Measuring Methane Emissions from Agricultural Sources

In 2001, CH₄ emissions accounted for 45% of the GHG emissions from agricultural sources in Canada. The two main sources were from enteric fermentation by ruminants and from manure management. This estimate which is primarily based on the IPCC methodology needs to be verified. Field measurements were recently undertaken to improve CH₄ emission estimates. The backward Lagrangian Stochastic (bLS) technique, which provides emission estimates using concentration measurements and turbulence data obtained from an open-path laser and a three dimensional sonic anemometer, was used in a series of field experiments. Under ideal conditions, the model overestimated CH₄ emissions by about 5% with a standard deviation of 21% and a standard error of 5%. For complex situations such as from a manure storage tank near a barn, where the confidence in the estimates based on the bLS technique is poor, CH₄ was released from the manure storage tank, as a tracer gas, to calibrate the model. Results compare poorly with estimates of methane emissions based on the IPCC Tier 1 and Tier 2 methodologies. For a larger source, such as a feedlot, CO₂ concentration measurements obtained with an aircraft-based system were used to estimate CO₂ emissions. A mean value of 3.7 kg CO₂ head⁻¹ day⁻¹ was obtained using a Gaussian plume model. With an appropriate CO₂:CH₄ production ratio, CH₄ emissions from thousands of animals could be estimated.