I'm sure you've all seen or used an air cleaner in your home or seen one advertised on tv...it sucks in air and removes dust and particulates etc...I have thought that it would be very adventageous to mount an air cleaner (perhaps "Oreck" style with washable filter)on top of every car. It would be inside a streamlined cowling that you could open with a key in order to wash the filter. Every automobile would be sucking in dirty air and returning clean air.With reduced emmissions this would go a long way towards cleaning up the air...

The atmosphere is pretty good at cleaning itself - most of what we put up there falls back down in a couple of days. A filter on the car exhaust might be a good idea.