Increase and seasonal cycles of nitrous oxide in the earth’s atmosphere

Based on about nine thousand ground-level measurements at Cape Meares, Oregon (45° N), and Cape Grim, Tasmania (42°S), spanning three years, it is shown that nitrous oxide (N₂O) is increasing at about 0.9 p.p.b. yr^-1 (0.6, 1.1) in the northern hemisphere, and at 0.7 p.p.b. yr^-1 (±0.2 p.p.b. yr^-1) in the southern hemisphere. It is also shown that N₂O concentrations vary with the season. On average, northern hemisphere concentrations are 0.8 p.p.b.v. higher during April, May, and June compared to the rest of the year, and southern hemisphere concentrations are about 0.5 p.p.b.v. lower during March, April, and May compared to the rest of the year. Based on the existing estimates of natural and anthropogenic sources of N₂O, the increase is explained by a sizeable anthropogenically-controlled land-based source. Mass-balance calculations also indicate that a natural land-based source, peaking in spring, would explain the main features of the observed seasonal cycle. The observed increase of N₂O is extrapolated into the future using a limited growth model and the results compared with exponential extrapolations which are sometimes used.