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Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland

Authors :
C. Buizert
P. Martinerie
V. V. Petrenko
J. P. Severinghaus
C. M. Trudinger
E. Witrant
J. L. Rosen
A. J. Orsi
M. Rubino
D. M. Etheridge
L. P. Steele
C. Hogan
J. C. Laube
W. T. Sturges
V. A. Levchenko
A. M. Smith
I. Levin
T. J. Conway
E. J. Dlugokencky
P. M. Lang
K. Kawamura
T. M. Jenk
J. W. C. White
T. Sowers
J. Schwander
T. Blunier
Source :
Atmospheric Chemistry and Physics, Vol 12, Iss 9, Pp 4259-4277 (2012)
Publication Year :
2012
Publisher :
Copernicus Publications, 2012.

Abstract

Air was sampled from the porous firn layer at the NEEM site in Northern Greenland. We use an ensemble of ten reference tracers of known atmospheric history to characterise the transport properties of the site. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the depth-diffusivity reconstruction. We define an objective root mean square criterion that is minimised in the model tuning procedure. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1σ Gaussian distribution. A comparison between two replicate boreholes drilled 64 m apart shows differences in measured mixing ratio profiles that exceed the experimental error. We find evidence that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. The ice age- gas age difference (Δage) at the firn-ice transition is calculated to be 182+3−9 yr. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore, diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records.

Subjects

Subjects :
Physics
QC1-999
Chemistry
QD1-999

Details

Language :
English
ISSN :
16807316 and 16807324
Volume :
12
Issue :
9
Database :
Directory of Open Access Journals
Journal :
Atmospheric Chemistry and Physics
Publication Type :
Academic Journal
Accession number :
edsdoj.6f7431feb1bc4c1e80ba7746c1688aae
Document Type :
article
Full Text :
https://doi.org/10.5194/acp-12-4259-2012