Use of FTIR spectroscopy for the measurement of CO₂ carbon stable isotope ratios

Istituto Nazionale di Ricerca Metrologica, INRIM, Strada delle Cacce 91, 10135, Torino (Italy)

^* Corresponding author: f.rolle@inrim.it

Abstract

Carbon dioxide (CO₂) is one of the most important long-lived anthropogenic greenhouse gases. Ocean, land and biosphere contribute to take up CO₂ emissions, but approximately half of fossil fuel CO₂ accumulates in the atmosphere. The study of isotopic composition of CO₂ can give useful information for assessing and quantifying the uptake of CO₂ in the environmental compartments, as well as for distinguishing natural from anthropogenic carbon in the atmosphere. In this work, an activity for the development of a Fourier Transform Infrared spectroscopy (FTIR) based method for δ¹³C-CO₂ determination in CO₂ in air mixtures is presented. The FTIR can be calibrated by a classical approach based on primary calibration gas standards, but an alternative calibration can be based on the generation of synthetic spectra, by means of radiative transfer calculation codes such as the Multiple Atmospheric Layer Transmission (MALT University of Wollongong, Australia). Another software (B-FOS) developed at the Bureau International des Poids et Mesures (BIPM) allows to interface MALT and the FTIR management software. This calibration approach is fast and reliable and can be used when the classical calibration based on reference gas mixtures might be demanding. The uncertainty obtained for δ¹³C-CO₂ measurements is around 0.1 ‰, at a nominal CO₂ mole fraction of 400 μmol mol^-1 in air.