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The troposphere is a complex multiphase and multicomponent environment with simultaneously occurring gas and aqueous phase as well as heterogeneous chemical transformations, which can potentially alter the physico-chemical composition of aerosols. In this multicomponent system, phase transfer processes take place, which can influence the particle and droplet formation as well as particle growth. In the modeling of such multiphase processes it is necessary to consider non-ideal conditions in deliquescent particles. Such, highly concentrated solutions are typical when small amounts of water available in aqueous aerosol particles. The behavior of this non-ideal mixed solvent-electrolyte solutions including inorganic and organic chemistry is not yet described satisfyingly in existing multiphase models. In light of this, a modeling framework has been developed in the context of multiphase air parcel model to treat the kinetic description of phase transfer processes considering complex multiphase chemistry and an extended description of non-ideal solutions for the aqueous phase chemistry by means of activity coefficient models. Activity coefficient models, such as AIOMFAC, mod. LIFAC and Pitzer-ion interaction models were implemented in this work. The current available activity coefficient models developed for electrolyte-organic-water mixtures are evaluated in the first part of the thesis. For that, the model investigations cover a scale, ranging from very simple to complex simulations. The implemented module can use different combinations of mixing rules and activity coefficient models flexibly. In the second part of the thesis, the extended activity coefficient model was implemented in the parcel model framework SPACCIM, in order to investigate the influence of treatment of non-ideality on multiphase chemistry. The effect of considering non-ideal solutions was studied for two different aerosol types (remote, urban) regarding complex multiphase chemistry. The advanced model simulations have shown under which circumstances it is important to consider non-ideal solutions and how they will affect the simulated particle/droplet growth and chemical transformations. The modeled activity coefficients and their pattern, which decides the multiphase chemical transformations, were investigated for inorganic and organic systems. The present results have shown that, the inclusion of the treatment of non-ideality can substantially extend our ability to model complex multiphase chemistry especially in the particle phase.
Production of atmospheric sulfate from SO2 emitted into the troposphere is the key question we have to answer for assessing main problems like acid rain, forest decline and negative climate forcing which is believed to counteract the green house effect. About one decade ago many researchers agreed that sulfate formation occurs dominantly (80-90 %) via the aqueous phase chemical transformation, where the SO2 dissociation is the first step. However, there is still a high uncertainty on the amount of sulfite (dissolved SO2) being oxidized and on that removed by wet deposition in the reduced form S(IV) (sulfite). This important question, whose answer gives climate modellers an essential input on the percentage of emitted SO2 converted into sulfate, was the aim of this work. This work presents experimental and theoretical results from studies of the ratio sulfite/sulfate in rainwater and cloudwater to assess the contribution of S(IV) to the total sulfur amount in the aqueous phase. The wet deposition of S(IV) in rainwater was studied by collecting rainwater samples from two different levels using a 324 m high tower. The increase of S(IV) wet deposition flux from the 324 m level to the ground level via sub-cloud scavenging of SO2 is significant. 13-51 % (36 % in average) of sulfur in rainwater on the ground level was found to be in the form of S(IV). The result that S(IV) is an important form of sulfur in rainwater was further confirmed by our theoretical study using a one-dimensional time-dependant physical-chemical cloud model. Model calculations show that most of sub-cloud scavenged SO2 will remain as free S(IV) in rainwater. In highly polluted areas the ratio can be as high as 0.9. This ratio in cloudwater is much less than that in rainwater according to our field experiment carried out at Mt. Brocken. Neverthless, under some special conditions, this ratio can be as high as 0.2, which means that the role of S(IV) in cloudwater is not ignorable. Thus, this study has confirmed the very few S(IV) measurements found in literature, suggesting the importance of S(IV) wet deposition. Our findings suggest that considerable part of emitted SO2 will not be transformed to sulfate especially in the sub-cloud layer. Therefore, the production of climate affecting sulfate aerosol via aqueous phase transformation of dissolved SO2 is more limited than believed by climate modellers.