Observing System Simulation Experiment (OSSE) of future ALTIUS ozone profiles

ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere) is a satellite mission dedicated to continue Earth limb measurements for atmospheric sciences (Fussen et al., JQSRT, 2019). It is an element of the ESA Earth Watch programme and is expected to be launched in 2024 on a low earth polar orbit. The instrument is based on three spectral imagers that will measure in UV-vis-NIR wavelength range and will operate in different viewing geometry: limb scattering and occultation of the sun, the moon, the planets and the stars. ALTIUS will retrieve vertical profiles of ozone, nitrogen dioxide, aerosol extinction, among others.

In this study, we present an Observing System Simulation Experiment (OSSE) of ALTIUS ozone profiles that we have compared with the existing observations from Aura Microwave Limb Sounder (MLS). For this purpose, we have created a stratospheric ozone reference dataset between June and September 2008 based on the assimilation of MLS data with the Belgian Assimilation System for Chemical Observations (BASCOE). During the MLS assimilation experiment, the ozone state is saved in the space of ALTIUS previously determined with the ALTIUS orbit simulator, then perturbed according to the ALTIUS error budget, which creates ALTIUS synthetic observations. The assimilation of these ALTIUS ozone profiles agrees well with those of MLS. The assimilation of the different modes of ALTIUS reveals that all modes are necessary to constrain ozone during the polar night: solar and stellar occultations are the most constraining during the June-August period while limb scattering profiles are the most constraining from September onward.