Development and field testing of a rapid and ultra-stable atmospheric carbon dioxide spectrometer

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.