Satellite and Ground-based Measurements of X CO 2 in a Remote Semi-Arid Region of Australia

Abstract. In this study, we present ground-based measurements of column-averaged dry-air mole fractions (DMFs) of CO 2 (or X CO 2 ) from an EM27/SUN portable spectrometer, equipped with an automated clam shell cover, taken in a semi-arid region of Australia. We compared these measurements to space-based X CO 2 retrievals from the Greenhouse Gases Observing Satellite (GOSAT). Side-by-side measurements of EM27/SUN with the Total Carbon Column Observing Network (TCCON) instrument at the University of Wollongong were conducted in 2015–2016 to derive an X CO 2 calibration factor of 0.9954 relative to TCCON. Although we found a slight drift of .0125 % per month in the calibration curve of the EM27/SUN vs TCCON X CO 2 , the alignment of the EM27/SUN proved stable enough for a campaign, keeping the retrieved X air values, a measure of stability, to within 0.5 % and the modulation efficiency to within 2 %. From the measurements in Alice Springs, we confirm a small bias of around 2 ppm in the GOSAT M-gain to H-gain X CO 2 retrievals, as reported by the NIES GOSAT validation team. Based on the reported random errors from GOSAT, we estimate the required duration of a future campaign in order to improve the estimated bias between the EM27/SUN and GOSAT. The dataset from the Alice Springs measurements is accessible at http://dx.doi.org/10.4225/48/5b21f16ce69bc (Velazco et al., 2018).