TRANSMITTANCE OF THE ATMOSPHERE
The earth is enveloped by a layer of atmosphere consisting of a mixture of gases and other solid and liquid particles. Though there is no well defined boundary or upper limit of the atmosphere, the gaseous materials extend several hundred kilometers above the earth’s surface,. The first 80 km of the atmosphere contains more than 99% of the total mass of the earth's atmosphere.
Vertical Structure of the Atmosphere
The vertical profile of the atmosphere is divided into four layers: troposphere, stratosphere, mesosphere and thermosphere. The tops of these layers are known as the tropopause, stratopause, mesopause and thermopause, respectively.
· Troposphere: The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% of the atmosphere's mass and almost all of its water vapor and aerosols. The average thickness of the troposphere is about 11 km in the middle latitudes. It is deeper in the tropical regions (up to 20 km) and shallower near the poles (about 7 km in summer, indistinct in winter). This layer is characterized by a decrease in temperature with respect to height, at a rate of about 6.5ºC per kilometer. All the weather activities (water vapour, clouds, precipitation) are confined to this layer. A layer of aerosol particles normally exists near to the earth surface.
· Stratosphere: The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler higher up and warmer farther down. The temperature at the lower 20 km of the stratosphere is approximately constant, after which the temperature increases with height, up to an altitude of about 50 km. Ozone exists mainly at the stratopause. The troposphere and the stratosphere together account for more than 99% of the total mass of the atmosphere.
· Mesosphere: The temperature decreases in this layer from an altitude of about 50 km to 85 km.
· Thermosphere: This layer extends from about 85 km upward to several hundred kilometers. The temperature may range from 500 K to 2000 K. The gases exist mainly in the form of thin plasma, i.e. they are ionized due to bombardment by solar ultraviolet radiation and energetic cosmic rays.
The term upper atmosphere usually refers to the region of the atmosphere above the troposphere.
Many remote sensing satellites follow the near polar sun-synchronous orbits at a height around 800 km, which is well above the thermopause.
The atmosphere consists of the following components:
· Permanent Gases: They are gases present in nearly constant concentration, with little spatial variation. About 78% by volume of the atmosphere is nitrogen while the life-sustaining oxygen accounts for 21%. The remaining one percent consists of the inert gases, carbon dioxide and other gases.
· Gases with Variable Concentration: The concentration of these gases may vary greatly over space and time. They consist of water vapour, ozone, nitrogeneous and sulphurous compounds.
· Solid and liquid particulates: Other than the gases, the atmosphere also contains solid and liquid particles such as aerosols, water droplets and ice crystals. These particles may congregate to form clouds and haze.
When electromagnetic radiation travels through the atmosphere, it may be absorbed or scattered by the constituent particles of the atmosphere. Molecular absorption converts the radiation energy into excitation energy of the molecules. Scattering redistributes the energy of the incident beam to all directions. The overall effect is the removal of energy from the incident radiation. The various effects of absorption and scattering are outlined in the following sections.
Absorption of EM Radiation by Atmospheric Gases
Absorption by Gaseous Molecules
The energy of a gaseous molecule can exist in various forms:
· Translational Energy: Energy due to translational motion of the centre of mass of the molecule. The average translational kinetic energy of a molecule is equal to kT/2 where k is the Boltzmann's constant and T is the absolute temperature of the gas.
· Rotational Energy: Energy due to rotation of the molecule about an axis through its centre of mass.
· Vibrational Energy: Energy due to vibration of the component atoms of a molecule about their equilibrium positions. This vibration is associated with stretching of chemical bonds between the atoms.
· Electronic Energy: Energy due to the energy states of the electrons of the molecule.
The last three forms are quantized, i.e. the energy can change only in discrete amount, known as the transitional energy. A photon of electromagnetic radiation can be absorbed by a molecule when its frequency matches one of the available transitional energies.
Absorption of ultraviolet (UV) in the atmosphere is chiefly due to electronic transitions of the atomic and molecular oxygen and nitrogen. Due to the ultraviolet absorption, some of the oxygen and nitrogen molecules in the upper atmosphere undergo photochemical dissociation to become atomic oxygen and nitrogen. These atoms play an important role in the absorption of solar ultraviolet radiation in the thermosphere. The photochemical dissociation of oxygen is also responsible for the formation of the ozone layer in the stratosphere.
Ozone in the stratosphere absorbs about 99% of the harmful solar UV radiation shorter than 320 nm. It is formed in three-body collisions of atomic oxygen (O) with molecular oxygen (O2) in the presence of a third atom or molecule. The ozone molecules also undergo photochemical dissociation to atomic O and molecular O2. When the formation and dissociation processes are in equilibrium, ozone exists at a constant concentration level. However, existence of certain atoms (such as atomic chlorine) will catalyse the dissociation of O3 back to O2 and the ozone concentration will decrease.
It has been observed by measurement from space platforms that the ozone layers are depleting over time, causing a small increase in solar ultraviolet radiation reaching the earth. In recent years, increasing use of the flurocarbon compounds in aerosol sprays and refrigerant results in the release of atomic chlorine into the upper atmosphere due to photochemical dissociation of the fluorocarbon compounds, contributing to the depletion of the ozone layers.
There is little absorption of the electromagnetic radiation in the visible part of the spectrum.
The absorption in the infrared (IR) region is mainly due to rotational and vibrational transitions of the molecules. The main atmospheric constituents responsible for infrared absorption are water vapour (H2O) and carbon dioxide (CO2) molecules. The water and carbon dioxide molecules have absorption bands centred at the wavelengths from near to long wave infrared (0.7 to 15 µm).
In the far infrared region, most of the radiation is absorbed by the atmosphere.
The atmosphere is practically transparent to the microwave radiation.
Scattering of Electromagnetic Radiation by Atmosphere
Scattering of electromagnetic radiation is caused by the interaction of radiation with matter resulting in the reradiation of part of the energy to other directions not along the path of the incidint radiation. Scattering effectively removes energy from the incident beam. Unlike absorption, this energy is not lost, but is redistributed to other directions.
Both the gaseous and aerosol components of the atmosphere cause scattering in the atmosphere.
Scattering by gaseous molecules
The law of scattering by air molecules was discovered by Rayleigh in 1871, and hence this scattering is named Rayleigh Scattering. Rayleigh scattering occurs when the size of the particle responsible for the scattering event is much smaller than the wavelength of the radiation. The scattered light intensity is inversely proportional to the fourth power of the wavelength. Hence, blue light is scattered more than red light. This phenomenon explains why the sky is blue and why the setting sun is red.
The scattered light intensity in Rayleigh scattering for unpolarized light is proportional to (1 + cos2 s) where s is the scattering angle, i.e. the angle between the directions of the incident and scattered rays.
Scattering by Aerosols
Scattering by aerosol particles depends on the shapes, sizes and the materials of the particles. If the size of the particle is similar to or larger than the radiation wavelength, the scattering is named Mie Scattering. The scattering intensity and its angular distribution may be calculated numerically for a spherical particle. However, for irregular particles, the calculation can become very complicated.
In general, the scattered radiation in Mie scattering is mainly confined within a small angle about the forward direction. The radiation is said to be very strongly forward scattered.
Each type of molecule has its own set of absorption bands in various parts of the electromagnetic spectrum. As a result, only the wavelength regions outside the main absorption bands of the atmospheric gases can be used for remote sensing. These regions are known as the Atmospheric Transmission Windows.
The wavelength bands used in remote sensing systems are usually designed to fall within these windows to minimize the atmospheric absorption effects. These windows are found in the visible, near-infrared, certain bands in thermal infrared and the microwave regions.
Effects of Atmospheric Absorption on Remote Sensing Images
Atmospheric absorption affects mainly the visible and infrared bands. Optical remote sensing depends on solar radiation as the source of illumination. Absorption reduces the solar radiance within the absorption bands of the atmospheric gases. The reflected radiance is also attenuated after passing through the atmosphere. This attenuation is wavelength dependent. Hence, atmospheric absorption will alter the apparent spectral signature of the target being observed.
Effects of Atmospheric Scattering on Remote Sensing Images
Atmospheric scattering is important only in the visible and near infrared regions. Scattering of radiation by the constituent gases and aerosols in the atmosphere causes degradation of the remotely sensed images. Most noticeably, the solar radiation scattered by the atmosphere towards the sensor without first reaching the ground produces a hazy appearance of the image. This effect is particularly severe in the blue end of the visible spectrum due to the stronger Rayleigh Scattering for shorter wavelength radiation.
Furthermore, the light from a target outside the field of view of the sensor may be scattered into the field of view of the sensor. This effect is known as the adjacency effect. Near to the boundary between two regions of different brightness, the adjacency effect results in an increase in the apparent brightness of the darker region while the apparent brightness of the brighter region is reduced. Scattering also produces blurring of the targets in remotely sensed images due to spreading of the reflected radiation by scattering, resulting in a reduced resolution image.
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