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Aerosols have important impacts on air quality and climate, but the processes\udaffecting their removal from the atmosphere are not fully understood and are\udpoorly constrained by observations. This makes modelled aerosol lifetimes\uduncertain. In this study, we make use of an observational constraint on\udaerosol lifetimes provided by radionuclide measurements and investigate the\udcauses of differences within a set of global models. During the Fukushima\udDai-Ichi nuclear power plant accident of March 2011, the radioactive isotopes\udcesium-137 (137Cs) and xenon-133 (133Xe) were released in large\udquantities. Cesium attached to particles in the ambient air, approximately\udaccording to their available aerosol surface area. 137Cs size\uddistribution measurements taken close to the power plant suggested that\udaccumulation-mode (AM) sulfate aerosols were the main carriers of\udcesium. Hence, 137Cs can be used as a proxy tracer for the AM sulfate\udaerosol's fate in the atmosphere. In contrast, the noble gas 133Xe\udbehaves almost like a passive transport tracer. Global surface measurements\udof the two radioactive isotopes taken over several months after the release\udallow the derivation of a lifetime of the carrier aerosol. We compare this to\udthe lifetimes simulated by 19 different atmospheric transport models\udinitialized with identical emissions of 137Cs that were assigned to an\udaerosol tracer with each model's default properties of AM sulfate, and\ud133Xe emissions that were assigned to a passive tracer. We investigate\udto what extent the modelled sulfate tracer can reproduce the measurements,\udespecially with respect to the observed loss of aerosol mass with time.\udModelled 137Cs and 133Xe concentrations sampled at the same location\udand times as station measurements allow a direct comparison between measured\udand modelled aerosol lifetime. The e-folding lifetime τe, calculated\udfrom station measurement data taken between 2 and 9 weeks after the\udstart of the emissions, is 14.3 days (95 % confidence interval\ud13.1–15.7 days). The equivalent modelled τe lifetimes have a\udlarge spread, varying between 4.8 and 26.7 days with a model median of\ud9.4 ± 2.3 days, indicating too fast a removal in most models. Because\udsufficient measurement data were only available from about 2 weeks after\udthe release, the estimated lifetimes apply to aerosols that have undergone\udlong-range transport, i.e. not for freshly emitted aerosol. However, modelled\udinstantaneous lifetimes show that the initial removal in the first 2 weeks\udwas quicker (lifetimes between 1 and 5 days) due to the emissions occurring at\udlow altitudes and co-location of the fresh plume with strong precipitation.\udDeviations between measured and modelled aerosol lifetimes are largest for\udthe northernmost stations and at later time periods, suggesting that models\uddo not transport enough of the aerosol towards the Arctic. The models\udunderestimate passive tracer (133Xe) concentrations in the Arctic as\udwell but to a smaller extent than for the aerosol (137Cs) tracer. This\udindicates that in addition to too fast an aerosol removal in the models, errors\udin simulated atmospheric transport towards the Arctic in most models also\udcontribute to the underestimation of the Arctic aerosol concentrations

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