Observations of CH₄ and N₂O by the NIMBUS 7 SAMS: A comparison with in situ data and two‐dimensional numerical model calculations

Monthly mean, zonal mean measurements of CH 4 and N 2 O for 1979 made by the stratospheric and mesospheric sounder (SAMS) on the NIMBUS 7 satellite are presented for the first time. Comparison with in situ and other available data confirms a general pattern of mixing ratios decreasing with height and of maxima, at a given pressure level, in low latitudes. The SAMS data, by virtue of its extensive coverage, reveals new features. These include during certain months a “double peak” when, along a constant pressure surface, mixing ratio maxima are found in low latitudes of both hemispheres with a local minimum at the equator. In the upper stratosphere are found regions of weak and strong horizontal gradients. Comparison with a two‐dimensional model shows many areas of agreement, especially for CH 4 , and the model is used to interpret atmospheric behavior. The model overestimates the observed N 2 O in the upper stratosphere. This is investigated in terms of a possible underestimation of the photochemical sink. We believe strongly, however, that uncertainties in transport representation cannot be ruled out.