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

Abstract

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. Plain Language SummaryWe present over two years of new observations of the vertical distribution of ozone in the atmosphere of Mars. The ExoMars Trace Gas Orbiter spacecraft has been recording observations of the Martian atmosphere since 2018 to map the presence and changes in abundance of gases such as ozone by using the "Nadir and Occultation for Mars Discovery (NOMAD)" instrument. NOMAD continually observes the change in ozone abundance (among other gases) at different heights across much of the planet. These abundance profiles have revealed the presence of distinct layers of ozone enhancement at high altitudes in the atmosphere of Mars toward the polar regions and between spring and autumn in the southern hemisphere of Mars, discussed in detail in the companion article. We observe broad periods where often the abundance of ozone follows the abundance of water from ∼10 km altitude up to ∼50 km altitude, and other times when the two appear to be opposite in their variation with height. Our retrievals of ozone from NOMAD data provide the first coincident observations of ozone and water and provide previously unavailable information on the photochemistry of Mars.

Seasonal Changes in the Vertical Structure of Ozone in the Martian Lower Atmosphere and Its Relationship to Water Vapor

“…In contrast to the south, the lower atmosphere has warmed, associated with northern summer, such that water vapour increases near the surface, and ozone is depleted below 20 km. The number densities reported byPatel et al (2021) do not reveal a decrease in O3 abundance and observe a more uniform O3 density up to 50 km. Most ACS observations in which ozone was observed occurred closer to the equinoxes, which was also observed with NOMAD.…”

“…This observation reveals a substantially colder atmosphere below 30 km, with very low water vapour, resulting in 200 ppbv O3 up to 25 km. NOMAD observations in the UV during the time period tend to be restricted to above 20 or 30 km, but reveal a measurable low number density at the lower altitude limits of their observations, and very low abundances above (Patel et al, 2021). Southern winter observations made during the aphelion period (panel g in Figure 2) are shown in Figure 4, which presents vertical profiles of temperature and the VMRs of O3 and H2O.…”

Planet‐Wide Ozone Destruction in the Middle Atmosphere on Mars During Global Dust Storm

“…We perform the retrievals of O 3 profiles in a manner that is equivalent to previous work (Khayat et al., 2021; Patel et al., 2021), but different in detail. First, we derive extinction profiles by applying the so‐called “onion‐peeling” method to the NOMAD/UVIS transmittance spectra, as was described for TGO/ACS observations (Stcherbinine et al., 2020).…”

“…At the same time, water vapor frequently deposits into ice clouds below ∼40 km (height changing with latitude and season) and becomes only sparsely available above the cloud zone. In that region, the O 3 formation is no longer suppressed, and its abundances will increase with height, although limited by the decreasing CO 2 density (Daerden et al., 2019; Khayat et al., 2021; Lefèvre et al., 2004; Patel et al., 2021; Clancy and Nair, 1996).…”

ExoMars TGO/NOMAD‐UVIS Vertical Profiles of Ozone: 2. The High‐Altitude Layers of Atmospheric Ozone

“…The retrieval process of the vertical distribution of ozone is discussed in detail in the companion paper (Patel et al., 2021) and is only briefly summarized here. The transmittance spectra (Figure 2) along the line‐of‐sight (LoS) at the respective tangent altitudes are converted during a two‐step process into ozone number densities (, molecules/cm 3 ).…”