How is nitrogen oxide formed in the atmosphere,and why is an excessive amount of nitrogen oxide harmful?

(N/A) At high altitudes,when lightning strikes,$N_2$ and $O_2$ combine to form nitrogen oxides ($NO$ and $NO_2$).
Excessive amounts of nitrogen oxides are harmful because:
$1$. Higher concentrations of $NO_2$ damage the leaves of plants and retard the rate of photosynthesis.
$2$. Nitrogen oxides are lung irritants that can lead to acute respiratory diseases in children.
$3$. They are toxic to living tissues and cause damage to various textile fibers and metals.