The international effort to understand and measure global sources and sinks of methane centers on an array of <i>in situ</i> sensors, terrestrial remote sensing platforms, and global monitoring satellites.  Achieving the World Meteorological Organization (WMO) target monitoring agreement of better than 2 ppb for methane requires highly accurate spectroscopic reference data and robust, SI traceable calibration. This link to the SI is necessary to remove physical artifacts from the calibration chain, which currently limits many remote sensing missions from achieving the WMO targets and introduces regional variation in concentration determination.   NIST offers unique capabilities in the effort to link methane spectroscopy to the SI through our direct access to primary standards in pressure, temperature, frequency, and amount-of-substance through the kilogram. Here we discuss ongoing work at NIST to produce high-accuracy, SI-linked spectroscopic reference data for methane.  These spectral models are linked to the SI through an atomic clock on the frequency axis and on the absorption axis through the kilogram using gravimetrically prepared gas samples.  Using advanced methods of cavity ring-down spectroscopy, we have demonstrated better than 0.1% uncertainty on spectroscopic reference data for carbon dioxide in the near-infrared and have achieved accuracy at the sub-permille level for other molecules.  We will discuss current results applying these methods for methane and future challenges in achieving the goals of accurate global monitoring.

Improving global atmospheric methane sensing through SI-linked spectroscopy

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ACS Spring 2023

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