The Darwin TCCON Site - CO2 Calibration and First Data

This paper presents the first measurements from the solar observatory deployed to Darwin, Australia in August 2005. The observatory is the second dedicated instrument in the Total Carbon Column Observing Network (TCCON). The first two years of column values are presented, as well as comparison to integrated in situ aircraft profiles obtained during the Tropical Warm Pool - International Cloud Experiment (TWP-ICE) in January - February 2006. Sites in TCCON will provide ground-based validation and calibration for upcoming space-based instruments, such as the Orbiting Carbon Observatory (OCO) and Greenhouse Gases Observing Satellite (GOSAT), which measure the same atmospheric quantities. The TCCON stations are calibrated against integrated columns measured from aircraft by instruments calibrated on the accepted WMO CO2 scale. Two CO2 microwindows are calibrated independently, and the correction factors are determined to be 1.0143 ± 0.0004 (fit ± 95% confidence interval) and 1.0152 ± 0.0003, respectively, for the 6228 cm-1 and 6348 cm-1 bands. These values are in good agreement with similar comparisons made at Park Falls, and show an improvement in the absolute calibration, due to improved CO2 line parameters. The first two years of CO2 data from the solar FTS are analysed for secular and seasonal trends in the column average mixing ratio. These are compared to some surface in situ data trends for the same period. The column data capture the secular trend as observed in surface in situ measurements, however, the seasonal cycle is dampened relative to the surface data illustrating the need for high precision measurements, but also the potential to remove rectifier effects.