Peroxyacyl nitrates

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The general structural formula of a peroxyacyl nitrate

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peroxyacetyl nitrate, the most common PAN

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), or by aldehydes and other oxygenated VOCs oxidizing in the presence of Template:Chem2.

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-methaneTemplate:Rp 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.

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These react reversibly with nitrogen dioxide (Template:Chem2) to form PANs:Template:Rp

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Night-time reaction of aldehydes with nitrogen trioxide is another possible source.Template:Rp

Since they dissociate quite slowly in the atmosphere into radicals and Template:Chem2, PANs are able to transport these unstable compounds far away from the urban and industrial origin. This is important for tropospheric ozone production as PANs transport NO_x to regions where it can more efficiently produce ozone.

Types

Peroxyacetyl nitrate is the most prevalent peroxyacyl nitrate (75–90% of total atmospheric emissions),Template:Rp followed by peroxypropionyl nitrate (PPN). Peroxybenzoyl nitrate (PBzN) and methacryloyl peroxynitrate (MPAN) have also been observed. The composition of PANs in a particular region depends heavily on which hydrocarbons are present in the atmosphere, with the exception of peroxyacetyl nitrate, which is able to be produced from a range of precursors.Template:Rp

Health effects

PANs are both toxic and irritating, as they dissolve more readily in water than ozone. They are lachrymators, causing eye irritation at concentrations of only a few parts per billion. At higher concentrations they cause extensive damage to vegetation. PANs are mutagenic, and are considered potential contributors to the development of skin cancer.