLEIL, L'Orme des Merisiers Saint-Aubin, France 4Biogeochemistry Department, Max Planck Institute for Chemistry, P.O. Box 3060, 55020, Mainz, Germany 5Institut für Angewandte Geowissenschaften, Technische Universität Darmstadt, Schnittspahnstr. 9, 64287 Darmstadt, Germany 6Climatology Research Group, University of the Witwatersrand, Johannesburg, South Africa 7Science department, College of Basic Education, Public Authority for Applied Education and Training, Al-Ardeya, Kuwait 8Walden University, Minneapolis, Minnesota, USA 9Massachusetts Institute of Technology, Cambridge, Massachusetts, USA 10LSCE, CNRS UMR 8212, CEA, Université de Versailles Saint-Quentin, Gif sur Yvette, France 11Geology and Geophysics Department, King Saud University, Riyadh, Saudi Arabia
Received: 02 Jan 2017 – Accepted for review: 19 Jan 2017 – Discussion started: 25 Jan 2017
Abstract. This paper presents new laboratory measurements of the mass absorption efficiency (MAE) between 375 and 850 nm for mineral dust of different origin in two size classes: PM10.6 (mass fraction of particles of aerodynamic diameter lower than 10.6 µm) and PM2.5 (mass fraction of particles of aerodynamic diameter lower than 2.5 µm). Experiments have been performed in the CESAM simulation chamber using generated mineral dust from natural parent soils, and optical and gravimetric analyses. Results show that the MAE values are lower for the PM10.6 mass fraction (range 37–135 × 10−3 m2 g−1 at 375 nm) than for the PM2.5 (range 95–711 × 10−3 m2 g−1 at 375 nm), and decrease with increasing wavelength as λ-AAE, where Angstrom Absorption Exponent (AAE) averages between 3.3–3.5, regardless of size. The size-independence of AAE suggests that, for a given size distribution, the possible variation of dust composition with size would not affect significantly the spectral behavior of shortwave absorption. Because of its high atmospheric concentration, light-absorption by mineral dust can be competitive to black and brown carbon even during atmospheric transport over heavy polluted regions, when dust concentrations are significantly lower than at emission. The AAE values of mineral dust are higher than for black carbon (~ 1), but in the same range as light-absorbing organic (brown) carbon. As a result, depending on the environment, there can be some ambiguity in apportioning the AAOD based on spectral dependence, which is relevant to the development of remote sensing of light-absorption aerosols from space, and their assimilation in climate models. We suggest that the sample-to-sample variability in our dataset of MAE values is related to regional differences of the mineralogical composition of the parent soils. Particularly in the PM2.5 fraction, we found a strong linear correlation between the dust light-absorption properties and elemental iron rather than the iron oxide fraction, which could ease the application and the validation of climate models that now start to include the representation of the dust composition, as well as for remote sensing of dust absorption in the UV-VIS spectral region.
Caponi, L., Formenti, P., Massabó, D., Di Biagio, C., Cazaunau, M., Pangui, E., Chevaillier, S., Landrot, G., Andreae, M. O., Kandler, K., Piketh, S., Saeed, T., Seibert, D., Williams, E., Balkanski, Y., Prati, P., and Doussin, J.-F.: Spectral- and size-resolved mass absorption efficiency of mineral dust aerosols in the shortwave: a simulation chamber study, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-5, in review, 2017.