Nitrous Oxide as a Greenhouse Gas - Essay Example

Literature Review Nitrous oxide is a greenhouse gas that also plays a role in the cycling of stratospheric ozone. Air samples from the lower stratosphere exhibit 15N/14N and 18O/16O enrichment in nitrous oxide, which can be accounted for with a simple model describing an irreversible destruction process. The observed enrichments are quite large and incompatible with those determined for the main stratospheric nitrous oxide loss processes of photolysis and reaction with excited atomic oxygen. Thus, although no stratospheric source needs to be invoked, the data indicate that present understanding of stratospheric nitrous oxide chemistry is incomplete. Nitrous oxide (N2O) is an atmospheric trace gas that contributes to the greenhouse effect. It is also involved in the catalytic destruction of ozone in the stratosphere and is increasing in concentration by about 0.25% per year. The increase is believed to result from fertilizer use, emissions from internal combustion engines, biomass burning, and industrial processes (Khalil 1995). It is naturally produced by nitrification and denitrification in soils and in the oceans, and is destroyed in the stratosphere via photolysis (90%) and reaction with excited atomic oxygen [O(1D)] (10%). Its atmospheric lifetime is between 100 and 150 years. Although the major sources and sinks of N2O are known, they are poorly quantified and inadequately balanced, both in terms of mass exchange and in their N and O isotopic composition. Stable isotopes have been used in the past to constrain sources and sinks of other atmospheric trace gases but have yet to be successfully applied to N2O. The isotopic approach to a global N2O budget is hindered by the wide range of observed isotopic values for each of the major natural sources, making it difficult to assign a unique value to each of the source terms. Soil flux samples have been shown to be variable but consistently depleted in both 15N and 18O relative to atmospheric N2O. Oceanic samples have exhibited a trend similar to typical nutrient profiles, with slightly depleted surface waters becoming progressively enriched along the nutricline and stabilizing with depth. Early analytical methods employed infrared absorption techniques (Wahlen 1985) or required decomposition of N2O with subsequent analyses of N2 and CO2. The use of direct injection techniques was introduced in 1993 when Kim and Craig reported heavy enrichment in both the N and O isotopes in two samples of stratospheric air. They proposed that a stratosphere to troposphere return flux of heavy N2O could balance the observed isotopically light source terms, although a simple mass-balance model showed that this led to a considerable overcorrection. Direct injection of N2O was subsequently shown to result in erroneous enrichment of 15N and Delta18O when contaminated by trace amounts of CO2. We present results for 15N and 18O of N2O obtained from samples collected in the lower stratosphere. Five samples were collected at midnorthern latitudes on board NASA's WB-57 aircraft, and two samples were collected at high northern latitude during the 1988 Juelich balloon campaign. We also measured, for comparison, the isotopic composition of tropospheric N2O sampled in La Jolla, California, under clean air conditions. Nitrous oxide mixing ratios decreased with height above the tropopause, whereas the heavy-isotope composition of the remnant N2O was found to be increasingly enriched. If the process responsible for this enrichment is an irreversible sink and if the fractionation factor remains constant, the data should obey what is known as a Rayleigh distillation, in which the resulting isotopic enrichment is related to the fraction remaining by the equation (1) R = R0 x fAlpha-1 where R and Ro are the residual (stratospheric) and initial (tropospheric) heavy-to-light isotope ratios, respectively; f is the fraction of N2O remaining (residual concentration divided by the initial concentration); and is the ratio of the heavy-to-light reaction or photolysis rates. This relationship can be approximated by the equation (2) Delta Congruent Delta0 + Epsilon x ln(f) where Epsilon = 1000(Alpha - 1) expressed in per mil; and o are the residual and initial delta values, respectively; and the slope of Eq. 2, Epsilon, is the enrichment factor. The stratospheric data from the WB-57 samples fit the Rayleigh distillation model well with a continuous fit through the tropospheric values. The derived enrichment factors of Epsilon = -14.5 per mil for 15N2O and Epsilon = -12.9 per mil for N218O are large and of similar magnitude for both isotopes. The low-altitude balloon data compare favorably with the WB-57 results, whereas the high-altitude balloon data lie above the least squares fits, by far for 15N and less so for Delta18O. Although the errors are significantly larger for the balloon samples because of their small sample size, we have no reason to regard these data differently. Therefore, we suggest that either competing loss processes (with different Alpha's) change relative strengths as a function of altitude or that enrichment factors are increasing above a certain altitude, possibly as a function of changes in incident radiation. The only stratospheric N2O data for comparison are a single datum for Delta 15N from Moore and the two results of Kim and Craig for both Delta 15N and 18O. The result of Moore was from a sample collected at a height of 20.8 km at 34 Degrees S with Delta 15N = 19.2 per mil (with no concentration indicated). This is intermediate with respect to the high-altitude balloon sample (27.3 per mil) and the upper limit of the WB-57 samples (13.9 per mil). Both samples are within the concentration range of the WB-57 data and yet are offset from the least squares fits. The possibility exists that the differences are real. However, inspection of the WB-57 data show that although they were collected over a period of 1 year and 3 months and at latitudes from 33 Degrees to 48 Degrees north, they are well correlated. This indicates that the processes involved are not latitude dependent (at least over the midlatitudes) nor do they appear to be seasonal in nature. Furthermore, extrapolation of data from Kim and Craig does not intercept tropospheric values for either 15N or 18O. This trend could be interpreted as a non-Rayleigh process, but the steep initial slopes (about 70 and 100 per mil for Delta 15N and Delta 18O, followed by abrupt transitions to slopes of 19 and 13 per mil) require that at least two different and isolated processes are operating and that the lower altitude one is associated with an extremely high enrichment factor. If a downward flux of N2O with high 15N/14N and 18O/16O ratios were responsible, the inflection would be expected to be reversed, steepening with altitude and the concomitant decrease in concentration. Finally, the balloon samples analyzed by Kim and Craig are contemporaneous with the balloon samples we analyzed. Although our high-altitude balloon data is elevated relative to the least squares fits, the lower altitude balloon data are consistent with the WB-57 data. At the time when the analyses of Kim and Craig were performed, the consequences of CO2 contamination had not been reported. Because no check for CO2 contamination was reported, the offset is most likely a result of trace amounts of contaminant CO2. Bibliography Khalil, M. A. K. and R. A. Rasmussen, J. Geophys. Res. 97, 14651 (1992); A. F. Bouwman, K. W. Van der Hoek, J. G. J. Olivier, ibid. 100, 2785 (1995). Wahlen, M. and T. Yoshinari, Nature 313, 780 (1985). T. Yoshinari and M. Wahlen, ibid. 317, 349 (1985). British Pharmacopoeia 2005, London: Stationery Office,2005 The Encyclopaedia of Analytical Science, Kidlington, Oxford:Elsevier Academic Press,c2005, 2nd ed.;editors:Paul Worsfold,Alan Townshend,Colin Poole Read More