Nitric acid decomposition
Generally, nitric acid is a strong oxidizing agent which get reduce to nitrogen oxides on reacting with metals which act as reducing agent thus forming a redox reaction. But in this case, only nitric acid is involved which get reduced to nitrogen oxides on reaction with air/light. So, this may be called oxidation reaction where nitric acid gets oxidized. From wikipedia article of nitric acid:


  Nitric acid is subject to thermal or light decomposition and for this reason it was often stored in brown glass bottles:
  
  $$\ce{ 4 HN^{V}O3 → 2 H2O + 4 N^{IV}O2 + O2}$$
  
  This reaction may give rise to some non-negligible variations in the
  vapor pressure above the liquid because the nitrogen oxides produced
  dissolve partly or completely in the acid.
  
  The nitrogen dioxide ($\ce{NO2}$) remains dissolved in the nitric acid
  coloring it yellow or even red at higher temperatures. While the pure
  acid tends to give off white fumes when exposed to air, acid with
  dissolved nitrogen dioxide gives off reddish-brown vapors, leading to
  the common name "red fuming acid" or "fuming nitric acid" – the most
  concentrated form of nitric acid at Standard Temperature and
  Pressure (STP). Nitrogen oxides ($\ce{NO_x}$) are soluble in nitric
  acid.


There are some old papers(here and here) which discuss the mechanism of nitric acid decomposition. Give a quick read on the papers and you will understand the mechanism of the reaction.