Laboratory Spectroscopy and Optical Metrology Approaches for Analysis of Atmospheric Constituents

Successful evaluation of atmospheric gas composition relies on complementary efforts from experimental campaigns, laboratory metrology, and quantum chemistry theory. When these three areas work in concert, we significantly improve our ability to accurately describe atmospheric composition and understand atmospheric chemical behavior. Here we will discuss techniques employed in NIST’s Optical Measurements Group to perform precise gas sample analysis and provide reference-grade spectroscopic data that is critical for Earth and exoplanet atmosphere observations.

First, we will discuss direct frequency comb spectroscopy (DFCS) methods. A cross-dispersed technique has been demonstrated in the mid-infrared spectral region for rapid and precise measurement of isotopic abundance of nitrous oxide [1] which is the third leading contributor to radiative forcing in Earth’s atmosphere. Leveraging fundamental molecular transitions near 4.5 µm, we can employ a small-volume gas cell with short (7 cm) optical pathlength to analyze pure gas samples. This presentation will introduce initial measurements of nitrous oxide community reference materials [2] which can be used for maintaining isotope abundance scales, a key metrology challenge when discerning and disseminating gas analysis results. Additionally, we will introduce near-infrared DFCS studies focused on benchmarking hydrogen cyanide (HCN) molecular line lists [3] which are relevant for Earth and exoplanetary observing systems that use HCN as a tracer for chemical or physical processes.

Further, we will highlight cavity ring-down (CRD) techniques that provide spectroscopic reference data that describe fundamental physical attributes of atmospheric constituents. These parameters, including molecular transition intensity, are necessary for accurate spectroscopic modelling and can impact the accuracy of experimental retrievals. Here, we will present ultra-precise mid-infrared CRD measurements enabled by state-of-the-art hybrid crystalline mirrors. [4] Recent experimental results for a carbon monoxide transition intensity in the fundamental band will be discussed.

[1] D. M. Bailey, G. Zhao, and A. J. Fleisher, Anal. Chem. 2020, 92 (20), 13759–13766

[2] J. Mohn et al., Rapid Commun. Mass Spectrom. 2022, 36 (13), e9296

[3] D. M. Bailey, E. M. Crump, J. T. Hodges, and A. J. Fleisher, Faraday Discuss. 2023, 245, 368-379

[4] GW. Truong et al., Nat. Comms. 2023, 14, 7846