Peroxyacyl nitrates
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 NO2.[1][2][3]
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.[4][5]
Formation
[edit]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-methane[6]: 2679 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.
- Hydrocarbons + O2 + light → RC(O)OO•
These react reversibly with nitrogen dioxide (NO2) to form PANs:[6]: 2680
- RC(O)OO• + NO2• ⇌ RC(O)OONO2
Night-time reaction of aldehydes with nitrogen trioxide is another possible source.[6]: 2680
Since they dissociate quite slowly in the atmosphere into radicals 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.
Types
[edit]Peroxyacetyl nitrate is the most prevalent peroxyacyl nitrate (75–90% of total atmospheric emissions),[6]: 2681 followed by peroxypropionyl nitrate (PPN).[7] Peroxybenzoyl nitrate (PBzN)[7] and methacryloyl peroxynitrate (MPAN)[4] 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.[4]: 7624
Health effects
[edit]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,[7] and are considered potential contributors to the development of skin cancer.[citation needed]
References
[edit]- ^ Cape, J.N. (2003). "Effects of airborne volatile organic compounds on plants". Environmental Pollution. 122 (1): 145–157. doi:10.1016/S0269-7491(02)00273-7. PMID 12535603.
- ^ Gaffney, Jeffrey S.; Marley, Nancy A. (2021). "The Impacts of Peroxyacetyl Nitrate in the Atmosphere of Megacities and Large Urban Areas: A Historical Perspective". ACS Earth and Space Chemistry. 5 (8): 1829–1841. Bibcode:2021ESC.....5.1829G. doi:10.1021/acsearthspacechem.1c00143. S2CID 238708473.
- ^ Jickells, T.; Baker, A. R.; Cape, J. N.; Cornell, S. E.; Nemitz, E. (2013). "The cycling of organic nitrogen through the atmosphere". Philosophical Transactions of the Royal Society B: Biological Sciences. 368 (1621). doi:10.1098/rstb.2013.0115. PMC 3682737. PMID 23713115.
- ^ a b c LaFranchi, B. W.; Wolfe, G. M. (2009). "Closing the peroxy acetyl nitrate budget: observations of acyl peroxy nitrates (PAN, PPN, and MPAN) during BEARPEX 200" (PDF). Atmos. Chem. Phys. 9 (19). Copernicus Publications: 7623–7641. Bibcode:2009ACP.....9.7623L. doi:10.5194/acp-9-7623-2009.
- ^ Thornton, Joel. "PANs". Department of Atmospheric Sciences, University of Washington. Retrieved 14 November 2010.
- ^ a b c d Fischer, E. V.; Jacob, D. J.; Yantosca, R. M.; Sulprizio, M. P.; Millet, D. B.; Mao, J.; Paulot, F.; Singh, H. B.; Roiger, A.; Ries, L.; Talbot, R.W.; Dzepina, K.; Pandey Deolal, S. (14 March 2014). "Atmospheric peroxyacetyl nitrate (PAN): a global budget and source attribution". Atmospheric Chemistry and Physics. 14 (5): 2679–2698. doi:10.5194/acp-14-2679-2014.
- ^ a b c Kleindienst, Tadeusz E.; Shepson, Paul B.; Smith, David F.; Hudgens, Edward E.; Nero, Chris M.; Cupitt, Larry T.; Bufalini, Joseph J.; Claxton, Larry D.; Nestman, F. R. (January 1990). "Comparison of mutagenic activities of several peroxyacyl nitrates". Environmental and Molecular Mutagenesis. 16 (2): 70–80. doi:10.1002/em.2850160204.