Nitric oxide (NO)
NO is a colourless, odourless, and non-flammable toxic gas. It is a by-product of combustion in air, as in automobile engines or fossil fuel power plants, and is produced naturally during the electrical discharges of lightning in thunderstorms. As a free radical it is very reactive and easily oxidized to nitrogen dioxide by oxygen.
Nitrogen dioxide (NO2)
NO2 is a reddish-brown, oxidizing, toxic gas with a pungent smell. It readily reacts with water to give nitric acid and nitrous acid.