Development and Deployment of a Field-Deployable Ambient Methane Carbon Isotope Analyzer Using Near-Ir Laser Absorption Spectroscopy
Abstract
The concentration of ambient methane in the atmosphere has more than doubled since pre-industrial times due to an imbalance between increasing sources of the gas and its sinks. The relative contribution of methane to the greenhouse effect is 15-fold greater than that of carbon dioxide over a period of 100 years. While it is very difficult to quantitatively identify the origin from measurements of concentration, many methane sources and sinks have distinct isotopic signatures δ13C in methane values). Therefore measurements of the stable C-isotopic distribution is essential in identifying sources and sinks of methane, and can aid in quantification of the magnitude of emissions from individual sources. To date, high precision isotope measurements of C-isotope in methane have been exclusively performed with isotope ratio mass spectrometry, which involves intensive sample treatment and processing of ambient air (cold trap, isolation, possible preconcentration, before combustion of methane to CO2, before analysis). Optical studies using conventional infrared laser spectroscopy have also been reported to measure isotopic ratios. However, the measurement precision reported using these optical analyses are typically very low unless used in conjunction with pre-concentration. We report on the development of a new portable analyzer, based on cavity enhanced laser absorption spectroscopy technique (called Off-Axis Integrated Cavity Output Spectroscopy) and inexpensive near-IR diode laser, for in situ measurements of the methane carbon isotope ratio 13C/12C (δ13C) and the methane mixing ratio [CH4] in ambient air without pre-concentration. A typical precision of less than 1 per mil in ambient air (containing 1.8-ppmv methane) may be achieved in a measurement time of less than 100 seconds. The novel and compact laser-based analyzer, which requires less than 150 watts of power, has been deployed for continuous measurements in the field and compared with measurements of discrete samples using conventional IRMS methods.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.A41C0094B
- Keywords:
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- 0315 ATMOSPHERIC COMPOSITION AND STRUCTURE / Biosphere/atmosphere interactions;
- 0394 ATMOSPHERIC COMPOSITION AND STRUCTURE / Instruments and techniques;
- 0426 BIOGEOSCIENCES / Biosphere/atmosphere interactions;
- 0454 BIOGEOSCIENCES / Isotopic composition and chemistry