2-Nitrofluoranthene has recently been demonstrated to be the most abundant nitrated fluoranthene in organic extracts of ambient air particulate matter collected both in Philadelphia (Nishioka et al., unpublished results) and in Southern California. Whereas the better-known isomer 3-nitrofluoranthene (3-NFA) is found in combustion emissions such as diesel exhaust (2) and arises from direct nitration by NO2/N204 or the NO(+2), 2-NFA appears to be formed from fluoranthene in the ambient air by a specific atmospheric transformation mechanism, possibly initiated by addition of an OH radical at the site of highest electron density. An analogous pathway would lead to the formation of 2-nitropyrene which was observed in rural ambient air particulate matter in Denmark. Other nitrated polycyclic aromatic hydrocarbons (nitroPAH) present in the environment are known to be potent bacterial mutagens. Their genotoxicity in prokaryotes is largely due to generation of active electrophiles (leading to formation of covalenty-bound C8-deoxyguanosine DNA adducts) by nitroreduction.