Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 16, 3525-3561, 2016
https://doi.org/10.5194/acp-16-3525-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
17 Mar 2016
Evaluation of observed and modelled aerosol lifetimes using radioactive tracers of opportunity and an ensemble of 19 global models
N. I. Kristiansen1, A. Stohl1, D. J. L. Olivié2, B. Croft3, O. A. Søvde4, H. Klein2, T. Christoudias5, D. Kunkel6, S. J. Leadbetter7, Y. H. Lee8, K. Zhang9, K. Tsigaridis10, T. Bergman11, N. Evangeliou1,12, H. Wang9, P.-L. Ma9, R. C. Easter9, P. J. Rasch9, X. Liu13, G. Pitari14, G. Di Genova14, S. Y. Zhao15, Y. Balkanski12, S. E. Bauer10, G. S. Faluvegi10, H. Kokkola11, R. V. Martin3, J. R. Pierce16,3, M. Schulz2, D. Shindell8, H. Tost6, and H. Zhang15 1NILU – Norwegian Institute for Air Research, Kjeller, Norway
2Norwegian Meteorological Institute, Oslo, Norway
3Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Canada
4Center for International Climate and Environmental Research – Oslo (CICERO), Oslo, Norway
5The Cyprus Institute, Nicosia, Cyprus
6Institute for Atmospheric Physics, Johannes Gutenberg University of Mainz, Mainz, Germany
7Met Office, Exeter, UK
8Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC, USA
9Pacific Northwest National Laboratory (PNNL), Richland, WA, USA
10Center for Climate Systems Research, Columbia University, and NASA Goddard Institute for Space Studies, New York, NY, USA
11Finnish Meteorological Institute, Kuopio, Finland
12Laboratoire des Sciences du Climat et de l'Environnement, CEA-CNRS-UVSQ, Gif-sur-Yvette, France
13Department of Atmospheric Science, University of Wyoming, Laramie, WY, USA
14University of L'Aquila, L'Aquila, Italy
15Laboratory for Climate Studies, National Climate Center, Chinese Meteorological Administration, Beijing, China
16Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA
Abstract. Aerosols have important impacts on air quality and climate, but the processes affecting their removal from the atmosphere are not fully understood and are poorly constrained by observations. This makes modelled aerosol lifetimes uncertain. In this study, we make use of an observational constraint on aerosol lifetimes provided by radionuclide measurements and investigate the causes of differences within a set of global models. During the Fukushima Dai-Ichi nuclear power plant accident of March 2011, the radioactive isotopes cesium-137 (137Cs) and xenon-133 (133Xe) were released in large quantities. Cesium attached to particles in the ambient air, approximately according to their available aerosol surface area. 137Cs size distribution measurements taken close to the power plant suggested that accumulation-mode (AM) sulfate aerosols were the main carriers of cesium. Hence, 137Cs can be used as a proxy tracer for the AM sulfate aerosol's fate in the atmosphere. In contrast, the noble gas 133Xe behaves almost like a passive transport tracer. Global surface measurements of the two radioactive isotopes taken over several months after the release allow the derivation of a lifetime of the carrier aerosol. We compare this to the lifetimes simulated by 19 different atmospheric transport models initialized with identical emissions of 137Cs that were assigned to an aerosol tracer with each model's default properties of AM sulfate, and 133Xe emissions that were assigned to a passive tracer. We investigate to what extent the modelled sulfate tracer can reproduce the measurements, especially with respect to the observed loss of aerosol mass with time. Modelled 137Cs and 133Xe concentrations sampled at the same location and times as station measurements allow a direct comparison between measured and modelled aerosol lifetime. The e-folding lifetime τe, calculated from station measurement data taken between 2 and 9 weeks after the start of the emissions, is 14.3 days (95 % confidence interval 13.1–15.7 days). The equivalent modelled τe lifetimes have a large spread, varying between 4.8 and 26.7 days with a model median of 9.4 ± 2.3 days, indicating too fast a removal in most models. Because sufficient measurement data were only available from about 2 weeks after the release, the estimated lifetimes apply to aerosols that have undergone long-range transport, i.e. not for freshly emitted aerosol. However, modelled instantaneous lifetimes show that the initial removal in the first 2 weeks was quicker (lifetimes between 1 and 5 days) due to the emissions occurring at low altitudes and co-location of the fresh plume with strong precipitation. Deviations between measured and modelled aerosol lifetimes are largest for the northernmost stations and at later time periods, suggesting that models do not transport enough of the aerosol towards the Arctic. The models underestimate passive tracer (133Xe) concentrations in the Arctic as well but to a smaller extent than for the aerosol (137Cs) tracer. This indicates that in addition to too fast an aerosol removal in the models, errors in simulated atmospheric transport towards the Arctic in most models also contribute to the underestimation of the Arctic aerosol concentrations.

Citation: Kristiansen, N. I., Stohl, A., Olivié, D. J. L., Croft, B., Søvde, O. A., Klein, H., Christoudias, T., Kunkel, D., Leadbetter, S. J., Lee, Y. H., Zhang, K., Tsigaridis, K., Bergman, T., Evangeliou, N., Wang, H., Ma, P.-L., Easter, R. C., Rasch, P. J., Liu, X., Pitari, G., Di Genova, G., Zhao, S. Y., Balkanski, Y., Bauer, S. E., Faluvegi, G. S., Kokkola, H., Martin, R. V., Pierce, J. R., Schulz, M., Shindell, D., Tost, H., and Zhang, H.: Evaluation of observed and modelled aerosol lifetimes using radioactive tracers of opportunity and an ensemble of 19 global models, Atmos. Chem. Phys., 16, 3525-3561, https://doi.org/10.5194/acp-16-3525-2016, 2016.
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Short summary
Processes affecting aerosol removal from the atmosphere are not fully understood. In this study we investigate to what extent atmospheric transport models can reproduce observed loss of aerosols. We compare measurements of radioactive isotopes, that attached to ambient sulfate aerosols during the 2011 Fukushima nuclear accident, to 19 models using identical emissions. Results indicate aerosol removal that is too fast in most models, and apply to aerosols that have undergone long-range transport.
Processes affecting aerosol removal from the atmosphere are not fully understood. In this study...
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