Source apportionment of methane using δ13C and δ2H 

Abstract

Use of both δ13C and δ2H measurements can be used to constrain methane sources. δ13C isotopes have been used to help identify the reasons for the continued growth in atmospheric methane, which since 2007 has coincided with a decline in δ13C. δ2H could offer a third dimension to help constrain the global methane budget, but its use has been limited because less data are available. There is a need for better identification of δ2H isotopic source signatures, and more long-term atmospheric data records. We present results of field campaigns carried out in a variety of source regions to characterise isotopic signatures and consider complexities in constraining source signatures for some categories. We also consider use of methane isotopic measurements at different scales for source partitioning. The isotopic signatures of urban emissions of methane have been characterised in London, Bucharest and Ho Chi Minh City. Methane sources in these cities are very different, with emissions being mostly from gas leaks in London, from wastewater and gas leaks in Bucharest, and from waste and traffic in Ho Chi Minh City. Measurements of cattle methane emissions in Jersey and Kenya show different isotopic signatures in methane from manure and eructation. Cattle diet, the age of manure and waste management practices cause variability in the isotopic signature of emitted methane. Wetland methane emissions from sites across Finland and Canada were collected in summer 2022. The Finnish boreal wetland methane isotopic signatures were δ2H -326 ± 19 ‰ and δ13C -68 ± 4 ‰, comparable with the results from Canada. Both δ2H and δ13C  in methane from boreal wetlands tends to be more depleted in the heavier isotope than in tropical wetland methane emissions. Both δ13C and δ2H can be used in the UM-UKCA chemistry climate model which includes multiple methane tracers tagged by isotopic composition and source type. It is hoped that better characterisation of the regional variability in isotopic signatures of some sources will help improve the ability to model the global methane budget.