Atmospheric CO_2 retrieved from ground-based near IR solar spectra

The column-averaged volume mixing ratio (VMR) of CO_2 over Kitt Peak, Arizona, has been retrieved from high-resolution solar absorption spectra obtained with the Fourier transform spectrometer on the McMath telescope. Simultaneous column measurements of CO_2 at ∼6300 cm^−1 and O_2 at ∼7900 cm^−1 were ratioed to minimize systematic errors. These column ratios were then scaled by the mean O_2 VMR (0.2095) to yield column-averaged vmrs of CO_2. These display similar behavior to the Mauna Loa in situ surface measurements. During the period 1977–1995, the column-averaged mixing ratio of CO_2 increased at an average rate of 1.49 ± 0.04 ppmv/yr with seasonal variations of ∼7 ppmv peak-to-peak. Our retrievals demonstrate that this remote technique is capable of precisions better than 0.5%