Partitioning of polychlorinated dibenzo-p-dioxins and dibenzofurans between the atmosphere and corn

Polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) are lipophilic atmospheric contaminants that accumulate in vegetation, a process which may lead to the contamination of food. Thus, knowledge of PCDD/F accumulation in vegetation is crucial to evaluating human exposure, in an effort to determine this accumulation mechanism, we analyzed matched atmosphere, corn kernel, and corn leaf samples from a private farm in Felicity, OH. Corn is an important feed crop in the United States; both corn kernels and corn leaves (silage) a re widely used. PCDD/F concentrations were bel ow the detection limits (0.5-1 pg/g lipids) in the corn kernels. However, we found PCDD/F concentrations of 510 +/- 75 fg/m(3), 1300 +/- 300 fg/m(3), and 4.2 +/- 1.2 ng/g lipids in the atmospheric gas phase,atmospheric particle phase, and corn leaves, respectively. For each set of air and corn leaf samples, we calculated corn leaf-atmosphere partition coefficients for both the atmospheric gas and particle phases, K-v,K-g and K-v,K-p respectively. On the basis of plots of the natural logarithm of these partition coefficients versus the reciprocal of the average, atmospheric temperature, we conclude that the partitioning of PCDD/F between the atmosphere and corn leaves is dependent on temperature. We calculated the enthalpies of phase change between the vegetation and air for all PCDD/F homologues; our values are comparable to literature values for polycyclic aromatic hydrocarbons and polychlorinated biphenyls. The ratio of K-v,K-p to K-v,K-g is a log-log function of the average vapor pressure for each PCDD/F homologue, indicating that vapor pressure plays an important role in the partitioning of both the gas and particle phases to corn leaves.