In Situ Measurement of Dissolved Methane and Carbon Dioxide in Freshwater Ecosystems by Off-Axis Integrated Cavity Output Spectroscopy

Abstract

A novel low-cost method for the combined, real-time, and in situ determination of dissolved methane and carbon dioxide concentrations in freshwater ecosystems was designed and developed. This method is based on the continuous sampling of water from a freshwater ecosystem to a gas/liquid exchange membrane. Dissolved gas is transferred through the membrane to a continuous flow of high purity nitrogen, which is then measured by an off-axis integrated cavity output spectrometer (OA-ICOS). This method, called M-ICOS, was carefully tested in a laboratory and was subsequently applied to four lakes in Mexico and Alaska with contrasting climates, ecologies, and morphologies. The M-ICOS method allowed for the determination of dissolved methane and carbon dioxide concentrations with a frequency of 1 Hz and with a method detection limit of 2.76 × 10(-10) mol L(-1) for methane and 1.5 × 10(-7) mol L(-1) for carbon dioxide. These detection limits are below saturated concentrations with respect to the atmosphere and significantly lower than the minimum concentrations previously reported in lakes. The method is easily operable by a single person from a small boat, and the small size of the suction probe allows the determination of dissolved gases with a minimized impact on shallow freshwater ecosystems.

Insight into chemical, biological, and physical processes in coastal waters from dissolved oxygen and inert gas tracers

“…It could be used for continuous, high-frequency monitoring of surface waters in order to improve understanding of physically-driven gas fluxes, and be used alongside instruments measuring the fluxes of biologically-active gases (Cassar et al, 2009;Rafelski et al, 2015). In recent years, numerous sensors for measuring dissolved gases have become available, including gas tension devices that measure the total gas pressure, and can be used to derive N 2 concentration when paired with an O 2 sensor (McNeil et al, 2005(McNeil et al, , 2006, and portable analyzers for individual dissolved gases including CO 2 , CH 4 , and N 2 O (Gonzalez-Valencia et al, 2014;Grefe and Kaiser, 2014;Bastviken et al, 2015). Simultaneous measurements of noble gases and bioactive gases would assist in separately quantifying the physical and biological fluxes for bioactive gases.…”

Toward High-Resolution Vertical Measurements of Dissolved Greenhouse Gases (Nitrous Oxide and Methane) and Nutrients in the Eastern South Pacific

“…Typical gas flow rate (SCFM) 0.004-0.4 coefficient, and C g,out and C g,in are the gas concentrations outside and inside of the membrane, respectively (Boutsiadou, 2012;Gonzalez-Valencia et al, 2014). Membrane concentrations (C m ) are related to gas phase concentrations based on the K p gas membrane distribution coefficient, as follows:…”

“…However, some progress has been made for continuous ongoing measurements in the field, using an equilibrator connected to CRDS for N 2 O and CH 4 (ArevaloMartinez et al, 2013;Grefe and Kaiser, 2014;O'Reilly et al, 2015;Roberts and Shiller, 2015;Brannon et al, 2016;Kock et al, 2016). Recently, polydimethylsiloxane membranes (PDMS) (Helixmark, Carpinteria, CA, USA), coupled with MIMS and CDRS, have shown promising advances for GHGs measurements in freshwater (Bell et al, 2007;Gonzalez-Valencia et al, 2014).…”

Groundwater data improve modelling of headwater stream CO<sub>2</sub> outgassing with a stable DIC isotope approach

“…Such direct in situ methods include submerged infrared gas analysis (Johnson et al, 2010), equilibrator systems (Polsenaere et al, 2013), an off-axis integrated cavity output spectrometer combined with a gas analyser (Gonzalez-Valencia et al, 2014), and non-dispersive infrared sensors inside floating chambers Lorke et al, 2015).…”