Use of argon to measure gas exchange in turbulent mountain streams
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Published:2018-05-18
Issue:10
Volume:15
Page:3085-3092
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ISSN:1726-4189
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Container-title:Biogeosciences
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language:en
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Short-container-title:Biogeosciences
Author:
Hall Jr. Robert O.ORCID, Madinger Hilary L.
Abstract
Abstract. Gas exchange is a parameter needed in stream metabolism and trace gas
emissions models. One way to estimate gas exchange is via measuring the
decline of added tracer gases such as sulfur hexafluoride (SF6).
Estimates of oxygen (O2) gas exchange derived from SF6 additions
require scaling via Schmidt number (Sc) ratio, but this scaling is
uncertain under conditions of high gas exchange via bubbles because scaling
depends on gas solubility as well as Sc. Because argon (Ar) and O2
have nearly identical Schmidt numbers and solubility, Ar may be a useful
tracer gas for estimating stream O2 exchange. Here we compared rates of
gas exchange measured via Ar and SF6 for turbulent mountain streams in
Wyoming, USA. We measured Ar as the ratio of Ar : N2 using a membrane inlet
mass spectrometer (MIMS). Normalizing to N2 confers higher precision than simply
measuring [Ar] alone. We consistently enriched streams with Ar from 1 to
18 % of ambient Ar concentration and could estimate gas exchange rate using
an exponential decline model. The mean ratio of gas exchange of Ar relative
to SF6 was 1.8 (credible interval 1.1 to 2.5) compared to the
theoretical estimate 1.35, showing that using SF6 would have
underestimated exchange of Ar. Steep streams (slopes 11–12 %) had high rates
of gas exchange velocity normalized to Sc=600 (k600, 57–210 m d−1),
and slope strongly predicted variation in k600 among all streams. We
suggest that Ar is a useful tracer because it is easily measured, requires no
scaling assumptions to estimate rates of O2 exchange, and is not an
intense greenhouse gas as is SF6. We caution that scaling from rates of
either Ar or SF6 gas exchange to CO2 is uncertain due to solubility
effects in conditions of bubble-mediated gas transfer.
Publisher
Copernicus GmbH
Subject
Earth-Surface Processes,Ecology, Evolution, Behavior and Systematics
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