Comparison of compositions of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) in air and soil samples collected in Ishikawa

Dioxins are present as impurities in agrochemicals applied to the soil. To examine the possibility that dioxins end up in the atmosphere, we compare the homologue composition and some characteristic isomer distributions of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in air and soil samples collected at sites in Ishikawa Prefecture, Japan. Tetrachloro dibenzo-p-dioxin (TeCDD) was the most abundant PCDD homologue in the air samples, while octachloro dibenzo-p-dioxin (OCDD) was the most abundant PCDD homologue in the soil samples. However, at several site, the most predominant homologue in the soil sample was not OCDD but TeCDD. The mean ratio of TeCDDs to PCDDs in the air samples was significantly smaller in winter than in other seasons, but no such seasonal variation was observed in the distribution of PCDF homologues. The abundance of PCDF homologues was inversely related to the number of chlorine substitutions in the air samples. The contribution of each toxic isomer (2,3,7,8-chlorine-substituted) to the total toxicity equivalency quantity (TEQ) in the air samples tended to be intermediate between their contributions to the TEQs in the soil and cinder samples. This result suggested that the air samples were influenced by both soil/dust suspension and combustion. In order to clarify the contributions of agrochemicals to dioxins in the air, we focused on the ratios of several characteristic isomers that are indicators of agrochemicals (chloronitrophen: CNP and pentachlorophenol: PCP) to their homologues. Significant decreases in the ratios of the isomers to their homologues in the air samples were observed only in winter, probably as a result of suppression of dust suspension by the snow cover. Therefore, the contribution of combustion to the dioxin concentration in the air was thought to be relatively large in winter.