Abstract Consecutive 12 hour day–night air samples (∼500 m 3 each) were taken over 7 days at three land-based sites and an over-water site in coastal New Jersey (NJ) in July 1998, in a campaign designed to shed light on factors controlling ambient PCDD/F concentrations. The sampling sites were chosen to reflect contrasting environments: urban/industrial from the center of the New York (NY)–NJ metropolitan area (Liberty Science Center, LSC); coastal Atlantic (Sandy Hook, SH); suburban NJ (New Brunswick, NB); over-water in Raritan Bay (RB). Despite proximity to the major NY/NJ conurbation, ambient PCDD/F concentrations in the region were low compared to literature data for other urban locations. Mean ∑Cl 4–8DD/Fs and ∑TEQ (in fg/m 3) were: 1400 and 16 at NB; 1000 and 9.5 over RB; 880 and 8.5 at LSC; and 830 and 6.6 at SH. Di- and tri-chlorinated dibenzo- p-dioxins and -furans (PCDD/Fs) were also measured and dominated the ∑Cl 2–8DD/F concentrations. Air–water exchange calculations demonstrated the relative importance of Cl 2–3DD volatilizing from the Lower Hudson River Estuary for ambient concentrations (∼25% of advection), but was of minor importance for the other PCDD/Fs. The study provides evidence that advective transport, local inputs and atmospheric processes combine in a complex manner to control ambient PCDD/F concentrations. These processes generally dominate any local diurnal influence of OH-radical-mediated depletion, which we had hypothesized would be detectable by measuring higher night- and day-time concentrations. Rather, it is implied that changes in the mixed boundary layer height resulted in higher night- than daytime concentrations at the urban and coastal sites. A strong diurnal signal, dominated by the lower chlorinated dioxins and furans, was detected at the rural site (NB) during a period of lower wind speeds.