Anyone know if its possible to collect all exhaust (or at least the harmful stuff) from cars right at the tailpipe, instead of having to deal with it in the atmosphere later? I know its at least possible to absorb carbon dioxide (http://www.azom.com/Details.asp?ArticleID=2052)...

I'm sure it's possible, but probably not feasible. You would need a scrubber of some sort.

Catalytic converters help eliminate a lot of the more hazardous stuff (unburnt fuel, CO, and some N-oxides), leaving you with CO2, H2O, and some N-oxides.

CO2 is easy to absorb/scrub with current tech. But Idk about scrubbing N-oxides. Apparently they are working on developing a NOx absorber (http://en.wikipedia.org/wiki/NOx_Adsorbers) for use in automobiles.

Now, if it is possible to scrub the CO2 and NOx, the question is just how big one would need to be for a car.

Totally impractical.

Gasoline weights about 750 grams/liter (about 6 pounds/gallon). It's composed mainly of hydrocarbon chains (CH2+2H) and hydrocarbon rings (CH). For the sake of arguement, let's just say that overall there's a ratio of one carbon atom to 2 hydrogen atoms (it's probably a bit lower, but not so much to make a difference). So just the mass of the carbon would be 12/14 of the total mass. But when you burn it that combines to CO2, which comes to a mass of 44/14 of the mass of the original gas, or 2357 grams from that one liter.. Under normal temperatures and pressures that's about 1200 liters of CO2, from one liter of gas.

Now, if you were even somehow able to compress it down to a solid, that's 1.5 liters. That's right, at it's densest, you'd need a storage tank 50% larger than your gas tank.