Nitric acid or nitrous oxide is subject to thermal or light decomposition: 4 HNO3 → 2 H2O + 4 NO2 + O2. This reaction can cause some variation in non-negligible in the vapor pressure above the liquid because the nitrogen oxides produced dissolve partly or completely in the acid.
The nitrogen dioxide (NO2) remains dissolved in the nitric acid coloring yellow or even red at higher temperatures. While the pure acid tends to pull out white smoke when exposed to air