Multiphase chemistry of orographic clouds: observations at subalpine mountain sites

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Observations out of two cloud events at elevated sites in the Black Forest/Germany and in the Vosges Mtn./France, are presented. The cloud-water, the particulate matter, and the gas-phase have been characterized chemically. Besides main ions, both inorganic and organic, the data sets include 17 trace elements. The air masses pollutant loadings indicated strong anthropogenic influence. The dominant cloud-water solutes were ammonium, nitrate and sulfate, as well as acidity. The latter was even more important than ammonium to counterbalance the strong acid anions in cloud-water collected at the Vosges Mtn. site. Acidity reached up to pH 2.8 in that case, whereas at the Black Forest site pH of cloud-water samples was between 4.9 and 4.0. Organic acids contributed a substantial part to cloud-water acidity at the Vosges Mtn. site, 11% by average. The fractions of the trace species incorporated into cloud-water, have been estimated based on cloud-water and aerosol loadings and differences have been identified. In particular, in one cloud a different behaviour of nitrate and sulfate was indicated in such a way, that a smaller fraction of sulfate was incorporated. The interstitial aerosol was enriched in carbon when compared to pre-cloud and after-cloud situations. Most of the elements specified, in particular those, which are thought to be dominated by crustal sources, were incorporated into the cloud-water to a higher extent, when compared to carbon. An increase of the free acid content of particulate matter (on a mass per volume of sampled air basis) was observed in both after-cloud situations when compared to the respective pre-cloud situations. No information is available, however, about the relative significance of transport from the source regions to the sites and source strength on one hand, and microphysical and chemical processes in the multiphase system on the other hand, which both may have contributed to changes in aerosol composition. When collecting cloud-water simultaneously with a pair of identical collectors beneath and above a conifer canopy, a higher liquid water content was found above the canopy, whereas higher concentrations of most of the solvents were found in cloud-water beneath the canopy.