Correcting δ13CH4 Analyses for Krypton Interference: The Potential Impact on Methane Budget Studies

Open Access
Author:
Doyle, Emily Ann
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 30, 2017
Committee Members:
  • Richard Alley, Thesis Advisor
  • Katherine Freeman, Committee Member
  • Lee Kump, Committee Member
Keywords:
  • Methane Budget
  • Krypton Interference
  • continuous-flow mass spectrometry
  • carbon isotope
  • gas chromatography
Abstract:
Methane (CH4) enters the atmosphere through numerous sources, each with a unique carbon and hydrogen isotope signature (δ13CH4 and δD(CH4), respectively). Methane budget studies use atmospheric CH4, δ13CH4, and δD(CH4) records to constrain methane sources over time. However, previous studies found that krypton (Kr) interferes with δ13CH4 values when measured via continuous flow isotope-ratio mass spectrometry (CF-IRMS). To account for this interference, we determined a Kr correction for the methane analytical system at The Pennsylvania State University (PSU). We measured this correction by adding an additional gas chromatography (GC) column to the preconcentration unit, which separated the Kr and CH4 peaks. By measuring three standards with both the original and modified systems, we quantified the Kr correction as a function of 1/[CH4] and applied it to previously published methane budgets. The Kr correction ranged from 0 to -0.4 ± 0.4‰. We used a simple two-box model to determine how this correction affected estimated methane emissions from biomass burning (BMB) and biogenic sources. Overall, the Kr correction equally decreased reconstructed BMB emissions and increased biogenic emissions, and the size of this shift differed between hemispheres. In the Southern Hemisphere, the maximum shift in emissions was 0.4 ± 0.5 Tg/year; in the Northern Hemisphere, the maximum shift was 1.5 ± 1.6 Tg/year. In all cases, the corrected budgets were indistinguishable from the originals within the combined measurement and computational uncertainties. However, Kr interference may be greater for other CF-IRMS systems and should be considered in future δ13CH4 analyses.