Evaporative emissions: These are produced from the evaporation of fuel, and are a large contributor to urban smog, since these heavier molecules stay closer to ground level. Fuel tends to evaporate in these ways:
Gas tank venting: The heating of the vehicle as the temperature rises from night to day, mean that gasoline in the tank evaporates, increasing the pressure inside the tank above atmospheric pressure. This pressure must be relieved and before emissions control it was simply vented into the atmosphere.
In 1996 and newer vehicles, the federal government required manufacturers to place a leak detection pump on vehicles. This pump services the purpose of putting a vacuum on the system to detect leaks. Because of this system, loose gas caps and filling the vehicle with gas while the engine is running can cause the light to come on.
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