12.7 Monitoring of Atmospheric Constituents

12.7 Monitoring of Atmospheric Constituents

Each atmospheric constituent such as water vapor, carbon-dioxidide, ozone, methane etc. has its own unique spectral characteristic of emission and absorption. With the use of these characteristics, the density of these atmospheric molecules can be monitored by measuring the spectral energy which transmits from the sun, the moon or the stars through the atmosphere, the scattering energy from the atmosphere or the clouds, the reflected energy from the earth surface and/or the thermal radiation emitted from the atmosphere and the earth surface. The spectral energy can be measured by two methods; absorption spectroscopy and emission spectroscopy.

These methods have been applied for many years ago for the measurement of the upper atmosphere from the ground. Recently the methods have been extended for measurements from aircraft, balloon and satellite. In addition, multi-spectral laser with variable wavelength, called laser rader or lidar, has been developed for the measurement of the spatial distribution of the atmospheric constituents.

Figure 12.7.1 shows the spectral transmittance of H2O, CO2, O3, N2O and CH4 in the infrared region.

Figure 12.7.2 shows the spectral attenuation of water vapor(H2O), and oxygen with a number of channels of the AMSU (Advanced Microwave Sounding Unit) instrument.

There are three methods used to measure the vertical distribution of atmospheric constituents; the occultation method which measures the attenuating light of the sun light at the sun rise and the sun sets, from a satellite, the limb scan method which measures the spectrum of atmosphere around the limb of the earth and the vertical viewing method, which measures the atmospheric emission from various altitudes and contribution ratio are analyzed with respect to the spectral absorption coefficient by the inversion method. The vertical look down method is operationally applied for carbon-dioxidide and water vapor in the infrared region and for ozone in the ultra-violet region.

Figure 12.7.3 shows the normalized contribution function at various wavelengths in the ultra-violet region where the vertical distribution of ozone is measured from the back scattering of blue ultra-violet (BUV) radiation.

Figure 12.7.4 shows the distribution of the integrated ozone which was measured with the TOMS (Total Ozone Mapping Spectrometer) on board Nimbus 7.