Evaluation of atmospheric PCDD/F depositions via automated and traditional water surface samplers in Taiwan.

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are a group of compounds of major environmental concern. Once emitted into the atmosphere, PCDD/Fs undergo photochemical reactions and enter other environmental compartments via wet and dry deposition. In this study, atmospheric PCDD/F depositions were collected via an automated PCDD/F deposition sampler and traditional cylindrical vessels, respectively, in northern, central, and southern Taiwan from 2008 to 2010. The automated PCDD/F precipitation sampler used in this study can prevent both resuspension and photodegradation of the PCDD/Fs collected and also effectively separates the PCDD/F samples into dry and wet contributions. The results indicate that the average atmospheric PCDD/F concentrations collected by the high-volume sampling trains were 13.6 ± 10 (n = 10), 15.6 ± 5.2 (n = 7), and 10.9 ± 6.3 (n = 6) fg I-TEQ/m(3) in northern, central, and southern Taiwan, respectively. In addition, the results also indicate that the PCDD/F deposition flux collected with an automated PCDD/F sampler (1.84 ± 0.90-8.68 ± 5.1 pg I-TEQ/m(2)/day, n = 23) is significantly higher than that sampled with cylindrical vessels (1.11 ± 0.69-5.64 ± 5.2 pg I-TEQ/m(2)/day, n = 23). Based on the Mann-Whitney statistical analysis, the p value (0.037) of PCDD/F deposition flux between those two samplers measurement is lower than 0.05. The difference is attributed to the fact that part of the PCDD/F depositions collected by traditional cylindrical vessels is photodegraded and revolatilized. In addition, the wet deposition flux of PCDD/Fs (3.66 to 470 pg I-TEQ/m(2)/rainy day, n = 23) observed in Taiwan is significantly higher than the dry deposition flux (0.38 to 4.55 pg I-TEQ/m(2)/sunny day, n = 23). The results demonstrate that the wet deposition is the major PCDD/F removal mechanism in the atmosphere. Furthermore, the overall PCDD/Fs deposition velocity and scavenging (rainout) coefficient in Taiwan are calculated as 0.20 ± 0.07 cm/s and 6.5 ± 0.2 × 10(4), respectively.