Das Projekt "Chemische Zusammensetzung und Prozesse in Wolken und Nebeln: Abhaengigkeit von der Partikel- und Troepfchengroesse" wird vom Umweltbundesamt gefördert und von Leibniz-Institut für Troposphärenforschung e.V. durchgeführt. A ground-based cloud experiment is to be conducted in November 1994 at the field station S Pietro Capofiume in the Po Valley to study the size dependence of chemical constituents in fog drops and aerosol particles. Four tasks have been singled out: 1) The size-dependent nucleation scavenging ratio of aerosol particles to be determined from the measured distribution of activated drops, interstitial particles and their residuals, using a system of differential mobility analysers; in parallel, the concentrations of soluble inorganic components and their size distribution in aerosol particles and droplets will be measured by means of cascade impactors and chemical analysis, 2) Gas-liquid phase partitioning of NH3, SO2 and organic acids and their anions in different aqueous phase size fractions, 3) Measurements of water-soluble Fe (II), Fe (III) and soluble iron in precursor and interstitial aerosol particles and in various size fractions of the aqueous phase, preferably as a function of time for long-lasting fogs, 4) The production of NO3 and HNO2 from NOX at high humidity and during fog events to differentiate between the two night-time oxidation pathways: I) Oxidation of NO2 by O3 to form NO3/N2O5 followed by their interaction with wet particles to form HNO3 and II) the heterogeneous reaction of NO2 with the aqueous surface of haze particles and fog drops to form HNO3 and HNO2 simultaneously: this process is expected to depend on particle and drop size.