It's so nice to see technical stuff here. Sorry I can't type chinese on this computer and I know only little french...
In diesel combustion mode, NOx is only formed in the periphery of the jet flame where the flame is stoichiometric. Temperature dominates the reaction rate of the Zeldovic NOx mechanism, and gasoline combustion temperature is higher than diesel one so it generates even higher raw NOx emissions. But gasoline engine can make use of TWC so NOx is not a problem.
For diesel engine to meet EU5 NOx restriction, EGR is enough for light duty cars. But still new technology such as SCR like Bluetec of Mercedes which uses ammonia, for example human urine, to eliminate the NOx. But this kind of SCR is usually hard to mounted on light duty cars since the lower exhaust temperature, compared to high duty trucks, require state of art thermal management to operate SCR efficiently. Maybe some day you can piss into your car's fuel tank to help the emissions.
About GDI, the efficiency is higher but will cause the same particulate problems like diesel engines. The TWC needs to be well taken care of and at high load combustion stability is a great difficulty too.
About biofuels, just like you said, we can't always grow crops to meet the requirement. So there is a second generation of biofuels come from wood, pardon me can't remember the tricky chemical names. The basic idea is to develop a fast growing cycle trees to make fuels. For current moment of enthanol based biofuels, a sentence is often mentioned which I think interesting, "Drink the best, Burn the rest!"
You are right about the pollutant generated from other industry like chemistry industry. But the major problem for automotive is that this kind of transportational pollutants is hard to control since it is everywhere. The amount is smaller but doesn't mean we should ignore it.
I am doing a master study in ENSPM (Ecole Nationale Superieure du Petrole et des Moteurs) but more focus on combustion and fluid stuff so I have little knowledge on engine control... I consider buying a car too but I don't have the need to drive to school and the transportation in Paris is really hard for me...
楼主 
