AdBlue is a chemical liquid used in Selective Catalytic Reduction (SCR) on diesel vehicles. Its purpose it to reduce NOx levels. The AdBlue fluid, composed of urea and demineralized water, is injected into the exhaust system just before a SCR catalyst. The introduction of AdBlue into the exhaust gases generates a chemical reaction inside the catalyst, which results in NOx being converted into nitrogen and water vapor.
AdBlue was introduced by vehicle manufacturers in order to hit the stringent emission targets set out by governmental bodies. It is sometimes referred to as DEF (Diesel Exhaust Fluid).