Straining pollution from the air
Recently Volvo began installing on some of its cars an air filter behind the front intake grill that cleans the air of more pollution than the vehicle itself emits. They've begun to sell models with this feature built-in; a fleet of mobile air cleaners will descend upon the world, cleaning it of smog -- in theory.
Installing these filters on each vehicle in the city bus fleet seems a logical step forward from this; my rusty grasp of airflow says 5280 cubic feet an hour of air is about 140 cubic feet per second -- presuming the air intake is a foot square. Given six hours averaging 5 miles an hour, the bus will sweep 15,840 cubic miles of air.
Data on the density of air particulates would be a valuable proof to this experiment; sampling the air repeatedly over a number of weeks before and after the filters are installed provides a way to show the decreasing air pollution at high-traffic areas such as the downtown bus station or the Franklin corridor.
One result of the experiments would be a partial map of the pollution index at a much finer granularity than is easily available; arguing that a bus station's air is cleaner due to the lack of cars and trucks might benefit public transportation and ecological concerns.
Neighborhoods with especially high levels of pollutants would be identified, with a solution available to the residents: installing a cleaning filter in their air conditioner would improve, slowly, the quality of the air they breathe each night.