Environmentally Persistent Free Radicals: Insights on a New Class of Pollutants

Environmentally persistent free radicals, EPFRs, exist in significant concentration in atmospheric particulate matter (PM). EPFRs are primarily emitted from combustion and thermal processing of organic materials, in which the organic combustion byproducts interact with transition metal-containing particles to form a free radical-particle pollutant. While the existence of persistent free radicals in combustion has been known for over half-a-century, only recently that their presence in environmental matrices and health effects have started significant research, but still in its infancy. Most of the experimental studies conducted to understand the origin and nature of EPFRs have focused primarily on nanoparticles that are supported on a larger micrometer-sized particle that mimics incidental nanoparticles formed during combustion. Less is known on the extent by which EPFRs may form on engineered nanomaterials (ENMs) during combustion or thermal treatment. In this critical and timely review, we summarize imp...