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GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 21, GB3017, doi:10.1029/2006GB002755, 2007
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Interannual and seasonal variability in atmospheric N2O Cynthia D. Nevison,1 Natalie M. Mahowald,1 Ray F. Weiss,2 and Ronald G. Prinn3 Received 15 May 2006; revised 16 April 2007; accepted 16 May 2007; published 11 September 2007.
[1] The increase in atmospheric N2O observed over the last century reflects large-scale
human perturbations to the global nitrogen cycle. High-precision measurements of atmospheric N2O over the last decade reveal subtle signals of interannual variability (IAV) superimposed upon the more prominent growth trend. Anthropogenic sources drive the underlying growth in N2O, but are probably too monotonic to explain most of the observed IAV. The causes of both seasonal and interannual variability in atmospheric N2O are explored on the basis of comparisons of a 1993–2004 atmospheric transport simulation to observations of N2O at five stations of the Advanced Global Atmospheric Gases Experiment (AGAGE). The complementary tracers chlorofluorocarbons (CFCs) 11 and 12 and SF6 also are examined. The model simulation does not include a stratospheric sink and thus isolates the effects of surface sources and tropospheric transport. Both model and observations yield correlations in seasonal and interannual variability among species, but only in a few cases are model and observed variability correlated to each other. The results suggest that tropospheric transport contributes substantially to observed variability, especially at Samoa station. However, some features of observed variability are not explained by the model simulation and appear more consistent with a stratospheric influence. At Mace Head, Ireland, N2O and CFC growth rate anomalies are weakly correlated to IAV in polar winter lower stratospheric temperature, a proxy for the strength of the mean meridional stratospheric circulation. Seasonal and interannual variability in the natural sources of N2O may also contribute to observed variability in atmospheric N2O. Citation: Nevison, C. D., N. M. Mahowald, R. F. Weiss, and R. G. Prinn (2007), Interannual and seasonal variability in atmospheric N2O, Global Biogeochem. Cycles, 21, GB3017, doi:10.1029/2006GB002755.
1. Introduction [2] Nitrous oxide (N2O) is a naturally occurring atmospheric greenhouse gas with a Global Warming Potential 300 times that of CO2 on a molecule per molecule basis [Prather et al., 2001]. The primary sink for N2O is photochemical destruction in the stratosphere, which releases reactive nitrogen that can catalyze ozone loss. Aside from N2, N2O is the only nitrogen gas that is sufficiently long-lived to become globally well mixed in the atmosphere. Since a small fraction of nitrogen involved in microbial N cycle transformations in both soils and oceans tends to leak off as N2O, the increase in atmospheric N2O from a preindustrial level of about 275 ppb to a 2005 value of 320 ppb (Figure 1a) signifies a large-scale perturbation to the global N cycle. When combined with the N2O atmospheric lifetime of 120 years and the total atmospheric burden of 1500 Tg N, the observed increase implies an anthropogenic source that now exceeds the natural microbial source by 1
National Center for Atmospheric Research, Boulder, Colorado, USA. Scripps Institution of Oceanography, La Jolla, California, USA. 3 Massachusetts Institute of Technology, Cambridge, Massachusetts, USA. 2
Copyright 2007 by the American Geophysical Union. 0886-6236/07/2006GB002755$12.00
50% and accounts for 1/3 of the total N2O source [Khalil et al., 2002; Hirsch et al., 2006]. [3] The global N2O concentration was first accurately determined in the late 1970s, at which time its global atmospheric increase was discovered and quantified [Weiss, 1981]. The growth in atmospheric N2O is attributed mainly to an increase in the amount and rate of N cycled globally in association with human agriculture. Agricultural N cycle perturbations include both industrial fixation of N2 to make synthetic fertilizer [Vitousek et al., 1997; Galloway et al., 2004], and the acceleration of nitrogen cycling through cultivation, land-use change, and the expanding global population of livestock (Figure 1b). A smaller amount of nitrogen is anthropogenically fixed as NOx by fossil fuel combustion and subsequently deposited on the biosphere [Holland et al., 2005a]. The assumption that 2% of anthropogenic N leaks off as N2O has been shown to reproduce the general shape of the increase in atmospheric N2O over the last 150 years [Nevison et al., 1996; McElroy and Wang, 2005] (also see Figure 1a). However, only a few studies have examined the finer-scale seasonal variability in atmospheric N2O [Bouwman and Taylor, 1996; Levin et al., 2002; Liao et al., 2004; Nevison et al., 2004, 2005] and still fewer have discussed N2O’s interannual variability [Schauffler and Daniel, 1994; Wong et al., 1999; Ishijima et al., 2001].
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Figure 1. (a) Two-box model based on work by Sowers et al. [2002] assuming an anthropogenic N2O source equal to 2% of the anthropogenic N fluxes in Figure 1b. The model reproduces the general shape of the observed atmospheric increase, as given by ice core, firn, and atmospheric data [Machida et al., 1995; Battle et al., 1996; Thompson et al., 2004]. (b) Increase in anthropogenic N in synthetic fertilizer, livestock manure and fossil NOx from 1860 to the present [Holland et al., 2005b].
[4] Previous studies of seasonal variability in atmospheric N2O have noted that the seasonal N2O minimum observed at Northern Hemisphere monitoring stations is out of phase with the predicted source minimum and may be caused in part by seasonal differences in the transmission of N2Odepleted air from the stratosphere [Bouwman and Taylor, 1996; Levin et al., 2002; Nevison et al., 2004; Liao et al., 2004]. The similarity in the minima of N2O and complementary halocarbon tracers (discussed below) provide support for this hypothesis. However, these studies could not rule out an additional or even dominant role for tropospheric transport variability for a variety of reasons. First, early modeling studies have found that all long-lived tracers tend to display ‘‘similar behavior’’ at sites remote from their sources and sinks [Plumb and McConalogue, 1988]. Second, trapping of polluted air in the thinner wintertime boundary layer is known to produce seasonality in Northern Hemisphere surface observations, regardless of changes in sources [Barrie and Hoff, 1984; Elkins et al., 1993]. Third, the seasonal amplitude ratios of N2O to other long-lived tracers are not quantitatively consistent with the inverse of their atmospheric lifetimes [Nevison et al., 2004], as one might predict for a purely stratospheric signal [Plumb and Ko, 1992]. [5] Much has been learned about the carbon cycle by analyzing seasonal and interannual variability in atmospheric CO2 [Prentice et al., 2001; Baker et al., 2006]. This paper aims to promote a similar discussion of atmospheric N2O, which is often considered the nitrogen cycle counterpart to CO2. The analysis presented here is based on model results and observations at 5 long-term monitoring stations. We begin by examining the potential of interannual changes in the anthropogenic N2O source to explain the observed atmospheric interannual variability (IAV). We next examine how transport acts upon N2O to create variability in the troposphere. For this analysis, we present the results of an atmospheric transport model simulation with realistic surface sources of N2O and IAV in atmospheric transport. We include several complementary tracers, discussed below, in the simulation. While our primary motivation is to understand IAV, we revisit the analysis of seasonal variability
presented by Nevison et al. [2004], since influences capable of creating seasonal cycles are likely to be relevant for IAV. Finally, we evaluate the influence of IAV in the cross tropopause exchange of stratospherically depleted air on tropospheric N2O and complementary tracers by comparing observed growth rates to IAV in lower stratospheric temperature, which is used as a proxy for variability in the mean stratospheric circulation. [6] We note that this paper does not attempt to evaluate IAV in natural N2O sources (with the exception of a brief discussion of the relationship between ENSO and the N2O ocean source in the tropical Pacific). Such an evaluation is beyond the scope of the paper because there are still substantial uncertainties in the mean annual natural N2O budget [Khalil et al., 2002]. Atmospheric N2O data potentially can provide top-down constraints to help quantify natural microbial ocean and soil sources and discern the regional imprint of anthropogenic sources. However, variability in tropospheric N2O can only be exploited fully to constrain and identify sources with an improved understanding of the influence of atmospheric transport and the natural stratospheric photochemical sink [Nevison et al., 2005; Hirsch et al., 2006].
2. Methods 2.1. Complementary Tracers (CFC-11, CFC-12, and SF6) [7] Chlorofluorocarbons (CFCs) 11 and 12 are used in this paper as complementary tracers to aid in the interpretation of variability in atmospheric N2O. Like N2O, the CFCs are long-lived, well-mixed species in the troposphere and are destroyed by photochemistry in the stratosphere. Thus CFCs can be used to examine both the influence of the stratospheric sink and of tropospheric transport variability. Unlike N2O, the CFCs are entirely man-made gases with relatively well-known surface sources that largely ceased in 1996. CFC-11, the shorter-lived of the two CFCs, has been declining in the troposphere since the early 1990s, while the longer-lived CFC-12 only began to level off in the early 2000s [Montzka et al., 1999; Prinn et al., 2000]. Despite
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these trends, substantial surface sources of CFCs may remain, either from illegal production, production in developing countries allowed under the Montreal Protocol, or the leakage of old refrigerators, air conditioners and closed-cell foams [Hurst et al., 2004]. [8] An additional complementary tracer considered in this study is sulfur hexafluoride (SF6). SF6 is a man-made compound that is used and released from electric power transmission equipment and other industrial applications. It has risen rapidly in the atmosphere from
