How significant is air decomposition on formation of nitrogen oxides? How significant is $\ce{NO$_x$}$ formation from $\ce{O2}$ (reactant) and $\ce{N2}$ decomposition from simply heating the two as opposed to from $\ce{CO2}$, $\ce{H2O}$, (and $\ce{N2}$) dissociation? In other words, which of the two pathways for $\ce{NO$_x$}$ formation is more significant (I imagine this will vary with equivalence ratio):


$\ce{O}$ and $\ce{N}$ radical from $\ce{N2}$ and $\ce{O2}$ dissociation
$\ce{O}$ radical from $\ce{CO2}$ and $\ce{H2O}$ dissociation with $\ce{N}$ radical


My feeling is that the second becomes more significant as equivalence ratio increases (more fuel means more $\ce{O2}$ is consumed in combustion).
In situations where NOx originiates form $\ce{N2}$, such as lightning or a high temperature furnace, the Zeldovich mechanism dominates:

$$\ce{O2 -&gt; O +O}$$

$$\ce{O + N2 -&gt; NO +N}$$

$$\ce{O2 + N-&gt; NO +O}$$

If a combusting fuel actual contains nitrogen, other mechanisms can be involved.