Remote sounding basics

How to measure the concentration of trace gases like ozone in the upper atmosphere ? Clearly, if you want to monitor the entire globe, you need to perform measurements from satellites by observing how the light emitted by the Sun or by other celestial bodies interacts with the Earth’s atmosphere. In particular, most instruments will measure the reflected or transmitted light intensity at different wavelengths (colors) from the UV to the far infrared. This is the principle of remote sounding.

It is easy to have an intuitive picture of this technique by illustrating remote sounding from the ground :

Just consider the Sun as seen by an observer at ground level. When the Sun is high in the sky (f. i . at noon), it is so bright that it is impossible to observe it with the naked eye. Later on, we can observe the solar disk at sunset without being blinded. Of course, the Sun itself has not changed during the day ! (Otherwise, people West of the observer could observe the zenith Sun with the naked eye).

Therefore, there is only one simple explanation : the Sunlight at sunset has been more absorbed by the atmosphere. Indeed, the path to space from the observer is shorter in the direction of the vertical (zenith) than toward the Earth’s horizon. Hence, any ray will pass through a larger amount of absorbing gas (here, just the air) at sunset. Consequently, to observe how the apparent Sun intensity is decreasing with daytime already gives information related to the gas quantity that is responsible for light absorption.

Another striking feature of the sunset or sunrise is the fact that the Sun now looks reddish whereas it was yellow at noon (although we know that the Sun spectrum is composed by much more than the visible light). Again, we are convinced that the Sun itself did not change. The explanation is due to the fact that light absorption and scattering is not the same at different wavelengths or colors. In particular, for very small particles like nitrogen and oxygen molecules that are the main constituents of air, the diffusion of light is more efficient in the blue than in the red. If the Sun looks more reddish (actually we should say that it is less bluish !) , it is just because the blue sunrays have been more scattered at sunset due to the larger amount of gas. ozonecross.png The essential point here is to recognize that the relative change of apparent color gives information on the nature of the scattering medium (here a gas composed of air molecules).

Finally, it becomes evident that the concentration of a molecule (like ozone) that is absorbing light at a specific wavelength or wavelength range can be retrieved by measuring the light intensity transmitted at this wavelength and for different viewing directions associated with different amounts of absorber.

In the figure, we see a physical property (called « cross section ») of the ozone molecule measuring its ability to absorb solar light at different wavelengths. The « Hartley » and « Huggins » bands are essential to protect life on Earth by absorbing the UV radiation with a strong efficiency. However, the « Chappuis » band is mostly used to investigate the stratospheric ozone layer.

How to perform atmospheric remote sounding from space? To obtain a good vertical resolution, two techniques co-exist: the occultation method and the limb scattering method.