A Cryogen-Free Automated Measurement System of Stable Carbon Isotope Ratio of Atmospheric Methane

Abstract

 Although methane plays an important role in climate change and atmospheric chemistry, its global budget remains quantitatively uncertain due mainly to a wide variety of source types. The stable carbon isotope ratio of atmospheric methane (δ¹³C-CH₄) is useful for separating contributions of different source categories, but due to the complex and laborious analysis, limited measurement data exists. We present a new system for δ¹³C-CH₄ measurement, optimized for the automated analysis of air samples. Although the system is designed in principle similarly to those in previous studies, we successfully set up the system with no use of cryogens (e.g. liquid nitrogen) and attained reproducibility sufficient to analyze atmospheric variations (∼ 0.1 ‰). We performed automated continuous measurements of ambient air outside our laboratory at about hourly intervals for 2 months, which characterized imprint of local methane sources well. Future measurement operation for flask air samples from existing atmospheric monitoring programs will provide a large number of atmospheric δ¹³C-CH₄ data.