Question

3.         Answer ALL parts of this question.

(a) Explain how greenhouse effect raises the temperature of the troposphere.

(5 marks)

           

(b) N₂O is a greenhouse gas. Draw the IR active vibrations of N₂O. The frequency of the bending vibration is 596 cm^-1, calculate the wavelength of light that N₂O can absorb due to this vibration (show your working).

(c) Discuss what is meant by the term “global dimming”.

(2 marks)

(d) Explain why chlorofluorocarbons make a significant contribution to the enhanced greenhouse effect even though their concentrations are in the order of 10 to 300 parts per trillion (ppt) compared to the 400 ppm concentration of carbon dioxide

(3 marks)

(e) Suggest reasons why the international community has not been successful at solving the problem of increased CO₂ emissions.

                                                                                                                          

Answer 1

3.

a.Earth receives energy from the Sun in the form UV, visible, and near IR radiation, most of which passes through the atmosphere without being absorbed or reflected. Of the total amount of energy available at the top of the atmosphere (TOA), about 26% is reflected back out to space by the atmosphere and clouds and 19% is absorbed by the atmosphere and clouds. Most of the remaining energy is absorbed at the surface of Earth. Because it is warm, the surface radiates far IR thermal radiation that consists of wavelengths that are much longer than the wavelengths that were absorbed (the overlap between the incident solar spectrum and the terrestrial thermal spectrum is small enough to be neglected for most purposes). Most of this thermal radiation is absorbed by the atmosphere, thereby warming it (in addition to sensible and latent heat fluxes from the surface). The atmosphere radiates energy both upwards and downwards; the part radiated downwards is absorbed by the surface of Earth. This leads to a higher equilibrium temperature than if the atmosphere were absent.

c. Global dimming is defined as the decrease in the amounts of solar radiation reaching the surface of the Earth. The by-product of fossil fuels are tiny particles or pollutants which absorb solar energy and reflect back sunlight into the space.

d. This anthropogenic compound is also a greenhouse gas, with a much higher potential to enhance the greenhouse effect than CO₂. Infrared absorption bands trap heat from escaping earth's atmosphere. In the case of CFCs, the strongest of these bands are located in the spectral region 7.8–15.3 µm ^[22] – referred to as an atmospheric window due to the relative transparency of the atmosphere within this region. The strength of CFC bands and the unique susceptibility of the atmosphere, at which the compound absorbs and emits radiation, are two factors that contribute to CFCs' "super" greenhouse effect.Another such factor is the low concentration of the compound. Because CO₂ is close to saturation with high concentrations, it takes more of the substance to enhance the greenhouse effect. Conversely, the low concentration of CFCs allow their effects to increase linearly with mass.