大学化学专业课件4地球的辐射平衡和温室效应英文版精编整理

SECTION 3 Radiation balance,The spectral distribution of radiation from the sun and from earth,The sun provides Earth with an enormous input of energy every day. Earth rids itself of energy at the same rate and thereby maintains a steady state, with a constant average temperature.,The spectral distribution of radiation from the sun and from earth is shown in Figure 2.1.,For a black body, the peak wavelength of the radiation is inversely proportional to the absolute temperature (Weins Law):,peak(nm) = 2.9106(nmK) / T(K),2. The relations between radiation wavelength and temperature of body,The sun is a very hot body; its peak wavelength is 483 nm, corresponding to a temperature of 6,000 K. Earth emits radiation with a peak wavelength of about 10,000 nm, corresponding to an average temperature of 288 K.,The sun is a very hot body; its peak wavelength is 483 nm, corresponding to a temperature of 6,000 K. Most of its rays fall between 400 and 700 nm, in the region of visible light; these wavelengths are visible because our eyes have evolved in response to sunlight. Earth emits radiation with a peak wavelength of about 10,000 nm, corresponding to an average temperature of 288 K. Thus, while Earth absorbs radiation mainly in the visible region, characteristic of the high temperature at the surface of the sun, it gives off radiation in the infrared region, which corresponds to the much longer wavelengths characteristic of Earths cooler surface temperature.,3. Can the energy balance be affected by human energy consumption?,In principle, if we keep increasing the rate of fossil and nuclear fuel burning, the global heat load might become significant. From equation, we can calculate how much the energy input to Earth would need to rise in order to increase the average temperature by 1 K.,Thus, human energy utilization would have to equal 1.57 percent of the solar input to produce a 1K rise in the average temperature.,to reach 1.57 percent of the suns energy would take about 240 years,Thus, direct heating of the planet through our increasing use of energy is not likely to become a serious problem. More serious is the potential for altering Earths temperature indirectly through changes induced by human activity in either the albedo(反照率) or the greenhouse effect. These matters are considered in the following sections.,The actual flows of energy through the atmosphere are quite complicated. Figure 2.2 shows the planets inputs and outputs of energy in units of 1020 kJ per year. About 54.4 units of solar energy impinge(撞击) on Earth and its atmosphere, but about 16.3 units(30 percent) are reflected to space, exerting no influence on Earths heat balance. Most of this light is reflected by clouds and the atmosphere; a smaller amount(2.2 units) is reflected by Earths surface. The remaining 38.1 units (70 percent) are absorbed, 13.0 units (24 percent) by the atmosphere and clouds, and 25.1 units (46 percent) by Earths surface.,4. Heat balance of earth,54.4=14.1+13.0+27.3,27.3=25.1+2.2,14.1+2.2=16.3,16.3+38.1=54.4,12.5+3.8=16.3,62.7=59.5+3.2,59.5=5.6+53.9,13.0+16.3+5.6=34.9,34.9+3.2=38.1,SECTION 4 Greenhouse effect,1. Infrared absorption and molecular vibrations,As mentioned above the greenhouse effect is trapping of heat below by the atmosphere. Earths atmosphere admits the rays from the sun, but traps the infrared rays emanating from Earths surface. But how is this trapping accomplished? Why do we worry about CO2 and other minor constituents of the atmosphere, when it is made up almost entirely of N2 and O2 (see Table 6.1).,The answer is that the major atmospheric gases are unable to absorb infrared light. They do not meet the two fundamental requirements for the absorption of electromagnetic radiation. In contrast, polyatomic molecules have numerous vibrations； at least some of these vibrations change the dipole（偶极） moment and are infrared-active. All the gases that contribute significantly to the greenhouse effect are polyatomic.,Symmetric stretch,vs,Asymmetric stretch,vas,Figure 6.12 Molecular vibrations of CO2 and H2O in units of wavelengths (nm).,Bend,It might appear that water and carbon dioxide would not be effective in heat trapping because Earths emissions cover a wide spectrum of wavelengths, whereas the molecular vibrations correspond to specific energies. However, the molecules can undergo not only vibrations but also rotations. For each molecular vibration, infrared photons can induce transitions to many different rotational levels (rates of rotation).,Consequently, each vibration has a broad absorption band. These bands are shown for water and carbon dioxide in figure 6.14. In the top panel of the figure, the absorptions for these two molecules are added up and superimposed on Earths emission spectrum. The combined absorption bands can be seen to block most of the terrestrial radiation.,There is however, a relatively unobstructed region of the spectrum between 8,000 and 12,000 nm through which radiation can escape. This region is called the atmospheric window.,This window can be filled by other polyatomic molecules such as the chlorofluorocarbons (CFCs) (see Figure 6.15), methane (CH4) and nitrous oxide(N2O). The CFCs are of major concern as destroyers of stratospheric ozone, but they are also important greenhouse gases. Their impact is significant ever though their concentration is about five orders of magnitude lower than that of carbo