Introduction

An understanding of the global cycle of N2O and the capacity to construct a balanced budget of global sources and sinks, requires knowledge of the concentration of the gas in earth's atmosphere and its average lifetime, how that concentration has changed with time, and the principal influences that affect the magnitude of the sources and sinks. An essential component of this knowledge is the estimation of the present N2O concentration in the atmosphere, by high-precision gas chromatographic analysis of air samples collected at monitoring stations around the world (for example CSIRO, 2009), and the determination of how the atmospheric concentration has varied over time.

This latter information comes from the analysis of air trapped in firn (the ice formed from relatively recent snowfalls on glaciers) and in deeper glacier ice of greater age (for example Wolff and Spahni, 2007). In recent years, drilling through the Antarctic and Greenland ice caps (Plate 4.2) has yielded cores of ice up to several hundred thousand years in age, and the trapped air has given the concentrations of all the major long-lived greenhouse gases -carbon dioxide, methane and N2O - through several glacial-interglacial cycles back to 650,000 yr BP. The N2O record is less complete than those of the other gases, because of suspected artifacts affecting the gas concentration in certain periods (Wolff and Spahni, 2007), but it is clear that the concentration varied from highs of around 270ppbv (270nmol mol-1) in interglacial periods to lows around 200ppbv in glacial periods (for example Sowers, 2001; Fluckiger et al, 2004). Over most of the last four millennia N2O mixing ratios (i.e. the concentrations in dry air) have been close to 270ppbv (Fluckiger et al, 2002), but in more recent times there has been a 20 per cent increase, beginning around AD1850. The mixing ratio exceeded 280ppbv for the first time in 1905, reached 300ppbv in the mid-1970s, 319ppbv in 2005 (IPCC, 2007), and now exceeds 320 ppbv. The trends, on different timescales, are shown in Figure 4.1 and Plate 4.3. This increase has no precedent over the last 650,000 years, and whereas in the past the sources and sinks were broadly in line, limiting the fluctuations in atmospheric mixing ratio to within a narrow range, the only rational explanation for the increase since 1850 is a change in the magnitude of sources, and/or of sinks, by human activity; these issues are explored in later sections. The rate of increase in atmospheric N2O concentrations increased from approximately 0.15ppb yr_1 between 1900 and 1955 to the current linear increase rate of about 0.7ppb yr_1.

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