Transmission Spectroscopy with the ACE-FTS Infrared Spectral Atlas of Earth: A Model Validation and Sensitivity Study

Kurzfassung

ACE-FTS, the Atmospheric Chemistry Experiment - Fourier Transform Spectrometer onboard the Canadian Earth observation satellite "SciSat" is recording solar occultation spectra for about fifteen years. Five infrared atmospheric atlases for arctic summer and winter, midlatitude summer and winter, and the tropics and 31 limb rays (6 - 128 km) have been created by co-adding hundreds of cloud-free infrared spectra (2.2 - 13.3 mue) (Hughes et al., JQSRT 2014). These spectra provide a unique opportunity for model validation and to study the impact of individual molecules, spectral resolution, molecular spectroscopy data (HITRAN, GEISA, continua, etc.), and auxiliary data. Here we use GARLIC - Generic Atmospheric Radiative Transfer Line-by-Line Infrared Code (Schreier et al., JQSRT 2014) and compare observed and modeled "effective height spectra" obtained by integrating (summming) the entire limb sequence. This kind of spectra are typically used for remote sensing of (exo-)planetary atmospheres by transit spectroscopy, where only disk-averaged observations are possible. The Earth effective height spectrum varies between a few kilometers (in atmospheric window regions) and about 50 km in the CO2 v2 and v3 bands with small variations due to season and latitude. The largest impact on the transit spectra is due to water, carbon dioxide, ozone, methane, nitrous oxide, nitrogen, nitric acid, oxygen, and some chlorofluorocarbons (CFC11 and CFC12). The effect of further molecules considered in the modeling is either marginal or absent. The impact of spectroscopic input data on the model spectra is small. The best matching model with 17 molecules absorbing has a mean residuum of 0.4 km and a maximum difference of 2 km to the measured effective height.