Peroxyacyl nitrates, or PANs, are powerful respiratory and eye irritants present in photochemical smog. They are formed from a peroxyacid radical and nitrogen dioxide, for example peroxyacetyl nitrate, CH3COOONO2.
PANs are both toxic and irritating. At concentrations of only a few parts per billion they cause eye irritation. At higher concentrations they cause extensive damage to vegetation.
PANs are secondary pollutants, which means they are not directly emitted as exhaust from power plants or internal combustion engines, but they are formed from other pollutants by chemical reactions in the atmosphere. Free radical reactions catalyzed by ultraviolet light from the sun oxidize unburned hydrocarbons to aldehydes, carboxylic acids, and finally peroxyacids, which combine with nitrogen dioxide to form peroxyacyl nitrates.