2.4 Energy Interactions in the Atmosphere
The atmosphere has different effects on the EM transfer at different wavelength. In this section, we will mainly introduce the fact that the atmosphere can have a profound effect on intensity and spectral composition of the radiation that reaches a remote sensing system. These effects are caused primarily by the atmospheric scattering and absorption.
Scattering: The redirection of EM energy by the suspended particles in the air.
Different particle sizes will have different effects on the EM energy propagation.
dp << l Rayleigh scattering Sr dp =l Mie scattering Sm dp >> l Non-selective scattering Sn
The atmosphere can be divided into a number of well marked horizontal layers on the basis of temperature.
It is the zone where weather phenomena and atmospheric turbulence are most marked. It contains 75% of the total molecular and gaseous mass of the atmosphere and virtually all the water vapour and aerosols.
height 8 - 16 km (pole to equator)
Stratosphere: 50 km Ozone
Mesosphere : 80 km
Thermosphere : 250 km
Exosphere : 500 km ~ 750 km
The atmosphere is a mixture of gases with constant proportions up to 80 km or more from ground. The exceptions are Ozone, which is concentrated in the lower stratosphere, and water vapor in the lower troposphere. Carbon dioxide is the principal atmosphere gas, with its concentration varying with time. It is increasing since the beginning of this century due to the burning of fossil fuels. Air is highly compressible. Half of its mass occurs in the lowest 5 km and pressure decreases logarithmically with height from an average sea-level value of 1013 mb.
Figure 2.3 Horizontal layers that divide the atmosphere (Barry and Chorley, 1982)
Scattering causes degradation of image quality for earth observation. At higher altitudes, images acquired in shorter wavelengths (ultraviolet, blue) contain a large amount of scattered noise which reduces the contrast of an image.
Absorption: Atmosphere selectively absorbs energy in different wavelengths with different intensity.
The atmosphere is composed of N2 (78%), O2 (21%), CO2, H2O, CO, SO2, etc. Since different chemical element has a different spectral property, regions with different intensity. As a result, the atmosphere has the combined absorption features of various atmospheric gases. Figure 2.4 shows the major absorption wavelengths by CO2, H2O, O2, O3 in the atmosphere.
Figure 2.4 Major absorption wavelengths by CO2, H2O, O2, O3 in the atmosphere
(Source: Lillesand and Kiefer, 1994)
Transmission: The remaining amount of energy after being absorbed and scattered by the atmosphere is transmitted.
H2O is most variable in the atmosphere.
CO2 varies seasonally.
Therefore, the absorpiton of EM energy by H2O and CO2 is the most difficult part to be characterized.
Atmospheric Window: It refers to the relatively transparent wavelength regions of the atmosphere.
Atmospheric absorption reduces the number of spectral regions that we can work with in observing the Earth. It affects our decision in selecting and designing sensor. We have to consider
1) the spectral sensitivity of sensors available;
2) the presence and absence of atmospheric windows;
3) the source, magnitude, and spectral composition of the energy available in these ranges.
For the third point, we have to base our decision of choosing sensors and spectral regions on the manner in which the energy interacts with the target under investigation.
On the other hand, although certain spectral regions may not be as transparent as others, they may be important spectral ranges in the remote sensing of the atmosphere.