Isotopic Signatures of Methane Emissions From Dairy Farms in California's San Joaquin Valley

In this study, we present seasonal atmospheric measurements of delta C-13(CH4) from dairy farms in the San Joaquin Valley of California. We used delta C-13(CH4) to characterize emissions from enteric fermentation by measuring downwind of cattle housing (e.g., freestall barns, corrals) and from manure management areas (e.g., anaerobic manure lagoons) with a mobile platform equipped with cavity ring-down spectrometers. Across seasons, the delta C-13(CH4) from enteric fermentation source areas ranged from -69.7 +/- 0.6 per mil (parts per thousand) to -51.6 +/- 0.1 parts per thousand while the delta C-13(CH4) from manure lagoons ranged from -49.5 +/- 0.1 parts per thousand to -40.5 +/- 0.2 parts per thousand. Measurements of delta C-13(CH4) of enteric CH4 suggest a greater than 10 parts per thousand difference between cattle production groups in accordance with diet. Isotopic signatures of CH4 were used to characterize enteric and manure CH4 from downwind plume sampling of dairies. Our findings show that delta C-13(CH4) measurements could improve the attribution of CH4 emissions from dairy sources at scales ranging from individual facilities to regions and help constrain the relative contributions from these different sources of emissions to the CH4 budget.