Vietnam part 5 CARL p1 55

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Part of the course deals with the complex interaction of Ox, HOx, and NOx in the presence of volatile organic compounds (VOCs) in the troposphere The global HOx tropospheric concentration is determined by the background chemistry just described for which the concentration of NOx is very low This is not the case for urban environments that release large amounts of NOx Here OH chemistry is dominated by interaction with NOx, leading to more complex chemistry As has been explained earlier, the Chapman mechanism gives an expression for the quasi steady-state concentration of O3 in the stratosphere based on the major formation and removal reactions, but neglecting any catalytic cycles In the troposphere, a steady state solution for ozone concentration can also be formulated based on its interaction with NOx The diagram is repeated on the extreme right and left for clarity The major loss process for O3 in the troposphere (certainly in the first km) is generally reaction with NO rather than photo-dissociation, which is much slower in the uv-depleted troposphere A quick calculation using typical concentrations of NO, and using the O3 + NO rate constant, show that it is by far the dominant process of the two From the process shown, a photo-stationary state (quasi-steady-state) expression can be formulated Which shows that [O3] is expected to be proportional to the NO2/NO ratio and to the amount of light available (JNO2) In previous pages, the idea of the photo-stationary state (pss) for ozone in the troposphere was developed Whilst this can be a reasonable prediction in some circumstances it more often fails to give the correct ozone concentrations And just as for stratospheric ozone, it is clear that there must be other sources or sinks, or that the mechanism of the pss is perturbed in some way One perturbation is the reaction of the hydroperoxy radical with NO to give NO2 and OH This reaction essentially creates NO2 without the loss of ozone; it will therefore contribute to net ozone formation The HO2 radical is created in the atmosphere following any reaction that gives HCO or H as a product: the large relative concentrations of O2 ensure that nearly all H or HCO is converted to HO2 Both NO and NO2 are toxic and damaging to the animals and plants in the concentrations found in the polluted PBL Of the two, NO2 has a greater environmental impact, although they are normally considered together, as NOx, in pollution control strategies due to their rapid photochemical interchange We now return to the possible perturbations of the photo-stationary state for ozone that was developed earlier As seen in the previous slide, the only fate of alkyl radicals in the atmosphere is reaction with molecular oxygen giving alkylperoxy radicals Alkylperoxy radicals, RO2 (including HO2) react readily with NO giving NO2 and an alkoxy radical At high concentrations of RO2, RO2 + R'O2 (self) reactions become significant leading to a variety of products that depend on the alkyl groups involved These self reactions will reduce the efficiency of ozone formation via the RO2 + NO2 reaction All of the reactions above are generally associated with polluted regions having both high VOC and high NOx levels Before we consider these regions we first look at some important chemistry associated with remote regions In polluted regions there is a significant interaction between VOC, NOx, HOx, and Ox species This interaction determines the loss or production rate of ozone The following diagrams are similar to those used for the simple photo-stationary state condition for O3 developed above but have been redrawn in a slightly different manner so that all of the relevant interactions can be accommodated The first cycle given above is the basic Ox cycle in which O3 is photo-dissociated, producing both ground-state O atoms and electronically-excited O(1D) atoms Most of the O atoms react with O2 giving again O3 Since the lifetime of O is very short, most (more than 99.99 %) of Ox resides as O3 in the troposphere In the presence of water vapour, a small fraction of O(1D) atoms reaction with H2O yielding two OH Thus the Ox cycle has a small loss process Note, the beginning mixing ratio for O3 in all of the following cases is 30 ppbv, which is the average background level for the remote troposphere If it is assumed that there is no water vapour present, but instead NOx, the photolysis of O3 is essentially by-passed since now there is a more efficient way in which to convert O3 to O atoms The O3 photolysis rate remains unchanged, but production of O and destruction of O3 is dominated now by the NOX cycle The addition of this cycle though shifts the Ox partitioning more in favour of O3 This has no significant effect on the O3 concentration since it is already large relative to that of O, but lowers the O concentration This, in fact, is another representation of the photo-stationary state for ozone given earlier The overall effect of the Ox, NOx coupling is that light is converted to heat Only at what extremely high concentrations of NO2 can there be any significant reaction involving O atoms other than O + O2 + M The NO2 + O reaction does not lead to O3 formation and therefore represents an overall loss rate for O3 Since this reaction is only significant under rather extreme conditions it is not further considered If HOx is now included in the picture, a catalytic cycle is now possible in which OH reacts with O3 giving HO2, which in turn reacts with O3 giving OH Note that the former reaction has the larger rate constant but in steady-state the rates [OH]k(OH + O3) and [HO2]k(HO2 + O3) are equal The inclusion of the HOx cycle leads to an O3 lifetime (half-life) of about hours, but this is not the whole picture since other reactions decrease the HOx concentration which would otherwise continue to increase due to its production in the reaction of O(1D) with H2O Consideration of the Ox/HOx coupling in isolation, the net effect is reaction of O with O3 giving two O2 molecules 10 This page illustrates that an apparent small change in structure can lead to rather different product yields For these compounds (and for many similar structures) isomerisation can occur via a 5-membered ring structure leading to a H transfer and formation of a hydroxy group The rate constant for reaction with O2 is similar for both radicals, however isomerization of 2-hexoxy is more efficient C-C bond fission in both cases is minor 41 42 43 Experimental determination of the rate constants of carbonyl reactions in the atmosphere is quite challenging due to the unusual temperature and pressure dependence of the rate constants The task in not aided also by the difficulties of ab initio calculations, as illustrated above 44 A good example of the complication of these reactions is given on this page for the reaction of OH with acetic acid as determined in our laboratories The Potential Energy Surface constructed shows that the key transition states TS1,TS2 and TS3 control the product distribution: At high temperatures, methyl-H abstraction via the looser TS1 and TS2 should dominate At very high temperatures, methyl-H abstraction should even gain in relative importance because it can readily occur directly, without proceeding through an initial pre-reactive complex At low temperatures on the other hand, acidic-H abstraction via the rigid but lower-lying TS3 should dominate, the more so because at low T one has a high concentration of this most stable pre-reactive complex, which facilitates extensive tunneling of the acidic-H through the barrier So the lower the temperature, the higher the branching fraction of the CO2-forming acidicH abstraction channel 45 The new kH data, the red squares, are within plus minus 15% of our earlier ones In the meantime there is also a new set of data by Vimal et al., the green dots 46 The impact of the very stable doubly-H bonded pre-reactive complex on the rate coefficients kH(T) at low temperatures is demonstrated by a comparison with the k(T) behavior for the OH reactions with acetone and ethane This figure displays the rate coefficients per methyl group At low temperature, kH(T) is over an order of magnitude higher than that for acetone, shows a much more pronounced negative T dependence and exhibits its minimum at Tmin = 530 K, that is, almost double the Tmin of ~250 K for acetone, reflecting the larger stability of the OH.acetic acid complex due to its double H bond compared to a single H bond for the OH.acetone complex At very high T > 1000 K, the title reaction is expected to proceed through direct methyl-H abstraction, as for the reaction of OH with ethane or OH with acetone Our kH at 800 K nearly approach the value of k(T) for OH + CH3CH3 and OH + acetone per CH3 group At T from 500 K to 750 K, our kH(T) results are substantially lower This suggests that up to about 800 K methyl- H abstraction remains adversely affected by the long range H-bonding attraction between the approaching OH radical and the carboxyl functionality 47 The CO2 yields at room T demonstrate that abstraction of the acidic hydrogen occurs preferentially despite the dissociation energy of the O-H bond being about 12 kcal.mol-1 greater than that of the C-H bond These results over wide temperature range reveal that the branching fraction of acidic hydrogen abstraction decreases sharply with increasing temperature such that it should be insignificant at temperatures higher than 500 K 48 49 50 51 52 53 54 55 [...]... dependence All of these charateristics have been observed 35 Alternative fates of RO2 radicals in the atmosphere, which are especially important under low NOx conditions, are reaction with HO2 or other peroxy radicals The rate constant at room temperature for RO2 + HO2 reactions is 5 x 10-12 cm3 s-1 for R = CH3, 8 x 10-12 cm3 s-1 for R = C2H5 and about 1 .5 x 10-11 cm3 s-1 for larger R groups The self reactions... background mixing ratio of O3 is only about 30 ppbv, then it is expected that polluted regions can often attain peak concentrations several times this value over a few days 16 17 18 19 20 21 22 23 24 25 The following two pages demonstrate the important coupling between meteorology and chemistry for pollutant formation Even though the chemical conditions were already in place ,the ozone episode given... however has some very important consequences 34 The Reactions of RO2 with NO are quite fast and do not vary significantly with the nature of the alkyl group The recommended rate constants at 298 K are 7 .5 x 10-12 cm3 s-1 for R = CH3 and (8 to 9) x 10-12 cm3 s-1 for all other R groups The major pathway for these reactions is production of NO2 For larger R groups isomerization to form alkyl nitrate is... significant The product of this reaction is nitric acid HNO3, which is removed from the atmosphere by both wet and dry deposition This process lowers [OH] and therefore limits the O3 production rate 15 The concentration of OH is actually the key in determining the rate of production of O3 as a function of NOx for a given concentration of VOC This slide clearly shows a very strong correlation between... by a number of different paths A third path, not given above as it is minor is ROOR + O2 Overall rate constants show an trend according to whether RO2 is primary (k = 1 x 10-13 s-1), secondary (k  10- 15 s-1), or tertiary (k  10-17 s-1) 36
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