Distribution of polychlorinated dibenzo-P-dioxins and dibenzofurans (PCDD/Fs) and polycyclic aromatic hydrocarbons (PAHs) within the full size range of atmospheric particles

Particle size distributions of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polycyclic aromatic hydrocarbons (PAHs) were measured at a rural location in northeastern Bavaria during summer. The complete size range of the atmospheric particles was sampled using cascade impactors and a rotary impactor. On an average 92% of the hepta- and octachlorinated PCDD/Fs and 97% of the PAHs with five or more rings, which were all more or less completely particle bound in the atmosphere, were associated with particles with aerodynamic diameters (dae)<2.9 μm. The substance loading decreased steadily with increasing particle size. On average only 3.4 and 1.2% of the Cl7–8DD/F and the PAHs with five or more rings, respectively, were found on large particles with dae>8.6 μm. The temporal variation of the size distributions was low. Despite similar measured size distributions the washout ratios of the PCDD/Fs and PAHs calculated using simultaneously collected precipitation samples were different, with those of the PAHs lying consistently lower than those of the PCDD/Fs. This may have been due to differences in the particle size distributions which were not resolved with the impactors used. In the case of the semivolatile compounds for which significant fractions were present in the gas phase, 60–95% of the particle bound substance mass was always found on particles with dae<2.9 μm. However, the concentrations did not decrease continuously with increasing particle size. Rather, increased shares were found in the particle size range 0.95–2.9 μm for the PAHs and 8.6–26 μm dae for both the PCDD/Fs and the PAHs. It is proposed that the secondary maxima in the 8.6–26 μm size range are attributable to resuspended biogenic material that had accumulated the contaminants from the gas phase, whereas the high levels of the semivolatile PAHs in the 0.95–2.9 μm size range were due to combustion source characteristics.