Ground-based solar absorption studies for the Carbon Cycle science by Fourier Transform Spectroscopy (CC-FTS) mission

Carbon cycle science by Fourier transform spectroscopy (CC-FTS) is an advanced study for a future satellite mission. The goal of the mission is to obtain a better understanding of the carbon cycle in the Earth's atmosphere by monitoring total and partial columns of CO2, CH4, N2O, and CO in the near infrared. CO2, CH4, and N2O are important greenhouse gases, and CO is produced by incomplete combustion. The molecular O2 column is also needed to obtain the effective optical path of the reflected sunlight and is used to normalize the column densities of the other gases. As part of this advanced study, ground-based Fourier transform spectra are used to evaluate the spectral region and resolution needed. Spectra in the 3950-7140 cm−1 region with a spectral resolution of 0.0042 cm−1 recorded at Kiruna (67.84°N, 20.41°E, and 419 m above sea level), Sweden, on 1 April 1998, were degraded to the resolutions of 0.01, 0.1, and 0.3 cm−1. The effect of spectral resolution on the retrievals has been investigated with these four Kiruna spectra. To obtain further information on the spectral resolution, optical components and spectroscopic parameters required by the future mission, high-resolution solar absorption spectra between 2000 and 15000 cm−1 were recorded using Fourier transform spectrometers at Kitt Peak (31.9°N, 111.6°W, and 2.1 km above sea level), Arizona, on 25 July 2005 and Waterloo (43.5°N, 80.6°W, and 0.3 km above sea level), Ontario, on 22 November 2006 with spectral resolutions of 0.01 and 0.1 cm−1, respectively. Dry air volume mixing ratios (VMRs) of CO2 and CH4 were retrieved from these ground-based observations. The HITRAN 2004 spectroscopic parameters are used with the SFIT2 package for the spectral analysis. The measurement precisions for CO2 and CH4 total columns are better than 1.07% and 1.13%, respectively, for our observations. Based on these results, a Fourier transform spectrometer (maximum spectral resolution of 0.1 cm−1 or 5 cm maximum optical path difference (MOPD)) operating between 2000 and 15000 cm−1 is suggested as the primary instrument for the mission. Further progress in improving the atmospheric retrievals for CO2, CH4, and O2 requires new laboratory measurements of the spectroscopic line parameters.