A rapid and ultra-stable atmospheric carbon dioxide spectrometer

We present a field-tested and commercially available spectrometer to measure atmospheric carbon dioxide (CO2) with high precision (0.02 ppm at 1 Hz) and stability (within 0.1 ppm) without resorting to calibration tanks during long-term operation. The instrument, ABsolute Carbon dioxide (ABC), has the technical novelty of the spectral null method using an internal quartz reference cell with known CO2 column densities. The instrument has been deployed and measuring continuously at a long-term ecological research site (Harvard Forest, USA) for over 6 months without any on-site calibrations, showing no signs of drifts or inaccuracies of more than 0.1 ppm during this period. This successful field demonstration implies that ABC is capable to perform high accuracy unattended continuous field measurements with much simplified reference standards, relieving the need for calibration cylinders.