Overview and update of the SPARC Data Initiative: Comparison of stratospheric composition measurements from satellite limb sounders

Abstract. The SPARC Data Initiative (SPARC, 2017) performed the first comprehensive assessment of currently available stratospheric composition measurements obtained from an international suite of space-based limb sounders. The initiative's main objectives were (1) to assess the state of data availability, (2) to compile vertically resolved, monthly zonal mean trace gas and aerosol climatologies, and (3) to perform a detailed inter-comparison of these climatologies, summarising useful information and highlighting differences among datasets. The vertically-resolved climatologies of 26 different atmospheric constituents extending over the region from the upper troposphere to the lower mesosphere (300–0.1 hPa) are provided on a common latitude-pressure grid and include most major long-lived trace gases (O₃, H₂O, N₂O, CH₄, CCl₃F, and CCl₂F₂), transport tracers (HF, SF₆, HCl, CO, HNO₃, NO_y), and shorter-lived trace gases important to stratospheric chemistry including nitrogens (NO, NO₂, NO_x, NO₅, and HNO₄), halogens (BrO, ClO, ClONO₂ and HOCl), and other minor species (OH, HO₂, CH₂O, CH₃CN), and aerosol. This overview of the SPARC Data Initiative introduces the updated versions of the SPARC Data Initiative climatologies for the extended time period 1979–2018 and provides information on the satellite instruments included in the assessment: LIMS, SAGE I/II/III, HALOE, UARS-MLS, POAM II/III, OSIRIS, SMR, MIPAS, GOMOS, SCIAMACHY, ACE-FTS, ACE-MAESTRO, Aura-MLS, HIRDLS, SMILES, OMPS-LP, and TES. It describes the Data Initiative's top-down approach to comparing stratospheric composition measurements based on zonal monthly mean climatologies which provides upper bounds to relative inter-instrument biases and an assessment of how well the instruments are able to capture geophysical features of the stratosphere. An update to previously published evaluations of ozone and water vapour monthly mean climatologies is provided. In addition, example trace gas evaluations of methane (CH₄), carbon monoxide (CO), a set of nitrogen species (NO, NO₂, and HNO₃), the reactive nitrogen family (NO_y), and hydroperoxyl (HO₂) are presented. The results highlight the quality, strengths and weaknesses, and representativeness of the different datasets. As intended summary, the current state of our knowledge of stratospheric composition and variability is provided based on the overall consistency between the datasets. The updated SPARC Data Initiative monthly zonal mean climatologies are publicly available and accessible via the Zenodo data archive (Hegglin et al., 2020, https://doi.org/10.5281/zenodo.4265393).