Nitrous oxide, tropospheric sources

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. 

Recent estimates indicate that the level of carbon dioxide in the atmosphere has increased by a third since the beginning of the industrial age, and that it contributes significantly to global warming. Other major contributors include methane, tropospheric ozone, and nitrous oxide. Methane is the principal component of natural gas, but it is also produced by other sources such as rice paddies and farm animals. Tropospheric ozone is generated naturally and by the sunlight-... 

Nitrous oxide is important not only as a greenhouse gas but, as discussed in Chapter 12, as the major natural source of NC/ in the stratosphere, where it is transported due to its long tropospheric lifetime (Crutzen, 1970). The major sources of N20 are nitrification and denitrification in soils and aquatic systems, with smaller amounts directly from anthropogenic processes such as sewage treatment and fossil fuel combustion (e.g., see Delwiche, 1981 Khalil and Rasmussen, 1992 Williams et al., 1992 Nevison et al., 1995, 1996 Prasad, 1994, 1997 Bouwman and Taylor, 1996 and Prasad et al., 1997). The use of fertilizers increases N20 emissions. For pastures at least, soil water content at the time of fertilization appears to be an important factor in determining emissions of N20 (and NO) (Veldkamp et al., 1998). 

In this case, N20 (called nitrous oxide or laughing gas) has natural sources, such as emissions from swamps and other oxygen-free ( anoxic ) waters and soils. The oxygen atoms in this reaction can come from several tropospheric photolytic reactions involving OH or OOH. Another source of NO is the thermal reaction between N2 and 02 ... 

Globally, the oxides of nitrogen, NO (nitric oxide), NO2 (nitrogen oxide), and N2O (nitrous oxide), are key species involved in the chemistry of the troposphere and stratosphere. NO and N2O are produced mostly by microbial soil activity, whereas biomass burning is also an important source of NO. Nitric oxide is a species involved in the photochemical production of ozone in the troposphere, is involved in the chemical produaion of nitric acid, and is an important component of acid precipitation. Nitrous oxide plays a key role in stratospheric ozone depletion and is an important greenhouse gas, with a global warming potential more than 200 times that of CO2. 

Here we can see a biological source for nitrous oxide (N20), an important and rather stable trace gas in the troposphere. In nature there are many other... 

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). 

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. 

While ozone is a good molecule high in the stratosphere, its reviews in the lower troposphere are decidedly mixed. It is the source of minuscule concentrations of hydroxyl radical (OH, 10 molecules/ cm ). Hydroxyl is so extraordinarily active that it reacts with hydrocarbons (methane, benzene, etc) and most atmospheric compounds. Some extremely stable molecules, including chlorofluorocarbons like CFCI3 and CF2CI2 and N2O (nitrous oxide from fertihzers, a source of stratospheric NO), do not react with OH and migrate to the upper stratosphere (see chapter 8 for the consequences). 

Dinitrogen monoxide N2O is also called nitrous oxide, and emitted from natural and anthropogenic sources from the ground. Since N2O does not dissipate in the troposphere, it reaches to the stratosphere and subjects photolysis to provide reactive oxides of nitrogen (odd nitrogen). For this reason, the photolysis of N2O in the stratosphere is very important. 

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