Heterogeneous chemistry in the atmosphere of Mars

We present the first comprehensive general circulation model study of water ice condensation and cloud formation in the Martian atmosphere. We focus on the effects of condensation in limiting the vertical distribution and transport of water and on the importance of condensation for the generation of the observed Martian water cycle. We do not treat cloud ice radiative effects, ice sedimentation rates are prescribed, and we do not treat interactions between dust and cloud ice. The model generates cloud in a manner consistent with earlier one‐dimensional (1‐D) model results, typically evolving a uniform (constant mass mixing ratio) vertical distribution of vapor, which is capped by cloud at the level where the condensation point temperature is reached. Because of this vertical distribution of water, the Martian atmosphere is generally very far from fully saturated, in contrast to suggestions based upon interpretation of Viking data. This discrepancy results from inaccurate representation of the diurnal cycle of air temperatures in the Viking Infrared Thermal Mapper (IRTM) data. In fact, the model suggests that only the northern polar atmosphere in summer is consistently near its column‐integrated holding capacity. In this case, the column amount is determined primarily by the temperature of the northern polar ice cap. Comparison of the water cycle generated by the model with and without atmospheric ice condensation and precipitation shows two major roles for water ice cloud. First, clouds are essential to the observed rapid return of atmospheric water to the surface in late northern summer, as ice sedimentation forces the water column to shrink in response to the downward motion of the condensation level, concentrating water near surface sinks. Second, ice sedimentation limits the amount of water that is transported between the hemispheres through the Hadley circulation. This latter effect is used to greatly improve the model simulation of the annual water cycle by increasing ice sedimentation rates. The model is thus shown to be able to reasonably reproduce the annual cycles of vapor and ice cloud as compared to Viking data. In addition, the model is shown able to reproduce near‐instantaneous maps of water ice derived from Hubble Space Telescope images. The seasonal evolution of the geographic distribution of water ice compares reasonably well with Viking and Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) observations, except in the prediction of a weak tropical cloud belt in southern summer. Finally, it is shown that the tropical cloud belt is generated in the model by the cooling of water vapor entrained in the upwelling branch of the Hadley cell. Decline of the tropical cloud belt in mid northern summer is shown to be related to an increase in air temperatures, rather than to decreases in water vapor supply or the vigor of Hadley cell ascent. By equinox, the cloud belt experiences a second major decline event, this time due to a reduction in vapor supply. The ability of the model to emulate many aspects of observed cloud behavior is encouraging, as is the ability of enhanced ice sedimentation to improve the overall quality of the water cycle simulation. However, significant work remains to be done before all observational constraints can be matched simultaneously. Specifically, in order for the generally good fit to all other data to be attained, cloud ice particle sizes about an order of magnitude too large must be used.

Introduction: Atmospheric hydrogen chloride (HCl) was first detected in the martian atmosphere in 2021 by the ExoMars Trace Gas Orbiter (TGO) [1], and more recently also studied by ground-based telescopes [2]. Observed HCl occurs in abundances of parts per billion by volume (ppbv) and shows strong seasonal and spatial variation. The vast majority of confirmed detections have been during Mars’ dustier perihelion season and there is a notable paucity of detections during the clearer aphelion season; HCl abundances show steep increases/decreases at the corresponding equinoxes [3,4].When confirmed to be present, observed atmospheric HCl shows a bias towards higher abundances in the southern (summer) hemisphere. Measured HCl profiles also display vertical structure across their retrieved altitudes (generally 10-50 km above the surface), including sharp drops in abundance coinciding with the presence of water ice clouds [5].Analysis of observed profiles has indicated a weak positive correlation between HCl abundance and dust loading and a stronger positive correlation between HCl and water vapour abundance [6]; similar correlations are apparent from analysis of ground telescope measurements [2].Consideration of the observed sharp temporal and spatial gradients in HCl, together with its apparent relationship to aerosol and vapour abundances, has led to the idea that heterogeneous chlorine chemistry might explain these strong variations. This is supported by modelling of gas-phase chlorine chemistry in a Mars global climate model (GCM), which shows that in the absence of heterogeneous reactions the HCl distribution remains more uniformly mixed in time, latitude, and height [7].Candidate heterogeneous chlorine chemical reactions, involving dust and water ice aerosol, have recently been incorporated into both 1D models [8,9] and a GCM [10]. The resulting modelled HCl profiles and global distributions show a marked improvement relative to observations and are able to reproduce important aspects of the observed distribution including the southern hemisphere bias, enhanced abundances during the perihelion season, and strong vertical structure. However, modelling also reveals features which either conflict with observations or have not yet been observed, and the exact nature of the significant reactions involved remains an open question.Results & Discussion: We discuss results of our recently published GCM modelling work [7,10] and the gas-phase and heterogeneous reactions involved. We also discuss results from ongoing GCM modelling to better constrain the nature of proposed heterogeneous chlorine reactions. Figure 1. Comparison of atmospheric HCl profiles for TGO/ACS observations, gas-phase only model outputs, and heterogeneous model outputs. All model data is masked to best match the observation times and locations. Subplot (a) shows mean (solid lines) and population standard deviation (shaded area) for all observations/data within the perihelion seasons of MY 34–36. Subplots (b–d) show comparisons for individual observed profiles in MY 34–36, with error bars representing standard errors. Adapted from [10]. Our results show that gas-phase chlorine chemistry alone cannot reproduce the observed global HCl distribution in a GCM [7]. Inclusion of idealised heterogeneous chlorine chemistry improves representation of HCl abundance and is able to reproduce key features of the observed distribution, suggesting a crucial role for heterogeneous chemistry in the contemporary chlorine cycle on Mars [10]. Figure 1 displays selected model profiles (with and without the idealised heterogeneous reactions) compared to TGO observations, showing the more realistic vertical structure obtained when heterogeneous chemistry is considered.Further investigation of more specific and realistic potential heterogeneous reactions, adapted from [8] and [9] for our GCM, indicates that some reactions are more plausible than others as significant drivers in the contemporary Mars chlorine cycle. In the wait for further laboratory and observational work to better characterise these reactions under martian conditions, we discuss which reactions are more likely to be prominent based on analysis of their structural effects.

The photochemistry of ozone in the Martian atmosphere is generally considered to be well understood. Ozone forms through a three-body reaction involving O and O₂, both products of CO₂ photolysis, and it is destroyed by odd-hydrogen species (HOₓ) generated from water vapour photolysis, which helps to explain the observed anticorrelation between ozone and water vapour [1,2]. However, current photochemical models cannot reproduce ozone observations from various missions (e.g., Trace Gas Orbiter (TGO), Mars Express), with models generally suffering from a negative bias [2,3]. This discrepancy highlights gaps in our knowledge of the photochemical links between odd-oxygen (Oₓ), odd-hydrogen, and water vapour in the Martian atmosphere. A recent study investigated different factors that could influence the ozone content and concluded that the underestimation of ozone in the MPCM might be due to heterogeneous uptake of HOₓ species on water ice clouds or an overestimation of HOₓ photochemistry efficiency in the model [2].We build on that explorative study and use the latest configuration of the Mars Planetary Climate Model (MPCM) with initial conditions from the Mars Climate Database (MCD) v6.1 [4] to investigate how different parameters could influence the ozone vertical profiles. We study data collected in MYs 34 and 35, including Ox, HOx, CO, and water vapour retrievals from the ACS (Atmospheric Chemistry Suite) and NOMAD (Nadir and Occultation for MArs Discovery) instruments aboard TGO. This approach allows us to examine any altitude-dependent changes in chemistry. We will present the results from a systematic investigation into the impact of various assumed model parameters, e.g., absorption cross sections, reaction rates, and heterogeneous chemistry, on these species. We will also consider the impact of introducing new chemistry into the model, e.g., chlorine photochemistry that was recently implemented in the MPCM by Benne et al. (2025) (in review). We will conclude our presentation by highlighting the parameters with the greatest impact on model ozone, the interactions and variations of ozone and its precursors across the two MYs, and prioritising the future research required to bridge the gap between model and observed Martian ozone.

The study of the modern martian atmosphere is (1) a key to the climate of Mars’s past; (2) useful for comparison with other terrestrial planets such as the Earth; and (3) can support hazard analysis and weather forecasting for future exploration and habitation of the planet. Recently, it was found that middle atmospheric downwelling near the south pole during southern winter is much more vigorous than predicted by most Mars general circulation models. This underestimate may be due to models erroneously representing the radiative forcings in the atmosphere due to aerosol and/or the mechanical forcings due to wave breaking. Errors of this kind would influence middle atmospheric dynamics and likely would result from incomplete understanding of lower atmospheric processes such as dust transport. Here, retrievals of vertical profiles of temperature, pressure, dust, and water ice from the Mars Climate Sounder (MCS) on Mars Reconnaissance Orbiter (MRO) are used to characterize the atmospheric circulation of Mars and its forcings. First, I consider the annual cycle of the thermal structure and aerosol distributions of the lower and middle atmosphere and investigate the degree of coupling between the lower and middle atmospheric mean meridional circulations. To evaluate the role of wave breaking, I look for local convective instabilities in the Martian middle atmosphere: a key indicator of saturating vertically propagating waves such as the gravity waves and the thermal tides, which are important sources of wave drag in the Earth’s mesosphere. I then characterize the vertical distribution of dust and its approximate radiative effects during northern spring and summer and show there is usually a maximum in dust mass mixing ratio at ~15—25 km above the tropics, which is not currently simulated by models. Next, I evaluate the relative importance of dust storm activity, pseudo-moist convection due to the solar heating of dust, orographic effects, and scavenging by water ice clouds in producing this maximum. Finally, I show that published models underestimate the thickness and altitude of water ice clouds in northern summer.

Water vapor saturation and ice cloud occurrence in the atmosphere of Mars

Abstract. A layer of aerosols has been identified in the upper troposphere and lower stratosphere above the Asian summer monsoon (ASM) region, typically referred to as the Asian Tropopause Aerosol Layer (ATAL). This layer is fed by atmospheric pollutants over southern and eastern Asia lifted to the upper troposphere by deep convection in summer. The radiative effects of this aerosol layer change local temperature, influence thermodynamic stability, and modulate the efficiency of air mass vertical transport near the tropopause. However, quantitative understanding of these effects is still very poor. To estimate aerosol radiative effects in the upper troposphere and above, a set of radiative kernels is constructed for the tropical upper troposphere and stratosphere to reduce the computational expense of decomposing the different contributions of atmospheric components to anomalies in radiative fluxes. The prototype aerosol kernels in this work are among the first to target vertically resolved heating rates, motivated by the linearity and separability of scattering and absorbing aerosol effects in the ATAL. Observationally derived lower boundary conditions and satellite observations of cloud ice within the upper troposphere and stratosphere are included and simplified in our Tropical Upper Troposphere–Stratosphere Model (TUTSM). Separate sets of kernels are derived and tested for the effects of absorbing aerosols, scattering aerosols, and cloud ice particles on both shortwave (solar) and longwave (thermal) radiative fluxes and heating rates. The results indicate that the kernels can reproduce aerosol radiative effects in the ATAL well. Similarly, these aerosol kernels could be used to simulate radiative effects of biomass burning and volcanic eruption above the troposphere. This approach substantially reduces computational expense while achieving good consistency with direct radiative transfer model calculations, and it can be applied to models that do not require high precision but have strict requirements for computing speed and storage space.

MOM completed nearly 8 years in the Mars Orbit (2014–2022). During this period Mars Colour Camera (MCC) captured images of different types of clouds formed in the Martian Atmosphere. In present work, we mainly concentrate on the clouds appearing in the morning and evening terminator or in short seen at twilight. We considered 15 cases of clouds observed at twilight, in which the geometry of the observations allows us to derive the minimum altitude of the clouds, revealing that many of these clouds are in the upper troposphere (above 25 km and up to 45 km). The majority of these upper tropospheric clouds were detected in mid-latitudes (30N - 60N) during the Martian autumn and early winter season (Ls 20 to 90 deg). In this mid-latitude region, we also reported a detached layer of dust and water ice cloud in our previous work (Kalita et al., 2021a; 2021b). We propose a plausible mechanism that enhances the probability of the formation of a high-altitude cloud through temperature variation. Our selection process is manual and based on the contrast enhancement through GIMP. Image geometry confirms the incidence angle in the range of 95 to 105 degrees for evening clouds and 75 to 85 degrees during morning and further, using the angle values we estimated the height of the twilight clouds. Further verification with MCS data concretizes our finding regarding upper tropospheric cloud formation.

The theme of this thesis is studying the outgoing thermal IR spectra of Earth and Mars. It is divided into two parts: the first part (Chapters 1-4) is focused on the variability seen in the outgoing thermal IR spectra and its application in validating model simulation, and the second part (Chapters 5-6) concentrates on the detection of cirrus (cirrus/dust aerosol) from terrestrial (Martian) outgoing thermal IR spectra. In Chapter 1, an example of climate change seen from two spectrometers seperated by 26 years is used to illustrate the singular importance of the outgoing thermal IR spectra in climate observations. The importance of testing the variability of models and the feasibility of using the outgoing thermal IR spectra in such tests are discussed. In Chapter 2, a study of the temporal variability at the tropical and midlatitude Pacific Oceans seen from IRIS (Infrared Interferometer Spectrometer) spectra and corresponding synthetic spectra based on simulations from two GCMs (UCLA GCM and NCAR CAM2) is presented. The discrepancies between modeled and observed temporal variability are substantial. The differences between two GCMs are also significant. Further examination shows that these discrepancies are insensitive to the parameterization of cloud optical properties and most likely due to deficiencies in simulating the seasonal and intraseasonal variations of the Walker Circulation in the tropical Pacific and the seasonal variations of boundary-layer temperature, low cloud, and stratospheric temperature in the midlatitude Pacific. In Chapter 3, a survey of the spatial variability seen from AIRS (Atmospheric Infrared Sounder) spectra and corresponding synthetic spectra based on NCAR CAM2 simulation is presented. To a large extent, the simulated spatial variability agrees well with the observed counterpart. The major discrepancies between model and observation can be attributed to the incorrect location of ITCZ in the western Pacific, the underrepresented dust aerosol at the Arabian Sea and off the Atlantic Coast of North Africa, and the overestimated spatial variation of stratospheric temperature in the model. Chapter 4 presents a comparative study of the temporal and spatial variability seen in the Martian outgoing thermal IR spectra collected by MGS-TES (Thermal Emission Spectrometer). Surface temperature variation is the dominant contributor to the temporal and spatial variability seen here. The variations of CO2 column abundance, dust aerosol and water ice cloud associated with topography, as well as the imprint of dust storms, can be also seen from such analysis. The negative correlation between dust and water ice spectral features seen from this analysis suggests that, to some extent, dust and water ice cloud are mutually exclusive of each other in the Martian atmosphere. Chapter 5 presents a sensitivity study of identifying optically thin cirrus from high-resolution (each individual absorption line is almost resolved) thermal IR spectra based on the line shapes of the residual spectra. This cirrus-detection approach is different from all previous cirrus-detection algorithms in the sense of making use of information content contained in the high-resolution measurements. Chapter 6 presents a tri-spectral algorithm to detect water ice cloud, dust, and surface anisothermality from low-resolution Martian outgoing thermal IR spectra, such as MGS-TES spectra. This algorithm is complementary to any more sophisticated retrieval scheme and can be used to screen large amounts of data to get a quick overview.

Limits to Paris compatibility of CO2 capture and utilization

<p>Aerosols are key components of the Martian radiative transfer. For instance, airborne dust is ubiquitous on the planet and influences the climate by absorbing shortwave radiation, resulting in a local warming of the atmosphere. While ice clouds are related to the water cycle, forming due to adiabatic cooling of upward flows where the water vapor condenses on dust particles as the temperature is low enough. Nadir UV measurements are commonly used to monitor the aerosol loading and the ozone abundance in the Martian atmosphere and study their seasonal cycles [e.g. Perrier et al., 2006; Mateshvili et al., 2009; Clancy et al., 2016; Willame et al., 2017; Wolff et al., 2019].</p> <p>The analysis of UV nadir measurements, from MEX/SPICAM in a previous work [Willame et al., 2017] and currently from the NOMAD/UVIS data, were performed using the canonical dust scattering properties obtained in [Wolff et al., 2010]. In the frame of the RoadMap project, we will assess the impact on the nadir retrievals of using new sets of dust properties derived from laboratory measurements (see related presentation/poster of Martikainen et al. about lab measurements). In the present work, we will present preliminary results of the retrieval analysis.</p> <p> </p> <p><strong>Acknowledgements</strong></p> <p>The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/V002295/1, ST/V005332/1 and ST/S00145X/1 and Italian Space Agency through grant 2018-2-HH.0. This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS under grant number 30442502 (ET_HOME). The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). US investigators were supported by the National Aeronautics and Space Administration. Canadian investigators were supported by the Canadian Space Agency. The ROADMAP project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101004052.</p> <p> </p> <p><strong>References</strong></p> <p>Clancy, R.T., Wolff, M.J., Lefvre, F., Cantor, B.A., Malin, M.C., Smith, M.D., 2016. Daily global mapping of Mars ozone column abundances with MARCI UV band imaging. Icarus 266, 112–133.</p> <p>Mateshvili, N., Fussen, D., Vanhellemont, F., Bingen, C., Dekemper, E., Loodts, N., Tetard, C., 2009. Water ice clouds in the Martian atmosphere: two Martian years of SPICAM nadir UV measurements. Planet. Space Sci. 57, 1022–1031.</p> <p>Perrier, S., Bertaux, J.L., Lefèvre, F., Lebonnois, S., Korablev, O., Fedorova, A., Montmessin, F., 2006. Global distribution of total ozone on Mars from SPICAM/MEX UV measurements. J. Geophys. Res. (Planets) 111, 9.</p> <p>Willame Y., A.C. Vandaele, C. Depiesse, F. Lefèvre, V. Letocart, D. Gillotay, F. Montmessin. Retrieving cloud, dust and ozone abundances in the Martian atmosphere using SPICAM/UV nadir spectra. Planetary and Space Science, 2017, Volume 142, Pages 9-25.</p> <p>Wolff, M.J., Clancy, R.T., Goguen, J.D., Malin, M.C., Cantor, B.A., 2010. Ultraviolet dust aerosol properties as observed by MARCI. Icarus 208, 143–155.</p> <p>Wolff M.J., R. Todd Clancy, Melinda A. Kahre, Robert M. Haberle, François Forget, Bruce A. Cantor, Michael C. Malin, 2019. Mapping water ice clouds on Mars with MRO/MARCI, Icarus, Volume 332.</p>

New insights into martian atmospheric chemistry

Abstract. This paper examines the mechanisms responsible for the production of ice in convective clouds influenced by mineral dust. Observations were made in the Ice in Clouds Experiment – Dust (ICE-D) field campaign which took place in the vicinity of Cape Verde during August 2015. Measurements made with instruments on the Facility for Airborne Atmospheric Measurements (FAAM) aircraft through the clouds on 21 August showed that ice particles were observed in high concentrations at temperatures greater than about −8 ∘C. Sensitivity studies were performed using existing parameterization schemes in a cloud model to explore the impact of the freezing onset temperature, the efficiency of freezing, mineral dust as efficient ice nuclei, and multi-thermals on secondary ice production by the rime-splintering process. The simulation with the default Morrison microphysics scheme (Morrison et al., 2005) that involved a single thermal produced a concentration of secondary ice that was much lower than the observed value of total ice number concentration. Relaxing the onset temperature to a higher value, enhancing the freezing efficiency, or combinations of these increased the secondary ice particle concentration but not by a sufficient amount. Simulations that involved only dust particles as ice-nucleating particles produced a lower concentration of secondary ice particles, since the freezing onset temperature is low. The simulations implicate that a higher concentration of ice-nucleating particles with a higher freezing onset temperature may explain some of the observed high concentrations of secondary ice. However, a simulation with two thermals that used the original Morrison scheme without enhancement of the freezing efficiency or relaxation of the onset temperature produced the greatest concentration of secondary ice particles. It did so because of the increased time that graupel particles were exposed to significant cloud liquid water in the Hallett–Mossop temperature zone. The forward-facing camera and measurements of the vertical wind in repeated passes of the same cloud suggested that these tropical clouds contained multiple thermals. It is possible of course that several mechanisms, some of them only recently discovered, may be responsible for producing the ice particles in clouds. This study highlights the fact that the dynamics of the clouds likely play an important role in producing high concentrations of secondary ice particles in clouds.

The atmospheric haze which usually obscures the surface of Mars in blue light is probably a condensate rather than a dust because it can clear away rapidly. Carbon dioxide and water vapor are both present in the atmosphere, and condensation of each into the solid phase has been suggested to explain the blue haze. Calculations are carried out by assuming a convective atmosphere with a surface temperature of +10 C for the subsolar point and a lapse rate of 3.7 C/km. When the optical properties of clouds produced in such an atmosphere are determined with the aid of the Mie theory of scattering, it is found that (1) carbon dioxide ice clouds are far too opaque to be the cause of the blue haze; (2) water ice clouds can cause the blue haze if the surface frost point lies near -90 C. The second result is consistent with the failure of all modern attempts to detect water vapor spectroscopically in the Martian atmosphere, since the total precipitable water content corresponding to such a frost point is only 2 These results indicate that the author's earlier suggestion of carbon dioxide ice clouds should be rejected.

<p>This project maps ozone and ice-water clouds detected in the martian atmosphere to assess the atmospheric chemistry between ozone, water-ice and hydroxyl radicals. Hydroxyl photochemistry may be indicated by a non-negative or fluctuating correlation between ozone and water-ice. This will contribute to understanding the stability of carbon dioxide and atmospheric chemistry of Mars.</p><p>Ozone (O<sub>3</sub>) can be used for tracking general circulation of the martian atmosphere and other trace chemicals, as well as acting as a proxy for water vapour. The photochemical break down of water vapour produces hydroxyl radicals known to participate in the destruction of ozone. The relationship between water vapour and ozone is therefore negatively correlated. Atmospheric water-ice concentrations may also follow this theory. The photochemical reactions between ozone, water-ice clouds and hydroxyl radicals are poorly understood in the martian atmosphere due to the short half-life and rapid reaction rates of hydroxyl radicals. These reactions destroy ozone, as well as indirectly contributing to the water cycle and stability of carbon dioxide (measured by the CO<sub>2</sub>–CO ratio). However, the detection of ozone in the presence of water-ice clouds suggests the relationship between them is not always anti-correlated. Global climate models (GCMs) struggle to describe the chemical processes occurring within water-ice clouds. For example, the heterogeneous photochemistry described in the LMD (Laboratoire de Météorologie Dynamique) GCM did not significantly improve the model. This leads to the following questions:<em> what is the relationship between water-ice clouds and ozone, and can the chemical reactions of hydroxyl radicals occurring within water-ice clouds be determined through this relationship?</em></p><p>This project aims to address these questions using nadir and occultation retrievals of ozone and water-ice clouds, potentially using retrievals from the UVIS instrument aboard NOMAD (Nadir and Occultation for Mars Discovery), ExoMars Trace Gas Orbiter. Analysis will include temporal and spatial binning of data to help identify any patterns present. Correlation tests will be conducted to determine the significance of any relationship at short term and seasonal scales along a range of zonally averaged latitude photochemical model from the LMD-UK GCM will be used to further explore the chemical processes.</p><p>Interactions between hydroxyl radicals and the surface of water-ice clouds are poorly understood. Ozone abundance is greatest in the winter at the polar regions, which also coincides with the appearance of the polar hood clouds. The use of nadir observations will enable the comparison between total column of ozone abundance at high latitudes (>60°S) in a range of varying water-ice cloud opacities, as well as the equatorial region (30°S – 30°N) during aphelion. Water-ice clouds may remove hydroxyl radicals responsible for the destruction of ozone and thus the previously assumed anticorrelation between ozone and water-ice will not hold. The project will therefore assess the hypothesis that: <em>water-ice clouds may act as a sink for hydroxyl radicals.</em></p>

Observations from spaceborne microwave (MW) and infrared (IR) passive sensors are the backbone of current satellite meteorology, essential for data assimilation into modern numerical weather prediction and climate benchmarking. In this context, over the last decades, the study and the analysis of cloud microphysics have received increasing attention to better understand cloud feedbacks on climate. MW and IR observations from space offer complementary features concerning cloud microphysics, and various tools have been developed to retrieve cloud parameters such as the effective radius of water and ice clouds. However, MW-IR synergy for cloud investigation is currently under-explored. In this framework, innovative processing methods, such as those based on the use of Artificial Intelligence (AI), which can run on large databases and can handle hundreds of input variables from different sensors, such as those operating in hyperspectral and multispectral channels of the infrared and the microwave bands, such as the New Generation Atmospheric Sounding Interferometer (IASI-NG) and the Microwave Sounder (MWS) of the EPS second generation (EPSSG) platforms whose forthcoming launch is scheduled from 2024 onwards. A regression framework has been implemented based on the combined use of Random Forest (RF) regression and the principal components analysis (PCA) of IASI-NG and MWS observations to input the RF regressors. The supervised learning of liquid and ice water clouds' effective radii was carried out based on this framework. In conclusion, the regression analysis shows good agreement between reference and retrieved effective radius, with 80% correlation and root-mean-square error (RMSE) of 0.68 μm for liquid and 11.6 μm for ice cloud effective radius.