Impact of partitioning and oxidative processing of PAH in fogs and clouds on atmospheric lifetimes of PAH

Abstract

The importance of the atmospheric aqueous phase of fogs and clouds, for the processing and removal of polycyclic aromatic hydrocarbons (PAHs) is not well known. A multiphase model was developed to determine the fate and lifetime of PAHs in fogs and clouds for a limited set of daytime conditions. The model describes partitioning between three phases (aqueous, liquid organic, and gas), experimental and estimated (photo)oxidation rates. Using a limited set of microphysical and chemical input conditions, the loss rates of PAHs in the complex threephase system are explored. At 25°C, PAHs with two, three and four rings were predicted to be primarily in the gas phase (fraction in the gas phase x g > 90%) while five-and six-ring PAHs partitioned significantly into droplets with aqueous phase fractions of 1 to 6% and liquid organic phase fractions of 31 to 91%, respectively. The predicted atmospheric chemical lifetimes of PAHs in the presence of fog or cloud droplets (< 8 hours) were significantly shorter than literature predictions of PAH lifetimes due to wet and dry deposition (1 to 14 days and 5 to 15 months, respectively) and shorter than or equal to predicted lifetimes due to chemical reactions in the gas and organic particulate phases (1 to 300 hours). Even though PAH solubilities are  4 ×10-2 g L-1 , the results of the current study show that often the condensed phase of fog and cloud droplets cannot be neglected as a PAH sink. When fogs and clouds are present, PAHs are found in the aqueous phase (e.g., BAP: ×10-7 g L-1 ; Herckes et al., 2002) despite the low water solubilities of PAHs (1.4 ×10-7 to 3.2 ×10-2 g L-1 ; Pearlman et al., 1984). Capel et al. (1991) proposed three mechanisms to account for PAH presence in fog water: dissolved organic compounds that act as co-solvents for PAHs, organic compounds acting as surfactants at the droplet surface or in colloids, and PAHs bound to scavenged particles. It was found that filtering fog water isolated most PAHs (Leuenberger et al., 1988) which lead Capel et al. (1991) to conclude that scavenged particles were the largest reservoir of PAHs in fog. Nonetheless, Capel et al. (1990) found that the surface tension in fog water is lower than in pure water, which is a manifestation of surfactant (surface film) and/or cosolvent behavior. Valsaraj (2004; 2009) demonstrated PAH adsorption to water surfaces and to surfactant-like organic matter on water surfaces (Donaldson and Valsaraj, 2010; Chen et al., 2011). There have been few atmospheric PAH multiphase (i.e., more than two phases) studies. Lei and Wania (2004) employed partitioning ratios to predict PAH distribution in clouds. Ehrenhauser et al. (2012) compared the observed PAH distribution in fog with simple predictions using partitioning ratios. While the phase distribution and reactions within a three-phase system are not well known for PAHs, partitioning ratios have been measured and estimated for octanol

Fossil Fuel-Derived Polycyclic Aromatic Hydrocarbons in the Taiwan Strait, China, and Fluxes across the Air–Water Interface

“…64 In addition, because of discontinuity and much shorter durations of rainfall, wet deposition is considered to be at least 10 times less contribution to PAHs in the air−water interface than dry deposition. 65 Therefore, examining the combined impacts of free air−water exchange and dry deposition (F a/w+d ), the transport fluxes of PAHs at the air− water interface in the central TWS showed significant differences with the JRDW and SCSWC (Figure 3d). In the central TWS, the transport of 3-and 4-ring PAHs across the air−water interface were dominated by volatilization and deposition processes, respectively, based on the F a/w values of these PAHs (Figure 3c,d).…”

“…In the JRDW, that was because of the local higher particulate PAH discharge in the atmosphere; but in the SCSWC, that could be due to the higher loadings of PAHs on the fine particulates with larger specific surface areas in the long-range transport of SCSWC . In addition, because of discontinuity and much shorter durations of rainfall, wet deposition is considered to be at least 10 times less contribution to PAHs in the air–water interface than dry deposition . Therefore, examining the combined impacts of free air–water exchange and dry deposition ( F a/w+d ), the transport fluxes of PAHs at the air–water interface in the central TWS showed significant differences with the JRDW and SCSWC (Figure d).…”

Anthracene and Pyrene Photooxidation Kinetics in Saltwater Environments

“…Polycyclic aromatic hydrocarbons (PAHs) are toxic molecules. These pollutants are ubiquitous in environmental and atmospheric aqueous phases including surface waters, fog and cloud droplets, and particulate matter. − Photooxidation is a common fate of PAHs in environmental and atmospheric waters. Since products are often even more toxic than the parent compounds, accurate photooxidation kinetics are needed in order to predict the health effects of PAHs .…”

Ozone Enhances the Inflammation Effects of PM2.5 Components Based on an Exposomic Approach in a Panel Study

“…In contrast, as the first layer at the lung/air interface, airway surface liquid (i.e., lung lining fluid above the membranes of the epithelial cells) can more easily host the process of oxidation of O 3 and CC bond-containing compounds due to its higher temperature (32–37 °C) compared with the temperature of air . Laboratory research has shown that the kinetic rate constants of aromatics, such as acenaphthene and phenanthrene, in the liquid phase increased by 3–4 orders of magnitude compared with those in the gas phase at 25 °C and pH 7. − Leveraging the neutral pH of EBC (7.9 ± 1.1) in the airways in our study, we found the relevant oxidation processes follow two major pathways: direct reaction with O 3 and indirect reaction with reactive oxygen species (e.g., hydroxyl radicals) produced through O 3 decomposition. Therefore, we speculate that inhaled active compounds, namely, aromatics and terpenoids, are predisposed to oxidation by O 3 , resulting in the formation of toxicologically functional groups (e.g., carbonyl groups) in the body .…”