AMT Atmospheric Measurement Techniques AMT Atmos. Meas. Tech. 1867-8548 Copernicus Publications Göttingen, Germany 10.5194/amt-6-1611-2013 Towards a stable and absolute atmospheric carbon dioxide instrument using spectroscopic null method Xiang B. 1 Nelson D. D. 2 McManus J. B. 2 Zahniser M. S. 2 Wofsy S. C. 1 School of Engineering and Applied Science and Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA Aerodyne Research, Inc., 45 Manning Road, Billerica, MA 01821-3976, USA 05 07 2013 6 7 1611 1621 Copyright: © 2013 B. Xiang et al. 2013 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://amt.copernicus.org/articles/6/1611/2013/amt-6-1611-2013.html The full text article is available as a PDF file from https://amt.copernicus.org/articles/6/1611/2013/amt-6-1611-2013.pdf

We present a novel spectral method to measure atmospheric carbon dioxide (CO<sub>2</sub>) with high precision and stability without resorting to calibration tanks during long-term operation. This spectral null method improves precision by reducing spectral proportional noise associated with laser emission instabilities. We employ sealed quartz cells with known CO<sub>2</sub> column densities to serve as the permanent internal references in the null method, which improve the instrument's stability and accuracy. A prototype instrument – <b>AB</b>solute <b>C</b>arbon dioxide (ABC) is developed using this new approach. The instrument has a one-second precision of 0.02 ppm, which averages down to 0.007 ppm within one minute. Long-term stability of within 0.1 ppm is achieved without any calibrations for over a one-month period. These results have the potential for eliminating the need for calibration cylinders for high accuracy field measurements of carbon dioxide.

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