The distribution of air pollution in urban environments is greatly influenced by its source and the complexity of the urban environment, which determine the pollution concentrations in individual locations. Traffic is one of the main pollution sources in cities and, especially particulate matter (PM) poses a serious risk for public health. This study applies magnetic biomonitoring using moss bags to establish an extensive urban canopy layer monitoring network (n = 52) to study the distributions and concentrations of magnetic PM. Moss bags were placed at street and roof levels within the grid plan area of Turku, Finland, for 62 days in late autumn. Samples were analysed for mass-specific magnetic susceptibility (χ), hysteresis parameters and elemental components. At street level, traffic was the primary source of magnetic PM and related elements. At higher altitudes, these concentrations were reduced and mainly replaced by crustal elements. Magnetic measurements were also applied to test for equivalence with the CAR-FMI dispersion model and to assess the representativeness of an air quality monitoring station in the city centre. Inconsistencies were observed between the model and magnetic PM concentrations at street level. At rooftop level, the model estimates were higher than indicated by the magnetic PM. The magnetic measurements complemented measurements derived from supplementary monitoring sensors and Turku monitoring station. The representativeness of the monitoring station should be re-evaluated, and recategorizing should be considered. Magnetic biomonitoring using moss bags is an unparalleled tool for monitoring airborne PM pollution and a reliable tool for evaluating monitoring stations and dispersion model performance.