Actually, they aren't THAT bad.
The EPA says only 2% of the NOx from mobile sources comes from Aircraft engines. Most of the air going through a turbine engine is just air. Only 8% is products of combustion and only .4% is "pollution". Large Turbofans have a compression ratio higher than a Diesel so there isn't much left to come out but soot and the impurities in the fuel/air, which aren't much.
The "typical jetliner" is also 20 years old. Compare a typical Jetliner to 1,000 or even 100, 20 year old cars and the jetliner is cleaner.
If you had said "particulates", you might have a point as most older jets emit tons of these, but they don't really have much to do with smog. Until the EPA cracked down on Diesels in 1997, trucks and trains emitted tons of this stuff, too. These older planes will go the way of the dinosaur over the next 15-20 years and the particulate problem (if there ever really was one) will go away, with them.
As for efficency, the AVERAGE revenue passenger miles for the US airlines is 44 mpg. I would guess LA traffic averages less than 20 revenue passenger mpg.
You keep on talking about twenty year old cars in order to compare them with twenty year old airplanes. But the fact of the matter is most cars on the road are five years old or less, so it really doesn't make a lot of sense to talk about the state of the art, circa 1985.
In any case, I was not talking about the automobiles of 20 years ago. I was talking about the mobile sources of 10 years from now. In 10 years, everything with a tailpipe is going to be regulated except aircraft. This is going to make aircraft emmissions really stand out as the last remaining area for improvement.
Eventually pollution controls are going to come down the pike for aircraft. What form they will take is anybody's guess at this point. I really don't see how conventional pollution controls can be applied to modern jet aircraft engines. Some other form of propulsion may become necessary.