Peroxyacyl nitrates

Peroxyacyl nitrates, or PANs, are powerful respiratory and eye irritants present in photochemical smog. They are formed from a peroxyacyl nitrogen dioxide, for example peroxyacetyl nitrate, CH₃COOONO₂:

Hydrocarbons + O₂ + NO₂ + light → CH₃COOONO₂

PANs are both toxic and irritating, as they dissolve more readily in water than ozone. At concentrations of only a few parts per billion they cause eye irritation. At higher concentrations they cause extensive damage to vegetation. Both PANs and their mutagenic, as they can be a factor causing skin cancer.

PANs are secondary pollutants, which means they are not directly emitted as exhaust from nitrogen dioxide to form peroxyacyl nitrates.

The most common peroxyacyl radical is peroxyacetyl, which can be formed from the free radical oxidation of diacetyl.

Since they dissociate quite slowly in the atmosphere into ozone.