the story of CFCs and stratospheric ozone

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Ngày đăng: 30/11/2016, 21:42

Phần 3 của môn học liên quan đến câu chuyện của CFC và quá trình mà nó tạo ra một thảm họa môi trường. Phần này còn nhấn mạnh tầm quang trọng của khí quyển đến hoạt động của con người và sự tuyệt vời của sinh quyển đến những thay đổi trong bầu không khí. Part of the course relates the story of CFCs and how an environmental catastrophe was avoided This section also emphasises the great sensitivity of the atmosphere to human activity, and in turn, the great sensitivity of the 'biosphere" to changes in our atmosphere As far as the atmosphere is concerned, essentially all energy received from the Sun is in the form of electromagnetic radiation Other sources of energy from, for example, fossil fuel combustion or transfer from the warm Earth's core, are so small in comparison that they can be neglected The average amount of electromagnetic flux reaching the Earth is quite precisely known at (1366  3) W m-2, (this is equivalent to about 14 100% efficient, 100 W household light bulbs per square meter) perpendicular to the direction of the photons Over the spherical surface of the Earth, this energy averages to 342 W per square meter of the Earth’s surface Solar radiation, more precisely, ultraviolet radiation (wavelengths less than 400 nm), is, of course, hugely important to the chemistry of the atmosphere Without it, the atmosphere would be inert and any substances released from the Earth's surface would not be removed by either chemical reaction or photo-dissociation, leading to their increasing atmospheric concentrations and the related environmental impact The value 1366 Wm-2 is known as the Solar constant, S The Sun behaves very much like a black-body radiator, which is an object that is able to absorb and emit photons of all wavelengths Its electromagnetic spectrum follows closely that of a black body of temperature 5800 K, with the greatest deviations occurring at very short wavelengths 5800 K is essentially the average surface temperature of the Sun The peak intensity of the emission is found in the visible region close to 500 nm, which happens to be close to the visual response peak of the human eye at 555 nm As can be seen in this graph, both the intensity and the shape of the spectrum of the radiation reaching the Earth’s surface is modified by absorption of (and scattering by) several atmospheric species Amongst these absorbers, both O2 and O3 are prominent in the ultraviolet (UV) and visible regions of the spectrum The hashed area shows the total photon flux that would reach the Earth's surface if atmospheric species did not absorb at all The difference between the average incoming flux of 342 W m-2 and the flux arriving at the Earth's surface is accounted for by reflection, mostly from water clouds and other aerosols Most of the atmosphere is composed of N2, O2, and H2O vapour The ratio of the former two is essentially constant in the troposphere, stratosphere, and mesosphere, while the concentration of water vapour changes spatially in three dimensions ranging from a fraction of one percent to about four percent by number of molecules per unit volume Although most of the interesting chemistry of the atmosphere occurs between those minor species that make up only a small fraction of the atmosphere (orange block, above), the macroscopic structure of the lower and middle atmosphere is governed by the interaction of O2 with sunlight, as will be discussed shortly In order to consider the fate of molecules when subjected to UV and visible radiation, one may begin by simply looking at the dissociation energies of typical molecular bonds In the case of diatomic molecules, the bond dissociation energy is equal to the difference in enthalpy of formation of XY and of X + Y N2 has one of the strongest molecular bonds encountered in nature The bond dissociation energy can be related to the minimum photon energy (and hence its associated wavelength) necessary to produce N atoms: for N2, 127 nm or less is required to dissociate N2 to N + N, assuming that there is no barrier to the dissociation process that would mean even shorter wavelengths were required for dissociation According to the Sun’s emission spectrum as observed from space, relatively few photons of wavelength less than 127 nm reach the Earth's atmosphere O2, on the other hand, may dissociate a longer wavelengths of up to 240 nm In this spectral region, many more photons are available The change in light intensity as it passes through a gas can be easily described by the BeerLambert expression, which predicts an exponential decrease in intensity with distance if the concentration of the absorbing species remains constant Please note the different units used for absorption and also to the fact that the product (, k or) cL is dimensionless In this course we will tend to use absorption-cross section with units of cm2 (per molecule) as we will normally use concentrations with units of (molecules) cm-3 Absorption of light in the atmosphere is interesting for three reasons (1) It shields animals and plants from harmful UV wavelengths (2) it heats the atmosphere (3) it produces highly reactive species In order to quantify the absorption process, one needs to know the absorption crosssection (or equivalently, the molar extinction coefficient, or molar absorption coefficient) of each molecule and what the resulting dissociation products are, if any Shown here is the absorption cross-section for O2 and H2O That of N2 is not shown, but it becomes significant compared to the other two only below 150 nm, though it does not dissociate until 127 nm or less, as already noted Note, the values (and units) of the absorption cross section The highest is of the order of 10-17 cm2 This is considered to be a very high value for a molecule in the atmosphere, although atoms can have much greater peak absorption cross-sections than this The essential physical interpretation of absorption cross-section of 10-17 cm2 is that, according to a photon corresponding to a particular wavelength, the molecule appears to have a surface area in the direction of the photons approach, of 10-17 cm2 Such a surface area corresponds to a diameter of (4 x 10-17/)0.5 = 35 Ǻ, which is several times greater than the collision diameter of O2 To put this into perspective, remember that Iabs/I0 = cL, where  is the molecular absorption cross-section (cm-2 per molecule), c is the concentration (molecules per cm-3), and L is the path length over which absorption occurs (cm) At ground level, O2 has a concentration of about x 1018 (molecules) cm-3 Thus for 20% absorption, L = 0.2/(10-17x x 1018) = 0.004 cm Therefore, a layer of air of only 0.1 cm thick would appear entirely "black" if the absorption cross section of air in the visible was 10-17 cm2 (only a fraction light of exp[-(10-17 cm2 x x 1018 cm-3 x 0.1 cm)] = 0.007 would penetrate such a layer At the other extreme, in order to view an object through air that is km away, we can say the light absorption needs to be minimal (no more than 10 %, for example) What should be the absorption cross section in this case? Here  = 0.9/(5 x 1018 cm2 x 1000 m x 100 cm/m) = x 10-24 cm2 These figures are good to bear in mind Naturally, one has to consider that the concentrations of most absorbing species in the atmosphere are orders of magnitude less than that of O2, and one must also take into account of the logarithmic change in pressure with altitude, which means that any given absorption path length in the atmosphere in the vertical direction will not usually have a constant concentration Also shown on this figure, is the actinic flux: the spectral irradiance of the Sun directly above the atmosphere (say at an altitude of 200 km) in linear units Since most of the light reaching the atmosphere is lies at wavelengths longer than the main absorption bands of O2 and H2O, a better representation of the influence of O2 and H2O on the solar spectrum is that of a log/linear plot This is given on the next page On the previous graph, it was difficult to see the very small absorption cross-sections associated with O2 and H2O at longer wavelengths where the Sun's radiation flux begins to increase rapidly A logarithmic scale for the y-axis shows these more clearly Shown also are two examples of the range of the effectiveness of O2 in reducing the Sun's light intensity It can be clearly seen that at 150 nm the absorption cross-section is so large that a path of O2 (at ground level) of only mm is necessary to reduce the incident intensity by a factor e (that is, a factor of 2.72) At 240 nm, km of O2 is required to achieve the same reduction factor It is quite clear then that due to absorption of O2 alone, light at 150 nm cannot reach the Earth’s surface Would the atmosphere be transparent at 240 nm due to absorption of O2 only? In order to answer this question one would need to take into account the exponential changing concentration of O2 with altitude, as already mentioned An example calculation is given later on the page relating air pressure to altitude This graph shows that absorption of radiation by O2 accounts for the removal of the Sun’s radiation only in a limited region up to about 260 nm (the 240 nm cut-off for the dissociation of O2, already mentioned, refers to zero Kelvin, at other temperatures there is always a so-called “Boltzmann tail”, which is due to transitions from small populations of vibrationally-excited levels and has the effect of extending this abrupt, zero Kelvin, cut-off point - common the many physical phenomena) You will notice that part of the relative DNA damage spectrum is also plotted This spectrum is a subjective estimate of the degree of damage induced in DNA when exposed to various wavelengths In order to calculate the total relative DNA damage, one must multiply the photon flux spectrum (orange) by the relative DNA damage spectrum Since these two quantities are approximately anticorrelated, the total DNA damage spectrum (sometimes referred to as the “DNA action spectrum”) will have a relatively sharp peak Under the conditions given above, the total DNA damage, if only considering the wavelengths between 310 nm and 305 nm, would be much beyond that which can be safely repaired by most living organisms Clearly another absorber is required to protect the Earth’s biosphere This protection is provided by a relatively minor atmospheric constituent, ozone Earlier it was noted that different wavelengths will be absorbed at different rates in the atmosphere depending on the corresponding absorption cross-section of the species involved and their concentration When light passes through an absorbing medium, it is also expected that the light intensity increases exponentially with distance according to the BeerLambert Law However, this is valid if the concentration of the absorbing species is not a function of distance For the atmosphere, concentration of O2 increases exponentially with distance (from space) This has interesting consequences for the position of maximum absorption In the slide above three situation are considered for which light passes through a series of slabs of increasing concentrations having fixed absorption cross-sections and thickness As the light passes through, the amount of light absorbed in each slab is calculated It can be seen that for some situations the maximum absorption can take place somewhere in the middle of the stack of slabs This often occurs in the atmosphere 81 In the near future called as the third generation, more HFEs, as well as HFCs with lower global warming potential (GWP) will be introduced in the market as appropriate alternatives of the existing HFCs A hydrofluoroolefin with a chemical structure of CH2=CF-CF3 (HFC-1234yf) has been developed as an example of HFC with lower GWP On developing alternative CFCs, two ways of approaches have been conducted in industry One is to develop HCFCs, HFCs, and HFEs by using a combination of available resources and reactions specific to each manufacturer In this approach, the resultant alternatives not necessarily exhibit the same performance of corresponding CFCs and are often required to modify the performance by blending with other appropriate chemicals or introducing some new ingredients to meet the industrial requirement The other is to develop alternative CFCs by a molecular design with the aid of computational chemistry before their synthetic process is considered The typical example of this approach is the development of HCFC-225s 83 [...]... atmospheric pressure (and 2 73 K) This gives about 2.7 x 1019 molecules cm -3 (using the ideal-gas law) L is the path length Since we have (artificially) compressed the gas, the path length is now only 3 mm (30 0 DU) Substituting these values into the above equation leads to Itr/Io = 6.6 x 10 -36 Thus light at 240 nm is substantially reduced- to effectively zero by the presence of O3 Most people associate... case, as will be shown later, F atoms are not effective in removing ozone When comparing the effectiveness of O3 and O2 in absorbing light at various wavelengths it is important to remember that the O2 concentration at 30 km is about 1 x 1017 cm -3, whereas that for O3 is only about 2 -3 x 1012 cm -3 Considering this large ratio, and the extent of O2 at higher altitudes, it can be recognised that the decrease... co-reactant He proposed that NO molecules react with O3 to produce NO2 The so formed NO2 then reacts with O to reform NO Thus a continuous O3 destruction route is established It must be emphasised here that introduction of a catalytic destruction process for O3 does not mean necessarily that the concentration of O3 will be reduced to zero It means that O3 steady-state concentrations (arising from an eventual... southern mid-latitudes than above the tropics, where most O3 is generated 16 How can we determine the concentration of O3 as a function of altitude? Let first see if we can estimate the rate of the various processes From process 1, the rate of production of O atoms is 3. 6 x 107 cm -3 s-1 This should be considered a maximum as the presence of O3 at higher altitudes will absorb the light and reduce the... latitude A quick estimate of the total amount of CFC-1 13 in the atmosphere reveal that it was about equal to all the CFC-1 13 that had been produced to date It seemed not to be removed from the atmosphere 23 Destruction of hydrocarbons in the atmosphere occurs dominantly by reaction with OH, having daytime tropospheric concentrations in the low 106 radicals cm -3 range The slowest reacting hydrocarbon of all... constituents The product of this reaction is O3 You will notice that the O + O2 reaction also requires a third collision partner, M This represents any atmospheric constituent that can take away the excess vibrational energy via collision with the initially-formed, vibrationally-excited O3 molecule If this extra collisions do not occur, then the vibrationally-excited O3 molecule will simply re-dissociate to... because significant DNA damage begins at about 30 5 nm The addition of ozone to the atmosphere blocks all wavelengths below about 30 0 nm (brown) Importantly though, small changes in O3 concentration (grey) have a relatively large effect on damage to DNA and other biomolecules because of the strong increase in their damage spectrum at wavelengths shorter than 30 0 nm So any process that can decrease the... mainly to the absorption of O2 and not O3, even though O3 has an absorption cross-section five orders magnitude larger than O2 at 200 nm 30 The large change in the number of photons in the “atmospheric window” around 205 nm is illustrated in this figure Notice the log scale for the number of photons per second per cm2 per nm At 20 km altitude the 205-nm peak value is 3 x 107 photons cm-2 s-1 nm-1 For a... rates due to a particular process or particular processes It might be the case (as with O3) that the molecule or radical that is being considered is, in fact, regenerated by another process This could imply that it isn't removed at all and its concentration remains nearly constant The removal rate therefore better gives the expected average lifetime of a single radical or molecule by a particular process... on the concentration of O3 The Chapman mechanism predicts that significant concentrations of O3 will be formed only between about 10 and 40 km above the Earth’s surface A typical profile of ozone is given in the figure above, but plotted in two different fashions The lowest profile gives the concentration of ozone whilst the upper profile gives the volume mixing ratio in ppm (parts per million) That
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