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Accurate greenhouse gas remote sensing using open-path dual-comb spectroscopy
by Dr. Kevin Cossel, Dr. Esther Baumann, Dr. Fabrizio Giorgetta, Dr. Nathan Malarich, Dr. Griffin Mead, Dr. Nathan Newbury, Dr. Brian Washburn, Dr. Ian Coddington


Continuous, accurate determination of greenhouse gas concentrations on a variety of length scales is a critical for determining carbon fluxes especially from large areas such as urban areas, agriculture, and wetlands. Numerous satellite missions to provide global monitoring are on orbit or planned in the near future by governmental agencies and private industry. Further, there are a host of new remote sensing systems developed for regional and airborne atmospheric measurements. However, these systems struggle to provide measurements with the high accuracy and precision (0.1% or better) necessary for future accurate monitoring and inventory activities. We have developed a long-path dual frequency comb spectroscopy (DCS) technique which provides integrated path measurements of several greenhouse gases (in particular, CO2, CH4, and H2O) simultaneously over km-scale open-air paths and with time resolution on the order of one minute. DCS has several unique advantages over traditional measurement methods: broad spectral coverage for multispecies detection; absolute frequency calibration with negligible instrument lineshape leading to accurate spectra, and the ability to span scales ranging from point sensor to long (>>km) open-air paths. These features make DCS well suited both as an accurate remote sensing instrument as well as a calibration standard for other instruments. Here, we present a characterization of the accuracy and precision of DCS for greenhouse gas measurements and demonstrate accuracy at the 0.2% level with no required calibration. We then present results from a field comparison between DCS and in situ sensors at the Mauna Loa Observatory and use these data to evaluate several different spectral databases and lineshape models used in the remote sensing community including HITRAN and ABSCO. These different databases show concentration retrieval differences on the order of 1-2% and are currently the leading source of error in the measurements. Finally, we demonstrate measurements over a >14-km path length and show how this can be used to observe emissions at the city scale.

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Topic : Theme 2: Novel GHG concentration and flux methods and sensors.
Reference : T2-C3

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