Estimating methane emissions from dairy cattle housed in a barn and feedlot using an atmospheric tracer

Methane emissions from a barn and feedlot/paddock housing dairy cattle were estimated using a tracer gas (SF6) to simulate the dispersion of CH4. The tracer gas was released from 16 point sources distributed within the barn or feedlot to simulate the CH4 sources (cows). Using a two-dimensional (y, z) sampling grid, the observed downwind SF6/CH4 crosswind concentration field was integrated to give the portion of the SF6/CH4 plume that was captured by the sampling grid. Assuming that both SF6 and CH4 underwent similar turbulent atmospheric transport, the ratio of the respective captured plumes was then used to estimate the emission strength of methane from the known release rate of SF6. The predicted source strength of CH4 from the barn was within 6% of the estimate made using a different technique whereby the entire barn was used as an enclosed chamber. The methane emissions predicted in the barn and feedlot experiments were 542 (+/- 30%) L CH4 d(-1) cow(-1) and 631 (+/- 30%) L CH4 d(-1) cow(-1), respectively. Using census data on the population of dairy cattle in Canada, a national estimate of 0.245 (+/- 50%) Tg CH4 yr(-1) was made.