New mid-IR semiconductor lasers enable measurements of ambient methane

Methane (CH₄) is an important greenhouse gas, behind water vapor and carbon dioxide. Until recently, it was thought that its atmospheric-mass budget was in good shape, in the sense that all sources and sinks had supposedly been identified. However, recent work by several groups has uncovered evidence for a previously unrecognized source of methane: up to 30% of atmospheric CH₄ may originate from tropical vegetation under some conditions.¹ In addition, the greenhouse gas is increasingly emitted from permafrost regions in the Arctic that have begun to thaw in response to global warming.² While methane levels in the atmosphere have been increasing, the rate of increase has been slowing. However, since 2007, levels have been increasing again, perhaps because of these new sources. To improve our understanding of processes that affect CH₄ levels, more measurements are needed close to sources, supported by satellite-based measurements of column-CH₄levels.³

Measurements of atmospheric methane can be made remotely by satellites, aircraft, or ground-based sensors. Field sensors require good sensitivity, high reliability, rugged packaging, and simple operation. The precision required to meet scientific demands is on the order of ±0.2% or ±3 parts per billion by volume (ppbv).⁴ Many sensors have been developed using semiconductor-diode lasers that probe the gas through spectral absorption of weak transitions in the near-IR regime. Sensors based on this approach are now commercially available. They use a so-called cavity-enhanced technique, similar to cavity ringdown, to provide an optical pathlength (∼1–10km) that is sufficient to produce the desired precision within a measurement time of ∼1min.

We developed a different type of prototype methane sensor. It still uses a laser-absorption spectrometer but is based on a quantum-cascade laser (QCL) as the source. QCLs, first announced in 1994, produce radiation in the mid-IR range. They generate substantially more energy and are much more robust than previous, lead-salt-diode lasers that also emit at mid-IR wavelengths. This allows access to much stronger, fundamental absorption features, which in turn enables shorter pathlengths than those associated with near-IR laser-based sensors. Our sensor design couples a type II QCL (an interband-cascade laser, ICL) with a compact optical multipass cell characterized by a 6.8m optical path. The laser operates near 3.3μm to monitor a well-isolated line in the ν₃ fundamental band of CH₄. The sensor's precision is 15ppbv for a 60s acquisition time. This precision represents a noise-equivalent absorption of 5.3×10⁻⁵.

We demonstrated the sensor at the University of New Hampshire's Sallie's Fen Environmental Station near Barrington (New Hampshire). Emissions were monitored using a network of 10 automatic chambers located around the site. When the chamber lid is closed, CH₄ emissions build up inside the chamber. Air from the chamber is pulled by a pump to a central measurement location, where we recorded the concentration-time profile. The lid is then opened to refresh the volume with ambient air. Air from each chamber was selected every 18min using a switching manifold. We compared the laser sensor's results with those from a gas chromatograph (see Figure 1) and obtained excellent agreement.

Figure 1. Ambient methane (CH₄) measured at Sallie's Fen Environmental Station by a gas chromatograph (GC) and laser sensors on 20 July 2006 at 12:00 Eastern Daylight Time (EDT). The numbers in the boxes above each peak represent the particular chamber from which the sample originated. ppmv: Parts per million by volume. UNH: University of New Hampshire. PSI: Physical Sciences Inc. ICL: Interband-cascade laser. HH:MM:SS: Hours:minutes:seconds.

The ICL used in our sensor operated at cryogenic temperatures and not at the most favorable wavelength for methane sensing. Recent advances in manufacture have resulted in wider availability of operating wavelengths, as well as in ICLs that operate in continuous-wave mode at room temperature. Continuing work involves incorporating the latest ICLs, maturing the sensor hardware, and demonstrating measurements of the ratio of the stable isotopologues (molecules that differ only in their isotopic composition), ¹³CH₄/¹²CH₄.

This work was supported by the Department of Energy (contract DE-FG02-02ER83429) and we acknowledge Roger Dahlmann.