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Aerosol removal processes control global aerosol abundance, but the rate ofthat removal remains uncertain. A recent study of aerosol-bound radionuclidemeasurements after the Fukushima Daiichi nuclear power plant accidentdocuments 137Cs removal (e-folding) times of 10.0–13.9 days,suggesting that mean aerosol lifetimes in the range of 3–7 days in globalmodels might be too short by a factor of two. In this study, we attributethis discrepancy to differences between the e-folding and mean aerosollifetimes. We implement a simulation of 137Cs and 133Xe into theGEOS-Chem chemical transport model and examine the removal rates for theFukushima case. We find a general consistency between modelled and measurede-folding times. The simulated 137Cs global burden e-folding time isabout 14 days. However, the simulated mean lifetime of aerosol-bound137Cs over a 6-month post-accident period is only 1.8 days. We find thatthe mean lifetime depends strongly on the removal rates in the first few daysafter emissions, before the aerosols leave the boundary layer and aretransported to altitudes and latitudes where lifetimes with respect to wetremoval are longer by a few orders of magnitude. We present sensitivity simulations that demonstrate the influence ofdifferences in altitude and location of the radionuclides on the meanlifetime. Global mean lifetimes are shown to strongly depend on the altitudeof injection. The global mean 137Cs lifetime is more than one order ofmagnitude greater for the injection at 7 km than into the boundary layerabove the Fukushima site. Instantaneous removal rates are slower during thefirst few days after the emissions for a free tropospheric versus boundarylayer injection and this strongly controls the mean lifetimes. Global meanaerosol lifetimes for the GEOS-Chem model are 3–6 days, which is longer thanthatfor the 137Cs injected at the Fukushima site (likely due toprecipitation shortly after Fukushima emissions), but similar to the meanlifetime of 3.9 days for the 137Cs emissions injected with a uniformspread through the model's Northern Hemisphere boundary layer. Simulatede-folding times were insensitive to emission parameters (altitude, location,and time), suggesting that these measurement-based e-folding times provide arobust constraint on simulated e-folding times. Despite the reasonable global mean agreement of GEOS-Chem with measuremente-folding times, site by site comparisons yield differences of up to a factorof two, which suggest possible deficiencies in either the model transport,removal processes or the representation of 137Cs removal, particularlyin the tropics and at high latitudes. There is an ongoing need to developconstraints on aerosol lifetimes, but these measurement-based constraintsmust be carefully interpreted given the sensitivity of mean lifetimes ande-folding times to both mixing and removal processes

Interpreting aerosol lifetimes using the GEOS-Chem model and constraints from radionuclide measurements