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 (CH₄) 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 CH₄ sources on a regional-farm scale. Peak barn CH₄ 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. CH₄, CO₂ (carbon dioxide) and δ¹³C-CH₄ were mapped across the 120 km² 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 δ¹³C-CH₄ and δ²H-CH₄ source signatures and also typical CO₂:CH₄ ratios proximal to cattle in barns, expressed as relative excess over background. Excess CO₂:CH₄ 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 δ¹³C-CH₄ and δ²H-CH₄. Isotopic source signatures were determined using Keeling plot analysis for 61 of these samples with δ¹³C-CH₄ results showing clear distinction between manure management and eructation, although there was farm to farm variation in some cases. δ¹³C-CH₄ 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, δ¹³C-CH₄ was a more consistent indicator than δ²H-CH₄.