I think you are comparing apples and oranges, it's relatively easy to limit particulate (which it what everyone can see). The real issue is NOX which is going to occur whenever you have high temperature combustion with excess O2 (regardless of fuel). There is no practical way to prevent/limit the production of NOX without limiting the supply of O2 (that's what a EGR is trying to do) or lowering combustion temperatures (which would reduce efficiency). Unfortunately, the production of NOX is inherent in a diesel engine. The current solution (ammonia reaction with the exhaust gases) has been used in industrial facilities for years. It is just a high temperature chemical reaction to break down the NOX.