EPIC Tropospheric Ozone Data Product The Earth Polychromatic Imaging Camera (EPIC) on the Deep Space Climate Observatory (DSCOVR) spacecraft provides measurements of Earth-reflected radiances from the entire sunlit portion of the Earth. The measurements from four EPIC UV (Ultraviolet) channels are used to reconstruct global distributions of total ozone. The tropospheric ozone columns (TCO) are then derived by subtracting independently measured stratospheric ozone columns from the EPIC total ozone. TCO data product files report gridded synoptic maps of TCO measured over the sunlit portion of the Earth disk on a 1-2 hour basis. Sampling times for these hourly TCO data files are the same as for the EPIC L2 total ozone product. This Version 1.0 of the TCO product is based on Version 3 of the EPIC L1 product and the Version 3 Total Ozone Column Product. The stratospheric columns were derived from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) ozone fields (Gelaro et al., 2017). In contrast to the EPIC total ozone maps that are reported at high spatial resolution of 18 × 18 km2 near the center of the image, the TCO maps are spatially averaged over several EPIC pixels and reported on a regular spatial grid (1° latitude x 1° longitude). Kramarova et al. (2021) provide a detailed description of the EPIC TCO product and its evaluation against independent sonde and satellite measurements. Table 1 lists all of the variables included in the TCO product files. Ozone arrays in the product files are integrated vertical columns in Dobson Units (DU; 1 DU = 2.69×1020 molecules m-2). Filename Convention The TCO product files are formatted HDF5 and represent a Level-4 (L4) product. The filenames have the following naming convention: ”DSCOVR_EPIC_L4_TrO3_01_YYYYMMDDHHMMSS_03.h5” where “TrO3” means tropospheric column ozone, “01” means that this is version 01 for this product, “YYYYMMDDHHMMSS” is the UTC measurement time with “YYYY” for year (2015-present), “MM” for month (01-12), “DD” for day of the month (1-31), and “HHMMSS” denotes hours-minutes-seconds, and “03” signifies that v3 L1b measurements were used to derive the EPIC total ozone and consequently TCO. Column Weighting Function Adjustment There are two TCO gridded arrays in each hourly data file for the user to choose from; one is denoted TroposphericColumnOzone and the other is TroposphericColumnOzoneAdjusted. The latter TCO array includes an adjustment to correct for reduced sensitivity of the EPIC UV measurements in detecting ozone in the low troposphere/boundary layer. The adjustment depends on latitude and season and was derived using simulated tropospheric ozone from the GEOS-Replay model (Strode et al. 2020) constrained by the MERRA-2 meteorology through so-called replay method. Our analysis (Kramarova et al., 2021) indicated that the adjusted TCO array is more accurate and precise. Flagging Bad Data Kramarova et al. (2021) notes that the preferred EPIC total ozone measurements used for scientific study are those where the L2 “AlgorithmFlag” parameter is equal to 1, 101, or 111. In this TCO product we have included only L2 total ozone pixels with these algorithm flag values. A gridded version of the AlgorithmFlag parameter is provided in the TCO product files, as a comparison reference, but it is not needed by the user for applying data quality filtering. Another parameter in the EPIC L2 total ozone files for filtering questionable data is the “ErrorFlag”. The TCO product files include a gridded version of this ErrorFlag parameter that the user should apply. Only TCO gridded pixels with ErrorFlag value of zero should be used. TCO measurements at high satellite look angles and/or high solar zenith angles should also be filtered out for analysis. The TCO files include a gridded version of the satellite look angle and the solar zenith angle denoted as “SatelliteLookAngle” and “SolarZenithAngle”, respectively.