Characterising methane emissions from dairy farm sources using mobile and dual-isotope measurements in Jersey, Channel Islands

Field campaigns in Jersey, Channel Islands (Crown Dependency of British Isles), were carried out to understand the distribution and scale of agricultural methane (CH 4 ) emissions. We used vehicle-mounted spectrometers and isotope analysis to fingerprint and map methane sources on Jersey dairy farms to test whether mobile dual-isotope surveys can quantitatively separate enteric and manure CH 4 sources on a regional-farm scale. Peak barn CH 4 mixing ratios ( ≤ 500 ppm), observed from continuous overnight monitoring in a confined cattle barn, fall within concentration windows targeted by catalytic-oxidation prototypes, suggesting potential for the successful implementation of removal techniques, subject to ventilation-rate and cost studies. CH 4 , CO 2 (carbon dioxide) and δ 13 C CH 4 were mapped across the 120 km 2 island of Jersey, visiting 11 dairy farms and one wastewater treatment works. Methane emissions from different sources at each farm were isolated in order to determine δ 13 C CH 4 and δ 2 H CH 4 source signatures and also typical CO 2 :CH 4 ratios proximal to cattle in barns, expressed as relative excess over background. Excess CO 2 :CH 4 ratios around 8–12 can be considered a cow barn signature. 138 grab samples were collected during two island-wide campaigns (November 2021, June 2023) and analysed for δ 13 C CH 4 and δ 2 H CH 4 . Isotopic source signatures were determined using Keeling plot analysis for 61 of these samples with δ 13 C CH 4 results showing clear distinction between manure management and eructation, although there was farm to farm variation in some cases. δ 13 C CH 4 source signatures also varied with the age of the manure, likely related to the progressive oxidation of the manure over time. In differentiating between methane from breath and manure, δ 13 C CH 4 was a more consistent indicator than δ 2 H CH 4 . • The atmospheric CH 4 distribution of Jersey, Channel Islands, was mapped. • Distinctive variability in δ 13 C-CH 4 signatures observed between eructation and manure • δ 2 H-CH 4 measurements less diagnostic than δ 13 C-CH 4 for eructation and manure. • CO 2 /CH 4 ratios can be used as a reliable tracer for cattle emissions. • Up to 500 ppm methane was observed overnight in barns, so CH 4 removal strategies may be viable.