Use of argon to measure gas exchange in turbulent mountain streams

Hall Jr., Robert O.; Madinger, Hilary L.

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 (inline-formulaSF6). Estimates of oxygen (inline-formulaO2) gas exchange derived from inline-formulaSF6 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 (inline-formulaAr) and inline-formulaO2 have nearly identical Schmidt numbers and solubility, inline-formulaAr may be a useful tracer gas for estimating stream inline-formulaO2 exchange. Here we compared rates of gas exchange measured via inline-formulaAr and inline-formulaSF6 for turbulent mountain streams in Wyoming, USA. We measured inline-formulaAr as the ratio of inline-formulaArinline-formula:inline-formulaN2 using a membrane inlet mass spectrometer (MIMS). Normalizing to inline-formulaN2 confers higher precision than simply measuring [inline-formulaAr] alone. We consistently enriched streams with inline-formulaAr from 1 to 18 inline-formula% of ambient inline-formulaAr concentration and could estimate gas exchange rate using an exponential decline model. The mean ratio of gas exchange of inline-formulaAr relative to inline-formulaSF6 was 1.8 (credible interval 1.1 to 2.5) compared to the theoretical estimate 1.35, showing that using inline-formulaSF6 would have underestimated exchange of inline-formulaAr. Steep streams (slopes 11–12 inline-formula%) had high rates of gas exchange velocity normalized to inline-formulaSc=600 (inline-formulak600, 57–210 inline-formulam d−1), and slope strongly predicted variation in inline-formulak600 among all streams. We suggest that inline-formulaAr is a useful tracer because it is easily measured, requires no scaling assumptions to estimate rates of inline-formulaO2 exchange, and is not an intense greenhouse gas as is inline-formulaSF6. We caution that scaling from rates of either inline-formulaAr or inline-formulaSF6 gas exchange to inline-formulaCO2 is uncertain due to solubility effects in conditions of bubble-mediated gas transfer.



Hall Jr., Robert O. / Madinger, Hilary L.: Use of argon to measure gas exchange in turbulent mountain streams. 2018. Copernicus Publications.


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