Mercury Optimax Models 135, 150, Direct Fuel Injection., Page 217Get this manual
EMISSIONS
Oxides of NitrogenNOx
NOx is slightly different byproduct of combustionNitrogen is one of the elements that makes up the air going into the engineUnder extremely high temperatures it combines with oxygen to form oxides of nitrogen (NOx)This happens in the engine's combustion chambers when temperatures are too highNOx itself is not harmful, but when exposed to sunlight it combines with unburned hydrocarbons to create the visible air pollutant known as smogSmog is serious problem in California as well as many other heavily populated areas of the United States