Mercury Mariner 4 and 5HP 4-Stroke Outboards Service Shop Manual 1999, Page 95Get 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 combi nes 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 su nlight 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