Measurements of δ¹³C in CH₄ and using particle dispersion modeling to characterize sources of Arctic methane within an air mass

Abstract

A stratified air mass enriched in methane (CH4) was sampled at ~600 m to ~2000 m altitude, between the north coast of Norway and Svalbard as part of the Methane in the Arctic: Measurements and Modelling campaign on board the UK's BAe‐146‐301 Atmospheric Research Aircraft. The approach used here, which combines interpretation of multiple tracers with transport modeling, enables better understanding of the emission sources that contribute to the background mixing ratios of CH4 in the Arctic. Importantly, it allows constraints to be placed on the location and isotopic bulk signature of the emission source(s). Measurements of δ13C in CH4 in whole air samples taken while traversing the air mass identified that the source(s) had a strongly depleted bulk δ13C CH4 isotopic signature of −70 (±2.1)‰. Combined Numerical Atmospheric‐dispersion Modeling Environment and inventory analysis indicates that the air mass was recently in the planetary boundary layer over northwest Russia and the Barents Sea, with the likely dominant source of methane being from wetlands in that region.

Measurement of the 13C isotopic signature of methane emissions from northern European wetlands

“…[]. The isotopic signature of −71‰ is similar to the signature of methane released in ebullition from northern Siberian lakes (−70‰ [ Walter et al ., ]), wetland emissions in western Siberia (−70‰) [ Umezawa et al ., ], and long‐range transported methane to the Arctic from northern Russia [ France et al ., ], suggesting that this isotopic signature may be representative of boreal wetland emissions, but this should be verified by aircraft campaigns over other regions in which wetlands emissions dominate. New measurements in Canadian wetland regions have been made using the diel sampling technique for comparison (see supporting information).…”

“…At 100 m above the wetlands, the aircraft measurements showed the same signature over wide tracts of wetland in well‐characterized air masses. This signature could also be identified in far‐traveled air arriving after multiday transport at the remote Zeppelin station [ Fisher et al ., ] and also sampled in air over the Arctic Ocean [ France et al ., ] in air transported from Siberia. The Canadian diel results found that the methane was slightly more enriched at −67 ± 1‰, but these measurements were at lower latitudes (54°21′N and 49°53′N) on the southern fringe of the boreal forest.…”

A cautionary tale: A study of a methane enhancement over the North Sea

“…[]. For a more detailed breakdown of source signatures, see, e.g., France et al [] and Zazzeri et al [].…”

“…A further possibility is that various mainland and offshore sources have already been well mixed by the time we sampled the air. For example, an approximation of the bulk signature of multiple sources that have mixed together can be made using the following equation [ France et al , ]: where δ 13 C bulk is the bulk signature (as determined by the Keeling plot), and X and Y represent different component sources of CH 4 . In this case, a simple calculation can be made to test whether a mixture of land‐based emissions could combine with gas rig emissions to plausibly generate the observed bulk signature of −52‰.…”

“…The average UK emission is −58 ± 3‰ based on values in Zazzeri et al [2017]. For a more detailed breakdown of source signatures, see, e.g., France et al [2016] and Zazzeri et al [2015].…”