The winter effect on formation of PCDD/Fs in Guangzhou by vehicles: A tunnel study

Prior studies showed that the polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) concentrations in the atmosphere are much higher in the winter than in the summer. This so called winter effect was explained via meteorology-dependent factors such as dispersion, mixing and photo chemical degradation or home heating related formation of PCDDs/Fs. In this study, we took vehicle emission as an example to investigate winter effect on PCDD/Fs formation by fossil fuel combustion. We hypothesized that vehicle emission of PCDDs/Fs may be elevated in the winter season due to the promoted supplies of Cl− (via particular matter) in winter. We collected particulate and gaseous samples from the Pearl River Tunnel and its adjacent open air during spring/summer and winter seasons. Chemical analyses of the tunnel samples showed that the PCDD/F concentrations in the tunnel ranged from 18.6 to 20.4 pg m−3 (1.28–1.39 pg I-TEQ m−3) in the winter, which were 3–5 times higher than in the spring/summer. In the open atmosphere adjacent to the tunnel, the PCDD/F concentrations were much lower than in the tunnel; e.g., approximately one fifth of the tunnel air concentrations during the winter. The emission factors (EFs) calculated based on the tunnel data were 3440 (or 230 I-TEQ) and 1580 (or 27.8 I-TEQ) pg km−1 vehicle−1 in winter and spring/summer season, respectively. The much higher PCDD/F concentrations in the tunnel air and much greater EF value during the winter are likely related to higher content of Cl− associated with small size particulates. This suggests that the winter effect observed in the open atmosphere is not only caused by meteorology-dependent factors and home heating, but also may partly results from much greater PCDD/F formation rates during the combustion processes of fossil fuels such as gasoline- and diesel-fuel in the winter.