The chemical processes that take place in the atmosphere occur in various phases, namely the gas, liquid or particle phase. These reactions within the multiphase system are of crucial importance for the transformation and degradation of trace gases, as well as for the formation of new particles in the atmosphere. Radicals such as hydroxyl (OH) and peroxy radicals play a central role in this. These are primarily generated by photochemical processes and act as ‘cleaning agents’ for the atmosphere, as they oxidise organic trace substances.
In the gas phase, the oxidation of volatile organic compounds (VOCs) leads to the formation of less volatile products, which can subsequently transfer to the liquid phase or attach themselves to existing aerosols, potentially resulting in the secondary formation of organic aerosols (Secondary Organic Aerosols, SOA).
In the liquid phase, for example in small droplets from fog or clouds, reactions can take place that would either not occur at all or proceed only very slowly in the gas phase. Water-soluble organic compounds are oxidised by dissolved oxidising agents, such as hydrogen peroxide or radicals.
In addition to the reactions within these phases, reactions at their interfaces also play a crucial role. Mass transfer takes place at these interfaces, and the reactions there are often more efficient than pure gas-phase or liquid-phase reactions, as the reactants are locally concentrated there.
These processes significantly alter the chemical composition of the atmosphere and thus influence air quality, climate and human health. To improve our understanding of tropospheric multiphase processes, numerous laboratory experiments are available at TROPOS within the Atmospheric Chemistry Department. Depending on the research questions of the various projects, processes taking place in the gas, liquid or particle phases themselves are investigated.
Laboratory equipment such as flow reactors or Knudsen cells can be used to investigate phase transfer processes of gaseous compounds at the interface between the gas phase and the liquid, solid or particle phase.
Using experiments in liquid-phase bulk reactors as well as in laser and lamp photolysis apparatus, radical reactions of radicals relevant to the atmosphere and excited states, as well as their kinetics, are investigated. For example, •OH, NO3• and SOx•, as well as triplet states of organic compounds, are examined. In addition, non-radical reactions of oxidising agents, such as ozone and hydrogen peroxide, in the aqueous phase are investigated. Numerous analytical methods, such as EPR spectroscopy and other offline techniques, are available for the identification and quantification of the reaction products.