The physicochemical characterization of gas and particle phase before, during and after clouds, chemical interactions of cloud droplets, gas and particle phase.

Clouds strongly impact phyical and chemical processes in the atmosphere. Studying cloud processes in a natural environment is challenging due to the altitude as well as the spatial and temporal variability of clouds. Ground-based hill-cap cloud experiments with associated upwind and downwind valley stations offer the possibility to physically and chemically characterise gas phase, aerosol particles, and cloud droplets before, during, and after the passage of an air mass through a cloud. This can help to better understand many different cloud processes. TROPOS is thus organising and leading such complex field experiments on aerosol-cloud interactions on a regular basis with international collaborations. These campaigns take place at Mount Schmücke in the Thuringian forest area, where a 20 m high measurement tower has been built for this purpose, which hosts many of the typically applied sampling techniques (Fig. 1). A multitude of measurement techniques is used to study gas phase, aerosol particles, and cloud water at the three campaign sites (Fig. 2). Interesting impacts of a cloud on physical and chemical properties of aerosol particles have been observed in these experiments:

• Clouds alter the chemical composition of aerosol particles. After their passage through a hill-cap cloud, aerosol particles were observed to possess a slightly different chemical composition. During some cloud events sulphate and organic material might be produced (from SO2 and organic precursors) in aqueous phase processes within the cloud.
  
  
• Clouds alter the physical properties of particles. After their passage through a cloud, aerosol particles were observed to be more hygroscopic, i.e. they take up water more easily and activate more quickly to cloud droplets in subsequent cloud formation processes. This likely relates to the modified chemical composition of the particles.
  
  
• Cloud droplets contain much higher concentrations of certain organic species (carbonyls, oxygenated volatile organic compounds) as expected based on the current understanding of the phase partitioning of these compounds. Possible reasons for this are adsorption of organics onto the droplet surface or even the formation of an organic film, which both could influence chemical processes in the droplets, as well as the exchange of further trace constituents between water and air.
  
  
• Clouds reduce the concentrations of important radical oxidants in the gas phase (OH and HO2) and thereby influence the oxidation capacity of the atmosphere. This was recently observed experimentally for the first time at the Schmücke.