Atmospheric emissions and trends of nitrous oxide deduced from 10 years of ALE–GAGE data

We present and interpret long‐term measurements of the chemically and radiatively important trace gas nitrous oxide (N 2 O) obtained during the Atmospheric Lifetime Experiment (ALE) and its successor the Global Atmospheric Gases Experiment (GAGE). The ALE/GAGE data for N 2 O 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 N 2 O 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 N 2 O 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 N 2 O, but they do not support the hypothesis that the temporal N 2 O increase is caused solely by increases in anthropogenic N 2 O emissions associated with fossil fuel combustion. Instead, the cause for the N 2 O 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 N 2 O in the northern hemisphere relative to the southern hemisphere. Accepting a theoretically‐calculated N 2 O lifetime of 166±16 years due to stratospheric destruction only, we deduce from the ALE/GAGE data a 10‐year average global N 2 O emission rate of (20.5±2.4) × 10 12 g N 2 O yr −1 , but with significant year‐to‐year variations in emissions associated perhaps with year‐to‐year variations in tropical land disturbance.