ExoMars TGO/NOMAD-UVIS Vertical Profiles of Ozone: 1. Seasonal Variation and Comparison to Water

https://doi.org/10.1029/2021JE006837

M. R. Patel, G. Sellers, J. P. Mason, J. A. Holmes, M. A. J. Brown, S. R. Lewis, K. Rajendran, P. M. Streeter, C. Marriner, B. G. Hathi, D. J. Slade, M. R. Leese, M. J. Wolff, A. S. J. Khayat, M. D. Smith, S. Aoki, A. Piccialli, A. C. Vandaele, S. Robert, F. Daerden, I. R. Thomas, B. Ristic, Y. Willame, C. Depiesse, G. Bellucci, J.-J. Lopez-Moreno

We present ∼1.5 Mars Years (MY) of ozone vertical profiles, covering L_S = 163° in MY34 to L_S = 320° in MY35, a period which includes the 2018 global dust storm. Since April 2018, the Ultraviolet and Visible Spectrometer channel of the Nadir and Occultation for Mars Discovery (NOMAD) instrument aboard the ExoMars Trace Gas Orbiter has observed the vertical, latitudinal and seasonal distributions of ozone. Around perihelion, the relative abundance of both ozone and water (from coincident NOMAD measurements) increases with decreasing altitude below ∼40 km. Around aphelion, localized decreases in ozone abundance exist between 25 and 35 km coincident with the location of modeled peak water abundances. High-latitude (>±55°), high altitude (40–55 km) equinoctial ozone enhancements are observed in both hemispheres (L_S ∼350°–40°) and discussed in the companion article to this work (Khayat et al., 2021). The descending branch of the main Hadley cell shapes the observed ozone distribution at L_S = 40°–60°, with the possible signature of a northern hemisphere thermally indirect cell identifiable from L_S = 40°–80°. Morning terminator observations show elevated ozone abundances with respect to evening observations, with average ozone abundances between 20 and 40 km an order of magnitude higher at sunrise compared to sunset, attributed to diurnal photochemical partitioning along the line of sight between ozone and O or fluctuations in water abundance. The ozone retrievals presented here provide the most complete global description of Mars ozone vertical distributions to date as a function of season and latitude.

patel o3

(Top) Northern hemisphere altitude profiles for (a) NOMAD retrievals of O3; (b) modeled O3 based on initial conditions derived from SPICAM observations and evolved through time considering assimilated temperature and dust observations from MCS; (c) retrieved and modeled H2O abundances (H2O is modeled after LS = 345°). NOMAD retrievals of H2O are from Aoki et al. (2019). (bottom) The equivalent as above, but for the southern hemisphere. The UVIS O3 retrieval profiles cover the period LS = 163° in MY34 to LS = 320° in MY35 and consist of 5,990 individual profiles and 677,473 individual spectra. The wide and narrow shaded periods represent the timing of the MY34 global dust storm and regional storm respectively. Altitude values are with reference to the Mars areoid, and the latitude that each profile is taken at is indicated by the top plot.