SCIAMACHY which is onboard the Envisat has 3 different modes of measurements. (link : http://www.iup.uni-bremen.de/sciamachy/instrument/modes/index.html) . In Nadir it is easy to understand that location is beneath the sensor. But in limb or occultation mode if we consider some measurements what location does they belong to (in the sense geo-location of the measurement) as the measurements here are tangent to atmosphere.
A rule of thumb is to use a tangent point in the layered atmosphere and to assign the retrieved parameter to this point, BUT:
the situation depends on the optical thickness of the spectral domain where the measurement is performed. Imagine that one observes a very optically thick atmosphere in limb mode - in this case the contribution function of the layers close to observer will be higher than in the case of optically thin atmosphere, for which the tangent point layer will be the main contributor (consider the density and geometrical length). In certain cases, the tangent point is not visible at all (!).
As for the occultation, larger absorption, in general, will correspond to lower altitudes (= tangent point layer is the main contributor), but I can imagine the situation when the concentration of a given component grows with altitude (secondary maximum of ozone, for example) and this growth outperforms the pressure decrease with height.
As for the nadir observations, the location is, indeed, below the sensor, but the signal contribution in altitude is not uniform - again, optically thicker spectral lines carry the information about the upper layers while the thinner ones allow "reaching" the ground.
Hope this helps.
P.S. These were general considerations. For an instrument like SCHIAMACHY, all the contribution functions are known - check the documentation for numbers.
For SCIAMACHY limb measurements, see e.g. this paper:
Accounting for the effect of horizontal gradients in limb measurements of scattered sunlight
J. Puķīte, S. Kühl, T. Deutschmann, U. Platt, and T. Wagner
Atmos. Chem. Phys., 8, 3045-3060, 2008
We hope to publish a paper soon that contains a discussion on the effects of inhomogeneous or "small" aerosol and cloud layers (small in comparison with the instruments field of view) on limb reflectance. I will keep you informed, if you're interested.
But even in nadir the situation is not as simple as you state - Mr. Feofilov already addressed the issue with weak and strong absorbers, but the situation becomes even more complicated when different surface types, clouds and aerosols come into play ; particularly if these are spatially inhomogeneous (as is often the case).
What, exactly, do you intend to do? The literature on this topic is exhaustive...!
Thank you Marloes and Feofilov for providing your views. @Marloes Thanks for being considerate, I would be keen to learn about your upcoming publication. Actually I am trying to understand the transport phenomena of the trace gases using RS data.
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