Peroxyacyl nitrates
Encyclopedia
Acyl peroxy nitrates, or Peroxyacyl nitrates (also known as APNs, or PANs), are powerful respiratory and eye irritants present in photochemical smog. They are produced 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 NO2. The final step is a combination of a peroxyacyl radical
and nitrogen dioxide
, for example peroxyacetyl nitrate
, CH3C(O)OONO2:
CH3C(O)OO· + NO2 → CH3C(O)OONO2
The general equation is:
Hydrocarbons + O2 + light → CxHyC(O)OO·
CxHyC(O)OO· + NO2 → CxHyC(O)OONO2
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.
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.
Both PANs and their chlorinated
derivates are said to be mutagen
ic, as they can be a factor causing skin cancer
.
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, ketones, and dicarbonyl compounds, whose secondary reactions create peroxyacyl radicals, which combine with nitrogen dioxide
to form peroxyacyl nitrates.
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
.
Since they dissociate quite slowly in the atmosphere into radical
s and NO2, 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 NOx
to regions where it can more efficiently produce ozone
.
References
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
and nitrogen dioxide
Nitrogen dioxide
Nitrogen dioxide is the chemical compound with the formula it is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is a prominent...
, for example peroxyacetyl nitrate
Peroxyacetyl nitrate
Peroxyacetyl nitrate is a peroxyacyl nitrate. It is a secondary pollutant present in photochemical smog. It is thermally unstable and decomposes into peroxyethanoyl radical and nitrogen dioxide gas. It is a lachrymatory substance....
, CH3C(O)OONO2:
CH3C(O)OO· + NO2 → CH3C(O)OONO2
The general equation is:
Hydrocarbons + O2 + light → CxHyC(O)OO·
CxHyC(O)OO· + NO2 → CxHyC(O)OONO2
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.
PANs are both toxic and irritating, as they dissolve more readily in water than ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
. They are lachrymators, causing eye irritation at concentrations of only a few parts per billion. At higher concentrations they cause extensive damage to vegetation.
Both PANs and their chlorinated
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
derivates are said to be mutagen
Mutagen
In genetics, a mutagen is a physical or chemical agent that changes the genetic material, usually DNA, of an organism and thus increases the frequency of mutations above the natural background level. As many mutations cause cancer, mutagens are therefore also likely to be carcinogens...
ic, as they can be a factor causing skin cancer
Skin cancer
Skin neoplasms are skin growths with differing causes and varying degrees of malignancy. The three most common malignant skin cancers are basal cell cancer, squamous cell cancer, and melanoma, each of which is named after the type of skin cell from which it arises...
.
PANs are secondary pollutants, which means they are not directly emitted as exhaust from power plants
Fossil fuel power plant
A fossil-fuel power station is a power station that burns fossil fuels such as coal, natural gas or petroleum to produce electricity. Central station fossil-fuel power plants are designed on a large scale for continuous operation...
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, ketones, and dicarbonyl compounds, whose secondary reactions create peroxyacyl radicals, which combine with nitrogen dioxide
Nitrogen dioxide
Nitrogen dioxide is the chemical compound with the formula it is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is a prominent...
to form peroxyacyl nitrates.
The most common peroxyacyl radical is peroxyacetyl, which can be formed from the free radical oxidation of acetaldehyde
Acetaldehyde
Acetaldehyde is an organic chemical compound with the formula CH3CHO or MeCHO. It is one of the most important aldehydes, occurring widely in nature and being produced on a large scale industrially. Acetaldehyde occurs naturally in coffee, bread, and ripe fruit, and is produced by plants as part...
, various ketones, or the photolysis of dicarbonyl compounds such as methylglyoxal
Methylglyoxal
Methylglyoxal, also called pyruvaldehyde or 2-oxopropanal is the aldehyde form of pyruvic acid. It has two carbonyl groups, so it is a dicarbonyl compound. Methylglyoxal is both an aldehyde and a ketone....
or diacetyl
Diacetyl
Diacetyl is a natural byproduct of fermentation. It is a vicinal diketone with the molecular formula C4H6O2...
.
Since they dissociate quite slowly in the atmosphere into radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
s and NO2, PANs are able to transport these unstable compounds far away from the urban and industrial origin. This is important for tropospheric ozone
Tropospheric ozone
Ozone is a constituent of the troposphere . Photochemical and chemical reactions involving it drive many of the chemical processes that occur in the atmosphere by day and by night...
production as PANs transport NOx
NOx
NOx is a generic term for the mono-nitrogen oxides NO and NO2 . They are produced from the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperatures...
to regions where it can more efficiently produce ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
.
References