Peroxyacetyl nitrate

Peroxyacetyl nitrate is a peroxyacyl nitrate.^[1] It is a secondary pollutant present in photochemical smog and PAN concentrations can be sensitive to precursor emissions.^[2][1] It is thermally unstable and decomposes into peroxyethanoyl radicals and nitrogen dioxide gas. It is a lachrymatory substance, meaning that it irritates the lungs and eyes.^[3]

Peroxyacetyl nitrate

Names
Preferred IUPAC name Acetic nitric peroxyanhydride
Other names PAN peroxyacetyl nitrate α-oxoethylperoxylnitrate
Identifiers
CAS Number	2278-22-0 Y
3D model (JSmol)	Interactive image
Abbreviations	PAN
ChemSpider	15907 N
ECHA InfoCard	100.017.187
EC Number	218-905-6
PubChem CID	16782
UNII	SQ8V0P4N89 Y
CompTox Dashboard (EPA)	DTXSID4062301
InChI InChI=1S/C2H3NO5/c1-2(4)7-8-3(5)6/h1H3 N Key: VGQXTTSVLMQFHM-UHFFFAOYSA-N N
SMILES CC(OO[N+]([O-])=O)=O
Properties
Chemical formula	C2H3NO5
Molar mass	121.05 g mol−1
Solubility in water	1.46 × 10 5 mg l−1 at 298 K
log P	−0.19
Vapor pressure	29.2 mmHg at 298 K
Henry's law constant (kH)	0.000278 m3 atm mol−1 at 298 K
Atmospheric OH rate constant	10−13 cm3 molecule−1 s−1 at 298 K
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Peroxyacetyl nitrate, or PAN, is an oxidant that is more stable than ozone.^[1] Hence, it is more capable of long-range transport than ozone.^[1] It serves as a carrier for oxides of nitrogen (NOx) into rural regions and causes ozone formation in the global troposphere.^[1]

Atmospheric chemistry

PAN is produced in the atmosphere via photochemical oxidation of hydrocarbons (e.g. Alkenes, Aromatics, and isoprenes).^[4][3] Carbonyls (oxidized VOCs) create acyl radicals which then become peroxyacetic acid (PA) radicals.^[4] 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.^[1][4] The PA radicals can reversibly react with nitrogen dioxide (NO2) to form PAN.^[1] Night-time reaction of acetaldehyde with nitrogen trioxide is another possible source.^[4] Since there are no direct PAN emissions, it is a secondary pollutant.^[1] Next to ozone and hydrogen peroxide (H₂O₂), it is an important component of photochemical smog.^[1]

${\displaystyle {\ce {CH3C(O)OO + NO2 + M <=> PAN + M}}}$^[1]

${\displaystyle {\ce {R2: CH3CHO + OH ->[O2] CH3C(O)OO + H2O}}}$

${\displaystyle {\ce {R3: CH3COCHO + hv ->[O2] CH3C(O)OO + HCO}}}$

Other peroxyacyl nitrates in the atmosphere are peroxypropionyl nitrate (PPN), peroxybutyryl nitrate (PBN), and peroxybenzoyl nitrate (PBzN). Chlorinated forms have also been observed.^[1] 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.^[1] At lower temperatures, these peroxy-nitrates are stable and can be transported over long distances,^[1] providing nitrogen oxides to otherwise unpolluted areas. At higher temperatures, they decompose into NO₂ and the peroxyacyl radica^[1] l.

The decay of PAN in the atmosphere is mainly thermal.^[1] Thus, the long-range transport occurs through cold regions of the atmosphere, whereas the decomposition takes place at warmer levels.^[1] PAN can also be photolyzed by UV radiation.^[1] It is a reservoir gas that serves both as a source and a sink of RO_x- and NO_x radicals.^[1] Nitrogen oxides from PAN decomposition enhance ozone production in the lower troposphere.^[1]

The natural concentration of PAN in the atmosphere is below 0.1 μg/m³.^[1] Measurements in German cities showed values up to 25 μg/m³.^[1] Peak values above 200 μg/m³ have been measured in Los Angeles in the second half of the 20th century (1 ppb of PAN corresponds to 4.37 μg/m³).^[1] Due to the complexity of the measurement setup, only sporadic measurements are available.^[2][5] The satellite based Cross-Track Infrared sounder (CrIS) instrument is able to provide mid-tropospheric PAN measurements on a global scale.^[5][2]

PAN is a greenhouse gas.

Sensitivity

PAN has a sensitivity to precursor emissions, mainly from VOCs and NOx.^[1][2][4] PANs sensitivity towards VOCs is greater than that of NOx.^[4] VOC reductions have more of an effect on PA radicals than on NOx.^[4] Notably, global emissions of precursor during Covid-19 demonstrated that PAN concentrations do not always decrease with a decrease in NOx concentrations.^[2][6] Similarly, PAN responds non-linearly to precursor changes.^[1][2] Alkenes and oxidized VOCs strongly influence the formation of PA radicals.^[4] Meteorological effects also influence the availability of these radicals and hence PAN formation.^[6]

Synthesis

PAN can be produced in a lipophilic solvent from peroxyacetic acid.^[7][8] 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.^[8][9]

As an alternative, PAN can also be synthesized in the gas phase via photolysis of acetone and NO₂ with a mercury lamp. Methyl nitrate (CH₃ONO₂) is created as a by-product.^[9]

Atmospheric effects

Seasonal cycles of PAN have been observed.^[1] Meteorological effects such as temperatures, wind patterns, and the availability of radicals influence PANs stability as well as transportation in the atmosphere.^[1][6] During the springtime in the northern hemisphere, high concentrations are attributed to an increase in photochemical activity.^[6] In addition, concentrations of PAN increase due to it having a relatively large lifetime against thermal decomposition.^[1] Transportation of PAN can also occur by wildfire smoke moving it into an otherwise unpolluted region.^[2] In the northern hemisphere winter however, PAN levels become limited when there is reduced hydrocarbons, NO2, and low solar radiation.^[1]

Toxicity

The toxicity of PAN is similar to that of NO2 but higher than sulfur dioxide (SO2).^[3] Populations with pulmonary disease tend to be more sensitive to the toxic effects of PAN.^[3] Eye irritation from photochemical smog can be caused by an increase in PAN levels.^[3] Concentrations at or above 0.64 mg/m³ increase the likelihood of eye irritation.^[3] PAN is a very weak mutagen.^[3]