Solutes can greatly affect pollutant photodegradation kinetics in atmospheric aqueous phases such as surface waters, atmospheric aerosols, and cloud and fog droplets. We have measured photooxidation rate constants of the polycyclic aromatic hydrocarbons (PAHs) pyrene and anthracene in aqueous solutions containing environmentally relevant concentrations of halide salts (NaCl, NaBr, and NaI) and in seawater. Chloride, bromide, and iodide did not affect pyrene photodegradation kinetics but increased anthracene photodegradation rate constants at low halide concentrations. The largest anthracene rate constant measured, in the presence of 0.2 M NaCl, was 3.4 times larger than that in deionized water. Smaller enhancements were observed in the presence of NaBr and NaI, with maximum rate constants observed at concentrations of 1 × 10 −5 and 1 × 10 −8 M, respectively. We determined that this enhancement was due to singlet oxygen ( 1 O 2 ) generation resulting from interactions between halides and electronically excited anthracene. Interactions between pyrene and halides also formed 1 O 2 , but this did not affect pyrene's observed photodegradation rate constant. At higher halide concentrations, anthracene photodegradation rate constants showed a weak negative dependence on halide concentration, likely due to quenching of anthracene's excited triplet state. Our results suggest that the fates of some PAHs in saline environments could differ from those predicted by kinetics measured in deionized water in the absence of solutes.
Heterogeneous processes can control atmospheric composition. Snow and ice present important, but poorly understood, reaction media that can greatly alter the composition of air in the cryosphere in polar and temperate regions. Atmospheric scientists struggle to reconcile model predictions with field observations in snow-covered regions due to experimental challenges associated with monitoring reactions at air-ice interfaces, and debate regarding reaction kinetics and mechanisms has persisted for over a decade. In this work, we use wavelength-resolved fluorescence microscopy to determine the distribution and chemical speciation of the pollutant anthracene at the surfaces of environmentally relevant frozen surfaces. We show that anthracene adsorbs to frozen surfaces in monomeric form, but that following lateral diffusion, molecules ultimately reside within brine channels at saltwater ice surfaces, and in micron-sized clusters at freshwater ice surfaces; emission profiles indicate extensive self-association. We also measure anthracene photodegradation kinetics in aqueous solution and artificial snow prepared from frozen freshwater and saltwater solutions and use the micro-spectroscopic observations to explain the rate constants measured in different environments. These results resolve long-standing debates and will improve predictions of pollutant fate in the cryosphere. The techniques used can be applied to numerous surfaces within and beyond the atmospheric sciences.
Cleaning surfaces with sodium hypochlorite (NaOCl) bleach can lead to high levels of gaseous chlorine (Cl2) and hypochlorous acid (HOCl); these have high oxidative capacities and are linked to respiratory issues.
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