Thirty-seven 2−3 day air samples were collected between September and December 1997 to investigate the factors controlling PCDD/F concentrations at a semirural site near Lancaster on the west coast of the U.K. ΣCl2-8DD/F (ΣTEQ) concentrations varied by a factor of 40, from 4.1 to 182 pg/m3 (5.5−220 fg ΣTEQ/m3) with a mean concentration of 37.2 pg/m3 (38 fg ΣTEQ/m3). Atmospheric PCDD/F concentrations depended strongly on air mass origin, with lowest ΣCl3-8DD/F concentrations coming from the west (off the Irish Sea) and higher ones arriving over land, suggestive of ongoing land-based sources. In marked contrast, Cl2DD/F levels were highest with westerly winds, suggestive of specific sources west of Lancaster. Most homologue groups and 2,3,7,8-substituted congeners showed a significant (at the P ≤ 0.05 level) inverse relationship with ambient temperature, consistent with domestic burning of coal/wood as an important source during colder periods. In contrast, the Cl2DD/Fs were positively correlated with temperature, rain, and wind direction. Principal components analysis, based on the relative homologue pattern of Cl3-8DD/Fs, discriminated few sample groups. Samples originating from the cleaner “ocean sector” were weathered and relatively depleted in PCDDs. An individual sample, taken around “Bonfire night” (a traditional festival), was dominated by Cl7/8DDs. However, the majority of samples was clustered together, regardless of air mass origin. It is hypothesized that diffuse land-based sources principally supply Cl3-8DD/Fs to the atmosphere and that their long atmospheric residence times mean that the PCDD/F mixture is only slowly altered during air mass movement.