Short-term methane emissions from two dairy farms in California estimated by different measurement techniques and US Environmental Protection Agency inventory methodology: A case study

Atmospheric top-down measurements have attributed up to twice the methane (CH4) emissions of bottom-up (BU) inventories to dairy production. We explored this discrepancy by estimating CH4 emissions of two dairy facilities in California with U.S. Environmental Protection Agency (USEPA) methodology, which is used for BU inventories, and three independent measurement techniques: 1) open-path measurements with inverse dispersion modeling (hereafter “open-path”); 2) vehicle measurements with tracer flux ratio method; and 3) aircraft measurements with closed-path method. All three techniques estimated whole farm CH4 emissions during one week in the summer of 2016. In addition, open-path also estimated whole farm CH4 emissions during two months in the winter of 2017. The objectives of the present study were: 1) to compare the different techniques to measure whole farm CH4 emissions from dairies, 2) to estimate CH4 emissions from animal housing and liquid manure storage, and compare them to USEPA inventory estimates, and 3) to compare CH4 emissions between the two dairies. Whole farm CH4 estimates were similar among measurement techniques. No seasonality was detected for CH4 emissions from animal housing, but CH4 emissions from liquid manure storage were three to six times greater during the summer than during the winter. Open-path estimates for liquid manure storage emissions were similar to monthly USEPA estimates during the summer but not during the winter, and neither open-path estimates from summer nor winter were similar to the annual USEPA estimate. Thus, CH4 emissions need to be measured throughout the year to evaluate annual inventories. Methane yields from housing and liquid manure storage were used to compare emissions between the farms. While CH4 yields from animal housing were similar (on average 20.9 g CH4/kg dry matter intake), CH4 yields from liquid manure storage at one dairy were 1.7 and 3.5 times greater than at the other dairy during summer (234 vs. 137 g CH4/kg volatile solids [VS]) and winter (78 vs. 22 g CH4/kg VS), respectively. This greater CH4 yield was attributed to the greater proportion of manure stored in liquid form, which suggests that the promotion of manure management practices that reduce the amount of manure solids stored in liquid form, such as manure separators, could significantly reduce CH4 emissions from dairies. These results demonstrate that multiple techniques for monitoring emissions on these farms were comparable