Table 2.

Evidence for the ability of EPFRs to generate reactive oxygen species.

Source	Finding	References
TSP (Athens); Urban street dusts; DEP; GEP	PM generates hydroxyl radical in aqueous suspension. Hydroxyl radical formation was linked with redox-active metal content.	[72]
Biochar	Biochar contains persistent free radicals evident by EPR. Biochar can activate H2O2 to produce hydroxyl radical.	[11]
DEP; Coal fly ash	Suspensions of DEP and coal fly ash produce hydroxyl radical. Metal ions and superoxide implicated in its production. Neither kaolinite nor silica produce ·OH.	[69]
Ambient air PM (California); DEP	In the presence of ascorbate, ambient air PM and DEP both generate ·OH. ·OH production is correlated with Cu content	[70]

Abbreviations: DEP = diesel exhaust particles, GEP = gasoline exhaust particles, TSP = total suspended particulates.