In climate and weather models, the quantitative description of aerosol and cloud processes relies on simplified assumptions. This contributes major uncertainties to the prediction of global and regional climate change. Therefore, models need good parameterizations for heterogeneous ice nucleation by atmospheric aerosols. Here the authors present a new parameterization of immersion freezing on desert dust particles derived from a large number of experiments carried out at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud chamber facility. The parameterization is valid in the temperature range between 2128 and 2368C at or above water saturation and can be used in atmospheric models that include information about the dust surface area. The new parameterization was applied to calculate distribution maps of ice nuclei during a Saharan dust event based on model results from the regional-scale model Consortium for Small-Scale Modelling-Aerosols and Reactive Trace Gases (COSMO-ART). The results were then compared to measurements at the Taunus Observatory on Mount Kleiner Feldberg, Germany, and to three other parameterizations applied to the dust outbreak. The aerosol number concentration and surface area from the COSMO-ART model simulation were taken as input to different parameterizations. Although the surface area from the model agreed well with aerosol measurements during the dust event at Kleiner Feldberg, the ice nuclei (IN) number concentration calculated from the new surface-area-based parameterization was about a factor of 13 less than IN measurements during the same event. Systematic differences of more than a factor of 10 in the IN number concentration were also found among the different parameterizations. Uncertainties in the modeled and measured parameters probably both contribute to this discrepancy and should be addressed in future studies.

[1] The impact of the heterogeneous hydrolysis of N 2 O 5 on tropospheric gas phase and particle phase chemistry was investigated by performing model simulations with two comprehensive model systems and taking into account recent findings on the heterogeneous reaction probability of N 2 O 5 . Hereby, we focused on photosmog conditions in the lower troposphere. Chemistry box model runs were carried out neglecting transport and deposition processes. The heterogeneous hydrolysis of N 2 O 5 leads to a decrease of ozone under low-NO x conditions and to a strong increase of ozone under high-NO x conditions. One-dimensional simulations were performed to take into account vertical mixing processes, deposition, and temporal changes of the emissions. The rate constant for the heterogeneous hydrolysis was determined depending on the simulated aerosol surface area density. A large impact of the heterogeneous hydrolysis on the nocturnal concentrations of N 2 O 5 , NO 3 , HNO 3 , and the surface area density and nitrate content of the aerosol is found. However, the effect of the hydrolysis of N 2 O 5 on ozone decreases considerably compared to the box model simulations. Three-dimensional simulations for a typical summer smog situation for the southwestern part of Germany and on the European scale, which cover a variety of atmospheric and emission conditions, confirm these findings. The impact of heterogeneous hydrolysis on ozone is small, but it causes remarkable changes in the nocturnal concentrations of nitrogen-containing species and on aerosol properties such as surface area density and nitrate content.

Abstract. A new fully online coupled model system developed for the evaluation of the interaction of aerosol particles with the atmosphere on the regional scale is described. The model system is based on the operational weather forecast model of the Deutscher Wetterdienst. Physical processes like transport, turbulent diffusion, and dry and wet deposition are treated together with photochemistry and aerosol dynamics using the modal approach. Based on detailed calculations we have developed parameterisations to examine the impact of aerosol particles on photolysis and on radiation. Currently the model allows feedback between natural and anthropogenic aerosol particles and the atmospheric variables that are initialized by the modification of the radiative fluxes. The model system is applied to two summer episodes, each lasting five days, with a model domain covering Western Europe and adjacent regions. The first episode is characterised by almost cloud free conditions and the second one by cloudy conditions. The simulated aerosol concentrations are compared to observations made at 700 stations distributed over Western Europe. For each episode two model runs are performed; one where the feedback between the aerosol particles and the atmosphere is taken into account and a second one where the feedback is neglected. Comparing these two sets of model runs, the radiative feedback on temperature and other variables is evaluated. In the cloud free case a clear correlation between the aerosol optical depth and changes in global radiation and temperature is found. In the case of cloudy conditions the pure radiative effects are superposed by changes in the liquid water content of the clouds due to changes in the thermodynamics of the atmosphere. In this case the correlation between the aerosol optical depth and its effects on temperature is low. However, on average a decrease in the 2 m temperature is still found. For the area of Germany we found on average for both cases a reduction in the global radiation of about 6 W m2, a decrease of the 2 m temperature of 0.1 K, and a reduction in the daily temperature range of −0.13 K.

Abstract. The aging of soot is one of the key uncertainties in the estimation of both the direct and indirect climate effect. While freshly emitted soot is initially hydrophobic and externally mixed, it can be transferred into an internal mixture by coagulation, condensation or photochemical processes. These aging processes affect the hygroscopic qualities and hence the growth behaviour, the optical properties and eventually the lifetime of the soot particles. However, due to computational limits the aging of soot in global climate models is often only parameterised by an estimated turnover rate resulting in a lifetime of soot of several days. Hence, the aging process of soot is one of the key uncertainties governing the burden and effect of black carbon. In this study, we discuss the time scale on which diesel soot is transferred from an external to an internal mixture based on the results of our simulations with a comprehensive mesoscale model. For daytime conditions during summer condensation of sulphuric acid is dominant and the aging process occurs on a time scale of τ =8 h close to the sources and τ =2 h above the source region. During winter comparable time scales are found but ammonium nitrate becomes more important. During night time condensation is not effective. Then coagulation is the most important aging process and our results show time scales between 10 h and 40 h.

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[1] The heterogeneous reaction probability g N 2 O 5 of N 2 O 5 depends largely on the aerosol chemical composition. Several recent laboratory studies have shown that the presence of organic coatings on aqueous aerosols can suppress heterogeneous N 2 O 5 hydrolysis. In this study we investigated the relative importance of organic coatings, formed via gas-toparticle conversion on aqueous aerosols, to N 2 O 5 hydrolysis on local and regional scales during summer in Europe. We present results of one-dimensional process studies and of regional-scale model simulations for Europe with the comprehensive model system COSMO-ART. To treat N 2 O 5 hydrolysis, we used recent results from laboratory studies that quantified g N 2 O 5 on the basis of the organic coating thickness. The simulations showed that during any episode the conditions for formation of N 2 O 5 and secondary organic compounds were very variable and depended strongly on the meteorological conditions. In regions of the model domain where both components were built-up, the formation of organic coatings could decrease particulate nitrate concentrations by up to 90%. Where these conditions were not met, the impact of the organic coating was negligible.

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Abstract.We investigated the impact of mineral dust particles on clouds, radiation and atmospheric state during a strong Saharan dust event over Europe in May 2008, applying a comprehensive online-coupled regional model framework that explicitly treats particle microphysics and chemical composition. Sophisticated parameterizations for aerosol activation and ice nucleation, together with two-moment cloud microphysics are used to calculate the interaction of the different particles with clouds depending on their physical and chemical properties.The impact of dust on cloud droplet number concentration was found to be low, with just a slight increase in cloud droplet number concentration for both uncoated and coated dust. For temperatures lower than the level of homogeneous freezing, no significant impact of dust on the number and mass concentration of ice crystals was found, though the concentration of frozen dust particles reached up to 100 l −1 during the ice nucleation events. Mineral dust particles were found to have the largest impact on clouds in a temperature range between freezing level and the level of homogeneous freezing, where they determined the number concentration of ice crystals due to efficient heterogeneous freezing of the dust particles and modified the glaciation of mixed phase clouds.Our simulations show that during the dust events, ice crystals concentrations were increased twofold in this temperature range (compared to if dust interactions are neglected).This had a significant impact on the cloud optical properties, causing a reduction in the incoming short-wave radiation at the surface up to −75 W m −2 . Including the direct interaction of dust with radiation caused an additional reduction in the incoming short-wave radiation by 40 to 80 W m −2 , and the incoming long-wave radiation at the surface was increased significantly in the order of +10 W m −2 .The strong radiative forcings associated with dust caused a reduction in surface temperature in the order of −0.2 to −0.5 K for most parts of France, Germany, and Italy during the dust event. The maximum difference in surface temperature was found in the East of France, the Benelux, and Western Germany with up to −1 K. This magnitude of temperature change was sufficient to explain a systematic bias in numerical weather forecasts during the period of the dust event.