Aerosols influence the Earth's energy balance directly by modifying the radiation transfer and indirectly by altering the cloud microphysics. Anthropogenic aerosol emissions dropped considerably when the global COVID-19 pandemic resulted in severe restraints on mobility, production, and public life in spring 2020. We assess the effects of these reduced emissions on direct and indirect aerosol radiative forcing over Europe, excluding contributions from contrails. We simulate the atmospheric composition with the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model in a baseline (business-as-usual) and a reduced emission scenario. The model results are compared to aircraft observations from the BLUESKY aircraft campaign performed in May–June 2020 over Europe. The model agrees well with most of the observations, except for sulfur dioxide, particulate sulfate, and nitrate in the upper troposphere, likely due to a biased representation of stratospheric aerosol chemistry and missing information about volcanic eruptions. The comparison with a baseline scenario shows that the largest relative differences for tracers and aerosols are found in the upper troposphere, around the aircraft cruise altitude, due to the reduced aircraft emissions, while the largest absolute changes are present at the surface. We also find an increase in all-sky shortwave radiation of 0.21 ± 0.05 W m⁻² at the surface in Europe for May 2020, solely attributable to the direct aerosol effect, which is dominated by decreased aerosol scattering of sunlight, followed by reduced aerosol absorption caused by lower concentrations of inorganic and black carbon aerosols in the troposphere. A further increase in shortwave radiation from aerosol indirect effects was found to be much smaller than its variability. Impacts on ice crystal concentrations, cloud droplet number concentrations, and effective crystal radii are found to be negligible.

We present the coupling of the Jena Scheme for Biosphere–Atmosphere Coupling in Hamburg version 4 (JSBACHv4) to the ECHAM/MESSy Atmospheric Chemistry (EMAC) model. With JSBACH, the soil water bucket model in EMAC is replaced by a diffusive hydrological transport model for soil water that includes water storage and infiltration in five soil layers, preventing soil from drying too rapidly and reducing biases in soil temperature and moisture. A three-layer soil scheme is implemented, and phase changes in water in the soil are considered. The leaf area index (LAI) climatology in EMAC has been substituted with a phenology module calculating the LAI. Multiple land cover types are included to provide a state-dependent surface albedo, which accounts for the absorption of solar radiation by vegetation. Plant net primary productivity, leaf area index and surface roughness are calculated according to the plant functional types. This paper provides a detailed evaluation of the new coupled model based on observations and reanalysis data, including ERA5/ERA5-Land datasets, Global Precipitation Climatology Project (GPCP) data and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data. Land surface temperature (LST), terrestrial water storage (TWS), surface albedo (α), net top-of-atmosphere radiation flux (Rad_TOA), precipitation (precip), leaf area index (LAI), fraction of absorbed photosynthetic active radiation (FAPAR) and gross primary productivity (GPP) are evaluated in particular. The strongest correlation (r) between reanalysis data and the newly coupled model is found for LST (r=0.985, with an average global bias of −1.546 K), α (r=0.947, with an average global bias of −0.015) and Rad_TOA (r=0.907, with an average global bias of 3.56 W m⁻²). Precipitation exhibits a correlation with the GPCP dataset of 0.523 and an average global bias of 0.042 mm d⁻¹. The LAI optimisation yields a correlation of 0.637 with observations and a global mean deviation of −0.212. FAPAR and GPP exemplify two of the many additional variables made available through JSBACH in EMAC. FAPAR and observations show a correlation of 0.663, with an average global difference of −0.223, while the correlation for GPP and observations is 0.564 and the average global difference is −0.001 kg carbon km⁻¹. Benefiting from the numerous added features within the simulated land system, the representation of soil moisture is improved, which is critical for vegetation modelling. This improvement can be attributed to a general increase in soil moisture and water storage in deeper soil layers and a closer alignment of simulated TWS with observations, mitigating the previously widespread problem of soil drought. We show that the numerous newly added components strongly improve the land surface, e.g. soil moisture, TWS and LAI, while surface parameters, such as LST, surface albedo or Rad_TOA, which were mostly prescribed according to climatologies, remain similar. The coupling of JSBACH brings EMAC a step closer towards a holistic comprehensive Earth system model and extends its versatility.

Aerosols influence the Earth’s energy balance through direct radiative effects and indirectly by altering the cloud micro-physics. Anthropogenic aerosol emissions dropped considerably when the global COVID–19 pandemic resulted in severe restraints on mobility, production, and public life in spring 2020. Here we assess the effects of these reduced emissions on direct and indirect aerosol radiative forcing over Europe, excluding contributions from contrails. We simulate the atmospheric com- position with the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model in a baseline (business as usual) and a reduced emission scenario. The model results are compared to aircraft observations from the BLUESKY aircraft campaign performed in May/June 2020 over Europe. The model agrees well with most of the observations, except for sulfur dioxide, particulate sulfate and nitrate in the upper troposphere, likely due to a somewhat biased representation of stratospheric aerosol chemistry and missing information about volcanic eruptions which could have influenced the campaign. The comparison with a business as usual scenario shows that the largest relative differences for tracers and aerosols are found in the upper troposphere, around the aircraft cruise altitude, due to the reduced aircraft emissions, while the largest absolute changes are present at the surface. We also find an increase in shortwave radiation of 0.327 ± 0.105 Wm⁻² at the surface in Europe for May 2020, solely attributable to the direct aerosol effect, which is dominated by decreased aerosol scattering of sunlight, followed by reduced aerosol absorption, caused by lower concentrations of inorganic and black carbon aerosols in the troposphere. A further in- crease in shortwave radiation from aerosol indirect effects was found to be much smaller than its variability. Impacts on ice crystal- and cloud droplet number concentrations and effective crystal radii are found to be negligible.

In this study, we use an earth system model with detailed atmospheric chemistry (EMAC v2.55.2) to undertake simulations of hydrogen (H2) atmospheric dynamics. Extensive global equilibrium simulations were performed with a horizontal resolution of 1.9°. The results of this simulation are compared with observational data from 56 stations in the National Oceanic and Atmospheric Administration (NOAA) Global Monitoring Laboratory (GML) Carbon Cycle Cooperative Global Air Sampling Network. We introduced H2 sources and sinks, the latter inclusive of a soil uptake scheme, that accounts for bacterial consumption. The model thus accounts for detailed H2 and methane (CH4) flux boundary conditions. Results from the EMAC model are accurate and predict the magnitude, amplitude and interhemispheric seasonality of the annual H2 cycle at most observational stations. Time series comparison of EMAC and observational data produces Pearson correlation coefficients (r) in excess of 0.9 at eight remote stations located in polar regions and on high mid-latitude islands. A further 23 stations yielded correlation coefficients between 0.7–0.9, predominantly located in remote marine stations across all latitudes and also in polar regions. The quality of model predictions (r<0.5, 9 stations) is reduced in anthropogenically highly polluted stations in east Asia and the Mediterranean region and stations impacted by peat fire emissions in Indonesia, as local and incidental emissions are difficult to capture. Our H2 budget corroborates bottom-up estimates in the literature in terms of source and sink strengths and overall atmospheric burden. By simulating hydroxyl radicals (OH) in the atmosphere leading to a CH4 lifetime in agreement with observationally constrained estimates, we show that the EMAC model is a capable tool for undertaking high accuracy simulation of H2 at global scale. Future research applications could target the impact of potentially significant natural and anthropogenic H2 sources on air quality and climate, reducing uncertainties in the H2 soil sink and impacts of H2 release on the future oxidising capacity of the atmosphere.

Aerosols and clouds are key external factors that significantly influence the light-use efficiency of plants and their primary productivity. Since photosynthesis and transpiration are tightly linked through leaf stomata, the diffuse fertilization effect (DFE) can also impact water-use efficiency (WUE). However, surface measurements have not yet been systematically employed to enhance the ability of global models to capture these effects, particularly across pan-tropical regions. Current global models remain uncalibrated, lacking validation datasets necessary to accurately represent the annual variations in gross primary productivity (GPP) and WUE driven by the DFE. In this study, we assessed the influence of DFE on annual carbon uptake and water dynamics using traditional methods widely applied across diverse forest ecosystems globally. Our analysis utilized long-term micrometeorological data from the FLUXNET dataset in six evergreen broadleaf forest (EBF) ecosystems, located in northern South America (SA), Central Africa (AF), South Asia (AS), and Oceania (OC). Preliminary comparisons were conducted with GPP and ET values simulated using the EMAC/JSBACH numerical systems. Here, we show the optimal physiological thresholds for GPP and WUE under overcast/smoky sky conditions and the typical tipping points linked to irradiance relative (f). Our preliminary results from the FLUXNET dataset indicated GPP and WUE rates as high as 0.4-0.6 g C m-2 h-1 and ET 0.3-0.5 mm h-1 for f between 1.1-0.7 (± 0.39) and solar zenith angles (SZA) ranging 0°–80°. For f ≤ 0.6, GPP and ET decreased rapidly in the studied areas, with a total breakdown of photosynthesis and evapotranspiration reaching around f 0.2 (± 0.36). The EMAC/JSBACH systems satisfactorily reproduced the behavior of the observed variables (aforementioned). However, we found systematic overestimations of temperature and solar radiation compared to FLUXNET measurements (~35%), which also can explain the GPP overestimations. Systematic calibrations in the EMAC/JSBACH are still necessary to achieve more accurate estimates of annual carbon and water losses due to DFE. Potential outcomes and benefits include: (i) improved physical representation and performance of Dynamic Global Vegetation Models (DGVMs) across different Plant Functional Types (PFTs) and landscapes; (ii) identification of the physiological optimum of forests under conditions affected by wildfires or extreme drought periods; and (iii) quantification of annual global water and carbon losses in tropical forests caused by wildfires; enhancements or photodamage caused by exposure to biomass-burning organic aerosol (BBOA), along with improved representation of global carbon cycling.