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 dibenz ofurans (PCDD/Fs) and polycyclic aromatic hydrocarbons (PAHs) were measured at a rural location in northeastern Bavaria during summer. The complete si ze range of the atmospheric particles was sampled using cascade impactors a nd 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 partic le bound in the atmosphere, were associated with particles with aerodynamic diameters (d(ae)) < 2.9 mu m. The substance loading decreased steadily wit h increasing particle size. On average only 3.4 and 1.2% of the Cl7-8DD/F a nd the PAHs with five or more rings, respectively, were found on large part icles with d(ae) > 8.6 mu m. The temporal variation of the size distributio ns was low. Despite similar measured size distributions the washout ratios of the PCDD/Fs and PAHs calculated using simultaneously collected precipita tion samples were different, with those of the PAHs lying consistently lowe r than those of the PCDD/Fs. This may have been due to differences in the p article size distributions which were not resolved with the impactors used. In the case of the semivolatile compounds for which significant fractions w ere present in the gas phase, 60-95% of the particle bound substance mass w as always found on particles with d(ae) < 2.9 mu m. However, the concentrat ions did not decrease continuously with increasing particle size. Rather, i ncreased shares were found in the particle size range 0.95-2.9 mu m for the PAHs and 8.6-26 mu m d(ae) for both the PCDD/Fs and the PAHs. It is propos ed that the secondary maxima in the 8.6-26 mu m size range are attributable to resuspended biogenic material that had accumulated the contaminants fro m the gas phase, whereas the high levels of the semivolatile PAHs in the 0. 95-2.9 mu m size range were due to combustion source characteristics. reser ved. (C) 1999 Elsevier Science Ltd. All rights reserved.