Environmentally persistent free radicals and their lifetimes in PM2.5.

For the first time, an expansive study into the concentration and extended decay behavior of environmentally persistent free radicals in PM2.5 was performed. Results from this study revealed three types of radical decay-a fast decay, slow decay, and no decay-following one of four decay patterns: a relatively fast decay exhibiting a 1/e lifetime of 1-21 days accompanied by a slow decay with a 1/e lifetime of 21-5028 days (47% of samples); a single slow decay including a 1/e lifetime of 4-2083 days (24% of samples); no decay (18% of samples); and a relatively fast decay displaying an average 1/e lifetime of 0.25-21 days followed by no decay (11% of samples). Phenol correlated well with the initial radical concentration and fast decay rate. Other correlations for common atmospheric pollutants (ozone, NOx, SO2, etc.) as well as meteorological conditions suggested photochemical processes impact the initial radical concentration and fast decay rate. The radical signal in PM2.5 was remarkably similar to semiquinones in cigarette smoke. Accordingly, radicals inhaled from PM2.5 were related to the radicals inhaled from smoking cigarettes, expressed as the number of equivalent cigarettes smoked. This calculated to 0.4-0.9 cigarettes per day for nonextreme air quality in the United States.