Evaluation of observed and modelled aerosol lifetimes using radioactive tracers of opportunity and an ensemble of 19 global models

Author(s)

N. I. Kristiansen, A. Stohl, D. J.L. Olivié, B. Croft, O. A. Søvde, H. Klein, T. Christoudias, D. Kunkel, S. J. Leadbetter, K. Zhang, K. Tsigaridis, T. Bergman, N. Evangeliou, H. Wang, P. L. Ma, R. C. Easter, P. J. Rasch, X. Liu, G. Pitari, G. Di Genova, S. Y. Zhao, Y. Balkanski, S. E. Bauer, G. S. Faluvegi, H. Kokkola, R. V. Martin, J. R. Pierce, M. Schulz, D. Shindell, H. Tost, H. Zhang

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 accumulationmode (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 τ_e, 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 τ_e 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.

Organisation(s)

Department of Meteorology and Geophysics, Center for Teaching and Learning

External organisation(s)

Norwegian Institute for Air Research, Norwegian Meteorological Institute, Dalhousie University, Centre for International Climate and Environmental Research, The Cyprus Institute, Johannes Gutenberg-Universität Mainz, Met Office, Durham University, Duke University, Pacific Northwest National Laboratory, Center for Climate Systems Research, Finnish Meteorological Institute, Université de Versailles-Saint-Quentin-en-Yvelines, University of Wyoming, Università degli Studi dell’Aquila, National Climate Center, Chinese Meteorological Agency, Colorado State University

Journal

Atmospheric Chemistry and Physics

Volume

16

Pages

3525-3561

No. of pages

37

ISSN

1680-7316

DOI

https://doi.org/10.5194/acp-16-3525-2016

Publication date

03-2016

Peer reviewed

Yes

Austrian Fields of Science 2012

105205 Climate change

ASJC Scopus subject areas

Atmospheric Science

Sustainable Development Goals

SDG 13 - Climate Action

Portal url

https://ucrisportal.univie.ac.at/en/publications/evaluation-of-observed-and-modelled-aerosol-lifetimes-using-radioactive-tracers-of-opportunity-and-an-ensemble-of-19-global-models(53617886-cc03-4dc6-a98e-a87795508e04).html