A Better Understanding of Atmospheric Methane Sources Using 13CH3D and 12CH2D2 Clumped Isotopes

We evaluate the use of clumped isotopes of methane (CH₄) to fingerprint local atmospheric sources of methane. We focus on a regenerative stormwater conveyance (RSC) stream wetland site running through the University of Maryland campus, which emits methane due to its engineering. Air samples in the RSC were collected at different heights above the surface and at different times of the day including both early in the morning, after methane accumulated below the nocturnal boundary layer, and late in the afternoon when convection mixed air to the cloud layer. Measured Δ¹²CH₂D₂ values of air samples record mixing between locally produced methane with low D/H and ambient air. The Δ¹²CH₂D₂ of the near surface air collected at the RSC during the early morning ranges from ∼+23‰ to ∼+35‰ which is lower than the ∼+50‰ values of tropospheric air. Mixing between background air (with Δ¹²CH₂D₂ ∼+50‰) and methane captured from chamber and bubble samples, as well as produced in incubation (all with negative Δ¹²CH₂D₂), explains the observed values of Δ¹²CH₂D₂ and Δ¹³CH₃D of near surface RSC air samples. The effect of mixing with biogenic sources on Δ¹³CH₃D is much smaller. The findings demonstrate how methane isotopologues can be used as a tool not only to fingerprint local contributions to these greenhouse gas emissions but also to identify sources of near-surface methane hot spots.