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

Abstract

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.

Processes controlling the annual cycle of Arctic aerosol number and size distributions

“…These wet scavenging developments were also implemented in a GEOS-Chem v9-03-01 simulation of 137 Cs (also using GEOS5 met fields) and evaluated against 137 Cs measurements taken for several weeks following the March 2011 Fukushima Daiichi nuclear power plant accident. Implementation of these scavenging revisions yielded improved agreement with the radionuclide measurements (median ratio of measured to modelled surface-layer concentrations changed from 5.53 to 0.52) and reduced efolding times from 21.8 to 13.2 days, which is close to the measurement value of 14.3 days (Kristiansen et al, 2016). These wet-removal revisions also slightly reduced the mean bias relative to measurements of the number of aerosols larger than 40 nm (N40), 80 nm (N80), and 150 nm (N150) for the same global set of 21 geographically diverse sites as described in D'Andrea et al (2013) (not shown).…”

FAIR v1.3: a simple emissions-based impulse response and carbon cycle model

“…Aerosols have a lifetime of the order of days (Kristiansen et al, 2016), and the emissions are converted to forcing without an intermediate concentration step.…”

Bayesian inverse modeling and source location of an unintended <sup>131</sup>I release in Europe in the fall of 2011

“…For example, both atmospheric transport models may simulate too short a lifetime of particulate iodine. This, as for many other models, was found for Cs-137 attached to particles after the Fukushima Dai-ichi accident (Kristiansen et al, 2016). The inversion would probably try to compensate a too strong loss of mass by increasing the emitted amount.…”