Total column CO ₂ measurements at Darwin, Australia – site description and calibration against in situ aircraft profiles

An automated Fourier Transform Spectroscopic (FTS) solar observatory was established in Darwin, Australia in August 2005. The laboratory is part of the Total Carbon Column Observing Network, and measures atmospheric column abundances of CO ₂ and O ₂ and other gases. Measured CO ₂ columns were calibrated against integrated aircraft profiles obtained during the TWP-ICE campaign in January–February 2006, and show good agreement with calibrations for a similar instrument in Park Falls, Wisconsin. A clear-sky low airmass relative precision of 0.1% is demonstrated in the CO ₂ and O ₂ retrieved column-averaged volume mixing ratios. The 1% negative bias in the FTS X_CO2 relative to the World Meteorological Organization (WMO) calibrated in situ scale is within the uncertainties of the NIR spectroscopy and analysis.