The importance of nitrous oxide (N2O) in atmospheric chemistry is well known. Not only is it a potent greenhouse gas, being some 200 times more effective than carbon dioxide (CO2) on a per molecule basis at absorbing infrared radiation (Ramaswamy, 2001), but its reaction with O^D) in the stratosphere is the major source of the ozone-destroying catalyst NO. Its concentration in the troposphere has increased by 9.4% since pre-industrial times and continues to increase by 0.6 ppbv/year (0.2% per year) (Khalil et al., 2002). It currently contributes ~6% to the total enhanced radiative forcing from greenhouse gases relative to pre-industrial times, and a doubling of its concentration would increase the global temperature by ~0.7°C (Ramaswamy, 2001). N2O is produced primarily through the biological processes of nitrification and denitri-fication in soils and the ocean. The reason for its increase is the additional release of N2O from agricultural activities, most notably the use of nitrogen fertilizers. As agriculture in the developing countries transitions from organic to synthetic nitrogen fertilizers, it is likely that the level of N2O in the atmosphere will continue to rise. As such it is important to understand the life history of this gas for an effective management policy.
The individual flux strengths of N2O sources are poorly constrained and contribute the largest uncertainties to N2O's global budget. Bottom-up estimates of N2O's global source range from ~7 to 37 Tg N/year (Prather and Ehhalt, 2001) with a likely value of ~18Tg N/year (Kroeze et al., 1999).
The global source can be constrained by using a simple one-box atmospheric model:
where dB/dt is the rate of change of the atmospheric N2O burden, and S and L are the respective global source and loss rates. If we use the recommended Intergovernmental Panel on Climate Change (IPCC) values of L = 12.6 Tg N/year and dB/dt = 3.8 Tg N/year (Prather and Ehhalt, 2001), the estimated source strength is 16.4 Tg N/year. Although this method can put a constraint on the aggregate source total, it does not provide information on fluxes for N2O's individual sources. However, advances in isotopic analysis on both theoretical and instrumental grounds have opened new pathways to a better understanding of trace gas budgets. Since the various production and removal processes isotopically fractionate N2O differently, they leave isotopic signatures in the troposphere and stratosphere, which can be used to put constraints on the source and sink fluxes.
After an initial discussion on the bulk removal processes of N2O in the stratosphere, we continue with a review of recent work on the measurement and analysis of its isotopic composition in the atmosphere. In this chapter, we restrict our discussion to the stratosphere, where the isotopic frac-tionation is limited to the sink processes of photolysis and reaction with O(1D), with perhaps some minor contributions from exotic processes. The stratosphere is a supplier of heavy N2O isotopes to the troposphere; this, combined with the surface emissions of relatively light isotopes from N2O's major sources, determines the isotopic composition of the troposphere.
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