Benzo[a]pyrene (BaP), a key polycyclic aromatic hydrocarbon (PAH) often associated with soot particles coated by organic compounds, is a known carcinogen and mutagen. When mixed with organics, the kinetics and mechanisms of chemical transformations of BaP by ozone in indoor and outdoor environments are still not fully elucidated. Using direct analysis in real-time mass spectrometry (DART-MS), kinetics studies of the ozonolysis of BaP in thin films exhibited fast initial loss of BaP followed by a slower decay at long exposure times. Kinetic multilayer modeling demonstrates that the slow decay of BaP over long times can be simulated if there is slow diffusion of BaP from the film interior to the surface, resolving long-standing unresolved observations of incomplete PAH decay upon prolonged ozone exposure. Phase separation drives the slow diffusion time scales in multicomponent systems. Specifically, thermodynamic modeling predicts that BaP phase separates from secondary organic aerosol material so that the BaP-rich layer at the surface shields the inner BaP from ozone. Also, BaP is miscible with organic oils such as squalane, linoleic acid, and cooking oil, but its oxidation products are virtually immiscible, resulting in the formation of a viscous surface crust that hinders diffusion of BaP from the film interior to the surface. These findings imply that phase separation and slow diffusion significantly prolong the chemical lifetime of PAHs, affecting long-range transport of PAHs in the atmosphere and their fates in indoor environments.

In urban environments, vehicle exhaust and nonexhaust emissions represent important sources of fine particulate matter with an aerodynamic diameter less than 2.5 μm (PM2.5), which plays a central role in adverse health effects and oxidative stress. We collected PM2.5 filter samples from two highway sites (Anaheim and Long Beach, CA) and an urban site (Irvine, CA) to quantify environmentally persistent free radicals (EPFRs) contained in PM2.5 and the generation of radical forms of reactive oxygen species (ROS) in water using electron paramagnetic resonance spectroscopy. The EPFR concentrations were 36 ± 14 pmol m–3 at highway sites, which were about two times higher than those at the urban site. EPFRs correlate positively well with CO, NOx, and elemental and organic carbon, indicating that EPFRs are emitted from vehicular exhaust. Good correlations of EPFRs and Fe and Cu may indicate that EPFRs are stabilized by Fe and Cu emitted from tire and brake wears. EPFRs are negatively correlated with ozone, suggesting that photochemistry does not play a large role in the formation of EPFRs and possibly also indicating that EPFRs are quenched by ozone. Highway PM2.5 are found to generate mainly OH and organic radicals in the aqueous phase. The generated ROS are correlated with PM2.5 mass concentrations and OH radicals show a good correlation with EPFRs, implying the role of EPFRs in aqueous OH radical generation. The PM2.5 oxidative potentials as quantified with the dithiothreitol (DTT) assay are correlated with ROS, OH, and organic radicals for PM2.5 collected in Anaheim, whereas little correlations are observed for Long Beach. These findings highlight the interplay of various PM redox-active chemical components and complex relationship between ROS formation and DTT activity.

Wildfires, which have been occurring increasingly in the era of climate change, emit massive amounts of particulate matter (PM) into the atmosphere, strongly affecting air quality and public health. Biomass...

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