Variation of CO/CO2 profiles in the Marian mesosphere and lower thermosphere retrieved from TGO/NOMAD

<p>CO is produced by the photodissociation of CO<sub>2</sub> and recycled to CO<sub>2</sub> by the catalytic cycle involving HOx in the Martian atmosphere [e.g., McElroy & Donahue, 1972]. In the mesosphere and lower thermosphere (MLT) region of Mars, the number density of CO is determined by photodissociation, diffusion, and atmospheric circulation. The increase of the CO mixing ratio in the MLT region and further enhancement in the polar region due to the transport of CO-enriched air via meridional circulation are predicted in the 3D models [Daerden et al., 2018; Holmes et al., 2019]. On the other hand, the decrease in the CO mixing ratio in the MLT region during a global dust storm is detected by TGO/ACS, which suggests that the increase in the hygropause altitude leads to the increase in the vertical range over which OH becomes available to convert into CO<sub>2</sub> [Olsen et al., 2021]. Additionally, a substantial variation of the homopause altitude has been investigated [Slipski et al., 2018; Jakosky et al., 2017; Yoshida et al., 2020], which suggests that the order of magnitude changes in the eddy diffusion coefficient at the homopause [Slipski et al., 2018], and then variations in the profile of CO mixing ratio in the MLT region. However, the effects of change in the eddy diffusion coefficient on the profile of CO mixing ratio have not been investigated. The variability of the CO mixing ratio profiles can be a clue for understanding the dynamical coupling between the lower and the upper atmospheres.</p> <p>To clarify the contributions of photochemistry, diffusion, and atmospheric circulation to the CO/CO<sub>2</sub> profiles in the MLT region, we use the Nadir and Occultation for MArs Discovery (NOMAD) instrument aboard Trace Gas Orbiter (TGO). NOMAD solar occultation is designed as the combination of the Acousto Optical Turnable Filter and echelle grating [Neefs et al., 2015; Thomas et al., 2016]. NOMAD solar occultation operates in the wavelength range of 2.2 - 4.3 μm (2320 to 4350 cm<sup>-1</sup>) with a high spectral resolution (λ/dλ = 20000) [Vandaele et al., 2018]. It provides us CO and CO<sub>2</sub> spectra below 100 km and 180 km altitudes, respectively.</p> <p>In this study, we applied the equivalent width technique [Chamberlain and Hunten, 1987; Krasnopolsky, 1986] to derive a new set of CO and CO<sub>2</sub> column densities, respectively, with the observed atmospheric transmittance spectra by NOMAD solar occultation. The absorption lines centered at 4285.0, 4288.2, and 4291.5 cm<sup>-1</sup> for CO (2-0) band and 3358.7, 3364.9, and 3366.4 cm<sup>-1</sup> for CO<sub>2</sub> (21102-00001) band are carefully selected for retrievals due to the contribution of nearby and central orders [cf. Liuzzi et al., 2019]. It is noted that the line strengths of the selected CO<sub>2</sub> have high sensitivity to the background temperature. In this study, we applied the vertical profiles of temperature simulated in the GEM-Mars model [Neary et al., 2018; Daerden et al., 2019]. We retrieve the CO and CO<sub>2</sub> slant column densities between 60 and ~100 km altitudes because those slant opacities are saturated below 60 km altitude. The CO and CO<sub>2</sub> spectra observed from April 2018 to September 2020, corresponding to from MY 34 Ls ~ 150 to MY 35 Ls ~ 280, are investigated.</p> <p>We found that the retrieved CO/CO<sub>2</sub> ratio between 60 and ~100 km increases with altitude. A behavior of the decrease in the CO/CO<sub>2</sub> ratio during the global dust storm corresponds to the previous observations [Olsen et al., 2021]. However, the CO/CO<sub>2</sub> profiles also vary with season and latitude. For interpretation, the 1D photochemical model will be compared with newly obtained CO/CO<sub>2</sub> profiles, especially in order to discuss the contributions from the variations in eddy diffusion coefficient and photochemistry in the MLT region on Mars.</p>

1D atmospheric modelling of K2-18b

<p>The atmospheric composition of exoplanets with masses between 2 and 10 M⊕ is poorly known. In that regard, the sub-Neptune K2-18b offers a valuable opportunity for the characterisation of such atmospheres under Earth-like stellar irradiation. Previous analyses of its transmission spectrum from the Kepler, Hubble and Spitzer space telescopes data using both retrieval algorithms and forward-modelling suggest the presence H<sub>2</sub>O, as well as a low amount of CH<sub>4</sub> in a H<sub>2</sub>–He atmosphere.</p> <p>We present here simulations of the atmosphere of K2-18 b using Exo-REM, our self-consistent 1D atmospheric model — recently adapted for transiting, high-metallicity giant exoplanets — to study the atmosphere of K2-18b. We compared the transmission spectra computed by our model with the above-mentionned data (0.4 to 5 μm) to infer the planet atmospheric composition assuming a H<sub>2</sub>–He dominated atmosphere. We investigated the effect of irradiation, eddy diffusion coefficient, internal source, clouds, C/O ratio and metallicity on the atmospheric structure and transit spectrum. <br />We will put an emphasis of the relative contributions of the various absorbers to the transmission spectrum. We will show that our simulations favor atmospheric metallicities from 100 to 200 times solar. We will also discuss the possibility of a CH<sub>4</sub>-depleted atmosphere and of liquid H<sub>2</sub>O cloud formation.</p>

Urban canopy meteorological forcing and its impact on ozone and PM_2.5: role of vertical turbulent transport

It is well known that the urban canopy (UC) layer, i.e., the layer of air corresponding to the assemblage of the buildings, roads, park, trees and other objects typical to cities, is characterized by specific meteorological conditions at city scales generally differing from those over rural surroundings. We refer to the forcing that acts on the meteorological variables over urbanized areas as the urban canopy meteorological forcing (UCMF). UCMF has multiple aspects, while one of the most studied is the generation of the urban heat island (UHI) as an excess of heat due to increased absorption and trapping of radiation in street canyons. However, enhanced drag plays important role too, reducing mean wind speeds and increasing vertical eddy mixing of pollutants. As air quality is strongly tied to meteorological conditions, the UCMF leads to modifications of air chemistry and transport of pollutants. Although it has been recognized in the last decade that the enhanced vertical mixing has a dominant role in the impact of the UCMF on air quality, very little is known about the uncertainty of vertical eddy diffusion arising from different representation in numerical models and how this uncertainty propagates to the final species concentrations as well as to the changes due to the UCMF. To bridge this knowledge gap, we set up the Regional Climate Model version 4 (RegCM4) coupled to the Comprehensive Air Quality Model with Extensions (CAMx) chemistry transport model over central Europe and designed a series of simulations to study how UC affects the vertical turbulent transport of selected pollutants through modifications of the vertical eddy diffusion coefficient (Kv) using six different methods for Kv calculation. The mean concentrations of ozone and PM2.5 in selected city canopies are analyzed. These are secondary pollutants or having secondary components, upon which turbulence acts in a much more complicated way than in the case of primary pollutants by influencing their concentrations not only directly but indirectly via precursors too. Calculations are performed over cascading domains (of 27, 9, and 3 km horizontal resolutions), which further enables to analyze the sensitivity of the numerical model to grid resolution. A number of model simulations are carried out where either urban canopies are considered or replaced by rural ones in order to isolate the UC meteorological forcing. Apart from the well-pronounced and expected impact on temperature (increases up to 2 ∘C) and wind (decreases by up to 2 ms−1), there is a strong impact on vertical eddy diffusion in all of the six Kv methods. The Kv enhancement ranges from less than 1 up to 30 m2 s−1 at the surface and from 1 to 100 m2 s−1 at higher levels depending on the methods. The largest impact is obtained for the turbulent kinetic energy (TKE)-based methods. The range of impact on the vertical eddy diffusion coefficient propagates to a range of ozone (O3) increase of 0.4 to 4 ppbv in both summer and winter (5 %–10 % relative change). In the case of PM2.5, we obtained decreases of up to 1 µg m−3 in summer and up to 2 µg m−3 in winter (up to 30 %–40 % relative change). Comparing these results to the “total-impact”, i.e., to the impact of all meteorological modifications due to UCMF, we can conclude that much of UCMF is explained by the enhanced vertical eddy diffusion, which counterbalances the opposing effects of other components of this forcing (temperature, humidity and wind). The results further show that this conclusion holds regardless of the resolution chosen and in both the warm and cold parts of the year.

The urban canopy meteorological forcing and its impact on ozone and PM_2.5: role of the vertical turbulent transport

Urban surfaces due to specific geometry and physical properties bring modified transport of momentum, moisture and heat between them and the air above and perturb the radiative, thermal and mechanical balance resulting in changed meteorological condition (e.g. the UHI – urban heat island phenomenon). From an air quality perspective, many studies argue that one of the most important changes is the increased turbulence enhancing vertical mixing of pollutants above cities, although increased temperatures and wind stilling play an important role too. Using the regional climate model RegCM4 coupled to chemistry transport model CAMx over central Europe we study how urban surfaces affect the vertical turbulent transport of selected pollutants through modifications of the vertical eddy diffusion coefficient (Kv). For the period of 2007–2011 and over central Europe numerous experiments are carried out in order to evaluate the impact of six different methods for Kv calculation on the surface concentrations as well as vertical profiles of ozone and PM2.5 over selected cities (Prague and Berlin). Three cascading domains are set up at 27 km, 9 km and 3 km resolutions, which further enables to analyze the sensitivity to model grid resolution. Numerous experiments are performed where urban surfaces are considered or replaced by rural ones in order to isolate the urban canopy meteorological forcing. Apart from the well pronounced and expected impact on temperature (increases up to 2 °C) and wind (decreases up to −2 m s−1) there is strong impact on vertical eddy diffusion in all of the six Kv methods. The Kv enhancement ranges from a few 0.5 up to 30 m2 s−1 at the surface and from 1 to 100 m2 s−1 at higher levels depending on the methods, while the turbulent kinetic energy (TKE) based methods produce the largest impact. The range of impact on the vertical eddy diffusion coefficient propagates to a range of ozone (O3) increase of 0.4 to 4 ppbv near the surface in both summer and winter, while at higher levels, decreases occur from a few −0.4 ppbv to as much as −2 ppbv. In case of PM2.5, enhanced vertical eddy diffusion leads to decrease of near surface concentrations ranging from almost zero to −1 μg m−3 in summer and to decreases from −0.5 to −2 μg m−3 in winter. Comparing these results to the total-impact, i.e. to the impact of all considered urban meteorological changes, we can conclude that much of the overall urban meteorological forcing is explained by acting of the enhanced vertical eddy diffusion, which counterweights the opposing effects of other components of this forcing (temperature, humidity and wind impact). The results further show that this conclusion holds regardless of the resolution chosen and in both the warm and cold part of the year. Our study demonstrates the dominant role of turbulent transport of pollutants above urban areas and stresses the need for further investigation how variation of urban land-use influence the pollutant transport from the urban canopy.

Comparison of GEOS-5 AGCM planetary boundary layer depths computed with various definitions

Accurate models of planetary boundary layer (PBL) processes are important for forecasting weather and climate. The present study compares seven methods of calculating PBL depth in the GEOS-5 atmospheric general circulation model (AGCM) over land. These methods depend on the eddy diffusion coefficients, bulk and local Richardson numbers, and the turbulent kinetic energy. The computed PBL depths are aggregated to the Köppen–Geiger climate classes, and some limited comparisons are made using radiosonde profiles. Most methods produce similar midday PBL depths, although in the warm, moist climate classes the bulk Richardson number method gives midday results that are lower than those given by the eddy diffusion coefficient methods. Additional analysis revealed that methods sensitive to turbulence driven by radiative cooling produce greater PBL depths, this effect being most significant during the evening transition. Nocturnal PBLs based on Richardson number methods are generally shallower than eddy diffusion coefficient based estimates. The bulk Richardson number estimate is recommended as the PBL height to inform the choice of the turbulent length scale, based on the similarity to other methods during the day, and the improved nighttime behavior.