The discovery of direct mutagenic activity, as determined by the Ames Salmonella typhimurium reversion assay, in the organic fractions of ambient aerosols collected throughout Southern California, led us to investigate the reactions of benzo(a)pyrene (BaP) deposited on glass fibre filters with ambient photochemical smog, as well as with O 3 , NO 2 , and peroxyacetyl nitrate (PAN) in simulated atmospheres. A variety of products are readily formed, including phenols, diphenols, dihydrodiols, etc. Directly mutagenic nitroderivatives are formed upon exposure of BaP (a carcinogen and pro-mutagen) and of perylene and pyrene (non-mutagens) to 1 part/10 6 of NO 2 and a trace of HNO 3 in air. If such reactions occur in urban atmospheres they may account in part for the ‘excess’ carcinogenicity (over BaP and certain other polycyclics) observed in organic particulates collected from smog and exhausts of spark ignition and diesel engines. However, such gas-solid interface processes may also have occurred on the filters commonly used in particulate sampling. Thus the possibility of ‘filter artefacts’ must be recognized. Carcinogenic N -nitrosamines have been detected in air at or near several industrial plants. Therefore, reactions of their possible precursors, e.g. secondary and tertiary methyl and ethylamines with NO x -HONO were studied both at levels of less than 1 part/10 6 in air in a 50 m 3 outdoor chamber and at levels around 4 parts/10 6 in a long path (720 m) outdoor infrared cell with the use of Fourier transform spectroscopic techniques. At concentrations less than 1 part/10 6 diethyl and triethylamine readily form photochemical smog (O 3 , PAN, aerosols, etc.). Additionally, small but significant amounts of diethylnitrosamine (C 2 H 5 ) 2 NNO are formed in the dark from diethylamine (DEA) but destroyed in sunlight. In contrast, (C 2 H 5 ) 2 NNO is initially formed on irradiation of triethylamine (TEA)-NO x mixtures, then photodecomposed. Other significant nitrogenous compounds formed in sunlight include dialkylnitramines (R 2 NNO 2 , a major product) and substituted amides; small amounts of acetamide are present in the particulate phase from DEA and TEA. Both (CH 3 ) 2 NNO 2 and CH 3 CONH 2 are carcinogens in animals, though less potent than nitrosamines; the activity of (C 2 H 5 ) 2 NNO 2 is not known. Environmental implications of both systems are discussed.