Atmos. Meas. Tech., 14, 4737–4753, 2021 https://doi.org/10.5194/amt-14-4737-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. On the capability of the future ALTIUS ultraviolet–visible–near-infrared limb sounder to constrain modelled stratospheric ozone Quentin Errera, Emmanuel Dekemper, Noel Baker, Jonas Debosscher, Philippe Demoulin, Nina Mateshvili, Didier Pieroux, Filip Vanhellemont, and Didier Fussen Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium Correspondence: Quentin Errera ([email protected]) Received: 22 December 2020 – Discussion started: 21 January 2021 Revised: 7 May 2021 – Accepted: 27 May 2021 – Published: 29 June 2021 Abstract. ALTIUS (Atmospheric Limb Tracker for the In- 1 Introduction vestigation of the Upcoming Stratosphere) is the upcoming stratospheric ozone monitoring limb sounder from ESA’s Stratospheric ozone (O3) is an essential component of the Earth Watch programme. Measuring in the ultraviolet– Earth’s system. By absorbing solar ultraviolet (UV) light in visible–near-infrared (UV–VIS–NIR) spectral regions, AL- the stratosphere, it protects the Earth’s surface from expo- TIUS will retrieve vertical profiles of ozone, aerosol extinc- sure to harmful radiation (Brasseur and Solomon, 2005). Sur- tion coefficients, nitrogen dioxide and other trace gases from face emissions of halogen compounds, whose production has the upper troposphere to the mesosphere. In order to maxi- been progressively banned after the implementation of the mize the geographical coverage, the instrument will observe Montreal Protocol in 1987, are responsible for the reduction limb-scattered solar light during daytime (i.e. bright limb of the ozone layer worldwide (WMO, 2018). Emissions of observations), solar occultations at the terminator and stel- long-lived greenhouse gases may also have direct – via the lar/lunar/planetary occultations during nighttime. This pa- emissions of methane and nitrous oxide – and indirect – via per evaluates the constraint of ALTIUS ozone profiles on change in atmospheric temperature – effects on the state of modelled stratospheric ozone by means of an observing sys- the ozone layer (SPARC/IO3C/GAW, 2019). Moreover, by tem simulation experiment (OSSE). In this effort, a refer- affecting the thermal structure of the atmosphere, changes ence atmosphere has been built and used to generate AL- in the stratospheric ozone will have an impact on the atmo- TIUS ozone profiles, along with an instrument simulator. spheric circulation (Hardiman et al., 2014). The monitoring These profiles are then assimilated to provide ozone analy- of the stratospheric composition is thus crucial. ses. A good agreement is found between the analyses and In the past 40 years, there have been a number of the reference atmosphere in the stratosphere and in the extra- limb-viewing satellite instruments dedicated to stratospheric tropical upper troposphere. In the tropical upper troposphere, observations (SPARC, 2017). They have provided high- although providing significant information in the analyses, resolution vertical profiles of ozone and other key parame- the assimilation of ozone profiles does not completely elim- ters (temperature, aerosol extinction, halogens, water vapour, inate the bias with respect to the reference atmosphere. The etc.) allowing us to understand the causes affecting the ozone impacts of the different modes of observations have also been layer and their consequences. Today, only a few instruments evaluated, showing that all of them are necessary to constrain are confirmed for a future launch. A rare example is the ozone during polar winters where solar/stellar occultations Ozone Mapping and Profiler Suite Limb Profiler (OMPS-LP, are the most important during the polar night and bright limb Flynn et al., 2006) onboard the Joint Polar Satellite System data are the most important during the development of the platform 2 (JPSS-2), scheduled for 2022, which will measure ozone hole in the polar spring. ozone profiles from bright limb observations. Published by Copernicus Publications on behalf of the European Geosciences Union. 4738 Q. Errera et al.: OSSE of ALTIUS O3 Another instrument, to be launched in 2024, is the Atmo- also want to measure the added value of the different AL- spheric Limb Tracker for the Investigation of the Upcom- TIUS modes of observation (solar, stellar, planetary and lu- ing Stratosphere (ALTIUS, Fussen et al., 2019). This mis- nar occultations, and bright limb measurements), in particu- sion was proposed by Belgium as an element of ESA’s Earth lar for the stellar occultations, which are the most complex Watch programme and was later supported by Canada, Lux- to implement in the mission scenario. embourg, and Romania. ALTIUS objectives are to observe To answer the first question, we have simulated 5 months the global distribution of stratospheric ozone, aerosol extinc- of ALTIUS observations using a chemical data assimilation tion, nitrogen dioxide and other trace gases at high verti- (DA) system dedicated to stratospheric ozone. In a first DA cal resolution. In addition, ALTIUS will also be a demon- experiment, called the nature run, MLS O3 profiles were as- stration mission to deliver near-real-time (NRT) ozone pro- similated and the analyses were saved in the simulated AL- files, i.e. with a latency of less than 3 h from the sensing TIUS space of observations. These simulated ALTIUS obser- to the delivery of the retrieval product to operational ser- vations were then perturbed according to the estimated AL- vices like the Copernicus Atmosphere Monitoring Service TIUS ozone error covariance matrices. Simulated ALTIUS (CAMS, Lefever et al., 2015; Flemming et al., 2017). data were then assimilated (the assimilation run). The impact Since its inception, ALTIUS was designed as an innovative of ALTIUS ozone profiles was evaluated by measuring how hyperspectral imager to be flown onboard a micro-satellite the assimilation run could reproduce the nature run. A control of the PROBA class (Vrancken et al., 2014) operating from run, without data assimilation, was also performed to check a sun-synchronous near-polar orbit. The original trade-off that the agreement between the nature and assimilation runs between cost and scientific return led to the selection of a is due to the constraint of ALTIUS profiles and not due to the passive remote-sensing instrument sensitive to the ultravio- model alone. let (UV), visible (VIS), and near-infrared (NIR) parts of the To answer the second question, several assimilation ex- electromagnetic spectrum (Fussen et al., 2019). In order to periments have been carried out using only one or two of the maximize the spatial coverage of the geophysical products, measurement modes of ALTIUS. Comparison of these exper- three observation geometries will be applied to every orbit: iments with the assimilation run emphasizes the importance bright limb measurements during daytime, solar occultations of each observation mode. at the terminator and stellar/lunar/planetary occultations dur- This paper is organized as follows: Sect. 2 provides ad- ing nighttime. ditional information on the ALTIUS mission. Section 3 in- The aim of this paper is to evaluate to what extent AL- troduces the DA system and its configurations for this study. TIUS ozone profiles are able to constrain modelled ozone in In Sect. 4, the setup of the OSSE is described, i.e. the na- a data assimilation system and to evaluate the added value ture run, the control run, the simulation of ALTIUS profiles, of the different modes of observations. This is done using the assimilation run and the runs with the selected ALTIUS an observing system simulation experiment (OSSE). OSSEs modes of observation. The evaluation of the assimilation run are typically designed to investigate the potential impacts of is presented in Sect. 5, while Sect. 6 evaluates the impact prospective observing systems using data assimilation tech- of the different ALTIUS observation modes. Conclusions are niques (Masutani et al., 2010). In OSSEs, simulated rather provided in Sect. 7. than real observations are used as the input of the data assimi- lation system. While OSSEs were usually applied to evaluate satellite instruments dedicated to meteorological use, several 2 The ALTIUS mission experiments also focused on satellite instruments measuring As mentioned in the Introduction, one of the objectives of tropospheric chemical composition (Claeyman et al., 2011; ALTIUS is the measurement of vertical ozone profiles by Timmermans et al., 2015; Abida et al., 2017). To the best of means of hyperspectral imagers operating in the UV, VIS and our knowledge, this paper is the first discussing an OSSE for NIR with three measurement geometries. a satellite instrument dedicated to stratospheric ozone profile measurements. 1. On the day side of the orbit, limb-scattered solar light In this paper, we set up several OSSEs to answer two ques- will be measured by looking above the Earth’s hori- tions related to ALTIUS observations. First, we would like to zon at tangent altitudes ranging from 0 to 100 km. This compare the constraint of assimilated ALTIUS ozone pro- method was successfully applied by previous missions files on modelled ozone fields in the stratosphere and the up- such as OSIRIS (Llewellyn et al., 2004), SCIAMACHY per troposphere against the constraint of ozone profiles mea- (Bovensmann et al., 1999), SAGE-III (Rault, 2005), and sured by the Microwave Limb Sounder (MLS, Waters et al., OMPS-LP (Flynn et al., 2006), where only instruments 2006). MLS has measured day and night O3 (and other trace measuring in UV, VIS and NIR are mentioned in this gases) with high accuracy and stability since 2004 (Hubert list. et al., 2016) and has been successfully assimilated by numer- ous chemical data assimilation systems (e.g. Wargan et al., 2. Close to the terminator of the orbit, the instrument will 2017; Inness et al., 2019; Errera et al., 2019). Second, we point at the Sun and track its occultation across the at- Atmos. Meas. Tech., 14, 4737–4753, 2021 https://doi.org/10.5194/amt-14-4737-2021 Q.