Publication Type:Journal Article
Source:Journal of Geophysical Research: Atmospheres, Volume 95, Issue D11, p.18369 - 18385 (1990)
Keywords:Biosphere/atmosphere interactions, Geochemical cycles, Middle atmosphere: composition and chemistry, Troposphere: composition and chemistry
We present and interpret long-term measurements of the chemically and radiatively important trace gas nitrous oxide (N2O) obtained during the Atmospheric Lifetime Experiment (ALE) and its successor the Global Atmospheric Gases Experiment (GAGE). The ALE/GAGE data for N2O comprise over 110,000 individual calibrated real-time air analyses carried out over a 10-year (July 1978–June 1988) time period. These measurements indicate that the average concentration in the northern hemisphere is persistently 0.75±0.16 ppbv higher than in the southern hemisphere and that the global average linear trend in N2O lies in the range from 0.25 to 0.31% yr−1, with the latter result contingent on certain assumptions about the long-term stability of the calibration gases used in the experiment. Interpretation of the data, using inverse theory and a 9-box (grid) model of the global atmosphere, indicates that the N2O surface emissions into the 90°N–30°N, 30°N–0°, 0°–30°S, and 30°S–90°S semihemispheres account for about 22–34, 32–39, 20–29 and 11–15% of the global total emissions, respectively. The measured trends and latitudinal distributions are consistent with the hypothesis that stratospheric photodissociation is the major atmospheric sink for N2O, but they do not support the hypothesis that the temporal N2O increase is caused solely by increases in anthropogenic N2O emissions associated with fossil fuel combustion. Instead, the cause for the N2O trend appears to be a combination of a growing tropical source (probably resulting from tropical land disturbance) and a growing northern mid-latitude source (probably resulting from a combination of fertilizer use and fossil fuel combustion). The exact combination of these sources which best fits the data depends on the assumed tropospheric-stratospheric exchange rates for N2O in the northern hemisphere relative to the southern hemisphere. Accepting a theoretically-calculated N2O lifetime of 166±16 years due to stratospheric destruction only, we deduce from the ALE/GAGE data a 10-year average global N2O emission rate of (20.5±2.4) × 1012 g N2O yr−1, but with significant year-to-year variations in emissions associated perhaps with year-to-year variations in tropical land disturbance.