In organic chemistry, peroxyacyl nitrates (also known as Acyl peroxy nitrates, APN or PANs) are powerful respiratory and eye irritants present in photochemical smog. They are nitrates produced in the thermal equilibrium between organic peroxy radicals by the gas-phase oxidation of a variety of volatile organic compounds (VOCs). Another way to produce PANs is by aldehydes and other oxygenated VOCs oxidizing in the presence of NO<sub>2</sub>.
They are good markers for the source of VOCs as either biogenic or anthropogenic, which is useful in the study of global and local effects of pollutants.
Formation
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 non-methane hydrocarbons to aldehydes, ketones, and dicarbonyls, whose secondary reactions create peroxyacyl radicals. The most common peroxyacyl radical is peroxyacetyl, which can be formed from the free radical oxidation of acetaldehyde, various ketones, or the photolysis of dicarbonyl compounds such as methylglyoxal or diacetyl.
: Hydrocarbons + O<sub>2</sub> + light → RC(O)OO<sup>•</sup>
These react reversibly with nitrogen dioxide (NO<sub>2</sub>) to form PANs: Since they dissociate quite slowly in the atmosphere into radicals and NO<sub>2</sub>, PANs are able to transport these unstable compounds far away from their urban and industrial origins. This causes a decrease in photochemical ozone production near sources of NO<sub>x</sub> as PANs are a NO<sub>x</sub> reservoir, reducing the amount of NO<sub>x</sub> photolyzed. This allows PANs to transport NO<sub>x</sub> to regions where it can more efficiently produce tropospheric ozone.
Types
Peroxyacetyl nitrate is the first member of PANs identified by scientists in the 1950s and the most prevalent peroxyacyl nitrate (75–90% of total atmospheric emissions), followed by peroxypropionyl nitrate (PPN), the second member of PANs discovered from synthetic mixtures.
thumb|right|200px|The general [[structural formula of a peroxyacyl nitrate]]
thumb|200px|[[peroxyacetyl nitrate|<span style="color:#000080;">peroxy</span><span style="color:#800000;">acetyl</span> <span style="color:#008000;">nitrate</span>, the most common PAN]]