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Seoul, the capital of South Korea, is one of the largest megacities in northeast Asia. Previous studies have monitored atmospheric polycyclic aromatic hydrocarbons (PAHs) in Seoul, but few have produced highly spatially resolved data. In this study, a total of 144 polyurethane foam-passive air samplers (PUF-PASs) were deployed at 36 sites (4 road, 26 urban, and 6 background sites) in Seoul during the summer of 2016 and the winter of 2017. The concentrations of Σ 13 PAHs fell within a range of 2.6–49.9 ng/m3 in summer and 15.5–77.8 ng/m3 in winter. Low-molecular-weight PAHs with 3–4 rings were dominant (accounting for about 97% of all PAHs) at all sampling sites in both seasons compared to high-molecular-weight PAHs with 5–6 rings. Phenanthrene was the most abundant individual compound, accounting for 35–54% and 42–52% of the total PAHs in summer and winter, respectively. The spatial distribution of PAHs was determined by traffic, domestic heating, topography, transport, and meteorological conditions. The concentration of PAHs at the road sites was higher than at the urban and background sites, indicating that vehicular exhaust was a major source in both seasons. In addition, diagnostic ratios and principal component analysis revealed the influence of various sources, such as gasoline and diesel emissions and coal and biomass combustion. Backward air trajectory analysis highlighted the influence of long-range transport from northern China and North Korea during the winter, whereas the influence of local emissions and satellite cities appeared to dominate during the summer. In conclusion, passive air sampling was successfully employed to identify major sources of PAHs in an Asian megacity, indicating that it could produce data with a high spatial resolution for use in human risk assessment and air monitoring networks. • Passive air sampling was conducted at 36 sites in Seoul, Korea in summer and winter. • The mean concentration of PAHs in winter was higher than that in summer. • Road sites exhibited higher levels of PAHs than urban and background sites. • The influence of long-range transport was evident during the winter. • The influence of local emissions and satellite cities was dominant during the summer. [ABSTRACT FROM AUTHOR]