Automobiles, trucks, construction equipment, stationary power generators and motorcycles are all going to be required to have pollution controls within the next 5 years. Why are aircraft exempt?
The fact of the matter is your typical jetliner emits more smog-producing pollutants in one minute of operation that 1,000 modern automobiles will emit in a year. And yet we are beating our brains out making LEVs and ULEVs and SULEVs, while jet aircraft fly overhead, without a care.
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.