Methane often appears in the atmosphere at an irregular rate, puzzling researchers challenged with the task of tracking the gas’s sources in the landscape. An advanced infrared camera developed by Linköping University and Stockholm University researchers, in collaboration with the infrared imaging manufacturer Telops, allows users to both film and photograph methane emissions.
Photonics & Imaging Technology spoke with Magnus Gålfalk, Assistant Professor at Linköping University, about why the instrument is a valuable complement to today’s current methane detection tools.
Photonics & Imaging Technology: How does the camera photograph and film methane in the air?
Magnus Gålfalk: The camera is an Imaging Fourier Transform Spectrometer (IFTS). It uses infrared light, with the background — trees, buildings, hills — acting as a source of infrared light due to its heat. For example, a sunlit tree is often at least a few degrees hotter than the air. The methane between the camera and the background then introduces absorption lines, specific to methane, that can be used to identify this gas and to make an image of its distribution.
P&IT: How does the infrared light produce the resulting image?
Gålfalk: Incoming infrared light goes through a beam splitter, with part of the light reflecting off a stationary mirror and another part reflecting off a moving mirror. The light is then recombined and the light interference imaged at a detector. Exposures are made for thousands of positions of the moving mirror. Mathematical calculations can then produce a data cube, where both spatial and spectral information is contained — yielding a spectrum for any point in the infrared image. So one can essentially pick anywhere in an image and get the spectra at that point. Radiative transfer models can then be used to calculate the amount of methane between the camera and the background, pixel by pixel.
P&IT: Why is it important to monitor for methane?
Gålfalk: Emission hotspots are not well known, as it is difficult to cover a large area with point measurements, especially with handheld instruments. Satellites are great for global coverage, but they have coarse spatial resolution; you’re going to get pictures that are kilometer × kilometer size. An intermediate, landscape-sized scale is a good complement for pinpointing hotspots in both natural and anthropogenic environments. You can identify sources in the landscape level, and then make large-scale models from that.
P&IT: What have been the challenges of detecting methane?
Gålfalk: There are much weaker concentrations of methane than carbon dioxide, for instance. In marsh environments like wetlands, there are really low levels. But there are lots of wetlands, so it still adds up globally. It’s difficult to find the hot spots, considering the different kinds of plants and different reflections of the lake.
P&IT: What are the dimensions of the camera?
Gålfalk: Its size is about 50 × 50 × 25 cm, and the weight is 35 kg. It’s rather heavy.
P&IT: How is the camera able to achieve a high resolution?
Gålfalk: The field of view is 25 × 20 degrees, and the detector has 320 × 256 pixels, giving a spatial resolution better than 1 m2 for distances up to 750 meters. Background and air temperatures, as well as the amount of methane, nitrous oxide, and water vapor, can then be calculated pixel by pixel, producing an image.
P&IT: What is the spectral resolution?
Gålfalk: The accuracy of the gas quantification also depends on the spectral resolution. This instrument has a settable spectral resolution, which can be as high as 0.25 cm-1, enough to clearly identify and quantify these gases.
An important advantage with this method is that, because of the thousands of exposures used to make a data cube, it is possible to make a video showing air motion (seen because of water vapor and methane) at high frequency, making it possible to calculate not only amounts of methane but also fluxes (the amount of methane emitted per time).
P&IT: How has it been used so far?
Gålfalk: We have tested the technique for controlled gas releases at the Linköping University campus, anthropogenic emissions (waste incineration, sewage sludge deposit, and cows in a barn), and natural emissions (wetlands and lakes).