Recent Progress in Development of a Laser Based, Ultra-High Precision Isotope Monitor for Carbon Dioxide

Abstract:

Greenhouse gas (GHG) emissions are the primary drivers of global climate change and hence there is a crucial need to quantify their sources and sinks. A general technique to help constrain source and sink strengths in GHG exchange processes is the analysis of the relative proportions of isotopic variants of GHG's. Very high precision measurements of isotopologue ratios are necessary in order to identify sources and sinks because the characteristic changes are small. The standard method of isotopologue measurement has been mass spectrometry, but this technique typically requires significant sample preparation and relatively high instrument maintenance. Laser spectroscopy has the potential to ease these burdens and also to allow easy separation and analysis of interfering isobars such as ¹³C-CO₂ and ¹⁷O-CO₂.

We present recent results demonstrating ultra-high precision measurements of carbon dioxide isotope ratios which rival the accuracy of mass spectrometric measurements. These measurements were performed using Tunable Infrared Laser Direct Absorption Spectroscopy (TILDAS). We have developed a method for analyzing air samples from canisters by alternately and rapidly trapping sample gas and working reference gas in the optical cell. Using this technique, we have obtained isotopic measurement precisions of ~7 per meg for both ¹³C-CO₂ and ¹⁸O-CO₂ while measuring trapped ambient air samples with volumes as small as 200 ml with a 16 minute measurement duration. The figure shows a histogram of 2 minute measurements. Our current measurement precision for ¹⁷O-CO₂ is 30 per meg, but we expect to reduce this to 10 per meg by working in a better spectral region. Our ultimate goal is to create an automated, ultra-high accuracy carbon dioxide isotope monitor able to quantify ¹³C-, ¹⁸O-, and ¹⁷O-CO₂at the 10 per meg level using small (~100 standard ml), discreet air samples.

We will also discuss recent progress in the measurement of the clumped isotopes of carbon dioxide in ambient air samples. Simultaneous measurements of the clumped isotopic abundances of ¹³C¹⁶O¹⁸O and ¹³C¹⁶O¹⁷O in ambient air samples will be presented.