Nitrous oxide stratosphere

The stratosphere contains, however, only small amounts--a few tenths of a ppb-of chlorine free radicals of natural origin. They are produced by the decomposition of methyl chloride, CH3Q. The nitrogen oxides (NO and NO2) are more abundant and are produced in the stratosphere by the decomposition of nitrous oxide, N2O. Both CH3CI and N2O are of biological origin these compounds, released at the Earth s surface, are sufficiently stable to reach the stratosphere in significant amounts. 

Nitrous oxide (N2O) is an important greenhonse gas with a radiative forcing effect 310 times that of CO2 and a lifetime in the troposphere of approximately 120 years. Part of the N2O is converted to NO in the stratosphere, and so contributes to depletion of ozone. Nitric oxide (NO) is very reactive in the atmosphere and has a lifetime of only 1-10 days. It contribntes to acidification and to reactions leading to the formation of ozone in the troposphere, and so also to global warming. 

Besides NOx oxides, there is nitrous oxide (N2O), which is of special interest in isotope geochemistry. N2O is present in air at around 300 ppb and increases by about 0.2% per year. Nitrous oxide is an important greenhouse gas that is, on a molecular basis, a much more effective contributor to global warming than CO2 and that is also a major chemical control on stratospheric ozone budgets. 

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. 

Similar effects have also been observed in stratospheric nitrous oxide. and 5 0 measurements by Cliff and Thiemens (1997) reveal that stratospheric... 

Clayton RN, Goldsmith JR, Karel KJ, Mayeda TK, Newton RP (1975) Limits on the effect of pressure in isotopic fractionation. Geochim Cosmochim Acta 39 1197-1201 Clayton RN, Onuma N, Grossman C, Mayeda TK (1977) Distribution of the presolar component in Allende and other carbonaceous chondrites. Earth Planet Sd Lett 34 209-224 Clayton RN, Goldsmith JR, Mayeda TK (1989) Oxygen isotope fractionation in quartz, albite, anorthite and caldte. Geochim Cosmochim Acta 53 725-733 Cliff SS, Thiemens MH (1997) The 0/ 0 and 0/ 0 ratios in atmospheric nitrous oxide a mass independent anomaly. Science 278 1774-1776 Cliff SS, Brenninkmeijer CAM, Thiemens MH (1999) First measurement of the 0/ 0 and ratios in stratospheric nitrous oxide a mass-independent anomaly. J Geophys Res 104 16171-16175... 

Rafter TA (1957) Sulphur isotopic variations in nature, P 1 the preparation of sulphur dioxide for mass spectrometer examination, N Z J Sci Tech B38 849 Rahn T, Wahlen M (1997) Stable isotope enrichment in stratospheric nitrous oxide. Science 278 1776-1778... 

Yung YE, Miller CE (1997) Isotopic fractionation of stratospheric nitrous oxide. Science 278 1778-1780... 

Selwyn, G., J. Podolske, and H. S. Johnston, Nitrous Oxide Ultraviolet Absorption Spectrum at Stratospheric Temperatures, Geophys. Res. Lett., 4, 427-430 (1977). 

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). 

Johnson, J. C., S. S. Cliff, and M. H. Thiemens, Measurement of Multioxygen Isotopic (SlsO and S170) Fractionation Factors in the Stratospheric Sink Reactions of Nitrous Oxide, J. Geophys. Res., 100, 16801-16804 (1995). 

Rahn, T., and M. Wahlen, Stable Isotope Enrichment in Stratospheric Nitrous Oxide, Science, 278, 1776-1778 (1997). 

A few comments on nitrous oxide Nitrous oxide (NzO) is a gas produced mainly through natural sources. However, generally, motor vehicles and combustion processes contribute to its formation in urban areas. Unfortunately, it is involved adversely in the two global environmental problems it contributes to the greenhouse effect and penetrates into the stratosphere, destroying the ozone layer. 

Nitrous oxide is nontoxic—it used as the propellant in whipped-cream spray cans—and so might seem to be an unlikely pollutant. However, as noted earlier, it may contribute significantly to greenhouse warming. Furthermore, on diffusing to the stratosphere, N20 becomes involved in the ozone cycle (reactions 8.2, 8.3, and 8.6) following its conversion to nitric oxide (NO) ... 

Nitrous oxide contributes severely to global warming and the depletion of ozone in the stratosphere (Crutzen 1981, Bouwman 1996). Almost 90% of the global atmospheric N2O is formed during the microbial transformation of nitrate (NO ) and ammonia (NH ) in soils and water. In OECD countries the agricultural contribution to N2O emissions is estimated at 58% (IPCC 2001). Soils fertilised with inorganic fertilisers and manure stores are seen as the largest sources (Chadwick et al. 1999, Brown ef al. 2002). 

The concentration of nitrous oxide (N2O) in the atmosphere is increasing. This is a concern, since N2O has been identified as a greenhouse gas and as a source of ozoneconsuming NO in the stratosphere. A significant source of N2O to the atmosphere is production of adipic acid (AA), which is used in the production of nylon. Adipic acid is formed from reaction of cyclohexanol with nitric acid (HNO3) according to the scheme in Fig. 13.13. 

The Airborne Submillimeter SIS Radiometer (ASUR), operated on-board the German research aircraft FALCON, measures thermal emission lines of stratospheric trace gases at submillimeter wavelength. Measurement campaigns with respect to ozone depletion in the Arctic winter stratosphere were carried out in yearly intervals from 1992-97 to investigate the distributions of the radical chlorine monoxide (CIO), the reservoir species hydrochloric acid (HC1), the chemically inert tracer nitrous oxide (N20), and ozone (O3). The high sensitivity of the receiver allowed to take spatially well resolved measurements inside, at the edge, and outside of the Arctic polar vortex. This paper focuses on the results obtained for CIO from... 

Nitrous oxide (N20) is produced by bacteria in the natural denitrification process. It is chemically inert in the troposphere, but in the stratosphere it is degraded photochemically. The average concentration of N20 in the troposphere is about 300 ppb, and its residence time there is 10 years. What is the global rate of production of N20 in units of kg/ year Assume that the volume of the stratosphere (at 0°C and 1 atm) is 10% that of the atmosphere. 

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). 

FIGURE 7.10 Wavelength dependent enrichment coefficients for the nitrous oxide isotopologues in the region of stratospheric photolysis. Temperature is set to 298 K. 

---