Abstract
AbstractWe present ∼1.5 Mars Years (MY) of ozone vertical profiles, covering LS = 163° in MY34 to LS = 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 (LS ∼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 LS = 40°–60°, with the possible signature of a northern hemisphere thermally indirect cell identifiable from LS = 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.
Highlights
Ozone (O3) is a highly reactive trace gas in the Martian atmosphere, where odd-hydrogen (OH, HO2, H, H2O2) catalytic products of water vapor photolysis dominate Mars photochemistry (McElroy & Donahue, 1972; Parkinson & Hunten, 1972), and contribute the primary loss mechanism for Mars O3
Given the strong photochemical anticorrelation of O3 with H2O, we present the water profiles simultaneously retrieved with the IR channel of Nadir and Occultation for Mars Discovery (NOMAD) (Aoki et al, 2019), providing the first direct comparison of contemporaneous and co-located O3 and H2O vertical profiles
At the time of writing NOMAD water profiles retrievals are only publicly available up to LS = 345° in MY34 (February 2019), and the water profiles shown in Figure 7 only contain NOMAD water retrievals to LS = 345°
Summary
Ozone (O3) is a highly reactive trace gas in the Martian atmosphere, where odd-hydrogen (OH, HO2, H, H2O2) catalytic products of water vapor photolysis dominate Mars photochemistry (McElroy & Donahue, 1972; Parkinson & Hunten, 1972), and contribute the primary loss mechanism for Mars O3. A distinct-low latitude, mid-altitude peak in O3 arises from ∼20 K colder atmospheric (and surface) temperatures present around Mars aphelion (Clancy & Nair, 1996), when Mars is furthest from the Sun in its eccentric orbit (at the current epoch, this occurs around Mars northern summer at a solar longitude, LS, of 71°) These basic temporal and spatial behaviors of Mars atmospheric O3 and H2O column abundances are qualitatively reproduced in Mars global circulation models (GCM; e.g., Daerden et al, 2019; Holmes et al, 2017, 2020, 2018; Lefèvre et al, 2008, 2004)
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