In an ideal world, every component of the fuel (in this case diesel) would be 100% burned, but the reality is different. 
AdBlue changes the components that do not burn properly (= the harmful gases from the engine) into harmless gases > nitrogen and steam (= water and carbon dioxide) through a chemical reaction.
Contrary to what some think, AdBlue is not an additive. It is not added to the diesel but is refueled separately and only injected into the exhaust (after the diesel has been burned in the engine).
AdBlue consists of 30% urea and water.

Click here to find a Q8 station near you that sells AdBlue.