Multiphase chemistry and acidity of clouds at Kleiner Feldberg

The chemistry of cloud multiphase systems was studied within the Kleiner Feldberg Cloud Experiment 1990. The clouds encountered during this experimental campaign could be divided into two categories according to the origin of air masses in which the clouds formed. From the chemical point of view, clouds passing the sampling site during the first period of the campaign (26 October-4 November) were characterized by lower pollutant loading and higher pH, as compared to clouds during the final period of the experimental campaign (10–13 November). The study of multiphase partitioning of the main chemical constituents of the cloud systems and of atmospheric acidity within the multiphase systems themselves (gas + interstitial aerosol + liquid droplets) are presented in this paper. A general lack of gaseous NH3 was found in these cloud systems, which caused a lack of buffer capacity toward acid addition. Evidence supports the hypothesis that the higher acidity of the cloud systems during this final period of the campaign was due to input of HNO3. Our measurements, however, could not determine whether the observed input was due to scavenging of gaseous HNO3 from the air feeding into the cloud, or to heterogeneous HNO3 formation via NO2 oxidation by O3 to NO3 and N2O5. Sulfate in cloud droplets mainly originated from aerosol SO 4 2− scavenging, since S(IV) to S(VI) liquid phase conversion was inhibited due to both lack of H2O2 and low pH of cloud droplets, which made O3 and metal catalyzed S(IV) oxidation inefficient.