Version 4 retrievals for the atmospheric chemistry experiment Fourier transform spectrometer (ACE-FTS) and imagers

The Atmospheric Chemistry Experiment is a satellite-based mission that has been probing the Earth's atmosphere via solar occultation since February 2004. Instruments on board include a high resolution Fourier transform spectrometer (ACE-FTS) and a pair of filtered imagers. A new processing version (version 4, with version 4.1 representing the most recent update) has been implemented for these instruments. Analysis for the ACE-FTS instrument makes use of the latest spectroscopic information and features improved accuracy in forward model calculations, including a new instrumental line shape and employing a 100 m altitude sub-grid within the tangent layer of the 1 km altitude grid employed in previous processing versions. Changes were made in the handling of solar and deep space calibration spectra to avoid systematic errors that impacted previous processing versions. Emphasis was placed on improving software robustness, as well as minimizing occurrences of unphysical oscillation in retrieved profiles. Seven new molecules and three new isotopologues were added to the list of atmospheric constituents retrieved from the previous processing version (version 3.5/3.6) for a total of 44 molecules and 24 isotopologues. For the imagers, forward model calculations were changed to a 100 m altitude grid (rather than a 1 km grid) in version 4 processing. (C) 2020 Elsevier Ltd. All rights reserved.