Nitrous Oxide

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Nitrous oxide (N2O) is produced both naturally and via human intervention. Natural sources of N2O result primarily from bacterial activity in soils. In the process of denitrification, the stripping of oxygen from nitrate, bacteria such as Pseudamonas denitrificans and Nitrobacter release both nitrous oxide and nitrogen gas to the atmosphere. Tropical soils, far more productive of N2O than temperate soils, add about 4 million tons of nitrous oxide to the atmosphere annually. Synthetic (human-made) emissions, many and varied, account for some 40% of the total of 15 million tons added each year. During fossil fuel combustion, oxygen and nitrogen combine to produce N2O; and during the production of nylon (the generic term for all synthetic polyamides fabricated into all manner of tubes, pipes, filaments, and coatings), adipic acid is produced, as is N2O, as a byproduct from the oxidation of nitric acid and cyclohexanol. Approximately 0.3 metric tons of N2O are released per metric ton of adipic acid produced. Considering that some 5 billion pounds are manufactured worldwide, N2O is emitted at appreciable levels.

CO O

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Figure 5.4. Atmospheric nitrous oxide concentrations during the years 1000-2000. [Figure reproduced with the kind permission of the Intergovernmental Panel on Climate Change (IPCC).]

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Figure 5.4. Atmospheric nitrous oxide concentrations during the years 1000-2000. [Figure reproduced with the kind permission of the Intergovernmental Panel on Climate Change (IPCC).]

The increasing application of nitrogen-based fertilizers is giving rise to ballooning amounts of N2O, as is the increasing production of nitric acid, a major ingredient in explosives and nylon, which is manufactured by the catalytic oxidation of ammonia, a process that produces N2O as a byproduct. The application of fertilizers and livestock manure to pasture and cropland adds significant levels of nitrogen to soils that will be converted to N2 O via bacterial metabolism. Also, although nitrous oxide has a short retention time in the atmosphere as it breaks down readily in the presence of sunlight, its continued and increasing abundance makes it a substantial climate forcer. As shown in Figure 5.4, it, too, has had a substantial increase since the mid-1950s. Perhaps of most concern is the fact that although currently N2 O contributes about 0.1 W/m2 to the total forcing, it is a strong IR absorber—200 times more absorbing than CO2. It is its additive contribution together with that of methane, ozone, and the halocarbons that provides the largest climate forcing [4].

Halocarbons

Halocarbons are compounds containing carbon and halogen atoms—chlorine, bromine, iodine, and fluorine. The most notorious of the halocarbons are the chlorofluorocarbons (CFCs), of which there are a large family. If any synthetic chemical could have been said to be perfect, it would be the CFCs. They were tailormade: a response to the need to replace ammonia, sulfur dioxide, and methylchloride, then (in 1928) common household refrigerants, with a far less explosive, corrosive, and nontoxic substance. Frigidaire (at the time a subsidiary of General Motors) rose to the challenge. In 2 years they solved the problem and delivered chloroflurocarbon — chemically stable, inert, nonflammable, nonexplosive, and nontoxic. Who could ask for anything more? Also, their use in refrigerators substantially reduced electric power demand, which meant less combustion of fossil fuel. These Freons, the name DuPont dubbed them, made automobile air conditioning possible, as well as aerosol spray cans, solvents, and blowing agents for packaging materials and foams. There was good reason for scientists to hail CFCs as the perfect solution to the world's cooling and propellant needs. By any set of criteria, they were a success—until James Lovelock discovered trichlorofluoromethane (TCFM) in the air over Adrigale, County Cork, Ireland, in 1971 [13]. The inertness and relative water insolubility of the dozen or so CFCs were being viewed as a bonus. They weren't. They were in fact a calamitous flaw. Because CFCs are not destroyed in the lower atmosphere, they drift into the stratosphere, which is surprising, as CFC molecules are heavier than air. The molecular weight of TCFM is 137; of air, only 29. In the stratosphere they contact a layer of intense, highly energetic ultraviolet (UV) radiation (electromagnetic radiation), which splits the TCFM, releasing chlorine. The free chlorine atoms attack ozone molecules, forming chlorine oxide and releasing a molecule of oxygen—result: loss of ozone. The three reactions are:

CfCls + UV ^ CfCl2 + Cl Cl + O3 ^ ClO + O2 ClO + O ^ Cl + O2

It takes about a decade for CFCs to reach the stratosphere. Once there, they can react with ozone for as long as 100 years, with a single CFC molecule destroying 100,000 molecules of ozone. This is the mechanism that thins the protective ozone layer and produces the holes in the ozone that permits harmful UV radiation to reach the citizens below in the troposphere—at the earth's surface. CFCs contribute to over 80% of ozone depletion.

Regretfully, the almost perfect CFCs had yet another inherent imperfection. They are tremendous absorbers of IR radiation—20,000 times more absorbing than CO2. Fortunately, they are present in concentrations of only parts per trillion. Nevertheless, it had become a new and significant greenhouse gas, a major climate forcer. I say, had become, because of the Montreal Protocol. In 1987, 57 nations established the Montreal Protocol, which set a target for reducing the global production of CFCs by 50% by 1998. In 1992, it was further agreed to phase out production of CFCs in developed countries by 1996, and in developing countries by 2010. Now, with 68 countries on board, we are witnessing a decline in atmospheric CFC levels. However, as noted earlier, CFCs are long-lived and can remain aloft for years, adversely affecting the ozone layer.

Too often, the air, like the ocean, is viewed as a bottomless pit: the recipient of all manner of industrial effluents. William T. Sturgis, an atmospheric chemist at the University of East Anglia, Norwich, England, discovered trifluoro-methylsulfurpentafluoride (Sf5Cf3)—a rare halocarbon, 18,000 times more IR - absorbing than CO2, residing in the atmosphere, apparently a byproduct of industrial processes using fluorine [ 14] : More recently another synthetic intense IR absorber, sulfur hexafluoride (SF6), was discovered and identified [15]. So much for human forcings, but what of natural?

One of the most extraordinary discoveries was the recent finding that certain types of common fungi can produce ozone-destroying methyl halide gases. For years, the source of these gases in the atmosphere had eluded scientists until Kathleen K. Tresedu, a biogeochemist at the University of Pennsylvania, studied gases produced by four types of ectomycorrhizal fungi, and found that each produced methylchloride, methylbromide, and methylio-dide. These ectomycorrhizal fungi envelope roots of trees, forming symbiotic relationships. The fungi remove nitrogen and phosphorus from the soil and pass a portion along to the trees. In return, the trees provide carbohydrate for the fungi, which are nonphotosynthetic. As these fungi are found in forests the world over, they make up as much as 15% of the soil's organic matter. Between them, methylchloride and methylbromide appear to be responsible for some 20% of the current ozone destruction. On the face of it, these fungal emissions do not appear preventable or manageable [16].

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Responses

  • Jole
    How is nitrous oxide produced by human activity?
    2 years ago
  • fatima
    Why is nitrous oxide significant in global warming and climate change?
    10 months ago
  • jaxon
    How do humans produce nitrous oxide?
    10 months ago

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