Vietnam part 4 CARL p1 50

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This section explores some of the important interactions of HOx (OH + HO2) in the atmosphere Due to several rapid oxidation and reduction reactions OH and HO2 rapidly reach a quasi steady state As will be demonstrated later, the characteristic time to reach quasi-steady state is dependent on the average lifetime of the species It is worthwhile looking again at the most abundant species in the Earth’s lower atmosphere Just as with the interaction of the Sun’s radiation with the Atmosphere, one needs to approach reactivity by considering first any possible reactions between a given radical and the most abundant species since, even if the reaction is quite inefficient (having a rate bi-molecular rate constant below, say, x 10-16 cm3 s-1) , the actual rate of reaction – rate constant multiplied by the co-reactant concentration – could still the larger than for other more efficient reactions 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 metre) 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 cross-section (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 0.1 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 In order to work out the amount of light (or number of photons in this case) absorbed by the atmosphere, three item of information are required (1) the absorption cross-section as a function of wavelength (2) the initial light intensity as a function of wavelength (3) the concentration of the absorption species as a function of distance (for this we also assume a constant T of 250 K) It is also more convenient to simplify the absorption cross section and the photon flux data as indicated by the red lines In practice this means having a table of absorption cross section values for say each nm and the same for the photon flux Amounts of a species are reported in several manners For those interested in kinetics and reaction rates, concentrations are the most often units Modellers of the atmospheres often speak in terms of part per million (billion or trillion), or use partial pressures It is becoming increasingly popular however the use mass per unit volume when referring to harmful gases at ground level This page gives an example of all the units in common use 36 37 This report by the European Environmental Agency of 2006 gives ozone concentration threshold values for EU countries together with the number of days that these values are exceeded It is also worth noting the maximum ozone concentrations observed through the year and their duration Clearly more effort is required to ensure O3 remains at safe levels 38 Ozone in the troposphere has a very different origin to that in the stratosphere although the fundamental reaction is the same, O + O2 + M  O3 + M At the tropopause there is also some exchange of ozone between the troposphere and stratosphere Unlike in the stratosphere, ozone concentrations in the troposphere vary fairly rapidly and over different time scales In urban areas where ozone is generated, ozone concentrations tend to peak in the middle of the day and dip to their lowest concentrations during the middle of the night This type of cycle is called diurnal Ozone concentrations vary from day to day depending on emissions and meteorology, but sometimes under certain meteorological conditions, episodes occur Episodes are periods of a few days when ozone concentrations are unusually high For U.S southern urban areas, episodes are caused by local stagnation, when wind speeds are slow This allows ozone concentrations to build up In northern U.S cities, episodes are more heavily influenced by transport of ozone and precursors from other areas Episodes are also associated with high temperatures and high humidity Because ozone concentrations are dependent on high temperature and sunlight, the highest concentrations during the year occur in the summer months The lowest concentrations, or the cleanest air, occurs during the winter months This type of cycle is called seasonal Can ozone be transported from city to city? Ozone and the compounds that help form it, NOx and VOCs can be transported significant distances from where they are emitted and can cause ozone problems in other areas The distance of ozone transport between the precursor emissions and O3 removal from the atmosphere can be up to about 700 km 39 JNO2 is actually quite large since NO2 has a broad absorption spectrum that spans the near UV and visible spectral region Its most important absorption band for atmospheric chemistry extends from 250 nm to 650 nm Though the spectrum above is split into two main parts, the absorption spectrum itself appears continuous The two parts are drawn in order to distinguish the wavelength region where photo-dissociation occurs from that where fluorescence occurs; the latter being of no interest to the chemistry of the atmosphere, though it does give the brown appearance of NO2 that contributes to the tinge of photochemical smog Notice that the peak absorption cross-section is about a factor of fifteen less than that for O3 The y axis of the plot has the term QY which means quantum yield It is a ratio that expressed the average number of a particular photolysis product produced per photon absorbed The QY values for dissociation (i.e production of NO) are unity between 250 nm and about 380 nm, after which it drops off rapidly to zero at 430 nm On the other hand the QY for fluorescence (production of a photon by re-emission) increases over this same region and remains unity up to 650 nm 40 As seen on the previous page, the peak absorption cross-section for NO2 is smaller than that of O3 but the photolysis rates of the former are much greater than those of the latter due to the different spectral regions over which dissociation takes places The NO2 spectrum lies in a region where the actinic flux increases rapidly The plots on the left show calculated J values for NO2 and O3 as a function of altitude for several surface albedos These values can be used directly to compare the rate of production of O atoms from the two sources for a given concentration of O3 and NO2 The right-hand plot demonstrates how the J values are calculated It shows the absorption cross section values for both O3 and NO2 multiplied by the actinic flux One also needs to take into account that not all photons absorbed will result in photo-dissociation This is expressed as a “quantum yield”,  (a value ranging from to 1), which, in this case, is the ratio of number of O atoms produced per photon absorbed on average The area under the plots gives the desired J values Thus it can be clearly seen that NO2 is more rapidly photo-dissociated than O3 in the troposphere The difference in the two areas is about a factor 800 On consideration that the concentration of O3 is about on average a factor of 50 greater than that of NO, the resulting average ratio of O atom production is about 16 in favour of the NO route 41 Just as with O3 in the troposphere, NOx shows quite rapid (3-D) spatial and temporal variations Most NOx is found at ground level, but there is a definite secondary peak in the upper troposphere in many parts of the world Also NOx changes very rapidly during the day in urban areas and the interchange between NO and NO2 depends also on the time of day (NO2 being photo-dissociated during the day) 42 43 Lightning occurs mainly over land However, the frequency of lighting strikes is not distributed evenly around the globe North America receives substantial lighting activity, whereas West Europe receives relatively little activity For Europe then the influence of NOx produced by commercial aircraft can be a more important contribution to mid altitude tropospheric NOx than it is for North America 44 45 46 47 48 49 50 [...]... photodissociation of O2 is 242 .2 nm, however O3 production has been observed in pure O2 irradiated with 248 nm The mechanism might involve (though this is not clear) absorption of light by a small fraction of O2 in its =1 state The O atom formed then forms O3 which photodisociates to O + O2(
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