1. Y. L. Yung, R. L. Shia, Y. C. Chen, C. Y. Leung Divisions of Geological and Planetary Sciecnes, California Institute of Technology Mao-Chang Liang Research Center for Environmental Changes, Academia Sinica Isotopic Constraints on the Global Budget Atmospheric Nitrous Oxide References References IPCC 2007 (AR4); Rahn and Wahlen (2000); Morgan et al. (2004); Roeckmann et al. (2003; 2006); Roeckmann and Levin (2005). Abstract We develop a simple box model for nitrous oxide (N 2 O) that is based on laboratory kinetics measurements and constrained to reproduce the age of air in the stratosphere from the Caltech/JPL two-dimensional model of the terrestrial atmosphere,. We study three types of models. The Baseline Model assumes that the primary sources of N 2 O are the land, the ocean and agriculture, and the primary sink is destruction in the stratosphere. The Standard Model includes additional N 2 O sources from rivers, estuaries and coastal zones as well as fossil fuel combustion and industrial processes, as recommended by IPCC [2007]. Extended Models explores the consequences of a climate-related acceleration of the Brewer-Dobson circulation that transports N 2 O from the troposphere to the stratosphere and projections of future concentrations of N 2 O. The model includes all the commonly studied isotopologues and isotopomers of N 2 O and can account for most of known observations. These observations include the abundances and trends of the isotopologues and isotopomers of N 2 O since the Pre- Industrial Era. The data suggest that the negative trends in the isotopic fractionations appear to slow down in recent decades, a result that can be explained by the Standard Model but not the Baseline Model. We also discuss more speculative results on future projections of N 2 O. Sources and sinks OC -- ocean LD -- land AN -- anthropogenic Total = OC+LD+AN SK -- sink TD -- trend Table 1. Sources of N2O from models Use the Caltech/JPL 2-D and MOZART/CAM-CHEM 3-D models for better estimates. Better measurements are needed as present data have large uncertainties. Troposphere F_oceanF_landF_anthro. F_sink Measurements used: 1. N 2 O trend 2.  18 O 3.  15 N (1) Ocean source = prescribed isotopic composition. (2) The isotopic composition of anthropogenic source = land source (3) Solve the mean isotopic composition of land for PRE. (4) Magnitude of the natural and anthropogenic sources are deduced. Method – A Simple Box Model Future Work Conclusion Model Result: N2O Time Series ocean land 2003 1970 1700 FF+BB+RE Figure 1. Comparison of N 2 O concentrations from 1700 to the present between the Baseline Model (solid line) and data. Crosses: Machida et al. [1995]. Khalil and Rasmussen [1992]. NOAA/CMDL. Figure 2. Sources and Sinks for N 2 O from 1700 to the present. OC = ocean; LD = land; AN = anthropogenic; Total = OC + LD + AN. SK = sink. TD = trend = dN 2 O/dt. Units are in Tg N yr -1. Figure 3a. Comparison of specific isotopic change for  15 N(N 2 O) between the Baseline Model (black line), Standard Model (red line) and Extended model (green line) and data (cross ; Bernard et al. 2006) from 1700 to 2003 A.D. Figure 3b.Comparison of specific isotopic change for  15 N(N 2 O) between the Baseline Model (black line), Standard Model (red line) and Extended model (green line) and data (Roeckmann and Levin 2005) from 1990 to 2002 A.D. Figure 3c. Comparison of specific isotopic change for  18 O(N 2 O) between the Baseline Model (black line), Standard Model (red line) and Extended model (green line) and data (cross; Bernard et al. 2006) from 1700 to 2003 A.D. Figure 3d. Comparison of specific isotopic change for  18 O(N 2 O) between the Baseline Model (black line), Standard Model (red line) and Extended model (green line) and data (Roeckmann and Levin 2005) from 1990 to 2002 A.D. Source of N2O (IPCC vs. Model) a. AG: Agriculture. HE: Human excreta. AD: Atmospheric deposition. b.FF: Fossil fuel combustion & industrial processes. BB: Biomass and biofuel burning. RE: Rivers, estuaries, coastal zones. Isotopic composition provides additional constraints on the budget of N 2 O. The results are consistent with and refines those of IPCC 2007 (AR4). Figure 4. Isotopic signature of the sources of N 2 O for  18 O(N 2 O) and  15 N(N 2 O) in the model. Cross: Ocean; Asterisk: Land; Triangle: 1700 A.D.; Square: 1970; Circle: 2003. Dot: Combustion + Runoff (see Table.1).