Have you ever stepped out on a sunny day in a large city? Spend enough time out of doors, especially in the urban center, and you may notice that your eyes grow irritated and begin to water uncontrollably. While the camouflage of general metropolitan bustle may hide it from notice, you might also note much of whatever foliage there may be is sickly and decayed. These two effects of modern industrialization are linked to a single molecule, forming an essential component of the ominous haze hovering above and around you. Peroxyacetyl nitrate, commonly abbreviated as PAN, is an important contributor to the phenomenon of photochemical smog. As a lacrimator and strong oxidizer, it wreaks general havoc with city residents and plant matter.
PAN is the most abundant representative of a family of organic compounds called peroxyacyl nitrates. Scientists were entirely unaware of these compounds until the 1960s, when they were first identified among the many components of urban smog. PAN forms in the atmosphere through a series of complicated chemical reactions deriving from hydrogen peroxide, hydrocarbons, and nitrogen oxides emitted from sources of urban pollution such as power plants, oil refineries, automobiles, lawnmowers, and aircraft. The total reaction begins with the introduction of hydrocarbons or aromatic compounds to hydrogen peroxide (OH) molecules. They form the compound acetaldehyde (CH3OH), which then reacts again with hydrogen peroxide to form CH3CO3 and water (H2O). This compound gains a dioxide. In the final step, CH3CO3 reacts with a nitrogen dioxide to form PAN. In formula terms, the reaction goes thus:
1) HYDROCARBONS + OH --> CH3CHO (acetyldehyde)
AROMATICS + OH --> CH3CO (acetyl radical)
2) CH3CHO + OH --> H2O + CH3CO
3) CH3CO + O2 --> CH3CO3
4) CH3CO3 + NO2 <---> CH3C(O)OONO2
This final reaction is reversible, which means that it can occur in either direction. PAN can degrade into a nitrogen dioxide and acetyl compound as easily as it formed. Which state it occupies depends upon the ambient temperature. In warmer air temperatures, the compound either reacts or breaks down into its components. In colder temperatures, PAN remains stable. As a consequence, the concentration of PAN tends to be greater during the night than during the day. When the sun is shining, convection currents carry those clouds of PAN that don't decompose high into the troposphere, where the colder temperatures allow it a modicum of stability. Due to its constant forming, deforming, and mutation, PAN is easily dispersed from its original point of catalyzation in a smog that extends beyond the city center.
Chemists have not yet discovered any natural source for PAN; it seems to have arisen only with the introduction of widespread fossil fuel use. Because its only source is pollution and it is the second most abundant molecule, next to ozone, in photochemical smog, measurement of PAN is a good indication of overall smog levels.
Due to its highly oxidizing nature, PAN is injurious to plants in any concentration greater than 50 parts per billion, a number easily exceeded in the world's most polluted cities. In smaller concentrations, it is still an eye irritant.
In less significant quantities, PAN can also form within buildings, exchanging sunlight for flourescent light to catalyze reactions from the same acetyl and hydroxy compounds. The concentrations formed, at most 14 p.p.b., are negligible for most purposes, but its presence is a subject of concern for historical preservationists. PAN does not cause the fading or deterioration of colored dyes, but its other possible effects on works of art and manuscripts are not known.
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Williams, Edwin L.; Grosjean, Eric; Grosjean, Daniel. "Exposure Of Artists' Colorants To Peroxyacetyl Nitrate," Journal of the American Institute for Conservation, vol. 32, Number 1, Art. 6, 1993.
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