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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.

Measurements of the scavenging efficiency of aerosol particles in fog are presented. The scavenging efficiency as a function of size for accumulation‐mode particles is presented, along with efficiencies for the total number, accumulation‐mode number, and accumulation‐mode volume. Particles below ca. 0.3 µm diameter were not efficiently scavenged in the fogs. The scavenging efficiency for accumulation‐mode particles showed two steps, indicating that the hygroscopic/hydrophobic nature of the aerosol appeared to have been a controlling factor in determining scavenging efficiencies. Observed changes in the aerosol size distributions are discussed in reference to the processes (i.e., in‐cloud scavenging, aqueous‐phase reactions) potentially influencing them.

Measurements of the scavenging efficiency of aerosol particles in fog are presented. The scavenging efficiency as a function of size for accumulation-mode particles is presented, along with efficiencies for the total number, accumulation-mode number, and accumulation-mode volume. Particles below ca. 0.3 µm diameter were not efficiently scavenged in the fogs. The scavenging efficiency for accumulation-mode particles showed two steps, indicating that the hygroscopic/ hydrophobic nature of the aerosol appeared to have been a controlling factor in determining scavenging efficiencies. Observed changes in the aerosol size distributions are discussed in reference to the processes (i.e., in-cloud scavenging, aqueous-phase reactions) potentially influencing them.

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