Urea is injected into the exhaust as a catalyst to reduce NOx. Putting more fuel into a relatively cool exhaust stream (at least as compared to the very high temperature combustion chamber, where fuel injected under very high pressure into highly compressed air, and combusts) couldn't possibly reduce emissions.
Diesels will always be dirty engines because combustion is far less complete than it is in a gasoline engine. Save it for big vehicles that need ungodly torque to start big loads.
Interestingly, the reason diesel vehicles go further on a gallon of fuel is because diesel contains more energy per gallon. They are not more efficient than a gasoline engine. Based on miles per BTU, they are basically identical.
The way the new ones are supposed to deal with those particulates is by burning them off at some point in the exhaust process, partly by injecting more fuel in there. This is actually why the 2007 and newer ones can't run bio. It gums up.