Peroxyacetyl nitrate

Atmospheric chemistry

PAN is produced in the atmosphere via photochemical oxidation of hydrocarbons (e.g. Alkenes, Aromatics, and isoprenes). Carbonyls (oxidized VOCs) create acyl radicals which then become peroxyacetic acid (PA) radicals. Acetaldehyde is the dominant carbonyl species to produce PA radicals followed by Methylglyoxal, combined they can account for up to 80% of PA radical formation. The PA radicals can reversibly react with nitrogen dioxide () to form PAN. Night-time reaction of acetaldehyde with nitrogen trioxide is another possible source. Since there are no direct PAN emissions, it is a secondary pollutant. Next to ozone and hydrogen peroxide (), it is one of the most important components of photochemical smog.

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Other peroxyacyl nitrates in the atmosphere are peroxypropionyl nitrate (PPN), peroxybutyryl nitrate (PBN), and peroxybenzoyl nitrate (PBzN). Chlorinated forms have also been observed. PAN is the most important peroxyacyl nitrate. PAN and its homologues reach about 5 to 20 percent of the concentration of ozone in urban areas. At lower temperatures, these peroxy-nitrates are stable and can be transported over long distances, providing nitrogen oxides to otherwise unpolluted areas. At higher temperatures, they decompose into and the peroxyacyl radical.

The decay of PAN in the atmosphere is mainly thermal. Thus, the long-range transport occurs through cold regions of the atmosphere, whereas the decomposition takes place at warmer levels. PAN can also be photolyzed by UV radiation. It is a reservoir gas that serves both as a source and a sink of and radicals. Nitrogen oxides from PAN decomposition enhance ozone production in the lower troposphere.

The natural concentration of PAN in the atmosphere is below . Measurements in German cities showed values up to . Peak values above have been measured in Los Angeles in the second half of the 20th century ( of PAN corresponds to one part per billion (ppb)). Due to the complexity of the measurement setup, only sporadic measurements are available. The satellite based Cross-Track Infrared sounder (CrIS) instrument is able to provide mid-tropospheric PAN measurements on a global scale.

PAN is a greenhouse gas.

Sensitivity

PAN has a sensitivity to precursor emissions, mainly from VOCs and . PANs sensitivity towards VOCs is greater than that of . VOC reductions have more of an effect on PA radicals than on . Notably, global emissions of precursor during Covid-19 demonstrated that PAN concentrations do not always decrease with a decrease in concentrations. Similarly, PAN responds non-linearly to precursor changes. Alkenes and oxidized VOCs strongly influence the formation of PA radicals. Meteorological effects also influence the availability of these radicals and hence PAN formation.

Synthesis

PAN can be produced in a lipophilic solvent from peroxyacetic acid. For the synthesis, concentrated sulfuric acid is added to degassed n-tridecane and peroxyacetic acid in an ice bath. Next, concentrated nitric acid is added.

As an alternative, PAN can also be synthesized in the gas phase via photolysis of acetone and with a mercury lamp. Methyl nitrate () is created as a by-product.

Atmospheric effects

Seasonal cycles of PAN have been observed. Meteorological effects such as temperatures, wind patterns, and the availability of radicals influence PANs stability as well as transportation in the atmosphere. During the springtime in the northern hemisphere, high concentrations are attributed to an increase in photochemical activity. In addition, concentrations of PAN increase due to it having a relatively large lifetime against thermal decomposition. Transportation of PAN can also occur by wildfire smoke moving it into an otherwise unpolluted region. In the northern hemisphere winter however, PAN levels become limited when there are reduced hydrocarbons, , and low solar radiation.

Toxicity

The toxicity of PAN is similar to that of but higher than sulfur dioxide (). Populations with pulmonary disease tend to be more sensitive to the toxic effects of PAN. Eye irritation from photochemical smog can be caused by an increase in PAN levels. Concentrations at or above increase the likelihood of eye irritation. PAN is a very weak mutagen.

References

References

1. E V, Fischer. (March 14, 2014). "Atmospheric Peroxyacetyl Nitrate (PAN): a global budget and source attribution". Atmospheric Chemistry and Physics.
2. Shogrin, Madison J.. (February 27, 2024). "Changes to Peroxyacyl Nitrates (PANs) Over Megacities in Response to COVID-19 Tropospheric NO2 Reductions Observed by the Cross-Track Infrared Sounder (CrIS)". Geophysical Research Letters.
3. Vyskocil, Adolf. (April 17, 1998). "Peroxyacetyl nitrate: review of toxicity". Human & Experimental Toxicology.
4. Xueqi, Qiao. (June 1, 2023). "Strong relations of peroxyacetyl nitrate (PAN) formation to alkene and nitrous acid during various episodes". Environmental Pollution.
5. Vivienne H., Payne. (June 10, 2022). "Satellite measurements of peroxyacetyl nitrate from the Cross-Track Infrared Sounder: comparison with ATom aircraft measurements". Atmospheric Measurement Techniques.
6. Yulu, Qiu. (September 16, 2020). "Markedly Enhanced Levels of Peroxyacetyl Nitrate (PAN) During COVID-19 in Beijing". Geophysical Research Letters.
7. (January 1984). "An improved procedure for high purity gaseous peroxyacyl nitrate production: Use of heavy lipid solvents". Atmospheric Environment.
8. (1995-07-20). "Investigation of the loss processes for peroxyacetyl nitrate in the atmosphere: UV photolysis and reaction with OH". Journal of Geophysical Research: Atmospheres.
9. (January 1982). "A convenient method for preparation of pure standards of peroxyacetyl nitrate for atmospheric analyses". Atmospheric Environment.