Frequency-comb-based remote sensing of greenhouse gases over kilometer air paths

Abstract

Increasing our understanding of regional greenhouse gas transport, sources, and sinks requires accurate, precise, continuous measurements of small gas enhancements over long ranges. We demonstrate a coherent dual frequency-comb spectroscopy technique capable of achieving these goals. Spectra are acquired spanning 5990 to $6260{\mathrm{cm}}^{-1}$ (1600–1670 nm) covering $\sim 700$ absorption features from ${\mathrm{CO}}_2$, ${\mathrm{CH}}_4$, ${\mathrm{H}}_2\mathrm{O}$, HDO, and ${}^{13}{\mathrm{CO}}_2$, across a 2 km path. The spectra have sub-1-kHz frequency accuracy, no instrument lineshape, and a $0.0033{\mathrm{cm}}^{-1}$ point spacing. They are fit with different absorption models to yield dry-air mole fractions of greenhouse gases. These results are compared with a point sensor under well-mixed conditions to evaluate the accuracy of models critical to global satellite-based trace gas monitoring. Under heterogeneous conditions, time-resolved data demonstrate tracking of small variations in mole fractions, with a precision $<1\mathrm{ppm}$ for ${\mathrm{CO}}_2$ and $<3\mathrm{ppb}$ for ${\mathrm{CH}}_4$ in 5 min. Portable systems could enable regional monitoring.