Towards a mechanistic understanding of particulate matter redox cycling at the molecular level
Particulate matter (PM) is a toxic component of air pollution that has been linked to asthma and other respiratory illnesses. Current environmental regulations limit the concentration of PM allowed in the air. But such regulations treat all PM to be created equally and is problematic because PM is chemically complex in reality. PM contains a variety of chemicals from organic compounds to transition metals. Furthermore, the composition of PM changes over time as particles age in the atmosphere.
Evidence from toxicology and epidemiology studies suggests there are links between PM composition and its toxicity in the human body. For example, the risk of cardiovascular disease and respiratory illnesses correlates with higher concentrations of elemental carbon, nickel and vanadium in the body.
The toxicity of PM may be linked to its redox activity, a chemical property of its molecular constituents. Certain molecules within the PM can cause oxidative stress— an imbalance of oxidants and antioxidants— in living cells.
In his talk, McWhinney discussed the redox activity of organic and combustion-derived particles and what these findings suggest about PM toxicity. In the first set of experiments, McWhinney evaluated the redox activity of exhaust particles from a two-stroke engine. He found that oxidizing the exhaust with ozone increases the redox activity of emission particles, and the redox activity is dependent on the amount of ozone rather than particle mass.
Results from the second set of experiments show the redox activity of the secondary organic aerosol particles cannot be determined just by measuring the concentration of the hypothesized atmospheric oxidation products (i.e. quinones) and other redox active species must also be present.
In the third set of experiments, McWhinney investigated diesel exhaust particles (DEP) emitted from motor vehicles. DEP were compared to the engine particles because DEP have a higher amount of black carbon and are highly redox active. McWhinney found the redox activity of DEP to be very insoluble and long lived, meaning toxicity from redox-active sites remains with the particle and could last long enough to move throughout the body and affect different areas.
More research is needed with respect to predicting the redox activity of PM and its link to toxicology and epidemiology. Whether there are better targets for limiting PM pollution besides mass also remains an important question.