| Summary: | Previous research demonstrated environmentally persistent free radicals (EPFRs) will form on particulate surfaces under combustion conditions (temperature range of 150-400 °C) from reactions of organic precursors with redox-active transition metals. With an understanding of how these EPFRs form, it is necessary to determine how they behave in a natural environment after emission. To better understand this, the nature of EPFRs in ambient PM2.5 under simulated atmospheric conditions was investigated.
Ambient PM2.5 samples were collected at a roadside ambient monitoring site near heavy interstate traffic and major industrial activity. The EPFR concentration and general radical structure were determined with EPR spectroscopy. Studies of EPFR decay in ambient air demonstrated four decay patterns to emerge from analysis: a fast followed by a slow decay (47% of samples), a slow decay (24% of samples), no decay (18% of samples), and a fast decay followed by no decay (11% of samples) with half-lives for the decays lasting from several days to several months. All decays were suggested to result from reaction with oxygen and strengthened from an overall shift in the EPR g-factor. This shift implied an increased presence of oxygen centered radicals.
The negative health impacts of PM2.5 were studied by the generation of hydroxyl radicals. These studies revealed dissolved oxygen coupled with the presence of PM2.5 necessary to generate significant levels of hydroxyl radicals without the addition of H2O2.
Exposure of PM2.5 to ozone and NO revealed no effect on the organic radical (EPFR) signal, while NO2 exhibited a 5-8 time increase. When these exposed EPFRs were evaluated by hydroxyl radical generation, the NO and ozone exposed samples maintained the same levels as
the unexposed sample, while NO2 exposed samples displayed a decreased ability due to the formation of acid.
When PM2.5 was exposed to simulated solar exposure, the EPFR concentration was observed to increase substantially in all samples. Decay from irradiation followed a 2 decay pattern with the shorter, solar decay demonstrating a half-life of 8 hours and the longer decay 9 days. Irradiation also increased the amount of hydroxyl radicals generated from PM2.5.
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