Here we review the definitions of various nitrogen compounds.
Nitrous oxide, also called laughing gas, is often used by dentists as an anesthetic. Although some of this gas escapes to the atmosphere, a more important source of N2O is found in the natural and agricultural cycling of the nitrogen that is necessary for the maintenance of living matter. N2O is particularly important because it is an unreactive gas which is not water soluble and does not absorb visible radiation. The atmospheric lifetime of N2O is estimated to be about 150 years. Its primary destruction path is transport to the stratosphere where it absorbs UV radiation to form N2 and an oxygen atom (O). A minor destruction pathway (about 3.5%) is reaction with O(1D) to form two NO molecules. N2O thus acts as a carrier of reactive nitrogen to the stratosphere. Reactive nitrogen species formed in the troposphere, such as NO or nitrogen dioxide (NO2), have very short atmospheric lifetimes. They are chemically converted to nitric acid (HNO3) and rained out with a lifetime of 5-10 days. Thus they do not represent a significant source of NOx (NO + NO2) for the stratosphere. This is a general principle for stratospheric chemistry. Relatively inert carriers are necessary to get reactive gases from the surface into the stratosphere.
N2O is one of a number of forms of nitrogen that participate in what we call the biogeochemical cycle of nitrogen. Nitrogen is a heavy enough atom that it cannot escape Earth's gravitational pull. An atom of nitrogen therefore is trapped on Earth and is continuously converted from one form to another by chemical and physical processes. The cycle of nitrogen is illustrated in Figure 10.02, which shows the various forms (reservoirs) which nitrogen can take.
Unlike most elements, the bulk of the nitrogen in the Earth system is in the atmosphere (4 x 109 teragrams (Tg) where one Tg is equal to 1012 grams or one trillion grams). A smaller amount (4-6 x 108 Tg) is in crustal rocks, while an even smaller amount (~106 Tg) is in organic matter in the soils and ocean.
Nitrogen is of fundamental importance to the biosphere as the key element in the formation of amino acids. Although nitrogen gas, N2, is the predominant molecule in the atmosphere, it is not directly useful to living things. The triple bond in N2 makes it very difficult to break the molecule into its constituent nitrogen atoms. Most plants make use of nitrogen in the form of ammonium (NH4+) or nitrate (NO3-) ions. The process of conversion from atmospheric N2 to available nitrogen is called nitrogen fixation. Fixation can be accomplished in a number of ways. One of these is biological fixation. Certain bacteria, including the blue-green algae, fix nitrogen by conversion from N2 to NH4+. Bacteria that live in symbiosis with legumes (beans, peas, clover, etc.) also fix nitrogen. Most agricultural crops are nitrogen limited, and higher yields are achieved by the application of nitrogen fertilizer. The manufacture of nitrogen fertilizer adds fixed nitrogen to the cycle.
Nitrogen fixation produces NH4+ which is used by a number of soil organisms as an energy source. These organisms convert the NH4+ to NO3- in a process called nitrification. It has also been shown that the process of nitrification can release N2O as a byproduct.
Fixed nitrogen which has been taken up by plants and incorporated into plant tissues as amino acids is eventually recycled when the plant is harvested, dies, or drops its leaves. Animals eat the plants and make use of the nitrogen. This nitrogen is also returned to the cycle by excretion (as urea or other amine containing compounds) or the death and decay of the animal. The excreta, leaves, and dead organisms are acted upon by bacteria which can denitrify biomolecules producing N2 or N2O which is returned to the atmosphere. If the system were in balance, the amount of nitrogen returned to the atmosphere would equal that removed through nitrogen fixation. The production and use of nitrogen fertilizer enhances the fixation of nitrogen while denitrification responds with a time delay, leading to an imbalance in the system. Measurements indicate that about 7% of denitrification leads to production of N2O. Thus, by increasing nitrogen fixation through the manufacture of fertilizers, the atmospheric content of nitrous oxide can be increased.
The equilibrium state for a mixture of N2, O2, and water would have most of the N2 and O2 reacted as NO3- in the water. Achieving this state is an extremely slow process. Life processes such as those described above are responsible for maintaining our atmosphere of N2 and O2 in a state far from equilibrium. Industrial input modifies this state of disequilibrium, changing its balance.
With the advent of industrial fertilizer production, an imbalance was introduced into the nitrogen cycling system. The atmosphere responds to such imbalances by changing concentrations of cycle components until a new balance is achieved. These changes occur on a time scale for the particular gases involved. For N2O, its lifetime is about 150 years. The present imbalance in the system, from the recent rapid increases in the amount of fertilizer production, is driving an upward trend in N2O that is measured to be about 0.2 %/year.
Because of its long atmospheric residence time, nitrous oxide is relatively well mixed throughout the lower atmosphere. The difference between the northern and southern hemisphere concentrations is about 1 part per billion by volume (ppbv)* out of about 300 ppbv, with the northern hemisphere having the higher concentrations. The long residence time also means that the concentration of N2O at a given location will be relatively steady with only small fluctuations about the mean. These facts imply that the measurement of changes in N2O are relatively easy to make. It is fortunate that this is so, because the changes that have been measured are also relatively slow changes.
Direct measurements of N2O have been taken since the 1960s. Recently, measurements have been made on air trapped in bubbles in Arctic and Antarctic ice. A core of ice is drilled to great depth and brought to the surface. Measurements are made of the gases trapped in the bubbles as a function of depth in the ice. The depth can be related to age of the trapped air. By this method the record of N2O concentrations can be extended back several hundred years as shown in Figure 10.03. The measured N2O concentration was steady at about 275 to 280 ppbv from 1750 to the late 1800s. It then began to rise so that a concentration of about 290 ppbv was reached by 1950. Since that time the increase has been about 0.6 ppbv/year or about 0.2%/year. The present concentration is a little over 310 ppbv.
Agricultural systems represent an important part of the global nitrogen cycle. Nitrogen which has been fixed is removed from the agricultural system when the crop is harvested. Future crops are then produced by adding fertilizer containing industrially fixed nitrogen. When the nitrogen fertilizer is added to an agricultural field, the intention is for all of that nitrogen to be taken up by the crop. It is inevitable, however, that some of it will be lost to the process of denitrification. A fraction of that denitrification will result in production of N2O. In this way the addition of nitrogen fertilizers enhances the source of N2O to the atmosphere.
Nitrous oxide is also a potential byproduct of fuel consumption in internal combustion engines. Early measurements indicated that this could be a primary contributor to the observed rate of increase of N2O in the atmosphere. Problems have been found in the system used to measure N2O in combustion processes, and it is now believed that combustion is only a minor contributor to the observed increase. There is clearly some remaining uncertainty in the role of combustion to N2O production.
The primary constituents of the atmosphere are N2 (79%) and O2 (20%). At normal atmospheric temperatures these molecules are stable and coexist without interacting. In combustion engines, air is heated to temperatures that can be several thousand degrees Celsius. At these temperatures decomposition of air begins to take place and nitrogen oxides are formed. The primary nitrogen oxide formed is nitric oxide (NO). This can be converted to nitrogen dioxide (NO2) by reaction with ozone. NO2 is a brick-red or brown gas which is often observed in cities during pollution events (the "brown cloud").
The amount of NOx produced during combustion is highly sensitive to the temperature within the combustion engine. The NOx produced can thus be controlled by controlling the temperature. Such low-NOx combustors are often achieved by using an air-to-fuel ratio that is not quite the most efficient for burning (nonstoichiometric). By burning either fuel-rich, fuel-poor, or a combination of fuel-rich and fuel-poor in sequence, the amount of NOx in the combustion process can be reduced.
2.8.1 NOx Production by Ground Level Sources -- NOx is produced at the ground in both fixed sources (e.g. power plants) and mobile sources (e.g. automobiles). This NOx is converted to soluble nitric acid (HNO3) and rained out of the atmosphere. However, thunderstorms which pass over polluted areas will pull in air with high amounts of NOx and will loft that air rapidly to altitudes of 8 km or more. This represents a potentially large source of NOx to the upper troposphere but contributes a negligible amount to the stratosphere. Lightning generated in these thunderstorms can also be a significant local source of NOx in the troposphere.
2.8.2 NOx Production by Aircraft -- The world now has a fleet of about 10,000 commercial aircraft flying several billion miles per year. These aircraft are powered by engines burning more than 100 billion kg of fuel per year at high temperatures. They inject NOx from this combustion at their cruise altitudes in the upper troposphere and lower stratosphere. Although the amount of NOx generated by aircraft is small compared to that generated at the ground by fossil fuel combustion, it may still be significant to the upper troposphere because only a small fraction of the ground-level NOx get transported upward in convective storms. The relative roles of these two sources to upper tropospheric NOx is uncertain and subject to debate. This is an active research area.
Neither subsonic aircraft nor ground level sources of NOx are significant for the stratosphere, nor is the production of NOx in the upper troposphere by lightning. The NOx is just too reactive to be able to survive the trip into the stratosphere. The small amount of nitrous oxide (N2O) produced during combustion is more important to the stratosphere than tropospheric NOx.
While subsonic aircraft fly in the upper troposphere and lowest part of the stratosphere, supersonic aircraft, such as the Concorde, which is shown in Figure 10.04, cruise exclusively in the stratosphere. The altitude at which they cruise depends on the speed, or mach number, at which they fly.
The possibility of a large fleet of supersonic aircraft has arisen from time to time since the early 1970s. The Concorde fleet is only 16 aircraft. In order to be economically feasible, the potential fleets currently under discussion would number about 500 or more. The amount of NOx that such a fleet would inject directly into the stratosphere depends upon the number of aircraft, the amount of fuel they burn, and the amount of NOx produced per unit of fuel burned. The amount of NOx produced per unit of fuel burned is called the emission index (EI). Current subsonic aircraft have an average EI of about 15 g of NOx per kilogram of fuel burned. As newer engines are introduced, the average EI for NOx is decreasing. For a proposed fleet of 500 aircraft with an EI of 5 g NOx/kg fuel, the NOx concentrations in the lower stratosphere are estimated to increase by a few tens of percent.