Nitrous oxide, tropospheric sinks

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 last step in the current manufacture of adipic acid involves oxidation by nitric acid, which results in the formation of nitrous oxide (N2O) that is released into the atmosphere. Given that N2O has no tropospheric sinks, it can rise to the stratosphere and be a factor in the destruction of the ozone layer. It also acts as a greenhouse gas (see Section 8.4.1). 

A ground level source, and stratospheric sink for N2O, is consistent with the observed vertical concentration gradient. McElroy et al. (1976) estimate that an ozone reduction of up to 20% is possible, based upon the above catalytic cycle and increasing anthropogenic emissions of N2O, assuming no other sinks for atmospheric nitrous oxide are found. More recent model predictions suggest a total column ozone reduction of 4% based on the current increase in tropospheric N2O concentrations (National Research Council, 1984). 

There are no sinks for nitrous oxide in the troposphere. Instead, all of it rises eventually in the stratosphere where each molecule absorbs UV light and decomposes, usually to N2 and atomic oxygen (90%) or reacts with atomic oxygen (10%). More details about the role of N2O as ozone-depleting species are contained in Chapter 6. 

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. 

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