The occurrence of selected nitro-polycyclic aromatic hydrocarbons (nitro-PAHs) associated with atmospheric particulate matter has been investigated at an urban site and at a semi-rural site. For this purpose an analysis method based on gas chromatography and tandem ion trap mass spectrometry has been developed and applied. The nitro-PAK levels have been compared with levels of other air pollutants including unsubstituted PAHs, inorganic gases and particulate matter, as well as with meteorological parameters. Correlations and concentration ratios suggest that the dominant source of 9-nitroanthracene at the urban site is direct emissions, whereas at the semirural site its dominant source is atmospheric formation. The atmospheric formation of 2-nitrofluoranthene and 2-nitropyrene generally seems to be initiated by OH radicals during the day rather than by NO3 radicals at night. The average contribution of the OH initiated formation is estimated to be in the range of 90-100%. However, under wintertime conditions with cloudy weather implying low OH radical production, NO3 radicals may also be important as initiators of nitro-PAH formation. Samples influenced by transport of polluted air masses from the European continent have significantly elevated concentrations of atmospherically formed nitro-PAHs. The directly emitted nitro-PAHs, 1-nitropyrene and 3-nitrofluoranthene, do not exhibit elevated levels during such long-range transport episodes. (C) 2000 Elsevier Science Ltd. All rights reserved.