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| Titel |
Interpreting aerosol lifetimes using the GEOS-Chem model and constraints from radionuclide measurements |
| VerfasserIn |
B. Croft, J. R. Pierce, R. V. Martin |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 14, no. 8 ; Nr. 14, no. 8 (2014-04-30), S.4313-4325 |
| Datensatznummer |
250118648
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| Publikation (Nr.) |
 copernicus.org/acp-14-4313-2014.pdf |
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| Zusammenfassung |
Aerosol removal processes control global aerosol abundance, but the rate of
that removal remains uncertain. A recent study of aerosol-bound radionuclide
measurements after the Fukushima Daiichi nuclear power plant accident
documents 137Cs removal (e-folding) times of 10.0–13.9 days,
suggesting that mean aerosol lifetimes in the range of 3–7 days in global
models might be too short by a factor of two. In this study, we attribute
this discrepancy to differences between the e-folding and mean aerosol
lifetimes. We implement a simulation of 137Cs and 133Xe into the
GEOS-Chem chemical transport model and examine the removal rates for the
Fukushima case. We find a general consistency between modelled and measured
e-folding times. The simulated 137Cs global burden e-folding time is
about 14 days. However, the simulated mean lifetime of aerosol-bound
137Cs over a 6-month post-accident period is only 1.8 days. We find that
the mean lifetime depends strongly on the removal rates in the first few days
after emissions, before the aerosols leave the boundary layer and are
transported to altitudes and latitudes where lifetimes with respect to wet
removal are longer by a few orders of magnitude.
We present sensitivity simulations that demonstrate the influence of
differences in altitude and location of the radionuclides on the mean
lifetime. Global mean lifetimes are shown to strongly depend on the altitude
of injection. The global mean 137Cs lifetime is more than one order of
magnitude greater for the injection at 7 km than into the boundary layer
above the Fukushima site. Instantaneous removal rates are slower during the
first few days after the emissions for a free tropospheric versus boundary
layer injection and this strongly controls the mean lifetimes. Global mean
aerosol lifetimes for the GEOS-Chem model are 3–6 days, which is longer than
that
for the 137Cs injected at the Fukushima site (likely due to
precipitation shortly after Fukushima emissions), but similar to the mean
lifetime of 3.9 days for the 137Cs emissions injected with a uniform
spread through the model's Northern Hemisphere boundary layer. Simulated
e-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 measurement
e-folding times, site by site comparisons yield differences of up to a factor
of two, which suggest possible deficiencies in either the model transport,
removal processes or the representation of 137Cs removal, particularly
in the tropics and at high latitudes. There is an ongoing need to develop
constraints on aerosol lifetimes, but these measurement-based constraints
must be carefully interpreted given the sensitivity of mean lifetimes and
e-folding times to both mixing and removal processes. |
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