Despite its low concentration in the atmosphere, N2O is the third largest greenhouse gas contributor to overall global warming, after CO2 and CH4. N2O is also the major source of ozone-depleting nitric oxide (NO) and nitrogen dioxide (NO2) in the stratosphere (Crutzen 1970). Control of N2O is part of efforts to curb greenhouse gas emissions under the Kyoto Protocol. Once emitted, N2O remains in the atmosphere for approximately 120 years before removal, mainly by destruction in the stratosphere (IPCC 2007).
The global atmospheric N2O concentration has increased by 9.4% since pre-industrial times and continues to increase by 0.6 ppbv yr-1 (Khalil et al. 2002). From a pre-industrial value of about 270 ppb, its atmospheric level has reached to 319 ± 0.12ppb in 2005 (IPCC 2007). Global emissions are about 17.3 (15.8-18.4) and 17.7 (8.5-27.7) Tg N yr-1 estimated by top-down and bottom-up methods, respectively. The growth rate of about 0.3% per year (WMO 2003) has been approximately constant for the past few decades. The annual source of N2O from the Earth's surface has increased by about 40 to 50% over pre-industrial levels as a result of human activity (Khalil and Rasmussen 1992; Hirsch et al. 2006). More than a third of all N2O emissions are anthropogenic and are primarily due to agriculture.
The individual N2O source strengths are poorly understood at present and hence contribute to the largest uncertainties in N2O's global budget. This is because the anthropogenic emissions are from many small sources, each one is very uncertain, as is the case with distributed small sources of any gas. Moreover, it is also possible that there are undiscovered sources and sinks, but the sink side seems to be better defined. The destruction of N2O in the stratosphere causes enrichment of its heavier isotopomers and isotopologues, which helps to differentiate stratospheric and surface flux influences on tropospheric N2O (Butenhoff and Khalil 2007; Morgan et al. 2004). A current estimates of N2O emission from various sources have been in Table 6.2.
Agriculture remains the single biggest anthropogenic N2O source (Bouwman et al. 2002a; Smith and Conen 2004; Del Grosso et al. 2005). Human activity has increased N supply to coastal and open oceans, resulting in N2O emissions (Naqvi et al. 2000; Nevison et al. 2004; Kroeze et al. 2005). The results of various studies, that quantified the global N2O emissions from coastal upwelling areas, continental shelves, estuaries and rivers, suggest that the coastal areas contribute 0.3-6.6 TgNyr-1 of N2O or 7-61% of the total oceanic emissions (Nevison et al. 2004; Kroeze et al. 2005). Land use change continues to affect N2O and NO emissions (Neill et al. 2005).
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