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Originally Posted by 6600gt
There is no such thing as a ZERO emissions vehicle, unless you start the timer after it has rolled of the production line. Manufacturing a car generates quite a bit of pollution. So don't trust the commercials or the media. Maybe the car can suck up the pollution...
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My father tells this story of an emissions testing facility in Los Angeles during the 1970s. They'd hook up some equipment to the exhaust pipe of a car and read out the emissions to make sure they were within legal limits. One particularly smoggy day someone got cute and stuck the test nozzle out the window. The results: the air outside the building failed the emissions test. The cars using that air to run their engines passed.
There are also semi-serious proposals to put catalytic-converter-like coatings on the outside of car radiators to remove pollutants from the air while the cars drive down the highway.
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In case of hydrogen, we are not even solving our current problem... If you don't know, Water Vapor is a green house gas!
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The water vapor content of the bulk of the atmosphere is already in equilibrium at a given temperature. Add more, and more precipitates out. Add less, and there's room for more to evaporate from surface water. Emitting steam from an engine's exhaust does not increase the total amount of water vapor in the air. We can't really affect it much, and it can't be considered a pollutant any more than sand on the beach can be.
Increasing temperature, however, means the atmosphere can hold more water vapor. More water vapor tends to hold in more heat, increasing the temperature. Positive feedback -- make it warmer by a little, and it ends up being warmer by more than you expect. It's an important factor in modeling the climate. But we can't affect it directly; only by manipulating the global temperature through other means can we change the amount of water vapor in the air.