The major effects of the nitrogen oxides are indirect In the presence of water vapor in the atmosphere, they are partially converted to HNO3, which can cause acid rain Nitrogen oxides also play important role in photochemical reactions and the formation of smog through following reaction NO2+ sunlight NO+o (origin of the reddish-brown color of NO2) O+O2→O3 O3+NO→NO2+O2 More ingredients are needed to fully explain the formation of photochemical smog, for example, hydrocarbonsThe major effects of the nitrogen oxides are indirect. In the presence of water vapor in the atmosphere, they are partially converted to HNO3 , which can cause acid rain. Nitrogen oxides also play important role in photochemical reactions and the formation of smog through following reaction: NO2 + sunlight → NO + O (origin of the reddish-brown color of NO2 ) O + O2 → O3 O3 + NO → NO2 + O2 More ingredients are needed to fully explain the formation of photochemical smog, for example, hydrocarbons