Directly active mutagens are formed on exposure of the promutagen benzo[a]pyrene to gaseous pollutants in smog. In simulated atmospheres containing 1 part per million nitrogen dioxide and traces of nitric acid, directly mutagenic nitro derivatives are readily formed from both benzo[a]pyrene and perylene, a non-mutagen in the Ames reversion assay. Possible formation of direct mutagens by such reactions on sample collection filters, in exhaust effluents, and in the atmosphere should be recognized.

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ficient quality to be used for matched spatial filter recognition of the object, which is the ultimate goal. There is a considerable reduction in terms of time and cost with this system in comparison to other input methods available for coherent image processing. AcknowledgmentWe thank Aaron Gara and General Motors Corp. for the use of their liquid crystal image panel.

Real-time and integrated measurements of gaseous and particulate pollutants were conducted inside five conventional diesel school buses, a diesel bus with a particulate trap, and a bus powered by compressed natural gas (CNG) to determine the range of children's exposures during school bus commutes and conditions leading to high exposures. Measurements were made during 24 morning and afternoon commutes on two Los Angeles Unified School District bus routes from South to West Los Angeles, with seven additional runs on a rural/suburban route, and three runs to test the effect of window position. For these commutes, the mean concentrations of diesel vehicle-related pollutants ranged from 0.9 to 19 mg/m 3 for black carbon, 23 to 400 ng/m 3 for particle-bound polycyclic aromatic hydrocarbon (PB-PAH), and 64 to 220 mg/m 3 for NO 2 . Concentrations of benzene and formaldehyde ranged from 0.1 to 11 mg/m 3 and 0.3 to 5 mg/m 3 , respectively. The highest real-time concentrations of black carbon, PB-PAH and NO 2 inside the buses were 52 mg/m 3 , 2000 ng/m 3 , and 370 mg/m 3 , respectively. These pollutants were significantly higher inside conventional diesel buses compared to the CNG bus, although formaldehyde concentrations were higher inside the CNG bus. Mean black carbon, PB-PAH, benzene and formaldehyde concentrations were higher when the windows were closed, compared with partially open, in part, due to intrusion of the bus's own exhaust into the bus cabin, as demonstrated through the use of a tracer gas added to each bus's exhaust. These same pollutants tended to be higher on urban routes compared to the rural/suburban route, and substantially higher inside the bus cabins compared to ambient measurements. Mean concentrations of pollutants with substantial secondary formation, such as PM 2.5 , showed smaller differences between open and closed window conditions and between bus routes. Type of bus, traffic congestion levels, and encounters with other diesel vehicles contributed to high exposure variability between runs.

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