Diel (24-h) variations of concentrations of polychlorinated biphenyls (PCBs) in air are reported at two sites in Zurich, Switzerland, a city surrounded by hills. One site was located in the valley near the city center and the other site was on a hill called Uetliberg, 411 m higher and about 5 km distant from the city center site. Air samples were collected simultaneously at both sites over 4-h time periods for 3 consecutive days under stable meteorological conditions during a high pressure system in August 2007. PCB concentrations at the city site were markedly higher than those at Uetliberg, indicating that the city site is influenced by urban sources of PCBs. Concentrations measured at both sites show a clear diel cycle but have opposite phases: in the city concentrations were lower during the day and higher at night, while at Uetliberg concentrations were higher during the day and lower at night. These observations are explained and interpreted using a multimedia mass balance model that includes a stable night-time inversion layer that formed over the city but below the hilltop site. At Uetliberg the concentration of PCBs is consistent with background levels and the diel concentration pattern can be explained by temperature-mediated air−surface exchange and the influence of nearby woodland canopies. The diel pattern and concentrations in the city are attributable to volatilization from urban sources. We have developed a new method to estimate the strength of the urban PCB source using the model scenarios for the two sampling sites. Emission estimates derived from this method are in good agreement with earlier estimates derived from PCB production, consumption, and usage estimates. This study demonstrates the potential for estimating the strength of diffuse, regional sources of persistent organic pollutants (POPs) using a combined measurement and modeling approach. Such studies can provide important information to derive regional and national POPs emission inventories as required under the Stockholm Convention, and to quantify the effectiveness of actions to reduce POP emissions.