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Title: EVALUATION OF A MICROMETEOROLOGICAL MASS BALANCE METHOD EMPLOYING OPEN-PATH LASER FOR MEASURING METHANE EMISSIONS

Author
item DESJARDINS, R - AG AND AGRI-FOOD CANADA
item DENMEAD, O - CSIRO, AUSTRAILIA
item Harper, Lowry
item MCBAIN, M - AG AND AGRI-FOOD CANADA
item MASSE, D - AG AND AGRI-FOOD CANADA
item KAHARABATA, S - AG AND AGRI-FOOD CANADA

Submitted to: Atmospheric Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/23/2004
Publication Date: 12/1/2004
Citation: Desjardins, R., Denmead, O.T., Harper, L.A., Mcbain, M., Masse, D., Kaharabata, S. 2004. Evaluation of a micrometeorological mass balance method employing open-path laser for measuring methane emissions. Atmospheric Environment. 38:6855-6866.

Interpretive Summary: Confined animal farm operations, such as dairy farms, provide increased production efficiency, improved economics, and a better industry support system for individual producers. However, concentrating large numbers of animals in relatively small geographical areas may also present a challenge to manage wastes to minimize air-quality and global-change gaseous emissions. A large part of the problem is the inability to evaluate gaseous losses accurately and to determine if concentrated animal production does increase emissions. The purpose of these studies is to develop an accurate, non-interfering technique to evaluate gases emitted from animals and their wastes. In trials of a unique technique for measuring global change gas emissions from animal sources, measurements were made by researchers at the USDA-ARS, Watkinsville, GA, and Agriculture and Agri-Food Canada, Ottawa, to determine concentrations of a global change gas which was released from an artificial, ground-level (simulated animal) source. The main conclusions from the 23 trials were: (1) The emissions calculated from this new technique overestimated the true emissions by 5%. (2) The minimum sample size required for animal trials would be about 10 dairy cows. (3) Under normal measurement conditions, the technique could detect a change in an emission rate by the cows of 9%. This technique will allow managers and regulators the ability to accurately determine if emissions might be environmentally detrimental and evaluate if management changes will reduce gas emissions from animal production.

Technical Abstract: In trials of a mass balance method for measuring methane emissions, sonic anemometers and open-path lasers were used to measure the transport of methane released from a ground-level source across a downwind face 50m long and 6m high. Release rates matched simulated emissions from dairy herds of 2 to 40 cows. The long laser paths permitted inferences from measurements in 2 planes, one upwind, and one downwind. Along with the sonic anemometers, the fast-response instruments allowed calculation of instantaneous horizontal fluxes rather than fluxes calculated from mean wind speeds and mean concentrations. The detection limit of the lasers was 0.02 ppmv with a path-length of 50.2 m. The main conclusions from the 23 trials were: (1) Emissions calculated from mean wind speeds and concentrations overestimated the true emissions calculated from instantaneous measurements by 5%. (2) Because of small changes in methane concentration, the minimum sample size in animal trials would be 10 dairy cows, producing about 40 mg CH4 s-1. (3) For release rates greater than 40 mg CH4 s-1 and with sufficient replication, the technique could detect a change in production rate of 9% (P = 0.05). (4) Attention to perceived weaknesses in the present technique should help towards detecting changes of 5%.