This idea sort of falls under environment, but I bet more people would look at it here, so here she goes.
Busses travel in most cities for at least 15 hours of the day. During that time the bus may cover up to 300km. A filter placed on the roof of the bus would see a fair amount of air in one of those days. For example, if the dimensions of the filter were 2.5 x .5 meters then in the course of the day one bus could filter approximately 250,000 cubic meters of air. Filters could be washed and pellets made of the dirt collected. These pellets could be disposed of properly. This would make the cities a cleaner place. The governments could subsidies the bus companies and every one would be happy.
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