Nitrous oxide photodissociation

The 8 N- and 8 0-values of atmospheric N2O today, range from 6.4 to 7.0%c and 43 to 45.5%c (Sowers 2001). Terrestrial emissions have generally lower 8-values than marine sources. The 8 N and 8 0-values of stratospheric N2O gradually increase with altitude due to preferential photodissociation of the lighter isotopes (Rahn and Wahlen 1997). Oxygen isotope values of atmospheric nitrous oxide exhibit a mass-independent component (Cliff and Thiemens 1997 Clifif et al. 1999), which increases with altitude and distance from the source. The responsible process has not been discovered so far. First isotope measurements of N2O from the Vostok ice core by Sowers (2001) indicate large and 0 variations with time (8 N from 10 to 25%c and 8 0 from 30 to 50%c), which have been interpreted to result from in situ N2O production via nitrification. 

The photochemical processes of triatomic molecules have been extensively studied in recent years, particularly those of water, carbon dioxide, nitrous oxide, nitrogen dioxide, ozone, and sulfur dioxide, as they are important minor constituents of the earth s atmosphere. (Probably more than 200 papers on ozone photolysis alone have been published in the last decade.) Carbon dioxide is the major component of the Mars and Venus atmospheres. The primary photofragments produced and their subsequent reactions are well understood for the above-mentioned six triatomic molecules as the photodissociation involves only two bonds to be ruptured and two fragments formed in various electronic states. The photochemical processes of these six molecules are discussed in detail in the following sections. They illustrate how the knowledge of primary products and their subsequent reactions have aided in interpreting the results obtained by the traditional end product analysis and quantum yield measurements. 

For the major atmospheric oxide of nitrogen—nitrous oxide—the source is biological activity at the surface, and the sink is transport into the stratosphere, where it is destroyed by photodissociation and reaction with 0( D). There are no important photochemical reactions for nitrous oxide in the troposphere. 

The main photodissociation process for nitrous oxide is N20— N2 + O( D). It occurs with a quantum yield of nearly unity (Cvetanovic, 1965 Greiner, 1967a Paraskepopoulos and Cvetanovic, 1969). The photoproducts N2 + 0(5P) and NO + N(4S), although energetically allowed, would violate the spin conservation rule and are expected to be generated with low probability. For the second of these processes Preston and Barr (1971) have established a quantum yield of less than 2%. 

Nitrous oxide is inert in the troposphere its major atmospheric sink is photodissociation in the stratosphere (about 90%) and reaction with excited oxygen atoms, 0(1D) (about 10%). Oxidation of N20 by 0(1D) yields NO, providing the major input of NO to the stratosphere. We will return to this process in Chapter 5. Sources of N20 exceed estimated sinks by 3.8Tg(N)yr 1. 

Of special interest here is the radical generation during the early part of the irradiation, before the oxidant concentration has developed photodissociation of nitrous acid, HONO, and aldehydes is very important. The concentration of nitrous acid in the atmosphere due to the reaction... 

Inorganic Reactions. Photooxidation of propylene in the presence of oxides of nitrogen involves numerous inorganic reactions. The role of the NO2 photolysis in initiating O- and Og-reactions has already been discussed. Another inorganic compound of photochemical interest is nitrous acid, HONO, since it provides another source for OH radicals. A reaction scheme for the formation and subsequent photodissociation... 

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