Due to the complex interplay between burner aerodynamics and fuel oxidation and nitrogen species chemistry, no clear rules exist for determining which NOx formation mechanism is dominant in a given stationary burner configuration.

However, immediate NO is important near the inlet burner, where reacting fuel fragments mix with oxidizing air. Thus, both immediate and thermal NO contribute to the formation of nitrogen oxides in natural gas burners.

The term nitrogen oxides (NOx) is used to describe the sum of NO, NO2 and other nitrogen oxides that contribute to the formation of ozone, particulate matter, smog and acid rain.

Emissions of nitrogen oxides have been a concern since they are the main pollutants in photochemical smog and contribute to acidic wet and dry deposition. Nitrogen oxides are also important because they have indirect effects on climate by affecting global ozone concentrations. Although NOx species are relatively short-lived, they can chemically react with hydrocarbons to form PAN. The constituent PAN provides a reservoir for nitrogen oxides that can be transported over long distances to affect ozone chemistry downstream of the source. The lifetime of PAN is highly dependent on temperature.

There is considerable uncertainty in the sources of nitrogen oxides, with total sources estimated at about 42–47 TgN yr−1. The largest sources of reactive nitrogen in the troposphere are fossil fuel combustion, biomass burning, lightning discharges, microbial activity.