Nitrate radical

History

The existence of the radical was postulated in 1881-1882 by Hautefeuille and Chappuis to explain the absorption spectrum of air subjected to a silent electrical discharge.

Structure and properties

The radical does not react with water.

The absorption spectrum of has a broad band for light with wavelengths from about 500 to 680 nm, with three maxima in the visible at 590, 662, and 623 nm. Absorption in the range 640–680 nm does not lead to dissociation but to fluorescence: specifically, from about 605 to 800 nm following excitation at 604.4 nm, and from about 662 to 800 nm following excitation at 661.8 nm. In water solution, another absorption band appears at about 330 nm (ultraviolet). An excited state can be achieved by photons of wavelength less than 595 nm.

Preparation

Nitrogen trioxide can be prepared in the gas phase by mixing nitrogen dioxide and ozone: : + → + This reaction can be performed also in the solid phase or water solutions, by irradiating frozen gas mixtures, flash photolysis and radiolysis of nitrate salts and nitric acid, and several other methods.

Nitrogen trioxide is a product of the photolysis of dinitrogen pentoxide , chlorine nitrate , and peroxynitric acid and its salts.

:N2O5 → NO2 + NO3

:2 ClONO2 → Cl2 + 2 NO3

Atmospheric occurrence

Formation at night

In the atmosphere, the formation of nitrate radical also occurs from the reaction:NO2 + O3 - NO3 + O2NO3 concentrations maximize during the night due to the absence of photolysis. In its nightly abundance, the nitrate radical can oxidize volatile organic compounds (VOCs).

When NO3 is formed, its subsequent reaction with nitrogen dioxide NO2 produces dinitrogen pentoxide N2O5, which serves as a reservoir for NO3: NO2 + NO3 + M - N2O5 + M

Reaction with VOCs

As a dominant nighttime oxidizing agent, NO3 reacts with a variety of VOCs, in which the mechanism and the rate of the reaction will depend on the species. As an example, oxygenates such as aldehydes are oxidized by abstracting an H-atom, resulting in nitric acid HNO3 as one of the products. The lifetime of NO3 in the presence of these compounds typically ranges from minutes to days.

Conversely, unsaturated hydrocarbons (e.g. alkenes) are commonly oxidized by adding the nitrate radical to a carbon double bond. The reaction rates with these species are higher compared to those of oxygenates, notably affecting the abundance of the unsaturated hydrocarbons. In forested areas, for instance, NO3 can rapidly oxidize biogenic volatile organic compounds (BVOCs), such as terpenes and isoprene, which have been studied to contribute as a source of secondary organic aerosols (SOAs).

Rapid daytime photolysis

The reaction that efficiently removes the nitrate radical from the troposphere during the day is its photolysis:NO3 + hv - NO2 + ONO3 + hv - NO + O2in which the pathway followed will depend on the wavelength. When the sun is overhead, the photodissociation reaches maximum rates ranging from J = 0.02 to 0.2 s-1. Therefore, the daytime lifetime of NO3 is usually less than 5 seconds.

References

References

1. R. P. Wayne, I. Barnes, P. Biggs, J. P. Burrows, C. E. Canosa-Mas, J. Hjorth, G. Le Bras. G. K. Moortgat, D. Perner, G. Poulet, G. Restelli, and H. Sidebottom (1991): "The nitrate radical: Physics, chemistry, and the atmosphere". ''Atmospheric Environment. Part A. General Topics''. volume 25, issue 1, pages 1-203. {{doi. 10.1016/0960-1686(91)90192-A
2. Richard A. Graham and Harold S. Johnston (1978): "The photochemistry of the nitrate radical and the kinetics of the nitrogen pentoxide-ozone system". ''Journal of Physical Chemistry'', volume 82, issue 3, pages 254-268. {{doi. 10.1021/j100492a002
3. (2012). "Nighttime radical observations and chemistry". Chemical Society Reviews.
4. (2017-02-13). "Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol". [[Atmospheric Chemistry and Physics]].
5. (2024-01-23). "Measurement report: Atmospheric nitrate radical chemistry in the South China Sea influenced by the urban outflow of the Pearl River Delta". Atmospheric Chemistry and Physics.