Evolution of particle concentration and size distribution observed upwind, inside and downwind hill cap clouds at connected flow conditions during FEBUKO

The concentration and size distribution of atmospheric particles were traced upwind, inside and downwind a hill cap cloud within the ground-based cloud passage experiment FEBUKO, which was carried out at the mountain ridge Thüringer Wald (Germany) during October 2001 and 2002. Three cloud events were examined in detail for which the connected flow between all three sites was meteorologically confirmed and the influence of entrainment and local sources on particle concentration and size distribution was only of minor importance. Modifications of the number size distributions between 60 and 300 nm particle diameter that are attributed to cloud processing were observed comparing upwind and downwind dry particle size distributions. Despite a small interference of droplet deposition, a maximum in-cloud mass production of 0.38 μg m−3 was found accounting for 5% of the upwind aerosol mass. The corresponding mass production rate was estimated to 1.16 μg m−3 h−1. A formation of new ultra-fine particles was detected in the outflow of the orographic clouds during nighttime yielding number concentrations up to 300 cm−3 at a pre-existing dry particle surface area of about 300 μm2 cm−3. At a more than twice as high particle surface area, i.e. when adsorption of condensable gases on existing particles became more important, no new particle formation was observed. Whereas the in-cloud mass production proceeded rather continuously throughout the cloud events, the particle production occurred during periods of 15 min up to 2 h within the cloud events. In the absence of actinic radiation, ternary nucleation of gaseous substances outgassing from the evaporating droplets at high relative humidity is hypothesized as the most likely particle formation mechanism but the reason for the temporary appearance of the particle production is not known.