Final Activity Report Summary - PURAT (Particles in the urban atmosphere: behaviour of fine and ultrafine particles, their spatial variation, and relationships with local policy action) This project was concerned with exploring the spatial variability of fine (> 1 µm) and ultrafine (> 0.1 µm) particulates in urban atmospheric air. In the run-up phase of the project, quality assurance measures were developed for the field application of Particle size spectrometers (DMPS). Next, five specialised field experiments were conducted to collect urban aerosols at multiple points within the same city. The long-term experiment PURAT-0 in Leipzig established systematic differences in aerosol size distributions between an urban background and a roadside observation site. The distribution of aerosol particles in a narrow street canyon is a strong function of traffic density but also wind direction. During the experiment PURAT-1, ambient size distributions were collected at a total of eleven observation sites, revealing differences in ultrafine particle number concentration of up to two orders of magnitude within the same city. Particle number concentrations in the size range 100-400 nm correlated well between different urban background sites. However, the correlation degraded for ultrafine particles, and when correlations to roadside sites were considered. During the three-month experiment PURAT-2 in the vicinity of the Berlin urban motorway, concentrations of ultrafine particles were measured that range among the highest that have been reported in Germany. A three-dimensional emission and transport simulation confirmed the spatial patterns of aerosol dispersion around the motorway. Results from PURAT-3 and 4 in Leipzig, confirmed that the particle size distribution is far from spatially homogeneous in the microscale environment around a densely built-up street canyon. The general result is that the number of ultrafine particles tends to be far less homogeneously distributed than particle mass concentrations. The data collected will be of considerable importance for future works, such as an assessment of urban residents' exposure to ultrafine particles, the determination of vehicle emission factors as well as the validation of urban microscale dispersion models.