Gas-particle partitioning and air-water exchange of polycyclic aromatic hydrocarbons in the Three Gorges Reservoir, southwest China

Abstract

A 30 m vertical drawdown water level fluctuation zone (WLFZ) was formed annually due to seasonal water impoundment in the Three Gorges Reservoir (TGR) in Southwest China. In this study, a super site in downstream of WLFZ, ∼200 km off the Three Gorges Dam, was used to investigate the seasonally gas-particle partitioning and air-water exchange of USEPA 16 polycyclic aromatic hydrocarbons (16 PAHs). The average concentrations of 16 PAHs in the particle phase were 66.63 ± 9.15 ng/m3 in winter and 8.43 ± 2.95 ng/m3 in summer. In the gas phase, they were 28.47 ± 4.79 ng/m3 in winter and 10.57 ± 1.51 ng/m3 in summer. In the dissolved phase of surface water, they were 38.65 ± 6.37 ng/L in winter and 56.53 ± 8.86 ng/L in summer. The logarithmic gas-particle partitioning coefficient (lgKp) was negatively correlated with the logarithmic subcooled liquid vapor pressure (lgPL0). While the lgKp was positively correlated with logarithmic octanol-air distribution coefficient (lgKOA). These two regressions both indicated un-equilibrium of gas-particle partitioning of PAHs in the atmosphere. Applying “Whitman two-film resistance model” to our datasets shows that 3-ring PAHs had a net volatilization from water to air (2.74–6.43 ng/m2/d), and 4 ∼ 5-ring PAHs favored deposition from air to water (−0.614 to −0.413 ng/m2/d). The water as a potential ‘source’ for 3-ring and a ‘sink’ for 4 ∼ 5-ring PAHs was thereby revealed. The results of this study are crucial for understanding atmospheric gas-particle partitioning of PAHs and for revealing the factors and mechanisms governing their geochemical cycling at air and water interfaces.