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

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 three-phase system are explored. At 25 °C, PAHs with two, three and four rings are predicted to be primarily in the gas phase (fraction in the gas phase x_g > 90%) while five- and six-ring PAHs partition significantly into droplets with aqueous phase fractions of 1–6% and liquid organic phase fractions of 31–91%, respectively. The predicted atmospheric chemical lifetimes of PAHs in the presence of fog or cloud droplets (<8 h) are significantly shorter than literature predictions of PAH lifetimes due to wet and dry deposition (1–14 days and 5–15 months, respectively) and shorter than or equal to predicted lifetimes due to chemical reactions in the gas and organic particulate phases (1–300 h). 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.