A total of 16 global chemistry transport models and general circulation models have participated in this study; 14 models have been evaluated with regard to their ability to reproduce the ...near-surface observed number concentration of aerosol particles and cloud condensation nuclei (CCN), as well as derived cloud droplet number concentration (CDNC). Model results for the period 2011-2015 are compared with aerosol measurements (aerosol particle number, CCN and aerosol particle composition in the submicron fraction) from nine surface stations located in Europe and Japan. The evaluation focuses on the ability of models to simulate the average across time state in diverse environments and on the seasonal and short-term variability in the aerosol properties. There is no single model that systematically performs best across all environments represented by the observations. Models tend to underestimate the observed aerosol particle and CCN number concentrations, with average normalized mean bias (NMB) of all models and for all stations, where data are available, of -24% and -35% for particles with dry diameters > 50 and > 120nm, as well as -36% and -34% for CCN at supersaturations of 0.2% and 1.0%, respectively. However, they seem to behave differently for particles activating at very low supersaturations (< 0.1%) than at higher ones. A total of 15 models have been used to produce ensemble annual median distributions of relevant parameters. The model diversity (defined as the ratio of standard deviation to mean) is up to about 3 for simulated N3 (number concentration of particles with dry diameters larger than 3 nm) and up to about 1 for simulated CCN in the extra-polar regions. A global mean reduction of a factor of about 2 is found in the model diversity for CCN at a supersaturation of 0.2% (CCN(0.2)) compared to that for N3, maximizing over regions where new particle formation is important. An additional model has been used to investigate potential causes of model diversity in CCN and bias compared to the observations by performing a perturbed parameter ensemble (PPE) accounting for uncertainties in 26 aerosol-related model input parameters. This PPE suggests that biogenic secondary organic aerosol formation and the hygroscopic properties of the organic material are likely to be the major sources of CCN uncertainty in summer, with dry deposition and cloud processing being dominant in winter. Models capture the relative amplitude of the seasonal variability of the aerosol particle number concentration for all studied particle sizes with available observations (dry diameters larger than 50, 80 and 120nm). The short-term persistence time (on the order of a few days) of CCN concentrations, which is a measure of aerosol dynamic behavior in the models, is underestimated on average by the models by 40% during winter and 20% in summer.
Biogenic volatile organic compounds (BVOCs) emitted from vegetation are oxidised in the atmosphere and can form aerosol particles either by contributing to new particle formation or by condensing ...onto existing aerosol particles. As the understanding of the importance of BVOCs for aerosol formation has increased over the years, these processes have made their way into Earth system models (ESMs). In this study, sensitivity experiments are run with three different ESMs (the Norwegian Earth System Model (NorESM), EC-Earth and ECHAM) to investigate how the direct and indirect aerosol radiative effects are affected by changes in the formation of secondary organic aerosol (SOA) from BVOCs. In the first two sensitivity model experiments, the yields of SOA precursors from oxidation of BVOCs are changed by ±50 %. For the third sensitivity test, the formed oxidation products do not participate in the formation of new particles but are only allowed to condense onto existing aerosols. In the last two sensitivity experiments, the emissions of BVOC compounds (isoprene and monoterpenes) are turned off, one at a time. The goal of the study is to investigate whether it is of importance to treat SOA formation processes correctly in the models rather than to evaluate the correctness of the current treatment in the models. The results show that the impact on the direct radiative effect (DRE) is linked to the changes in the SOA production in the models, where more SOA leads to a stronger DRE and vice versa. However, the magnitude by which the DRE changes (maximally 0.15 W m−2 globally averaged) in response to the SOA changes varies between the models, with EC-Earth displaying the largest changes. The results for the cloud radiative effects (CREs) are more complicated than for the DRE. The changes in CRE differ more among the ESMs, and for some sensitivity experiments they even have different signs. The most sensitive models are NorESM and EC-Earth, which have CRE changes of up to 0.82 W m−2. The varying responses in the different models are connected to where in the aerosol size distributions the changes in mass and number due to SOA formation occur, in combination with the aerosol number concentration levels in the models. We also find that interactive gas-phase chemistry as well as the new particle formation parameterisation has important implications for the DRE and CRE in some of the sensitivity experiments. The results from this study indicate that BVOC-SOA treatment in ESMs can have a substantial impact on the modelled climate but that the sensitivity varies greatly between the models. Since BVOC emissions have changed historically and will continue to change in the future, the spread in model results found in this study implies uncertainty into ESM estimates of aerosol forcing from land-use change and BVOC feedback strengths.
This paper documents the global climate model EC-Earth3-AerChem,
one of the members of the EC-Earth3 family of models participating in the
Coupled Model Intercomparison Project Phase 6 (CMIP6). ...EC-Earth3-AerChem has
interactive aerosols and atmospheric chemistry and contributes to the
Aerosols and Chemistry Model Intercomparison Project (AerChemMIP). In this
paper, we give an overview of the model, describe in detail how it
differs from the other EC-Earth3 configurations, and outline the new features compared with the previously documented version of the model (EC-Earth
2.4). We explain how the model was tuned and spun up under preindustrial
conditions and characterize the model's general performance on the basis of
a selection of coupled simulations conducted for CMIP6. The net energy
imbalance at the top of the atmosphere in the preindustrial control
simulation is on average −0.09 W m−2 with a standard deviation due to
interannual variability of 0.25 W m−2, showing no significant drift.
The global surface air temperature in the simulation is on average 14.08 ∘C with an interannual standard deviation of 0.17 ∘C, exhibiting a small drift of 0.015 ± 0.005 ∘C per century.
The model's effective equilibrium climate sensitivity is estimated at 3.9 ∘C, and its transient climate response is estimated at 2.1 ∘C. The
CMIP6 historical simulation displays spurious interdecadal variability in
Northern Hemisphere temperatures, resulting in a large spread across
ensemble members and a tendency to underestimate observed annual surface
temperature anomalies from the early 20th century onwards. The observed
warming of the Southern Hemisphere is well reproduced by the model. Compared
with the ECMWF (European Centre for Medium-Range Weather
Forecasts) Reanalysis version 5 (ERA5), the surface air temperature climatology for 1995–2014 has an
average bias of −0.86 ± 0.05 ∘C with a standard deviation
across ensemble members of 0.35 ∘C in the Northern Hemisphere and
1.29 ± 0.02 ∘C with a corresponding standard deviation of
0.05 ∘C in the Southern Hemisphere. The Southern Hemisphere warm
bias is largely caused by errors in shortwave cloud radiative effects over
the Southern Ocean, a deficiency of many climate models. Changes in the
emissions of near-term climate forcers (NTCFs) have significant effects on
the global climate from the second half of the 20th century onwards. For the
SSP3-7.0 Shared Socioeconomic Pathway, the model gives a global warming at
the end of the 21st century (2091–2100) of 4.9 ∘C above the
preindustrial mean. A 0.5 ∘C stronger warming is obtained for
the AerChemMIP scenario with reduced emissions of NTCFs. With concurrent
reductions of future methane concentrations, the warming is projected to be
reduced by 0.5 ∘C.
Clouds and aerosols contribute the largest uncertainty to current estimates and interpretations of the Earth's changing energy budget. Here we use a new-generation large-domain large-eddy model, ...ICON-LEM (ICOsahedral Non-hydrostatic Large Eddy Model), to simulate the response of clouds to realistic anthropogenic perturbations in aerosols serving as cloud condensation nuclei (CCN). The novelty compared to previous studies is that (i) the LEM is run in weather prediction mode and with fully interactive land surface over a large domain and (ii) a large range of data from various sources are used for the detection and attribution. The aerosol perturbation was chosen as peak-aerosol conditions over Europe in 1985, with more than fivefold more sulfate than in 2013. Observational data from various satellite and ground-based remote sensing instruments are used, aiming at the detection and attribution of this response. The simulation was run for a selected day (2 May 2013) in which a large variety of cloud regimes was present over the selected domain of central Europe.
We have implemented and evaluated a secondary organic aerosol scheme within the chemistry transport model TM5-MP in this work. In earlier versions of TM5-MP the secondary organic aerosol (SOA) was ...emitted as Aitken-sized particle mass emulating the condensation. In the current scheme we simulate the formation of secondary organic aerosol from oxidation of isoprene and monoterpenes by ozone and hydroxyl radicals, which produce semi-volatile organic compounds (SVOCs) and extremely low-volatility compounds (EVOCs). Subsequently, SVOCs and ELVOCs can condense on particles. Furthermore, we have introduced a new particle formation mechanism depending on the concentration of ELVOCs. For evaluation purposes, we have simulated the year 2010 with the old and new scheme; we see an increase in simulated production of SOA from 39.9 Tg yr−1 with the old scheme to 52.5 Tg yr−1 with the new scheme. For more detailed analysis, the particle mass and number concentrations and their influence on the simulated aerosol optical depth are compared to observations. Phenomenologically, the new particle formation scheme implemented here is able to reproduce the occurrence of observed particle formation events. However, the modelled concentrations of formed particles are clearly lower than in observations, as is the subsequent growth to larger sizes. Compared to the old scheme, the new scheme increases the number concentrations across the observation stations while still underestimating the observations. The organic aerosol mass concentrations in the US show a much better seasonal cycle and no clear overestimation of mass concentrations anymore. In Europe the mass concentrations are lowered, leading to a larger underestimation of observations. Aerosol optical depth (AOD) is generally slightly increased except in the northern high latitudes. This brings the simulated annual global mean AOD closer to the observational estimate. However, as the increase is rather uniform, biases tend to be reduced only in regions where the model underestimates the AOD. Furthermore, the correlations with satellite retrievals and ground-based sun-photometer observations of AOD are improved. Although the process-based approach to SOA formation causes a reduction in model performance in some areas, overall the new scheme improves the simulated aerosol fields.
A condensed multiphase halogen and dimethyl sulfide (DMS)
chemistry mechanism for application in chemistry transport models is
developed by reducing the CAPRAM DMS module 1.0 (CAPRAM-DM1.0) and the
...CAPRAM halogen module 3.0 (CAPRAM-HM3.0). The reduction is achieved by
determining the main oxidation pathways from analysing the mass fluxes of
complex multiphase chemistry simulations with the air parcel model SPACCIM (SPectral Aerosol Cloud Chemistry Interaction Model).
These simulations are designed to cover both pristine and polluted marine
boundary layer conditions. Overall, the reduced CAPRAM-DM1.0 contains 32
gas-phase reactions, 5 phase transfers, and 12 aqueous-phase reactions, of
which two processes are described as equilibrium reactions. The reduced
CAPRAM-HM3.0 contains 199 gas-phase reactions, 23 phase transfers, and 87
aqueous-phase reactions. For the aqueous-phase chemistry, 39 processes are
described as chemical equilibrium reactions. A comparison of simulations
using the complete CAPRAM-DM1.0 and CAPRAM-HM3.0 mechanisms against the
reduced ones indicates that the relative deviations are below 5 % for
important inorganic and organic air pollutants and key reactive species
under pristine ocean and polluted conditions. The reduced mechanism has been
implemented into the chemical transport model COSMO-MUSCAT and tested by
performing 2D simulations under prescribed meteorological conditions that
investigate the effect of stable (stratiform cloud) and more unstable
meteorological conditions (convective clouds) on marine multiphase
chemistry. The simulated maximum concentration of HCl is of the order of
109 molecules cm−3 and that of BrO is around
1×107 molecules cm−3, reproducing the range of
ambient measurements. Afterwards, the oxidation pathways of DMS in a cloudy
marine atmosphere have been investigated in detail. The simulations
demonstrate that clouds have both a direct and an indirect photochemical
effect on the multiphase processing of DMS and its oxidation products. The
direct photochemical effect is related to in-cloud chemistry that leads to
high dimethyl
sulfoxide (DMSO) oxidation rates and a subsequently enhanced formation of methane
sulfonic acid compared to aerosol chemistry. The indirect photochemical
effect is characterized by cloud shading, which occurs particularly in the
case of stratiform clouds. The lower photolysis rate affects the activation
of Br atoms and consequently lowers the formation of BrO radicals. The
corresponding DMS oxidation flux is lowered by up to 30 % under thick
optical clouds. Moreover, high updraught velocities lead to a strong vertical
mixing of DMS into the free troposphere predominately under cloudy
conditions. The photolysis of hypohalous acids (HOX, X = Cl, Br, or I) is
reduced as well, resulting in higher HOX-driven sulfite-to-sulfate oxidation
in aerosol particles below stratiform clouds. Altogether, the present model
simulations have demonstrated the ability of the reduced mechanism to be
applied in studying marine aerosol–cloud processing effects in regional
models such as COSMO-MUSCAT. The reduced mechanism can be used also by other
regional models for more adequate interpretations of complex marine field
measurement data.
Oceans dominate emissions of dimethyl sulfide (DMS), the major natural sulfur source. DMS is important for the formation of non-sea salt sulfate (nss-SO₄2−) aerosols and secondary particulate matter ...over oceans and thus, significantly influence global climate. The mechanism of DMS oxidation has accordingly been investigated in several different model studies in the past. However, these studies had restricted oxidation mechanisms that mostly underrepresented important aqueous-phase chemical processes. These neglected but highly effective processes strongly impact direct product yields of DMS oxidation, thereby affecting the climatic influence of aerosols. To address these shortfalls, an extensive multiphase DMS chemistry mechanism, the Chemical Aqueous Phase Radical Mechanism DMS Module 1.0, was developed and used in detailed model investigations of multiphase DMS chemistry in the marine boundary layer. The performed model studies confirmed the importance of aqueous-phase chemistry for the fate of DMS and its oxidation products. Aqueous-phase processes significantly reduce the yield of sulfur dioxide and increase that of methyl sulfonic acid (MSA),which is needed to close the gap between modeled and measured MSA concentrations. Finally, the simulations imply that multiphase DMS oxidation produces equal amounts of MSA and sulfate, a result that has significant implications for nss-SO₄2− aerosol formation, cloud condensation nuclei concentration, and cloud albedo over oceans. Our findings show the deficiencies of parameterizations currently used in higher-scale models, which only treat gas-phase chemistry. Overall, this study shows that treatment of DMS chemistry in both gas and aqueous phases is essential to improve the accuracy of model predictions.
Model evaluation studies are essential for determining model performance as well as assessing model deficiencies, and are the focus of the Air Quality Model Evaluation International Initiative ...(AQMEII). The chemistry-transport model system COSMO–MUSCAT participates in this initiative. In this paper the robustness and variability of the model results against changes in the model setup are analyzed. Special focus is given to the formation of secondary particulate matter and the ability to reproduce unusually high levels of PM10 in Central Europe caused by long-range transported smoke of fires in western Russia. Seven different model configurations are investigated in this study. The COSMO–MUSCAT results are evaluated in comparison with ground-based measurements in Central Europe. The analysis is performed for two selected periods in April/May 2006 and October 2006 which are characterized by elevated concentrations of PM. Furthermore, the sensitivity of the results is studied against the used grid resolution and the meteorological forcing. Here, COSMO–MUSCAT is applied with different horizontal grid sizes and, alternatively, forced by reanalysis data with finer resolution. The use of finer grid resolutions in COSMO–MUSCAT has direct consequences on the meteorological forcing as well as on the calculated emission and deposition rates. The presented results suggest a large impact of the meteorological effects on the PM concentrations. The more accurate spatial appointment of the emissions and deposition fluxes seems to be of little consequence compared to the meteorological forcing.
Mechanism reductions of the detailed aqueous phase chemistry mechanism CAPRAM 3.0i are performed. Manual methods and automatic techniques are both applied in order to provide a less computationally ...intensive mechanism which is operational in regional chemistry transport models (CTMs). The finally reduced mechanism contains less than 200 reactions (4 times smaller than the detailed CAPRAM 3.0i) and describes the main characteristics of inorganic and organic aqueous phase processes occurring in tropospheric warm clouds. Most of the chemical reduction potential is realized in the CAPRAM 3.0i organic chemistry. The number of aqueous phase species decreases from 380 in the full mechanism to 130 in the final reduced version. The calculated percentage deviations between the full and reduced mechanism are on average below 5% for the most important organic and inorganic target compounds such as oxidants, inorganic and organic acids, carbonyls and alcohols. Comparisons of the required CPU times between the full and reduced mechanisms show reductions of approximately 40%. 2-D test simulations with the CTM MUSCAT were performed using prescribed meteorological conditions in order to examine the applicability of the reduced mechanism at regional scale. Simulations with the reduced CAPRAM 3.0i mechanism and a much less complex mechanism with only limited inorganic chemistry (INORG) were compared to evaluate the effects of more detailed chemistry. The model results show large differences in the level of oxidants and the inorganic and organic mass processing. Prospectively, the reduced mechanism represents the basis for studying aerosol cloud processing effects at regional scale with future CTMs and will allow more adequate interpretation of field data.
PDF
