Characterising Arctic Wetland Methane Emissions - Isotope studies from the ground to tropopause

Many methane sources have distinct isotopic signatures and hence atmospheric measurements can be used to apportion emissions from different source categories (wetlands, gas leaks, biomass burning etc). However within source categories there are often temporal and geographic variations in the emitted signature. Production, transport and oxidation of methane from wetlands before emission to the atmosphere can vary according to the trophic status, temperature, water table and vegetation type. The δ13C signature of produced methane depends on the distribution of and the ratio of methane from acetate dissimilation and CO2 reduction. The signature of methane released to the atmosphere will also depend on the transport pathway and the amount of oxidation that has occurred. Flux and isotopic signature from one chamber to the next and from one region to another are highly variable. This study presents measurements from the MAMM campaigns of 2012 and 2013 which incorporated ground and aircraft sampling of methane across northern Scandinavia for isotopic analysis. On the ground isotopic signature was measured in chambers at Sodankylä (Finland) and Abisko (Sweden) and diurnal measurements were made within five wetland areas. Concurrent tower and aircraft measurements considered emissions over a larger region. Wetland emissions dominate the methane source measured over the European Arctic in the summer. In the July 2012 aircraft campaign elevated methane concentrations were measured in the low altitude samples over land with an isotopic source signature (δ13C) of -72 × 4 ‰, comparable to the isotopic signature of -71 × 2 ‰ measured on the ground in emissions from wetlands. The isotopic composition of methane measured over the same area in an aircraft campaign in March 2012 was significantly more enriched in 13C at -53 ‰. High latitude wetland methane emissions tend to be more depleted in 13C than those from wetlands further south. Fluxes are highly seasonal with emissions occurring from northern Scandinavia between May and October. The wetland source is visible in atmospheric methane δ13C records measured at high latitude atmospheric background stations as a late summer depletion in 13C which contrasts with the more enriched 13C signature of winter emissions from northern sources such as gas fields.