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Name: Barbara
Status: other
Grade: other
Location: N/A 
Country: N/A
Date: 4/2/2005

My question relates to the car exhaust extractors that are used in traffic tunnels. When the exhaust fumes are discharged into the atmosphere, do they rise or fall?

The engineering part, my guess would be that they use fans with the blades reversed. In winter, when one wants to keep the heat down, the recommendation is to reverse the blades on the ceiling fan. Hence my guess that is how the exhaust fan works, drawing the air out from the tunnel. The winter analogy is to use the blades to draw the heat from the ceiling down to the room.

Now once the tunnel air is excavated from the tunnel, it obviously has particles suspended, but there are other properties, the heat from the exhaust, the other emissions. Maybe the easier approach would be to ask the question, "What could rise?" To a layman when it comes to molecules and atoms, the guess is only gases lighter than air. The logical first thing coming to mind would be helium, a gas that rises. But, a next question, "How far?" Again the layman, there is the ozone "layer". So using this reverse reasoning (often a good tool). There can be parts of the exhaust that could rise, I doubt helium is produced, but there may be elements in the fumes with potential. Everything else should fall to earth. One frustration of vacations in Los Angeles is the smog, in the morning the smog, or murk would hang around, untill the locals would say ,"it burns off". My reasoning the sun temperature causes the smog to disperse, some rising, as the ozone, but the guess, more falling to earth.

James Przewoznik

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