Wednesday, August 25, 2021 - 1:00pm

Abstract: Reactive oxygen species (ROS) including hydroxyl radical (OH·), superoxide anion (O2·-), ozone (O3) and oxygenated organic radicals play an important role in atmospheric and physiological processes. Polycyclic aromatic hydrocarbons (PAHs), including benzo[a]pyrene (BaP), are among the most prominent toxic compounds that can be found in indoor and outdoor environments. Their chemical lifetimes are highly determined by ROS such as OH· and O3, but the chemical degradation mechanism and kinetics of PAHs against these ROS remain to be elucidated.

Emerging health-related studies suggest that inhaled ambient particles serve as exogenous ROS sources. The ROS generated from the particles can potentially disturb the physiological functions in our body which can lead to oxidative stress and adverse health effects.  Recent advancement in analytical techniques such as electron magnetic resonance (EPR) spectrometer provided additional information on the type of ROS that can be generated from ambient particles. Past studies reported environmentally persistent free radicals (EPFRs) can be found in car exhaust and biomass burning particles. These particles containing EPFRs have been shown to generate ROS in aqueous solution, but the EPFR measurements in ambient particles are still limited.

In chapter 2 and 3 focus on the EPFR measurements and the type of ROS detected from aqueous extract of particulate matter (PM) collected at two different highway sites and during the wildfire events using EPR spectrometer. The OH· and carbon-centered organic radical formation have been detected in aqueous extracts of highway particle. In contrast, OH·, O2·-, carbon and oxygen-centered organic radicals have been found to be generated from wildfire PM1 and PM10 depending on the wildfire events that occurred in Southern California. The OH· and carbon-centered organic radical were detected in aqueous extract of coarse particles collected from wildfire events, which demonstrates the redox-active chemical components in wildfire PM are size-dependent. Chapter 2 discusses the correlations between the measured EPFR, ROS, and traffic-related pollutants. The chemical and EPFR measurements at the Long Beach highway (Interstate-710) show positive correlations between EPFR in highway fine PM and CO, NO2, and elemental carbon (EC), which are pollutants typically found in exhaust emissions.  The negative correlation between EPFR and O3 was also observed at the Long Beach, which suggests EPFR is most likely emitted from primary source at the highway. Positive correlation between EPFR and OH· generated from the highway fine PM was also observed at the Long Beach suggesting OH· are generated from similar source as EPFR. Lastly, the toxicity of highway particle was assessed using DTT (dithiothreitol) activity and explored correlation with ROS. High positive correlation between DTT activity and ROS was observed at the Anaheim highway (Interestate-5). Chapter 3 also investigates size-dependent EPFR and ROS measurements. EPFR was mainly found in wildfire PM1 and found to be approximately 10 times higher compared to the highway and urban background. O2·- was also found to be size-dependent and only detected in yellow-brownish extracts of wildfire samples suggesting redox-active chemical components in brown carbon may play a role in O2·- formation. These findings highlight the interplay of various PM redox active chemical components and the complex relationship between ROS formation and DTT activity.

Ozone (O3) is a major oxidant inducing chemical transformation of organic compounds in the atmosphere and organic films found in indoor environments. Chapter 4 discusses the ozonolysis of BaP in multicomponent thin films that can be found in indoor environments and in the atmosphere. Rapid degradation of BaP in thin films against O3 was initially observed but found to decay much slower at longer exposure time in direct analysis in real-time mass spectrometry (DART-MS) kinetic experiments. The kinetic multilayer modeling revealed that the diffusivity of BaP from bulk to the surface limits the decay of BaP, resolving long-standing unresolved observations of incomplete PAH decay upon prolonged ozone exposure. The chemical kinetics of BaP in alpha-pinene secondary organic aerosol (SOA) material against O3 under dry and humid conditions were also explored. The thermodynamic modeling predicted BaP is immiscible in the SOA mixtures and these results were implemented in the kinetic model. The kinetic model simulations showed that the slow decays of BaP in SOA mixtures under dry and humid conditions are also dependent on the bulk diffusivity of BaP in immiscible BaP layer upon prolonged ozone exposure. Lastly, BaP was found to be miscible in organic oils such as squalene, linoleic acid, and cooking oil based on visual inspection and thermodynamic model. However, the kinetic and thermodynamic model results show oxidation products forms a viscous crust in the organic films and hinders diffusion of BaP from the film interior to the surface. These findings demonstrate that phase separation and slow diffusion play a key role in the long-range transport of PAHs in the atmosphere and their fates in indoor environments.

Speaker: 

Brian Hwang

Institution: 

Shiraiwa Group

Location: 

RH 390