High Temporal Resolution Measurements and Modeling of the Isotopic Composition of Methane in Europe

Abstract:

Isotope measurements can help constraining the atmospheric budget of methane because different sources emit methane with slightly different isotopic composition. In the past, high precision isotope measurements have primarily been carried out by isotope ratio mass spectrometry on flask samples that are usually collected at relatively low temporal resolution. During the EU project INGOS, we have deployed a fully automated gas chromatography - isotope ratio mass spectrometry system (GC-IRMS), together with two laser instruments, during a 4-months campaign in the field at the Cabauw Experimental Site for Atmospheric Research (CESAR). More than 1600 measurements for δ¹³C and δD were obtained with IRMS during this period. Measurements show clear isotope signals associated with methane elevations both on the diurnal as well as the synoptic scale.

In order to assess the added value of such measurements for constraining the CH₄ budget, we performed coupled simulations of CH₄ and δ¹³C-CH₄ using the chemistry transport model TM5. We specifically assessed the relative impact of uncertainties in i) CH₄ emissions, ii) CH₄ isotope source signatures and iii) methane transport and chemistry throughout the atmosphere. By randomly perturbing CH₄ emissions and δ¹³C source signatures, we identified areas where simulated variations are dominated by uncertainties in the emission strength and areas where uncertainties in the isotope signatures dominate. At observation sites where the uncertainties in CH₄ emissions dominate the other sources of uncertainty, isotope observations should provide useful additional constraints on CH₄ emissions. At locations where uncertainties in the isotope signatures dominate, the isotope measurements will be useful to better constrain the source signatures themselves.