Ultrafine particles (UFPs) are distributed highly unevenly in the lower troposphere. Although these UFPs are positively detectable and have been studied for more than a century, their three-dimensional distribution, formation, and budget in the atmosphere remain largely uncertain, despite their obvious climate relevance. This is due to their short lifetime and the fact that they are invisible to the human eye and to remote sensing techniques. From the moment of their emission or generation, their spatial distribution is a result of meteorological processes, regional-scale transport, local thermal convection, and rapid loss by interaction with clouds as cloud condensation nuclei. Here, we report about three-dimensional airborne in situ studies aimed at investigating UFP sources, distribution, and behavior on different spatial and temporal scales. We identified fossil fuel–burning power stations, refineries, and smelters as major anthropogenic UFP sources. On a regional scale, their emissions are significantly higher than urban emissions. Particle emissions from such power stations are released typically at altitudes between 200 and 300 m AGL. Detailed in situ measurements of particle concentration and related parameters, together with meteorological measurements and analyses, enable reliable source attribution even over several hundred kilometers downwind from the emitter. Comprehensive meteorological analysis is required to understand the highly variable 3D concentration patterns generated by advective transport and thermal convection. Knowledge of primary emission strength, together with size distributions and atmospheric 3D transport of UFPs derived from airborne measurements, makes it possible to estimate the aerosols’ impact on meteorology, hydrological cycles, and climate.