Measuring Methane Emitted from Arctic Tundra
Why measure methane?
The "Greenhouse Effect" refers to warming of the Earth's surface as a result of infrared radiation from the sky. There is no scientific doubt that this effect is real; without it, the surface of the Earth would be very cold, like Mars. "Global Warming" refers to the idea that the amount of infrared radiation can change due to human activities. There is debate about whether global warming will occur, how much warming would occur if it did, and how the changes would be felt around the globe. Southwest Sciences methane emission measurements are designed to add information to that debate.
Air is mostly nitrogen and oxygen, which do not absorb or emit infrared radiation, so they do not contribute to the greenhouse effect or global warming. Thus, the greenhouse effect and the global warming debate focus entirely on trace components of the atmosphere: trace gases, clouds, and aerosol particles. Although the most important gases responsible for this warming are water vapor and carbon dioxide, other trace gases, including methane, also contribute to the greenhouse effect.
Making up only about 16 molecules out of every ten million of air, methane is almost as rare as a winning lottery ticket. That amount reflects the balance between annual production and destruction by processes around the earth. Thus, one reason to measure methane emissions is to understand and quantify these processes, so that we can calculate how the concentration of this important greenhouse gas will change in the future. Another reason is to understand how carbon atoms cycle through an ecosystem, where they can be stored as part of the matter that makes up plants and animals, or emitted as gases such as methane, and thus leave the ecosystem.
Where does methane come from?
Methane is produced by anaerobic (meaning "in the absence of oxygen") decomposition of organic matter. So to look for methane production, look for a low oxygen environment with something around to decay. These conditions are found in a variety of places, including wet areas such as bogs, marshes, and rice fields, the insides of cattle and other animals, and in the buried garbage of landfills. Methane is also produced by geological processes related to oil production.
The Arctic tundra is a vast and interesting ecosystem. Permafrost prevents water from draining into the soil, so that the ground is boggy in the summer. Dead plant matter covered by water can decay anaerobically to release methane.
How do we measure it?
Working with Prof. W. Oechel's research group from San Diego State University, Southwest Sciences set up a methane flux meter. The flux (the amount of gas emitted from an area in a given time, with units of mg m-2 day-1) was measured using a diode-laser based concentration sensor, shown in the accompanying photograph. The light from a miniature laser is brought to a sensor head on the tower using fiber optics, where the beam passes through the air, making 100 bounces between a pair of mirrors. The laser sensor uses the infrared absorption of methane to distinguish it from air and other gases; the strength of the infrared absorption is proportional to methane concentration. To determine the emission rate, the concentration measurements are combined with three dimensional wind speed measurements using a technique called eddy correlation, because it takes advantages of the natural movements or eddies of the air. If there is more methane in the air that blows up from the surface, then the surface is a source of methane.
And now for the results...
The instrumentation was deployed at a site 100 km south of Prudhoe Bay, AK, for ten days in late July, 1994. Equipment for the experiments was brought in by helicopter and operated from a portable generator. The measured flux — a source of 75 mg m-2 day-1 — agreed well with the emission rates as measured by other techiques.
Boggy areas are also good breeding grounds for mosquitoes, and the tundra in July was full of them. They appear in the photograph as small blurry spots. Occaisonally, a mosquito would fly through the laser beam and block it for a few seconds. In the future, we'll put insect repellent on our sensors!
This research was sponsored by the Department of Energy and the National Science Foundation.