Reactions In The Upper Atmosphere 521 Nitrogen

We have seen that molecular nitrogen is the most prevalent species in the atmosphere. The reason that nitrogen is not so important photochemically is its large bond energy (7.373 eV, corresponding to a photon of wavelength X = 169 nm), which limits its photodissociation chemistry to areas above the ozone layer. Molecular nitrogen absorbs only weakly between 169 and 200 nm, absorption in this region leading to a spin-forbidden excitation from ground-state N2 to the (3X+) excited state

1The selection rules are given here without proof or justification, which can be found in standard quantum mechanics and spectra reference books, such as M. D. Harmony, Introduction to Molecular Energies and Spectra, Holt, Reinhart, & Winston, New York, 1972, pp. 450-455.

Strictly speaking, these selection rules are valid ("allowed" transitions) only for small molecules made up of light atoms; "forbidden" transitions can occur in larger molecules, particularly those containing heavy atoms, but they are generally of very low intensity.

that is a source of highly excited oxygen atoms by transfer of energy through collisions with ground-state O:

With photons of wavelengths below lOOnm, photodissociation occurs, possibly through photoionization and dissociative recombination involving ground-state O2 and producing nitric oxide (NO)2:

An energetically excited nitrogen atom can be collisionally deactivated to the ground state by O2, producing singlet oxygen (discussed in the following section),

or it can react with O2 to produce NO:

Nitric oxide might also be expected to form by direct combination of ground-state nitrogen and oxygen atoms,

where M can be any other atomic or molecular species such as N, O, N2, or O2. This is a kinetically simple trimolecular reaction that occurs at essentially every triple collision between an N, an O, and an M species with a rate constant of the order of 1O"32 cm6 molecule"2 s"1 (see Section 4.3). However, since photodissociation of N2 only occurs at high altitudes above the ozone layer where the total concentration of M is very small, this recombination reaction is not a significant source of atmospheric NO.

As we will see in Sections 5.2.3, 5.3.2, and 5.3.3, NO plays a significant role in stratospheric photochemistry and it is also an important constituent in photochemical smog in the troposphere. At stratospheric heights it comes

2NO has an odd number of electrons (15), and therefore one of them must be unpaired. This unpaired electron occupies an antibonding w* orbital and is spread over the whole molecule. Nitrogen dioxide (NO2) also has an odd number of electrons (23) and thus also an unpaired electron, but in this case the electron is mostly localized on the N atom. Although NO and NO2 are technically free radicals and are involved in many free-radical reactions, in this book we will not indicate the lone (unpaired) electron on each of them as a dot; this will also be the case for atoms with odd numbers of electrons, such as hydrogen or the halogens. The dot will be used on other free radicals in this book.

mainly from nitrous oxide, N2O, by reaction with excited O(*D2) oxygen atoms:

It is generally accepted that the N2O is not produced directly in the atmosphere, but rather in the biosphere, primarily by microorganism reactions in soils and oceans (roughly two-thirds natural, one-third anthropogenic). Nitrous oxide is a strong greenhouse gas (Table 3-2). It does not react with the hydroxyl radical (.OH, the major initiator of removal of most trace gases in the atmosphere—see Section 5.3.2), nor does it absorb light above 260 nm, and therefore it is very stable in the troposphere. It is transported to the stratosphere primarily by large-scale movements of air masses within the troposphere. The concentration of N2O in the atmosphere is now increasing above the preindustrial level at a rate of approximately 0.25%/year, due in large part to anthropogenic biomass burning and bacterial oxidation of fertilizer nitrogen (NH|).3 In addition to reaction (5-9), N2O is destroyed in the stratosphere by photolysis. It absorbs light in a broad continuum starting at 260 nm, with a maximum at about 182 nm. The major photodissociation reaction is

providing one stratospheric source of excited 1D2 oxygen atoms. Other sources are discussed in the following section.

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