Nadir measurements from ACE of column amounts of atmospheric gases

The feasibility of using nadir observations to make column measurements of several stratospheric gases will be evaluated for the Atmospheric Chemistry Experiment (ACE) Fourier-transform spectrometer (FTS), which is scheduled for launch in 2002 on the SCISAT-1 platform. The measurement technique is based on using FTIR spectroscopy to measure the atmospheric absorption of cold gases below the satellite against the thermal emission background from the warm Earth. The FASCOD3 and MODTRAN4 transmission codes are used to simulate the background emission spectra above the Earth; the measured spectra are processed to yield the column concentration of a particular gas in the atmosphere. The gases that can be successfully measured with this technique include ozone, carbon dioxide, carbon monoxide and nitrous oxide. This technique will be demonstrated for nadir IMG spectra obtained in 1997 at a resolution of 0.1 cm^-1. Since ACE points at the Sun throughout its orbit, even when the Earth is in the way, nadir FTS measurements will automatically be taken in addition to the occultation measurements, if enough power is available. A nadir observation will consist of 100 co-added scans at a resolution of 0.4 cm^-1, which will require a total time of 16 seconds and achieve a signal-to-noise ratio of 80:1. Using the spectroscopic structure obtained from nadir IMG spectra at a resolution of 1 cm^-1 over an extended spectral interval, it will be demonstrated that a signal-to-noise ratio of 50:1 gives ozone columns with an error of less than 2%.