Ultrafine particles in the atmosphere are key factors for aerosol cloud interaction as they affect cloud droplet size distributions, latent heat transport into elevated layers via droplet evaporation and precipitation properties via delayed raindrop generation and possibly invigoration of torrential rains. They are spatially and temporarily highly uneven distributed, suggesting the presence of strong sources either for primary particle emissions or for particle precursor materials. Airborne investigations now allowed to identify major anthropogenic primary particle sources, their strength and contribution to the ultrafine particle budget. Current industrial flue gas cleaning technologies favor the production of nucleation mode aerosol by gas to particle conversion which is emitted into mid elevations of the planetary boundary layer, transported over hundreds of km and vertically mixed by thermal convection on different time scales. These sources also emit a suitable mixture of sulphur and nitrogen compounds and ammonia, key components for aerosol nucleation from gas to particle conversion. Meteorological transport and the results of three dimensional in situ measurements explain both the spatial and temporal patterns of number and size distributions of ultrafine nucleation mode particles observed. ... mehrBudget studies allow a comparison with recent emission szenarios. Results from clean and polluted environments and the potential impact of these ultrafine particles on the hydrological cycle will be discussed.