The Great Oxidation Event (GOE) was a 200 Myr transition circa 2.4 billion years ago that converted the Earth's anoxic atmosphere to one where molecular oxygen (O-2) was abundant (volume mixing ratio >10(-4)). This significant rise in O-2 is thought to have substantially throttled hydrogen (H) escape and the associated water (H2O) loss. Atmospheric estimations from the GOE onward place O-2 concentrations ranging between 0.1 % to 150 % PAL, where PAL is the present atmospheric level of 21 % by volume. In this study we use WACCM6, a three-dimensional Earth System Model to simulate Earth's atmosphere and predict the diffusion-limited escape rate of hydrogen due to varying O-2 post-GOE. We find that O2 indirectly acts as a control valve on the amount of hydrogen atoms reaching the homopause in the simulations: less O-2 leads to decreased O3 densities that reduce local tropical tropopause temperatures by up to 17 K, which increases H2O freeze-drying and thus reduces the primary source of hydrogen in the considered scenarios. The maximum differences between all simulations in the total H mixing ratio at the homopause and the associated diffusion-limited escape rates are a factor of 3.2 and 4.7, respectively. The prescribed CH4 mixing ratio (0.8 ppmv) sets a minimum diffusion escape rate of approximate to 2 & times;10(10) mol H yr(-1), effectively a negligible rate when compared to pre-GOE estimates (similar to 10(12)-10(13) mol H yr(-1)). Because the changes in our predicted escape rates are comparatively minor, our numerical predictions support geological evidence that the majority of Earth's hydrogen escape occurred prior to the GOE. Our work demonstrates that estimations of how the tropical tropopause layer and the associated hydrogen escape rate evolved through Earth's history requires 3D chemistry-climate models which include a global treatment of water vapour microphysics.

The strategies that policymakers take to mitigate climate change will have considerable implications for human exposure to air quality, with air quality co‐benefits anticipated from climate change mitigation. Few studies try to model these co‐benefits at a regional scale and even fewer consider health inequalities in their analyses. We analyze the health impacts across Western and Central Europe from exposure to fine particulate matter () and surface level ozone () in 2014 and in 2050 using three scenarios with different levels of climate change mitigation, using a high‐resolution atmospheric chemistry model to simulate future air quality. We use recent health functions to estimate mortality related to the aforementioned pollutants. We also analyze the relationship between air quality mortality rate per 100,000 people and Human Development Index to establish if reductions in air quality mortality are achieved equitably. We find that air quality‐related mortality ( + mortality) will only reduce in the future following a high‐mitigation scenario (54%). It could increase by 7.5% following a medium‐mitigation scenario and by 8.3% following a weak mitigation scenario. The differences are driven by larger reductions in ‐related mortality and a small reduction in ‐related mortality following the high‐mitigation scenario, whereas for the other scenarios, smaller improvements in ‐related mortality are masked by worsening ‐related mortality. We find that less developed regions of European countries have higher mortality rates from and exposure in the present day, but that this inequity is reduced following greater climate change mitigation.

Estimates of aviation effective radiative forcing (ERF) indicate that contrail cirrus is currently its largest contributor, although with a substantial associated uncertainty of ∼ 70 %. Here, we implement the contrail parameterisation developed for the Community Atmosphere Model (CAM) in the UK Met Office Unified Model (UM), allowing us to compare, for the first time, the impact of key features of the host climate model on contrail cirrus ERF. We find that differences in background humidity between the models result in the UM-simulated contrail fractions being 2 to 3 times larger than in CAM. Additionally, the models show contrasting responses in overall global cloud fraction, with contrails increasing the total cloud fraction in the UM and decreasing it in CAM. Differences in the complexity of the cloud microphysics schemes lead to significant differences in simulated changes to cloud ice water content due to aviation. After compensating for the unrealistically low contrail optical depth in the UM, we estimate the 2018 contrail cirrus ERF to be 40.8 mW m−2 in the UM, compared to 60.1 mW m−2 in CAM. These values highlight the substantial uncertainty in contrail cirrus ERF due to differences in microphysics and radiation schemes between the two models. We also find a factor-of-8 uncertainty in contrail cirrus ERF due to existing uncertainty in contrail cirrus optical depth. Future research should focus on better representing microphysical and radiative contrail characteristics in climate models and on improved observational constraints.

Solar driven energetic particle precipitation (EPP) is an important factor in polar atmospheric ozone balance and has been linked to ground-level regional climate variability. However, the linking mechanism has remained ambiguous. The observed and simulated ground-level changes start well before the processes from the main candidate, the so-called EPP-indirect effect, would start. Here we show that initial reduction of polar mesospheric ozone and the resulting change in atmospheric heating rapidly couples to dynamics, transferring the signal downwards, shifting the tropospheric jet polewards. This pathway is not constrained to the polar vortex. Rather, a subtropical route initiated by a changing wind shear plays a key role. Our results show that the signal propagates downwards in timescales consistent with observed tropospheric level climatic changes linked to EPP. This pathway, from mesospheric ozone to regional climate, is independent of the EPP-indirect effect, and solves the long-standing mechanism problem for EPP effects on climate.

The solar forcing dataset prepared for the 6th round of the Coupled Model Intercomparison Project (CMIP6) has been used extensively in climate model experiments. Recently, an International Space Science Institute (ISSI) Working Group was established to revisit the solar forcing recommendations in order to define a roadmap for building a revised solar forcing dataset for the upcoming 7th round of CMIP (Funke et al., 2023). This new dataset will introduce changes in the radiative forcing of climate either directly, or indirectly via changes in atmospheric composition. In CMIP6, the solar forcing consisted of both a total solar irradiance (TSI), along with a spectrally resolved solar irradiance (SSI). The TSI for solar minimum was set to 1360.8±0.5Wm-2 and the SSI covered the 10nm to 100mm spectral region. A similar approach is proposed for CMIP7 except for two major aspects of the reconstruction: 1) the definition of the reference spectrum for the quite Sun; 2) the temporal variability. The major difference between the proposed CMIP7 SSI quite sun reference spectrum and that used for CMIP6 is the spectral shape. The new SSI spectrum has an irradiance that is 1-5% higher in the visible band and lower by 1-2% in the Near-IR wavelength range (1000-2000nm). The solar temporal variability in the CMIP6 and CMIP7 reconstructions are based on both the NRLSSI2 and SATIRE reconstructions. These reconstructions have been improved in preparation for CMIP7 and the aim is for both reconstructions to use the same reference spectrum and be driven by the same solar proxies. In this work we used the Whole Atmosphere Community Climate Model (WACCM) to examine the chemical and climate implications of the proposed CMIP7 solar forcing updates compared to the CMIP6 approach. WACCM is a chemistry-climate model that extends from the surface to 140km. The horizontal resolution is ~1degree. WACCM has a detailed representation of chemical and dynamical processes from the troposphere through the lower thermosphere. We examined the “chemical only” impacts of the solar forcing choice by running WACCM in the specified dynamics mode using NASA Modern-Era Retrospective analysis for Research and Applications Version 2 (MERRA2). The “climate” impacts were derived by running the model with interactive dynamics coupled to a deep ocean. Conclusions from this work will support the development of the next version of WACCM for participation in the CMIP7 assessment.Funke, B., Dudok de Wit, T., Ermolli, I., Haberreiter, M., Kinnison, D., Marsh, D., Nesse, H., Seppälä, A., Sinnhuber, M., and Usoskin, I.: Towards the definition of a solar forcing dataset for CMIP7, Geosci. Model Dev. Discuss. https://doi.org/10.5194/gmd-2023-100.

This study presents an analysis of sporadic-E (Es) structures within WACCM-X (the Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension), including electrodynamical transport of metallic ions. A set of selection criteria have been developed to identify Es layers in WACCM-X output based on the total metal ion density in each model grid box. These criteria are used to create a climatology of Es, which is compared to Es occurrence rates derived from FORMOSAT/COSMIC-1 (Constellation Observing System for Meteorology, Ionosphere, and Climate) radio-occultation measurements. The novel identification algorithm analyses 2-week time slices between altitudes of 90-150 km, with Es layer events identified where the three selection criteria are met. Distinct seasonal distributions in Es occurrence were observed that are consistent with previous studies, with peaks during summer and reduced frequencies during winter, alignment of Es with geomagnetic contours, and layers descending in altitude as a function of local time. While discrepancies exist between WACCM-X and COSMIC data (WACCM-X occurrence rates are a factor of similar to 2 lower than COSMIC-derived occurrence rates at mid-latitudes), highlighting the ongoing challenges in modeling Es layers, this study enhances the modeling capabilities of sporadic Es and deepens our understanding of their formation; it establishes a basis for their enhanced integration into global climate models and facilitates further investigation of Es behavior under different atmospheric conditions, paving the way to improved prediction of the occurrence of Es.

State-of-the-art global chemistry-climate models such as WACCM cannot practically resolve the small-scale gravity waves (GWs) that are important in the mesosphere and lower thermosphere (MLT, ≈ 70-120km). A solution is the use of parametrizations that represent subgrid dissipating GWs (see e.g. Garcia et al., 2007). To reproduce key MLT features such as mesospheric jet reversals, pole-to-pole circulation and the summer mesopause, models rely on such schemes (McLandress, 1997; Holton & Alexander, 2000), though more development is needed. For example, WACCM tends to underestimate observed mesospheric densities of O, O3 and NO, and overestimate observed densities of the Na and Fe layers produced from cosmic dust ablation. Increasing evidence suggests a reason for this is a missing vertical transport from subgrid propagating GWs, and a solution has recently been achieved when these effects were included in the WACCM GW scheme (Guarino et al., 2023). In the current work, we resolve subgrid waves natively using WACCM with Regional Refinement (WACCM-RR). WACCM-RR provides the unprecedented opportunity to model the global climate up to altitudes of 140 km, and resolve individual regions down to as far as 1/32° at a low computational cost compared to global high resolution models. Trends from a model using a 1/8° grid over the Continental US (1° elsewhere), when compared to a global 1° model, are consistent with comparisons of standard WACCM models, to models using our updated GW scheme. For example, mesospheric densities of O, O3 and CO2 are increased, as predicted. A surprising contrast is a globally warmer atmosphere, likely due to sensitivity of the meridional circulation to GW activity in the refined region. The results point to the applicability of WACCM-RR for detailed investigations of wave-transport processes, and their impact on MLT dynamics and composition. We point out remaining questions and challenges.

Sporadic E (Es) layers are transient ionospheric phenomena that represent an important aspect of atmospheric dynamics, exerting influences on space weather and communication systems. They occur in the E region (~90-150 km) and are characterised by thin, localised layers of enhanced electron density. Their formation is linked to interactions involving atmospheric waves and tides, wind shear and/or electric field and plasma instabilities. Metal ions are tightly coupled with electrons through ionization and neutralisation processes and play a central role in the formation of Es layers1.Recently, Wu et al. [2021]2 examined the full transport of three metal ions (Fe+, Mg+ and Na+) in the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X) - a self-consistent global model including their full neutral and ion-molecule chemistry and the injection of metals from meteoric ablation3,4. The work of Wu et al. [2021] significantly improved the modelled global distribution and seasonal dependence of the metal ions in WACCM-X; since it captures the complex interactions between numerous atmospheric components, this extended WACCM-X provides a useful framework for the study of Es layers on a global scale.Although modelling of parameters relevant to Es layers (winds, temperatures, chemical constituents) has been carried out using whole atmosphere models2,5, modelling of Es layer occurrence has not been carried out self-consistently using a global climate model with metal ion transport. In this study we present a novel method to identify Es layers in WACCM-X with full transport of metal ions. We present a detailed account of the methodology employed for the identification of Es layers within WACCM-X and the resulting climatology of Es occurrence. The derived climatology is compared to observations from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellite6, which provides global high-resolution ionospheric observations. This comparison enables us to evaluate the performance of the model and identifies potential areas for future development.By better understanding the complex interplay between atmospheric variability and Es layer behaviour, we aim to improve our understanding of Es layers and their role in atmospheric dynamics. The insights gained from this research advance modelling capabilities, and could support space weather forecasting and communication systems, as well as contributing to the broader understanding of Es layers and their significance in atmospheric science. 1. Yu, B., et al. (2021) Atmospheric Chemistry and Physics, 21(5), 4219-42302. Wu, J., W. Feng, H. L. i. Liu, X. Xue, D. R. Marsh, and J. M. C. Plane (2021), Atmospheric Chemistry and Physics, 21(20), 15619-156303. Liu, H.-L., et al. (2018), Journal of Advances in Modelling Earth Systems, 10(2), 381-4024. Carrillo-Sánchez, J. D., J. C. Gómez-Martín, D. L. Bones, D. Nesvorný, P. Pokorný, M. Benna, G. J. Flynn, and J. M. C. Plane (2020), Icarus, 335, 1133955. Chu, Y. H., C. Y. Wang, K. H. Wu, K. T. Chen, K. J. Tzeng, C. L. Su, W. Feng, and J. M. C. Plane (2014), Journal of Geophysical Research: Space Physics, 119(3), 2117-21366. https://www.cosmic.ucar.edu/global-navigation-satellite-system-gnss-background/cosmic-

The cause of the inhomogeneous near-ultraviolet absorption observed in the upper clouds of Venus remains a key question in Venusian research. One possible candidate in the literature is ferric chloride. The absorption spectrum of ferric chloride currently in use by models uses ethyl acetate as a solvent and does not reproduce the absorption features observed on Venus. The study of the optical properties and chemistry of ferric chloride in the sulphuric acid cloud droplets is required to draw valid conclusions regarding its suitability as a candidate for the near-UV absorption. In this study, we measure the absorption spectrum of ferric chloride in sulphuric acid from 200 – 600 nm at a range of temperatures and measure the rate of conversion of the ferric chloride ions into ferric sulphate ions. We then use the resulting ferric chloride absorption coefficients in a 1D radiative transfer model and estimate the required concentration of ferric chloride in the clouds to be 0.6 – 0.9 wt% in the mode 1 (~0.3 µm radius) cloud droplets to match observations. We also predict the atmospheric concentrations of ferric chloride formed from the reaction of iron ablating from cosmic dust entering Venus’ atmosphere around 120 km with hydrogen chloride emitted by volcanic activity, and estimate the accumulation timescale of ferric chloride to produce the required concentrations in the clouds.

Changes in the Brewer-Dobson circulation (BDC) in response to increasing CO2 concentrations can arise from the direct effect of radiative cooling in the stratosphere or the indirect effects induced by warmer sea surface temperatures (SSTs). This study aims to disentangle these two contributions in the Whole Atmosphere Community Climate Model (WACCM) by analyzing the timescales of the tropical upwelling response to an abrupt quadrupling of CO2. Transient atmosphere-ocean climate model simulations of 100 years under 4xCO2 conditions are compared to preindustrial control simulations and to simulations with an atmosphere-only version of WACCM that uses preindustrial SSTs. We find that most of the response in both shallow and deep branches of the BDC occurs on fast timescales (first 2-3 decades). In the shallow branch of the BDC, the response is mainly driven by changes in SSTs in the well-mixed shallow ocean, which cause tropospheric warming and an intensification and upward displacement of the subtropical jets, and alter wave forcing in their vicinity. The contribution from stratospheric radiative cooling is almost negligible. In the upper stratosphere, the response of tropical upwelling begins earlier and develops faster than in the shallow branch, owing to the larger contribution from the rapid adjustments. At 1 hPa, 70% of the fast response relates to stratospheric radiative cooling and 30% to warmer SSTs. The modulation of the filtering of non-orographic gravity waves (mainly of frontal origin) in the subtropics explain most of the response in tropical upwelling in the deep branch, on both fast and slow timescales.

It has been proposed that two isomers of the SO dimer (cis- and trans-OSSO) are candidates for the unknown UV absorber in Venus' atmosphere because they have a good spectral match with the absorber, despite the low concentrations predicted by 1D photochemical models. Here OSSO chemistry (production from SO and loss by photolysis, thermal decomposition, and reaction with O and Cl) has been included in the photochemistry scheme of a 3D planetary climate model (PCM-Venus) along with sulfur injection due to meteoric ablation. 1D multiple scattering radiative transfer modeling is then used to predict the resulting top-of-the-atmosphere reflectance produced by OSSO. The modeled OSSO concentrations are shown to be similar to 3 orders of magnitude too low to explain the observed absorbance levels, and the predicted ratio of the OSSO isomers provides an unsatisfactory match to the spectral shape of the unknown absorber.

The Community Earth System Model currently contains two primary atmospheric configurations: the Community Atmosphere Model 6 (CAM6, 32 levels, 40-km top); and the Whole Atmosphere Community Climate Model 6 (WACCM6, 70 levels, 140-km top). For CAM7, a number of factors motivate a raising of the model top and enhancement of the vertical resolution and this study documents the decision making process toward this next generation vertical grid. As vertical resolution in the troposphere/lower stratosphere is increased, the role of the resolved waves in driving the Quasi-Biennial Oscillation (QBO) is enhanced, becoming more similar in magnitude to ERA5 reanalysis. This can be traced to improved equatorial Kelvin waves and their vertical momentum fluxes. It is further shown that a model lid at 80-km does not have detrimental impacts on the representation of the QBO compared to a 140-km top. Based on this analysis, the vertical grid for CAM7 will have an 80-km top with 93 levels, 500-m grid spacing in the troposphere and lower stratosphere, and 10 additional levels in the boundary layer compared to CAM6. A 58-level/40-km low-top option will also be available. We further introduce new coupled simulations using CAM6 but with CAM7's vertical grid above the boundary layer and use these to demonstrate that basic features of the stratospheric circulation are similar to WACCM6, despite the lower model top. These simulations further show that despite the higher fidelity of the QBO, the observed connection between the QBO and the Madden-Julian Oscillation is absent.

Ozone (O _3 ) is important for the survival of life on Earth because it shields the surface from ionizing ultraviolet radiation. However, the existence of O _3 in Earth’s atmosphere is not always beneficial. Resulting from anthropogenic activity, O _3 exists as a biologically harmful pollutant at the surface when it forms in the presence of sunlight and other pollutants. As a strong oxidizer, O _3 can be lethal to several different organisms; thus, when assessing the potential habitability of an exoplanet, a key part is determining whether toxic gases could be present at its surface. Using the Whole Atmosphere Community Climate Model version 6 (WACCM6; a three-dimensional chemistry-climate model), 12 atmospheric simulations of the terrestrial exoplanet TRAPPIST-1 e are performed with a variety of O _2 concentrations and assuming two different stellar spectra proposed in the literature. Four atmospheric simulations of the exoplanet Proxima Centauri b are also included. Some scenarios for both exoplanets exhibit time-averaged surface O _3 mixing ratios exceeding harmful levels of 40 ppbv, with 2120 ppbv the maximum concentration found in the cases simulated. These concentrations are toxic and can be fatal to most life on Earth. In other scenarios O _3 remains under harmful limits over a significant fraction of the surface, despite there being present regions that may prove inhospitable. In the case in which O _3 is detected in a terrestrial exoplanet’s atmosphere, determining the surface concentration is an important step when evaluating a planet’s habitability.

The solar forcing prepared for Phase 6 of the Coupled Model Intercomparison Project (CMIP6) has been used extensively in climate model experiments and has been tested in various intercomparison studies. Recently, an International Space Science Institute (ISSI) working group has been established to revisit the solar forcing recommendations, based on the lessons learned from CMIP6, and to assess new datasets that have become available, in order to define a road map for building a revised and extended historical solar forcing dataset for the upcoming Phase 7 of CMIP. This paper identifies the possible improvements required and outlines a strategy to address them in the planned new solar forcing dataset. Proposed major changes include the adoption of the new Total and Spectral Solar Irradiance Sensor (TSIS-1) solar reference spectrum for solar spectral irradiance and an improved description of top-of-the-atmosphere energetic electron fluxes, as well as their reconstruction back to 1850 by means of geomagnetic proxy data. In addition, there is an urgent need to consider the proposed updates in the ozone forcing dataset in order to ensure a self-consistent solar forcing in coupled models without interactive chemistry. Regarding future solar forcing, we propose consideration of stochastic ensemble forcing scenarios, ideally in concert with other natural forcings, in order to allow for realistic projections of natural forcing uncertainties.

The detection and characterization of Earth-like planets around Sun-like stars is an important goal of exoplanetary research, given their promise for hosting potentially habitable conditions. Key orbital parameters, such as eccentricity, can influence a planet's climate response and, as a consequence, affect its potential habitability. Utilizing the Earth System Model - the Whole Atmosphere Community Climate Model (WACCM6), we simulated Earth-like exoplanets with two different orbital parameters: one circular (e=0) and another highly eccentric (e=0.4), both with zero obliquity but fixing the annual mean insolation. The highly eccentric case exhibits a 1.9 K warmer surface temperature due to lower surface and cloud albedo and a weaker longwave cloud forcing. Exploring the annual global mean climate difference, we analysed latitudinal and seasonal variations in hydrological cycle variables, such as sea ice, land snow, and clouds. Land habitability metrics based on temperature and precipitation reveal that the e=0.4 case has over 25 per cent more habitable land area for more than 80 per cent of its orbit, compared with the e=0 case. Additionally, the global circulation pattern shifts from a three-cell to a two-cell system in the e=0.4 case, expanding the Hadley cell to higher latitudes, enhancing meridional latent heat transport, and improving land habitability at higher latitudes. Our study suggests that Earth-like exoplanets with high eccentricity orbiting Sun-like stars may have greater land habitability than their circular counterparts, due to seasonally warmer surface temperatures and more evenly distributed precipitation over land.

Understanding the composition and distribution of the unknown UV absorber (Figure 1) in the Venusian atmosphere has been an open question in planetary science for close to 100 years. Many candidates for the absorber have been proposed over the years. We focus on two possibilities: ferric chloride (FeCl3)[1] and the cis- and trans- forms of the SO dimer (OSSO)[2]. Figure 1: A false colour image of Venus. Regions of UV absorption appear orange. The cause of this absorption is unknown. Image credit: NASA/JPL-Caltech. FeCl3 has been proposed to exist within the sulphuric acid cloud droplets, but the absorption spectrum of FeCl3 generally used in the literature was measured in ethyl acetate, which is not present on Venus and produces an absorption spectrum with little similarity to the Venusian absorber[3]. We have measured the absorption spectrum of FeCl3 in sulphuric acid with small quantities of HCl added, and we found that it is much more similar in shape to the observed spectrum of the unknown absorber than prior FeCl3 spectra available in the literature. The spectra measured in sulphuric acid contain contributions from both ferric chloride and ferric sulphate ions. We estimated the molar partitioning of the species in the mixtures by performing least squared fitting to reproduce each measured spectrum from spectra of pure ferric sulphate (measured for Fe2(SO4)3 in 75-87 wt% aqueous H2SO4) and pure ferric chloride (measured for FeCl3 in 5-37 wt% aqueous HCl). We estimated the rate of the reaction converting FeCl3 toward Fe2(SO4)3 using this method at several temperatures and extrapolate to Venusian temperatures to predict the lifetime of FeCl3 within the sulphuric acid cloud droplets. To test if either FeCl3 or OSSO are viable candidates we employ three models: the global Planetary Climate Model for Venus (PCM-Venus) to model the photochemistry and 3D transport of the candidates in the atmosphere[4], a 1D sectional aerosol model to predict agglomeration and sedimentation as a transport mechanism of FeCl3 particles[5], and the 1D multiple scattering radiative transfer model SOCRATES[6]. We have added iron chemistry and updated sulphur chemistry into PCM-Venus in order to predict the abundance of gas-phase FeCl3 produced by the reaction of gas-phase HCl with iron produced by the ablation of cosmic dust particles around 115 km, and of OSSO produced from the recombination of two SO molecules. Mean gas and dynamical profiles from the PCM are used to initialise the agglomeration and sedimentation model, which is then run for many Venus years to reach steady state. The potential contributions of each species to the observed absorption were assessed using SOCRATES to predict the observed absorption from instantaneous dayside PCM-modelled concentrations of OSSO, and to estimate the required concentrations of FeCl3 in the different cloud modes and the required increase of OSSO from the PCM results to fully explain the absorption measured by MESSENGER/MASCS during its June 2007 Venus flyby (Pérez-Hoyos et al., 2018). Agglomeration and sedimentation modelling of FeCl3 suggests that the PCM-modelled FeCl3 column abundance above 60 km can account for more than 40% of the observed absorption. The full absorption can be explained by approximately 1 wt% FeCl3 in the mode 1 cloud droplets (Figure 2). Large ferric sulphate ion concentrations in the laboratory spectra used to model the absorption lead to higher than observed absorption near 300 nm. Work is ongoing to correct the iron partitioning between chloride and sulphate to expected Venusian ratios. Results of SOCRATES modelling suggest that an OSSO concentration profile 103 x larger than the PCM-modelled profile would be required to reproduce the depth of the observed absorption (Figure 2). The SO2 and SO profiles at 60 - 80km in the PCM are currently ~100 x smaller than observed concentrations, and so increase of 103 from current concentrations is not unreasonable (as OSSO rate of formation will increase with the square of SO concentration). However, the agreement of the spectral shape with the observations is significantly worse than has been previously reported by Frandsen et al. (2016)[2]. The spectral shape is better approximated when the concentration of trans-OSSO is increased relative to the cis-OSSO, though we have found no chemical justification to make this adjustment. Figure 2: Preliminary data comparing the MESSENGER/MASCS spectrum of the unknown absorber (*[3], grey crosses) to SOCRATES-modelled concentrations that best reproduce the depth of the absorption for cis- and trans-OSSO (purple dashed line) and FeCl3 in mode 1 (modal radius = 0.2 µm) sulphuric acid cloud droplets (orange and red dash-dotted lines). The modelled spectrum with neither OSSO or FeCl3 included (blue solid line) is included for reference.We conclude that both candidates show some merit as the unknown absorber and a mixture of both species is likely present on Venus and contributing to the absorption. Work is ongoing to decrease the ferric sulphate contribution in the laboratory-measured FeCl3 spectrum in line with expected Venusian concentrations (and thereby decrease the high absorption near 300 nm seen in current FeCl3 spectra shown in Figure 2), and to reproduce observed SO2 and SO concentrations with PCM-Venus, thereby likely increasing OSSO modelled concentrations. References [1] Zasova et al., 1981, https://doi.org/10.1016/0273-1177(81)90213-1 [2] Frandsen et al. 2016, https://doi.org/10.1002/2016GL070916 [3] Pérez-Hoyos et al. 2018, https://doi.org/10.1002/2017JE005406 [4] Martinez et al. (2024), doi.org/10.1016/j.icarus.2024.116035 [5] Frankland et al., 2017, https://doi.org/10.1016/j.icarus.2017.06.005 [6] Manners et al. (2022), SOCRATES Technical Guide, available at: https://code.metoffice.gov.uk/trac/socrates

TRAPPIST-1e is a tidally locked rocky exoplanet orbiting the habitable zone of an M dwarf star. Upcoming observations are expected to reveal new rocky exoplanets and their atmospheres around M dwarf stars. To interpret these future observations we need to model the atmospheres of such exoplanets. We configured Community Earth System Model version 2-Whole Atmosphere Community Climate Model version 6, a chemistry climate model, for the orbit and stellar irradiance of TRAPPIST-1e assuming an initial Earth-like atmospheric composition. Our aim is to characterize the possible ozone (O3) distribution and explore how this is influenced by the atmospheric circulation shaped by orography, using the Helmholtz wind decomposition and meridional mass streamfunction. The model included Earth-like orography, and the substellar point was located over the Pacific Ocean. For such a scenario, our analysis reveals a north-south asymmetry in the simulated O3 distribution. The O3 concentration is highest at pressures >10 hPa (below similar to 30 km) near the south pole. This asymmetry arises from the higher landmass fraction in the northern hemisphere, which causes drag in near-surface flows and leads to an asymmetric meridional overturning circulation. Catalytic species were roughly symmetrically distributed and were not found to be primary driver for the O3 asymmetry. The total O3 column density was higher for TRAPPIST-1e compared to Earth, with 8000 Dobson units (DUs) near the south pole and 2000 DU near the north pole. The results emphasize the sensitivity of O3 to model parameters, illustrating how incorporating Earth-like orography can affect atmospheric dynamics and O3 distribution. This link between surface features and atmospheric dynamics underlines the importance of how changing model parameters used to study exoplanet atmospheres can influence the interpretation of observations.

The gravity wave drag parametrization of the Whole Atmosphere Community Climate Model (WACCM) has been modified to include the wave-driven atmospheric vertical mixing caused by propagating, non-breaking, gravity waves. The strength of this atmospheric mixing is represented in the model via the “effective wave diffusivity” coefficient K_wave. Using K_wave, a new total dynamical diffusivity K_Dyn is defined. K_Dyn represents the vertical mixing of the atmosphere by both breaking (dissipating) and vertically propagating (non-dissipating) gravity waves. Here we show that, when the new diffusivity is used, the downward fluxes of Fe and Na between 80 and 100 km largely increase. Larger meteoric ablation injection rates of these metals (within a factor 2 of measurements) can now be used in WACCM, which produce Na and Fe layers in good agreement with lidar observations. Mesospheric CO is also significantly impacted, with the largest CO concentration increase occurring between 80-90 km, where model-observations agreement improves. However, in regions where the model overestimates CO concentration, the new parametrization exacerbates the model bias. The mesospheric cooling simulated by the new parametrization, while needed, is currently too strong almost everywhere. The summer mesopause in both hemispheres becomes too cold by about 30K compared to observations, but it shifts upward, partially correcting the WACCM low summer mesopause. Our results highlight the far-reaching implications and the necessity of representing vertically propagating gravity waves in climate models. This novel method of modelling gravity waves contributes to growing evidence that it is time to move away from dissipative-only gravity wave parametrizations.

There is considerable academic interest in the potential for air quality improvement as a co-benefit of climate change mitigation. Few studies use regional air quality models for simulating future co-benefits, but many use global chemistry-climate model output. Using regional atmospheric chemistry could provide a better representation of air quality changes than global chemistry-climate models, especially by improving the representation of elevated urban concentrations. We use a detailed regional atmospheric-chemistry model (WRF-Chem v4.2) to model European air quality in 2050 compared to 2014 following three climate change mitigation scenarios. We represent different climate futures by using air pollutant emissions and chemical boundary conditions (from CESM2-WACCM output) for three shared socioeconomic pathways (SSP1-2.6, SSP2-4.5 and SSP3-7.0: high-, medium- and low-mitigation pathways respectively). We find that in 2050, following SSP1-2.6, mean population-weighted PM2.5 concentrations across European countries are reduced by 52 % compared to 2014. Under SSP2-4.5, this average reduction is 34%. The smallest average reduction is 18 %, achieved following SSP3-7.0. Maximum 6-monthly-mean daily-maximum 8 h (6mDM8h) ozone (O-3) is reduced across Europe by 15 % following SSP1-2.6 and by 3 % following SSP2-4.5, but it increases by 13 % following SSP3-7.0. This demonstrates clear co-benefits of climate mitigation. The additional resolution allows us to analyse regional differences and identify key sectors. We find that the mitigation of agricultural emissions will be key for attaining meaningful co-benefits of mitigation policies, as evidenced by the importance of changes in NO3 aerosol mass to future PM2.5 air quality and changes in CH4 emissions to future O-3 air quality.

Interpretation of the ongoing efforts to simulate the atmospheres of potentially habitable terrestrial exoplanets requires that we understand the underlying dynamics and chemistry of such objects to a much greater degree than 1D or even simple 3D models enable. Here, for the tidally locked habitable-zone planet TRAPPIST-1e, we explore one effect which can shape the dynamics and chemistry of terrestrial planets: the inclusion of an Earth-like land-ocean distribution with orography. To do this we use the Earth-system model WACCM6/CESM2 to run a pair of TRAPPIST-1e models with N2-O2 atmospheres and with the substellar point fixed over either land or ocean. The presence of orography shapes atmospheric transport, and in the case of Earth-like orography, breaks the symmetry between the Northern and Southern Hemispheres which was previously found in slab ocean models. For example, peak zonal jet speeds in the Southern Hemisphere are 50%-100% faster than similar jets in the Northern Hemisphere. This also affects the meridional circulation, transporting equatorial material toward the south pole. As a result we also find significant changes in the atmospheric chemistry, including the accumulation of potentially lethal quantities of ozone at both the south pole and the surface. Future studies which investigate the effects of landmass distribution on the dynamics of exoplanetary atmospheres should pay close attention to both the dayside land fraction as well as the orography of the land. Simply modeling a flat landmass will not give a complete picture of its dynamical impact.