A Closed-Path Methane and Water Vapor Gas Analyzer

A closed-path methane and water vapor gas analyzer ¿ which is robust, portable, has low power consumption consistent with solar powered applications, uses room-temperature lasers, and is suitable for long-term and remote field measurements ¿ is not commercially available. Yet, such an analyzer is required to support the widespread deployment needed to achieve a better understanding of the global methane budget and to quantify exchanges between the atmosphere and terrestrial biosphere. Building on an open-path methane analyzer developed previously, this project will develop a tunable diode laser-based closed-path CH4 and H2O gas analyzer. The new closed-path instrument will use newly-developed laser technology: vertical cavity surface emitting lasers (VCSEL) and wavelength modulation spectroscopy (WMS). Compared to cavity ring-down spectroscopy (CRDS), VCSEL and WMS will provide better spectral performance (especially at ambient pressure), lower power consumption, high sensitivity, and less susceptibility to mirror contamination. Phase I demonstrated the feasibility of developing a closed-path CH4 and H2O gas analyzer with VCSEL and WMS. The resolution of this analyzer was better than 2 ppb CH4·(Hz)-1/2, which exceeded the required specification for methane flux measurement. Phase II will (1) develop a closed-path CH4 and H2O gas analyzer prototype, using a VCSEL, a dense-pattern multi-pass optical cell, and WMS; and (2) develop the instrument-embedded and application software. Commercial Applications and other Benefits as described by the awardee: Available methane analysis methods, such as gas chromatography and mass spectroscopy, are not suitable for long-term continuous field measurement, and laser-based CRDS requires main power, ultraclean optics, and isolation from the environment. The proposed analyzer could be used for net ecosystem methane exchange measurement. It also could be used for year-round measurements of atmospheric methane concentration, with minimum maintenance and with power from solar panels and batteries.