Validation and Application of Cavity-Enhanced, Near-Infrared Tunable Diode Laser Absorption Spectrometry for Measurements of Methane Carbon Isotopes at Ambient Concentrations

摘要

Methane is an effective greenhouse gas but has a short residence time in the atmosphere, and therefore, reductions in emissions can alleviate its greenhouse gas warming effect within a decadal time frame. Continuous and high temporal resolution measurements of methane concentrations and carbon isotopic ratios (δ~(13)CH_4) can inform on mechanisms of formation, provide constraints on emissions sources, and guide future mitigation efforts. We describe the development, validation, and deployment of a cavity-enhanced, near-infrared tunable diode laser absorption spectrometry system capable of quantifying δ~(13)CH_4 at ambient methane concentrations. Laboratory validation and testing show that the instrument is capable of operating over a wide dynamic range of methane concentration and provides a measurement precision for δ~(13)CH_4 of better than ±0.5‰ (1σ) over 1000 s of data averaging at ambient methane concentrations. The analyzer is accurate to better than ±0.5‰, as demonstrated by measurements of characterized methane/air samples with minimal dependence (＜1‰) of measured carbon isotope ratio on methane concentration. Deployment of the instrument at a marsh over multiple days demonstrated how methane fluxes varied by an order of magnitude over 2 day deployment periods, and showed a 17‰ variability in δ~(13)CH_4 of the emitted methane during the growing season.