Atmospheric Environment 38 (2004) 6855–6866 Evaluation of a micrometeorological mass balance method employing an open-path laser for measuring methane emissions R.L. Desjardins a,Ã , O.T. Denmead b , L. Harper c , M. McBain a , D. Masse´ d , S. Kaharabata a a Agriculture and Agri-Food Canada, Research Branch, K.W. Neatby Building, 960 Carling Avenue, Ottawa, ON, Canada K1A 0C6 b CSIRO Land and Water, GPO Box 1666, Canberra ACT 2601, Australia c Agriculture Research Service, United States Department of Agriculture, 1420 Experiment Station Road, Watkinsville, GA 30677, USA d Agriculture and Agri-Food Canada, Research Branch, 2000 Route 108 East, P.O. Box 90, Lennoxville, QC, Canada J1M 1Z3 Received 29 March 2004; accepted 23 August 2004 Abstract In trials of a mass balance method for measuring methane (CH 4 ) emissions, sonic anemometers and an open-path laser were used to measure the transport of CH 4 released from a ground-level source across a downwind face 50 m long and 6 m high. Release rates matched emissions expected from dairy herds of 2 to 40 cows. The long laser path permitted inferences from measurements in only two planes, one upwind and one downwind, while 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 the separation between the transmitters and reflectors being about 50 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 CH 4 s 1 . (3) For release rates greater than 40 mg CH 4 s 1 and with sufficient replication, the technique could detect a change in production rate of 9% (Po ¼ 0:05). (4) Attention to perceived weaknesses in the present technique should help towards detecting changes of 5%. Published by Elsevier Ltd. Keywords: Anthropogenic methane emissions; Mean horizontal flux; Recovery rates; Open-path laser-based gas analyzers; Mass balance method 1. Introduction Anthropogenic emissions of methane (CH 4 ) are estimated to contribute 20% of the world’s greenhouse gas emissions (IPCC, 2001). About 29% of the anthro- pogenic CH 4 emissions come from enteric fermentation in ruminant animals (notably cattle and sheep). Redu- cing these emissions is obviously important if countries hope to meet their obligations as a signatory to Kyoto. Various reduction measures have been proposed, but our ability to detect relatively small reductions of, say, 10–20% remains a challenge. Such differences are very ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.atmosenv.2004.09.008 Ã Corresponding author. Tel.: +16137591522; fax: +16137591432. E-mail address: desjardins@agr.gc.ca (R.L. Desjardins).