Technical note: Interferences of volatile organic compounds (VOCs) on methane concentration measurements

Abstract

Abstract. Studies that quantify plant methane (CH4) emission rely on the accurate measurement of small changes in the mixing ratio of CH4 that coincide with much larger changes in the mixing ratio of volatile organic compounds (VOCs). Here, we assessed whether 11 commonly occurring VOCs (e.g. methanol, α- and β-pinene, Δ3-carene) interfered with the quantitation of CH4 by five laser-absorption spectroscopy and Fourier-transformed infrared spectroscopy (FTIR) based CH4 analysers, and quantified the interference of seven compounds on three instruments. Our results showed minimal interference with laser-based analysers and underlined the importance of identifying and compensating for interferences with FTIR instruments. When VOCs were not included in the spectral library, they exerted a strong bias on FTIR-based instruments (64–1800 ppbv apparent CH4 ppmv−1 VOC). Minor (0.7–126 ppbv ppmv−1) interference with FTIR-based measurements were also detected when the spectrum of the interfering VOC was included in the library. In contrast, we detected only minor (<20 ppbv ppmv−1) and transient (< 1 min) VOC interferences on laser-absorption spectroscopy-based analysers. Overall, our results demonstrate that VOC interferences have only minor effects on CH4 flux measurements in soil chambers, but may severely impact stem and shoot flux measurements. Laser-absorption-based instruments are better suited for quantifying CH4 fluxes from plant leaves and stems than FTIR-based instruments; however, significant interferences in shoot chamber measurements could not be excluded for any of the tested instruments. Our results furthermore showed that FTIR can precisely quantify VOC mixing ratios and could therefore provide a method complementary to proton-transfer-reaction mass spectrometry (PTR-MS).

Towards reliable measurements of trace gas fluxes at plant surfaces

“…What particularly raised our concern regarding the results of the paper by Machacova and coauthors is that they report tree stem CH 4 uptake rates being similar in magnitude to that measured by our group in a mature spruce forest in Sweden, using the same instrumentation (Kohl et al, 2019). In Kohl et al (2019), we quantified the CH 4 fluxes from mature spruce stems by both Gasmet DX4040 and LGR UGGA analysers.…”

“…What particularly raised our concern regarding the results of the paper by Machacova and coauthors is that they report tree stem CH 4 uptake rates being similar in magnitude to that measured by our group in a mature spruce forest in Sweden, using the same instrumentation (Kohl et al, 2019). In Kohl et al (2019), we quantified the CH 4 fluxes from mature spruce stems by both Gasmet DX4040 and LGR UGGA analysers. In contrast with the Gasmet device, the LGR UGGA uses a narrow spectrum infrared laser to measure trace gases based on their absorbance at very specific wavelengths (Baer et al, 2002;Maddaloni et al, 2006).…”

“…This FTIR method measures the broad infrared spectrum of an air sample and uses spectral deconvolution to quantify the concentrations of multiple gas species simultaneously. In our technical note (Kohl et al, 2019), we demonstrate that this approach is particularly sensitive to interference from volatile organic compounds (VOCs). This interference is especially true for compounds missing in the spectral library used for deconvolution.…”

An automated system for trace gas flux measurements from plantfoliage and other plant compartments

“…Nevertheless, we consider them a good conservative estimate for assessing the potential impact of VOC emissions on CH 4 flux measurements. We converted these VOC mixing ratios to apparent CH 4 mixing ratios based on our recent quantification of upper limits to the spectral interference of various VOC in methane mixing ratio measurements with the Picarro G2301 and other methane analysers (Kohl et al, 2019a), using conservative uncertainty limits (±0.4 ppbv apparent CH 4 ppmv −1 methanol and ±0.2 ppbv apparent CH 4 ppmv −1 monoterpenes). Since the spectral interference of acetone was not quantified by Kohl et al (2019a), we applied the higher values value derived from methanol.…”

A Fast-response automated gas equilibrator (FaRAGE) for continuous in situ measurement of methane dissolved in water

“…Care should be taken when measuring in lakes with an anoxic hypolimnion where hydrogen sulfide is likely to accumulate. The performance of Cavity-Ring-Down gas analyzers can be potential affected by organics, ammonia, ethane, ethylene, or sulfur containing compounds (Kohl et al, 2019).…”