An outline is presented here of the Po Valley Fog Experiment 1989, carried out within the EUROTRAC‐GCE project. This experiment is a joint effort by several European research groups from 5 countries. The physical and chemical behaviour of the fog multiphase system was studied experimentally following the temporal evolution of the relevant chemical species in the different phases (gas, droplet, interstitial aerosol) and the evolution of micrometeorological and microphysical conditions, from the pre‐fog situation through the whole fog evolution, to the post‐fog period. Some general results, useful for describing the general features of the fog system, are presented here, while specific scientific questions on the different processes taking place within the system itself will be addressed in other companion papers of this same issue.
Samples of atmospheric aerosols, collected with cascade impactors in the urban area of Vienna (Austria) and at a coastal site on the North Sea, were investigated for black carbon (BC) as the main component of absorbing material and for mass. The size distributions are structured. The BC distributions of these samples show a predominant mode, the accumulation aerosol, in the upper submicron size range, a less distinct finer mode attributable to fresh emissions from combustion sources, and a distinct coarse mode of unclear origin. It is important to note that some parameters of the accumulation aerosol are related statistically, indicating the evolution of the atmospheric accumulation aerosol.
Simultaneous measurements of several non‐volatile species in unscavenged aerosol particles and in fog droplets have revealed differences in partitioning for different chemical species. The average scavenged fraction of sulphate was 18% and the corresponding fraction of elemental carbon was only 6%. This suggests that the aerosol was externally mixed, and that the chemical mixture of the aerosol as a function of size is important in the context of nucleation scavenging. The measurements obtained could not distinguish between the two primary hypotheses for explaining the observed differences, (a) that the particles had the same size distribution and their chemical composition was the controlling factor, and (b) that the elemental carbon was associated with smaller particles than the sulphate, so that the difference in scavenging efficiency was controlled by the size distribution of the particles.
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