High-accuracy measurements of N2O concentration and site-specific nitrogen isotopes in small or high concentration samples

Abstract:

Nitrous oxide (N₂O) gas is among the major contributors to global warming and ozone depletion in stratosphere. Quantitative estimate of N­₂O production in various pathways and N­₂O fluxes across different reservoirs is the key to understanding the role of N­₂O in the global change. To achieve this goal, accurate and concurrent measurement of both N₂O concentration ([N₂O]) and its site-specific isotopic composition (SP-δ¹⁵N), namely δ¹⁵N^α and δ¹⁵N^β, is desired. Recent developments in Cavity Ring-Down Spectroscopy (CRDS) have enabled high precision measurements of [N₂O] and SP-δ¹⁵N of a continuous gas flow. However, many N­­₂O samples are discrete with limited volume (< 500 ml), and/or high [N₂O] (> 2 ppm), and are not suitable for direct measurements by CRDS. Here we present results of a Small Sample Isotope Module 2 (SSIM2) which is coupled to and automatically coordinated with a Picarro isotopic N₂O CRDS analyzer to handle and measure high concentration and/or small volume samples. The SSIM2 requires 20 ml of sample per analysis, and transfers the sample to the CRDS for high precision measurement. When the sample injection is < 20 ml, a zero gas is optionally filled to make up the volume. We used the SSIM2 to dilute high [N₂O] samples and < 20 ml samples, and tested the effect of dilution on the measured SP-δ¹⁵N. In addition, we employed and tested a newly developed double injection method for samples adequate for two 20 ml injections. After the SSIM2 and the CRDS cavity was primed with the first injection, the second injection, which has negligible dilution of the sample, can be accurately measured for both [N₂O] and SP-δ¹⁵N. Results of these experiments indicate that the precision of SSIM2-CRDS is similar to that of the continuous measurements using the CRDS alone, and that dilution has minimal effect on SP-δ¹⁵N, as along as the [N₂O] is > 300 ppb after dilution. Overall, the precision of SP-δ¹⁵N measured using the SSIM2 is < 0.5 ‰.