Abstract
Abstract. We present field test results for a new spectroscopic instrument to measure atmospheric carbon dioxide (CO2) with high precision (0.02 μmol mol−1, or ppm at 1 Hz) and demonstrate high stability (within 0.1 ppm over more than 8 months), without the need for hourly, daily, or even monthly calibration against high-pressure gas cylinders. The technical novelty of this instrument (ABsolute Carbon dioxide, ABC) is the spectral null method using an internal quartz reference cell with known CO2 column density. Compared to a previously described prototype, the field instrument has better stability and benefits from more precise thermal control of the optics and more accurate pressure measurements in the sample cell (at the mTorr level). The instrument has been deployed at a long-term ecological research site (the Harvard Forest, USA), where it has measured for 8 months without on-site calibration and with minimal maintenance, showing drift bounds of less than 0.1 ppm. Field measurements agree well with those of a commercially available cavity ring-down CO2 instrument (Picarro G2301) run with a standard calibration protocol. This field test demonstrates that ABC is capable of performing high-accuracy, unattended, continuous field measurements with minimal use of reference gas cylinders.
Highlights
Accurate and precise measurement of atmospheric carbon dioxide (CO2) is key to monitoring and understanding anthropogenic impacts on atmospheric radiative forcing and climate
ABC relies on what we call the spectral null method, whereby the sample gas spectrum is divided by the spectrum in an internal, sealed quartz cell with a known CO2 optical depth close to that of the sample
We employ a variant of Tunable Infrared Laser Direct Absorption Spectroscopy (TILDAS) that incorporates a spectral null technique, comparing the sample spectra to spectra obtained from a permanently sealed quartz reference cell with known CO2 column density
Summary
Accurate and precise measurement of atmospheric carbon dioxide (CO2) is key to monitoring and understanding anthropogenic impacts on atmospheric radiative forcing and climate. Particular designs have targeted specific deployment purposes, such as long-term monitoring at remote sites, fast response eddy covariance measurement, mobile platform (i.e., car, ship, balloon, aircraft) in situ sampling, and atmospheric column concentration measurements. To address the problems presented by the need for periodic calibrations with gas standards, we developed an instrument called ABsolute Carbon dioxide, or ABC (Xiang et al, 2013). Results from periodic surveillance tank measurements and from ambient sampling comparisons to a calibrated CO2 instrument using CRDS (Picarro G2301) show stability within 0.1 ppm for the entire period. This performance puts the ABC instrument among the best CO2 instruments currently available. The new platform has the potential to be adopted for other stable gas measurements and eddy covariance flux sampling
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