G. B. Rieker, F. R. Giorgetta, W. C. Swann, J. Kofler, A. M. Zolot, L. C. Sinclair, E. Baumann, C. Cromer, G. Petron, C. Sweeney, P. P. Tans, I. Coddington, And N. R. Newbury, “Frequency-comb-based remote sensing of greenhouse gases over kilometer air paths”, Optica, 1(5), November 2014, 290-298, by the Optical Society of America, http://dx.doi.org/10.1364/OPTICA.1.000290, with supplemental information available at http://dx.doi.org/10.1364/optica.99.099999.s1.

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 (1600–1670 nm) covering ~700 absorption features from CO₂, CH₄, H₂O, HDO, and 13CO₂, across a 2 km path. The spectra have sub-1-kHz frequency accuracy, no instrument lineshape, and a 0.0033 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 less than one ppm for CO₂ and less than three ppb for CH₄ in 5 min. Portable systems could enable regional monitoring.

Such techniques are bound to be less expensive than satellite-based monitoring. Such systems are essential to support the happy day when emitters of carbon dioxide are assessed fines for doing so, in proportion to their emissions, whether these emitters are power plants, landfills, highway systems, cities, or suburban tracts.