Background matters with the SF6 tracer method for estimating enteric methane emissions from dairy cows: A critical evaluation of the SF6 procedure

Since its inception, the sulfur hexafluoride (SF6) tracer technique for estimating ruminal methane (CH4) emissions has undergone several refinements. One key divergence from the original description of the method has been its use with animals housed indoors. Given the different molecular masses of CH4 (16 g/mol) and SF6 (146 g/mol) it is possible that these gases could disperse and accumulate differentially within animal houses. The purpose of this study was to examine and compare the ambient outdoor concentrations of CH4 and SF6 with background concentrations measured during indoor experiments. A literature search found 52 scientific papers which reported use of the SF6 tracer technique with 17 reporting use indoors, 31 outdoors and 4 were desktop reviews or an uncommon implementation of SF6 as a tracer. Complete details of where background concentrations were measured, and how they were used, were not provided in any of the papers. Concentrations of CH4 in open air at Department of Primary Industries, Ellinbank, Victoria, Australia (38°14′S, 145°56′E) were variable at about 2.6 μmol/mol which was about 50% higher than those of 1.73 measured at the Cape Grim Baseline Air Pollution Station (40°41′S, 144°41′E). This difference was thought to be due to the CH4 emissions from cows in the Ellinbank area. During the same period, the SF6 concentration in open air at DPI Ellinbank increased from 4.9 pmol/mol in November 2003 to 6.8 pmol/mol in March 2010. This trend was similar to those measured at Cape Grim. Inside the DPI Ellinbank animal house, which is open to atmosphere on 2 sides, the accumulation of gases during experiments varied in a quadratic manner along the line of feeding stalls with the CH4 concentration ranging from 4 to 10 μmol/mol and SF6 ranging from 4 to 26 pmol/mol. Vertically, background concentration of CH4 trended from 4.6 μmol/mol at 225 mm above the floor to 12.3 μmol/mol at 1775 mm while SF6 trended from 8.2 to 14.9 pmol/mol at the same heights. Calculations showed that use of inappropriate background values to calculate CH4 emissions could lead to discrepancies ranging from −6.2% to +0.8% on an emission of 500 g CH4/cow/d. Thus, we recommend use of distributed sentinel canisters for monitoring accumulation of gases within animal houses, and using local background values to correct CH4 and SF6 measurements from individual animals.