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//2O 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//2O in the upper stratosphere. This is investigated in terms of a possible underestimation of the photochemical sink.