Modeling of Chemical Reactions (Comprehensive Chemical Kinetics)


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Introduction

King, M. Vernon Harcourt — ', Ambix 31 Part I , 16— Kuhn, T. Laidler, K. Lakatos, I. Laudan, L. Lewis, W. Logan, S. Lythcott, J. Matthews, M. McClelland, J. McCloskey, M. Gentner and A. Stevens eds. Mellor, J. Mierzecki, R. Nersessian, N. Hamilton eds. Niedderer, H. Ostwald, W. Morse , Longman Green, New York.

Partington, J. Piaget, J. Rice, F. Rice, O. Semenoff, N. Temkin, O. Treagust, D. Toulmin, S. Van't Hoff, J. Watts, D.

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Yablonskii, G. Crompton ed. Comprehensive Chemical Kinetics Vol. Gilbert 2 1. Personalised recommendations. Cite article How to cite? ENW EndNote. Secondary Reactions in the Atmosphere: The nature of atmospheric reactions initiated by HO- olefin reactions is, generally, even less certain than the corresponding primary processes. However, in the case of C2H,,, C3H6 and 2-C,,H8, for which the HO-addition has been shown to be the predomin- ant primary step, some significant progress has been made on the mechanistic interpretation of secondary reactions involving the HO-adducts in the presence of 02 and NO In particular, computer-aided numerical analyses of smog chamber data have played a major role.

In conclusion, there is a great need for kinetic and mechanistic data on the oxidation of free radicals formed from HO-olefin reactions. The above-mentioned studies illustrate a classical photochemical approach to these problems. Clearly, it is highly desirable to study these chemical systems with more direct experimental methods.

Thus, utmost accuracy is required for the kinetic and mechanistic information on olefin reactions for modeling purpose. Kinetics: Over the years, there have been numerous determinations of the rate constants for the reactions of 03 with a large variety of olefins. A summary of literature values for several olefins is given in table 4. Generally, the reported values agree reasonably well for terminal olefins, but scatter far beyond the estimated experimental precision for internally double-bonded olefins.

For instance, two of the most recent sets of extensive measurements by Huie and Herron [21] ref. The individual values in the latter set are all higher than those in the former Apparently, there are factors other than systematic measurement errors affecting one or both experiments. In these studies as well as most of the others, the rate constants were derived from the decay rates of 03 in the presence of excess olefins. Clearly, the degree of reliability of the above postulated mechanism depends on the accuracy of the input kinetic data for the series of secondary reactions initiated by the product H atoms.

On the basis of their modeling work, Herron and Huie [23] have pointed out the needs for improved data for several crucial free radical reactions occurring in C2H. In particular, the mechanistic knowledge of the HO-C2Hi, reaction is a prerequisite to unraveling the C2Hi, mechanism.

In general, the agreement should be considered fair considering the uncertainty in both measurements and model. Very recently, Dodge and Arnts [27] have developed a sufficiently detailed model including 26 steps for the reactions of 03 with methyl- substituted olefins, e. PB Kinetics, Symp. H, Kinetics 11, 45 Petroleum Inst. Kinetics 11, Kinetics 5, Summary of Session The discussion centered on the reactivity of OH and 03 and the mechanisms of reaction of OH and 03 with olefins. The mechanisms of OH-olefin reactions were treated by Cvetanovic.

In his work hydroxyl radicals were produced by the photo- lysis of N20 to produce O'D in the presence of H2 or H In the absence of 02, the major products of the reaction can be accounted for on the basis of an additions reaction followed by radical-radical and radical substrate reactions. There was no evidence that abstraction was important for ethylene or propylene, e. As pointed out by Niki in his review paper, larger olefins, particularly those with weak allylic bonds, all could react partially through an abstraction mechanism.

Kinetic data support this argument. Of greater interest is the question of the role of 02 in the reaction. If the initial reaction is addition, then in the presence of 02, a peroxy radical would be formed. The subsequent fate of this peroxy radical is one of the key problems in laboratory studies of atmospheric chemical reactions. The subsequent fate of the alkoxy radicals is not understood. In the mechanism used by Niki his fig. It was pointed out by Batt and by Golden, however, that the simpler alkoxy radicals were more likely to react with 02 see review paper of Golden and susequent discussion.

The rate constants for OH reactions are in relatively good shape as discussed by Atkinson. Rate constants for 03 reactions are still subject to some uncertainty because of problems in measurement methodology. These problems would be resolved if the mechanisms were better understood. The major lack of kinetic data is for cyclic olefins which as Niki pointed out are the most important class with respect to consumption of ozone by olefins. The mechanisms of ozone-olefin reactions have not been settled. Recent work, as reviewed by Niki, indicates that free radical yields are smaller than had been thought previously.

Whitten emphasized that in most cases the radical yields were the important information needed by modelers rather than specific reaction channels. Dodge presented a general model for the reactions based on both high and low pressure experimental observa- tions. This model appears to rule out the O'Neal- Blumstein mechanism. O'Neal agreed to this interpretation, pointing out the reasons for the discrepancy. Golden pointed out that the high and low pressure results used by Dodge might not be compatible because of different quenching rates etc.

In particular what are the rates of reaction with NO , SO , and aldehydes relative to isomerization and scission. Heicklen commented on the reactions of ozone with chlorinated ethylenes where a ir-complex may be involved. There was considerable interest expressed in extending observations to more complex systems. The importance of cyclozlkenes was noted above.

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Comprehensive modeling of bloodstain aging by multivariate Raman spectral resolution with kinetics

Stedman urged consideration of natural substances such as a-pinene and isoprene. O'Brien pointed out the difference between high and low molecular weight compounds particularly with respect to acid and aerosol formation. This raise unanswered questions as to the stabilization of the Criegee intermediate and their role in the formation of acids.

The origin of organic acids is also a puzzling problem in photooxidation studies of aldehydes. Acids are also found in low pressure ozonoalysis studies. The relative rate constant data of Cox [1] for ethane, propene and transbutene, obtained from the photolysis of HCHO-alkene mixtures in air at Torr total pressure, have not been included in table 1 as the stoichimetry factor was not known. The data from relative rate studies have been reevaluated on the basis of what is felt however biased by the authors to be the "best" rate constant for the reference reaction at the temperatures employed in the respective relative rate studies.

The rate in table 1 for ethene are limited to the high pressure re- sults, although the rate constants of Greiner [2] apparently obtained at Torr total pressure of helium have been included. For ethane and propene the limiting high pressure room temperature rate constants appear to be reasonably well defined table 1 at about 8 x 1Q cm3 molec'1 s-1 and 2.

For the higher alkenes the data do not appear to be as consistent. The rate constants obtained by Ravishankara et al. It is probable that wall adsorption problem as observed for propene in a metal reaction cell [18] in the static system used by Ravishankara et al. Total pressure not stated, but stated to be the same as in previous work [26], i. Calculated from the Arrhenius expression of reference [28] for T K. Rate constants at 20 Torr total pressure with helium as the diluent gas.

No pressure effects were observed over the total pressure range Torr 1-butene and cisbutene or Torr propene. It is evident that this data which is obviously subject to large uncertainties because of the assumptions made is in general agreement with that obtained by references [5. Rate constant data for the reaction of OH radicals with monoterpenes. Table 3. Rate constant data and Arrhenius parameters for the reaction of OH radicals with dialkenes. Calculated from the Arrhenius expression of reference [30] at K. This is especially so as this set of absolute rate constant data [14,20,22] also comprises the only temperature dependence studies for the alkenes other than for ethene.

Finally, it should also be noted that with the flash photolysis systems used to determine OH radical rate constants for the alkenes, problems have been encountered due to secondary reactions and due to wall absorption of the reactants. References [1] Cox, R. Faraday Trans.

Til] Stuhl, F. Health All, Thesis, University of California, Riverside, August Atkinson, R. H,, is addition. They found C2H5OH as the major product at high pressures and that this product became less important as the pressure was reduced, as expected for the pressure-sensitive addition reaction of HO with C2H4. They further estimated that in the high pressure limit, this fraction would further drop below 22 percent. Since then, the high pressure limit rate constant has been evaluated, and this fraction becomes 7 percent at the high pressure limit.

It is apparent that if the HO radicals become significantly energetic either thermally or by other means , and the pressure is low enough, then the abstraction reaction must become dominant. Thus pressure and temperature studies should be done to determine the conditions when the two competitive paths are important. Perry, R. Simonaitis, R.

William P. Carter, Statewide Air Pollution Research Center, University of California, Riverside, California At the present time, the major uncertainties we have found in developing and validating the mechanisms for the OH-olefin system concerns the rate constant for the decomposition of B-substitut- ed alkoxy radicals, relative to the rate of their reaction with Figure 2 in Dr. Niki's review paper is a revised version provided by Dr C The change does not significantly affect Dr Niki's conclusions.

The study of Niki et al. Riverside smog chamber data [3] that the acetaldehyde and formaldehyde yields in propene-containing NO -air systems are better fit by models which assume that reaction 1 is slow and reaction 2 is fast. The assumption that reaction 2 is much faster than reaction 1 is consistent with the theoretical estimates of Baldwin et al.

Our results [2] could be reconciled with those of Niki et al. However, the yields of these products monitored in n-butane-NO -air UCR chamber runs using the same techniques [3j agree well with our n-butane model predictions [2]. Since in the n-butane system, the photooxidation mechanism is less uncertain; this tends to indicate that the reported yields of these products in the recent UCR chamber experiments are probably not in error. It should be noted that our smog chamber modeling results are completely inconsistent with both reactions 1 and 2 being slow.

References [1] Niki, H. A [4] Baldwin, A. Niki in his figure 2. The predic- tions indicate that the decompositions compete with difficulty with their reaction with oxygen, if at all. See more detailed comments in session on free radical chemistry. Marcia C. Dodge, Environmental Protection Agency, Research Triangle Park, North Carolina We recently developed a mechanism for the propylene reaction to use in our modeling studies. The mechanism we formulated is based on the results of the two most recently published studies of olefin reactions. In our model, a fraction of the "hot" CH3CHOO radical is assumed to be collisionally-stabilized at atmospheric pressure.

The rest of the biradical undergoes rearrangement to form a "hot" acid and ester. The acid and ester subsequently decompose to various free radical species. This mechanism was used to model data collected in our laboratory on the ozonolysis of propylene. Four experiments were conducted in Teflon bags in air at atmospheric pressure.

An example of the type of fits obtained when we modeled these data is shown in figure 1. Experimental and simulated results using the new mechanism for olefin- ozone reactions. The fits obtained for the other three experiments were equally as good. Although the new mechanism adequately explains the observed decay of propylene and 03, the mechanism favored by many modelers in the past does not fit the data. In the last few years, many model ars have used a mechanism based on the O'Neal and Blumstein treatment of olefin-Oj reactions.

In this mechanism, the primary ozonide, after ring- opening, can undergo a number of rearrangements, the most likely of which is a-hydrogen abstraction to form unstable hydroperoxides. The simulated rate of propylene disappearance is significantly faster than the observed rate of TIME, minutes Fig. Experimental and simulated results using mechanism based on the O'Neal- Blumstein treatment of olefin-ozine reactions.

Clearly, the data do not support this treat- ment of propylene chemistry. Although the mechanism developed in this study adequately explains the observed decay of propylene and 03, the results should not be construed as definitive. Additional work is needed in order to fully elucidate the mechanism of ozone-olefin reactions.

Considering the non-existence of a proper data base for such an effort, it is not clear what such fits demonstrate. Certainly with the available number of adjustable parameters it does not take a very ingeneous investigator to fit the data.

To the unwary it may well appear that the entire problem has been solved. We know that this is not the case and it would be more worthwhile to high- light disagreements and inability to fit the data. This will immediately highlight the important questions that must be settled. Edward O'Neal, Department of Chemistry, San Diego State University, San Diego, California Concerning the O'Neal-Blumstein mechanism, it should be noted that in the original formulation, the rate of reaction from the molozonide to the Criegee intermediate was estimated using for an analogy the then available t-butoxy radical decomposition rate constant.

This is now known to have an A-factor about times higher than that used in the estimate. The Criegee reaction pathway is therefore corresponding more important, and relative to the competing intramolecular H-abstraction pathways, is now expected to be the dominant process under many reaction conditions. Gary Z. Whitten, Systems Applications, Inc. Our recent modeling work indicates that for ethylene the yield of free rad- icals should not exceed about 10 percent.

We were very pleased to see the results of Herron and Huie which confirm that estimate. In the case of the prophlene reaction we were pleased to see Niki's recent results which indicated a 30 percent radi- cal yield, and Dodge's estimate of about 38 per- cent. In our air models we are using a value of 30 to 35 percent. Anything greater leads to a marked decay of propylene. However 03 does react with olefins to form a reversible ir-complex E. Sanhueza, I.

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Hisatsune, and J. Heicklen, Chem. There is no evidence that this species plays any role in olefins containing only carbon and hydro- gen. This was interpreted as meaning that the reversible Tr-complex was the active species. Hisatsune, L. Kolopajilo, and J. Heicklen, J. The possible role of the ir-complex should be considered in 03 reactions involving substituted olefins.

Recommendations Reactions of Ozone and Hydroxyl Radicals with Olefins Reactions of olefins with ozone and hydroxyl radicals are of fundamental importance for the chemistry of photochemical smog and although sub- stantial progress is being made in this field, much further work remains to be done. This work should involve both determinations of rate constants and mechanistic studies. The latter should be based on detailed product analysis, supplemented by computer modeling. It is convenient to discuss ozone-olefin and OH-olefin reactions separately.

Reactions of Ozone with Qlefins A. Mechanism of ozone-olefin reactions While significant progress is being made in the investigation of the mechanism of ozone-olefin reactions, it is clear that the mechanism is not yet fully understood. Its full understanding is of crucial importance for an understanding of the chemistry of photochemical smog.


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Some of the chemistry involved in the olefin interaction in the gas phase is probably the same or is similar to the chemistry of the oxygenated free radicals reacting with 02, as for example the radicals formed by addition of OH to olefins in the presence of Further progress in this very difficult filed will require therefore imaginative studies not only of ozone-olefin reactions but also of the reactions of 02 with the oxygen containing free radicals produced in these systems. Photolysis of organic acids and esters and generation of selected oxygenated free radicals by other means are examples of the techniques which could be utilized for this purpose in future work.

Recommendations: 1 Selected ozone-olefin reactions, including cycloolefins, should be investigated under atmo- spheric conditions over a wide range of experiment- al parameters and with time resolved analysis of the concentrations of the reacting species and as many products as possible. Rates of ozone-olefin reactions The phenomenological "rate constants" of the reactions of 03 with a number of simple terminal olefins in the gas phase, show good mutual agree- ment and describe well the rates of consumption of these olefins.

However, their exact relation to the "true" bimolecular rate constants will only be resolved when the mechanism of ozone-olefin reactions in the gas phase becomes fully under- stood. The "rate constants" for internal olefins measured in different laboratories show greater discrepancies. These discrepancies may be largely due to the fact that the range of experimental conditions has not been sufficiently broad to establish potential trends in the values. A better understanding of the reaction mechanism will no doubt also help to resolve these discrepancies. The difference between the data obtained at high olefin concentrations in the gas phase and in non-polar solvents those obtained at low olefin concentrations in the gas phase and in non- polar solvents those obtained at low olefin concentrations under conditions similar to those in the polluted troposphere is puzzling.

Further work with the object of resolving this discrepancy, while not of the highest priority, could help in the understanding of the mechanism of the olefin reactions in the gas phase. Recommendations: 1 Measurements of the rates of olefin reactions in the gas phase should be extended to cover a substantially broader range of experimental conditions.

Mechanism of OH-olefin reactions The mechanism of the OH reactions with ethylene and propylene in the absence of 02 appears now to be reasonably well understood. Hydroxyl radicals add to these two olefins and there is little or no H atom abstraction at room temperature. A sugges- tion that there is approximately 8 percent H atom abstraction from ethylene at atmospheric pressure, possibly due to "hot" OH radicals, is given in a separate comment further below. The mechanism of OH-olefin reactions in the presence of 02, a process of crucial importance for the chemistry of photochemical smog, is unfortunately very in- completely understood.

Recommendations: 1 Study of the mechanism of the OH-olefin reactions in the absence of 62 should be extended to olefins other than C2Hi, and propylene, especially to the olefins known to be present in the polluted atmosphere. Rates of OH-olefin reactions Good experimental techniques for the determina- tion of OH-olefin reaction rates are now available. However, caution has to be exercised to assure accurate determination of the very small reactant concentrations used in some experiments and to establish the extent of the interfering secondary reactions, in particular of the OH-free radical secondary reactions.

The case of ethylene is of special interest in this respect because of a possibility of interception and consumption by reaction of the "hot" CH2CH2OH radicals by Such an interception could result in an appreciable increase in the rate constant of the OH-C2Hi, reaction in air relative to the value obtained in laboratory measurements in the absence of Rate constants for higher olefins and cycloolefins are less satisfactory, especially the values obtained by the competitive technique.

No values are available for some important naturally occurring olefins such as terpenus and isoprene, although the latter could be roughly estimated from the value of the rate constant for 1,3-butadiene. The range of the literature values of the rate constant for acetylene is large a factor of about and further determinations are required. Recommendations: 1 Further determinations of the rate constants are required for higher olefins, cycloolefins, isoprene, terpenes and acetylene.

Lloyd Environmental Research and Technology, Inc. Sources, ambient levels, photochemistry, and free radicals, reactions of these substances are treated. Keywords: Aldehyde; free radical; photolysis; reactions; review; troposphere 1. Introduction Aldehydes are major products in the oxidation of hydrocarbons and play a rather unique role in the photochemistry of the polluted troposphere.

For example, they can contribute to photochemical smog, eye irritation, and odor problems. Their importance has been recognized for over a decade Leighton, ; Altshuller and Cohen, ; Altshuller and Bufalini, While significant progress has been made in defining the photo- chemistry, kinetics, and mechanism of aldehyde photooxidation, much remains to be learned about their ambient concentrations as a function of time, season and location. Since aldehydes, both aliphatic and aromatic, occur as primary and secondary pollutants and are direct precursors of free radicals in the atmosphere, aldehyde chemistry represents an important subject area.

The understanding of this topic is necessary to meet the objective of modeling tropospheric chemical reactions. In this context, the major objective of this paper is to consider the historical interest in aldehydes; their sources and atmospheric concentrations; the photochemistry, kinetics and mechanism of their reactions and finally to delineate current measurement needs and recommend research priorities based on assessment of the current status of knowledge of the chemistry of aldehydes in the troposphere.

In addition, the role of other oxygenated hydrocarbons in tropospheric chemistry will be addressed briefly. Although aldehydes are the main oxygenated hydrocarbons generally considered, and will receive major considerations here, other classes of oxygenated hydrocarbons merit consideration and should be assessed in terms of their involvement in the chemistry of the polluted troposphere.

Thus ketones, esters, ethers and alcohols will be briefly considered to assess their possible importance in modeling the troposphere. The major areas of uncertainty will be discussed and research priorities suggested. This paper is an attempt to survey the current published literature on aldehydes and, to a lesser extent, other oxygenated hydrocarbons as the work relates to modeling the troposphere. It is hoped that the discussion periods will extend the coverage to include unpublished work, prelimi- nary results, and peripheral studies which have a direct bearing on the overall thrust of this paper.

Previous Work and Importance of Aldehydes Initial impetus for the interest in the role of aldehydes in photochemical air pollution stemmed largely from the possibility that they were connected with eye irritation which became a major phenomenon and problem in the Los Angeles basin during the 's. However, an early Stanford Research Institute study SRI concluded that "concentrations of aldehydes have rarely exceeded 0.

This lack of correla- tion tends to indicate that aldehydes alone are not responsible for eye irritation. Acrolein and formaldehyde were shown to be produced upon irradiation of dilute automobile exhaust and olefin-NOx mixtures Schuck, ; Schuck and Doyle, Aside from the possible relationship of aldehydes to eye irritation, it was subsequently proposed Leighton and Perkins, ; Leighton, that aldehydes could act as precursors to radicals which could either directly form oxidant or oxidize NO to N This possibility received support from the results of several experimental studies focused on the photooxidation of aldehydes under laboratory and simulated atmospheric conditions and generally employed formaldehyde and the lower molecular weight aliphatic aldehydes Haagen-Smit and Fox, ; Altshuller and Cohen, ; Altshuller, Cohen et al.

Recently Dimitriades et al. Figure 1 shows the significant impact of initial aldehyde concentrations on ozone formation in a nine-hour irradiation of a surrogate hydrocarbon mixture Pitts et al. Thus an approximately percent increase in initial formaldehyde concentration from 91 to ppb increases the maximum ozone concentration by approximately 25 percent from about 0. Clearly, the rate of formation of 03 is enhanced but it is possible that the 03 maximum value would not be significantly increased if the irradiations were carried out sufficiently long.

Effect of added HCHO on ozone formation in long-term irradiations of surrogate mix- ture from Pitts et al. Aldehydes can provide significant sources of radicals such as H02, OH and R02 which can influ- ence the rate at which photochemical oxidants are formed under ambient conditions.

With the advent of appropriate computer calculation facilities to handle complex kinetic mechanisms, a number of workers demonstrated this effect by carrying out computer simulations of atmospheric chemistry both with and without initial aldehydes Niki, Daby and Weinstock, ; Calvert et al. Many of these calculations have focused on formaldehyde which photodissociates to produce significant amounts of H02 radicals under ambient conditions. Thus Demerjian et al. Although there is some uncertainty attached to the quantum yields for photodissociation into radicals of HCHO as a function of wavelength vide infra , aldehydes are well established as important ingredients in photochemical smog formation.

The role of aldehydes as eye irritants and radical precursors has been given above. An additional role for aldehydes is as precursors to the formation of peroxyacyl nitrates. Stephens, ; Lonneman et al. Sources and Ambient Concentrations Sources. There are primary and secondary sources of aldehydes in the atmosphere. The primary sources are related to combustion and result from incomplete combustion in, for example, internal combustion engines, diesel engines and stationary sources, such as incinerators, etc.

Altshuller et al. Automobiles are a signifi- cant source of aldehydes and the latter account for up to one-tenth of the hydrocarbon emissions Black, Oberdorfer and Seizinger and Dimitriades have analyzed the individual aldehydes emanating from pre-controlled automobiles. Table 1 shows the percentage of aldehydes from automobile exhaust as determined by several workers Oberdorfer, ; Fracchio et al. It is evident from these emission sources that formaldehyde is the largest aldehyde component.

Similar but more extensive results are shown in table 2 which were obtained by Seizinger and Dimitriades In addition, benz- aldehyde and formaldehyde are produced, along with alcohols, ethers and ketones. One would of course expect variations in the relative amounts of these compounds depending on the fuel used, e q , see table 1. With the advent of hydrocarbon control measures for automobiles, the aldehyde concentrations have been reduced along with the hydrocarbons.

Table 3 shows a cssreducnf ve , class reductions for various automobiles employing different hydrocarbon control systems. Exhaust ialdehyde analyses adapted from National Academy of Sciences, Also includes acetone of unknown proportion. Table 2. Oxygenates in exhaust from simple hydro- carbon fuels from Seizinger and Dimi- triades, Oxygenate Acetaldehyde. Automobile exhaust hydrocarbon and aldehyde emission patterns from Black, Total exhaust Percentage of total hydrocarbon, wt. Typical emission of several classes of compounds including aldehydes from sta- tionary combustion sources from Na- tional Academy of Sciences, Irradia- tion of a propylene-NO-N02 mixture in air.

Initial experimental conditions Aldehydes have been measured in various parts of the world, but the most extensive body of data exists for Los Angeles Stanford Research Institute. More recently, the California Air Resources Board measured formaldehyde at levels up to 0. Acetaldehyde exhibited an average concentration of.

The Air Resources Board found that aldehyde levels in the eastern part of the Los Angeles basin were significantly lower than those in downtown Los Angeles: specifically, formaldehyde was found to average less than 0. Figure 3 shows hourly concentrations measured at two locations in the Los Angeles area in by Scott Research Laboratories. Both locations show sharp decreases in afternoon levels of total aliphatic aldehydes. The advent of Fourier transform infrared spectroscopy FTIR has added a significant new dimension to the measurement of trace pollutants, including aldehydes, in ambient air.

The technique is specific and sensitive. Hanst and coworkers first applied the method in Pasadena in Some of the results from this study are shown in figure 4. This is partly due to the improved absorptivities used in the study. In the last few years continuous measurements for formaldehyde have been undertaken in certain areas of New Jersey Cleveland et al. This continuous monitoring showed a correlation with vehicle traffic and a seasonal variation with higher levels in summer than in winter. Peak formaldehyde concentrations were in the range of 14 to 20 ppb at four sites monitored.

For example, figure 5 shows formaldehyde levels reported for Hourly aldehyde concentrations at two Los Angeles sites, October 22, from Air Fig. Quality Criteria Document for Hydrocarbons, Newark as a function of the day of the week. In Japan, Katou, , observed high levels of the unsaturated aldehyde, acrolein. The average concentration measured was 7.

With the advent of FT-IR spectroscopy employed by several groups of workers e. Of necessity, the geographical area covered will be limited in the near future by the complexity and expense of the instrumentation. Kinetics and Mechanism This section is divided into two parts -- the first discusses the primary attack of radicals on aldehydes and the second part discusses the fate in the atmosphere of the radicals produced. The aldehydic hydrogen in aldehydes is relatively weak C-H bond strength is 86 kcal mol"1, Trotman- Dickinson and Kerr, Consequently, this hydrogen atom will be susceptible to attack by radical species present under atmospheric conditions.

Of these OH is likely to be by far the most dominant. The two former studies were carried out at low pressure using a discharge flow-mass spectrometer technique for the generation of reactants and analysis of products respectively. In their latest study, Niki et al. In this way, rates of reaction of aldehydes relative to ethylene were determined. These values were placed on an absolute basis using the appropriate rate constant for the OH reaction with C2Hi, at atmospheric pressure Niki et al.

These latter workers used a flash photolysis- resonance fluorescence technqiue and carried out the first study of aldehydes over a range of temperature K. Arrhenius activation energies for the two aldehydes studied are small with acetaldehyde exhibiting a negative value. Table 5 allows a comparison among the results obtained by the various workers employing three different techniques.

The agreement between the earlier work of Niki and coworkers and the most recent study of Niki et al. Attention here will be focused on ground state atomic oxygen, 03P since this more abundant than 0 1D in the lower troposphere. No results are shown from purely high temperature studies such as shock tubes.

Inference for Stochastic Chemical Kinetics Using Moment Equations and System Size Expansion

The most extensive data are those obtained by Singleton et al. These workers used a phase shift technique and covered a temperature range of K. They re- ported that at the high end of their temperature range, abstraction of the alkyl group H atoms be- came significant for the higher molecular weight aldehydes. However, under atmospheric conditions, abstraction of the aldehydric H atom is likely to dominate. The room temperature rate constants in table 6 show that there is generally good agreement among the different workers for acetaldehyde but significant differences for propionaldehyde and butyraldehyde.

The technique used by Singleton et al. Table 5. Rate constant data and Arrhenius parameters for the reaction of OH radicals with aldehydes. Arrhenius parameters and rate constants for the reaction of oxygen atoms 03P with aldehydes. The rate constant appears to be about one third that of acetaldehyde which reflects the weaker aldehydric H bond in acetaldehyde. Thus this route will be unimportant for the atmospheric chemistry of aldehydes.

Cadle et al. The values shown in table 6 are in reasonable agreement with those of Gaffney et al. Product analysis was by gas chromatography. H02 Radicals. This rate would be about an order of magnitude smaller in the unpolluted troposphere. Although measurements of the rate constant for H02 reacting with HCHO would be desirable from a scientific viewpoint, unless current measurements at higher temperatures are grossly in error, it does not appear from an atmospheric modeling view- point that this reaction plays a significant role in the chemistry of the polluted troposphere.

Alkoxy Radicals. NO 3 Radicals. The results from studying the reaction of mixtures of N and CH3CHO were interpreted using numerical integration for the participating reactions.

Introduction

HSOi, Radicals. However, this reaction should be studied experimentally since thermochemistry is not always a reliable guide for estimating kinetic data. Atmospheric Reactions of Radicals Produced from Attack of Radicals on Aldehydes We have seen above that radicals of the form RCO are produced from the reaction of atoms and free radicals with aldehydes. In this section, the subsequent fate of these radicals under atmo- spheric conditions will be discussed.

Differentia- tion is made between the acyl radicals and their aromatic equivalents since there is evidence Niki et al. The simplest acyl radical is formyl produced from formaldehyde. Three reaction paths are possible for its reaction under atmospheric conditions. However, results from recent studies by Osif and Heicklen and Niki et al. Both of these studies used the Cl atom sensitized decomposition of formaldehyde in the presence of From the small yield of HCOOH and the observations of peroxynitric acid H02N02, they concluded that reaction 10 was unimportant in their system.

Cain: Stochastic Simulations for Chemical Kinetics

Horowitz et al. These workers photolyzed mixtures of HCHO at A at low pressures in the presence of 02 and added C02, and measured the quantum yields of formation of H2, CO and C02 and the loss of A lower limit for kio of 1. This would reduce the importance of Reaction 10 in the system used by Niki et al. Contrary to Osid and Heicklen and Niki et al. Clearly, further work is needed to clarify this discrepancy. Table 7. Rate constant, k8 Pressure cm3molec"1s-1 x Torr Reference 5. Washida et al. This value is essentially independent of pressure see table 7 and is in excellent agreement with that of Washida et al.

The kinetics and mechanism of PAN formation and thermal decomposition have been discussed recently Pate et al. Niki et al. However, they note that smog chamber studies show that benzaldehyde is far less reactive than the aliphatic aldehydes in producing ozone Dimitriades, They suggest that radicals formed by H abstraction from benzaldehyde are efficient NO scavengers. Support for these suggestions would significantly aid the understand- ing and computer modeling of aromatic hydrocarbon photooxidation. Photochemistry of Aldehydes The photodissociation of aldehydes is an import- ant radical generation mechanism in the formation of photochemical air pollution Leighton, ; Altshuller and Bufalini, , ; Pitts, ; Calvert et al.

Absorption spectra of formaldehyde 1 , acetaldehyde 2 , and propionaldehyde 3 from Calvert and Pitts, Figure 6 shows the absorption spectra for some common aldehydes which illustrates that they will absorb well beyond A. The rate constant for a particular primary process is an important quantity in assessing the importance of the process in the atmosphere.

It is given by k, 0,h - 1 J X,0,h. Considerable attention has been given to formaldehyde photolysis in recent years, partly because of its importance in photochemical air pollution. There appears to be general agreement that: a. The results of these studies have been superceded by results from more recent and definitive studies and can be discounted Horowitz and Calvert, ; Lewis and Lee, Consequently, only the recent studies post will be discussed here and emphasis will be on the radical production route, reaction This is illustrated in figure 7.

This figure is similar to that given by Horowitz and Calvert b but it has the additional results of Moortgat et al. Lewis et al. H atoms were produced in reaction It is this value which has been redetermined by Lewis and Lee and increased to 0. This change brings the earlier results of Lewis et al. Results of experiments at A with added oxygen lead the authors to conclude that little if any dissociation of HCHO into radicals occurred at A and longer wave- lengths, in contrast to earlier results. Marling photolyzed 4 Torr of HDCO using either a high pressure mercury arc coupled with a monochromater of monochromatic laser jrradiation in the wavelength range to A.

Marling found that radical production reached 55 percent at wavelengths less than a A, but no measurements of the absolute decomposition yield were reported. They find that, within the uncertainties introduced by isotopic differences in the HCHO and HDCO molecules, Marling's reduced results agree reasonably well with their own see fig. Different results were obtained in experiments with low NO and high NO. In view of the results of other recent studies, the suggested reworking of Clark's results appears valid. Moortgat et al. They employed two types of experiment.

Consequently, results from the latter should be directly applicable to modeling the lower troposphere. A Xenon arc mono- chromater was used to isolate the desired wave- lengths. Examination of figure 7 shows that results of recent studies show a consistent trend although there remains some scatter in the results of different studies. However, it is possible to draw a line or band which incorporates most of the results within their experimental error. Data of Marling and Clark plotted using interpretation of Horowitz and Calvert.

Photolysis of Acetaldehyde. Acetaldehyde is commonly used as a surrogate for aldehydes of higher molecular weight than formaldehyde. Its absorption spectrum is shown"in figure 6, which is taken from Calvert and Pitts As with formaldehyde, major uncertainty is concentrated in the quantum yields of the primary processes Calvert and Pitts, Parmenter and Noyes carried out emission studies and Archer et al.

These studies have been summarized by Weaver et al. In a comprehensive study, these latter workers obtained results which are consistent with the previous studies. The products formed at a function of pressure and added 02 were measured over the pressure range 20 to Torr. Weaver et al. Quantum yields of the primary processes in acetaldehyde photo-oxidation as a function of excitation wavelength Weaver et al. Rate constant data for the reaction of OH radicals with other oxygen-containing organics.

UV absorption spectra for acetone 1 , diethyl ketone 2 , MEK 3 , and methyl n-butyl ketone 4 from Calvert and Pitts, However, major uncertainty remains concerning the photolysis of ketones under ambient conditions including the quantun efficiency of radical production in the presence of If one compared the realtive rates of aldehydes and ketone photolysis under simulated atmospheric conditions given by Carter et al. Importance of Aldehydes and Other Oxygenates in Modeling Atmospheric Chemistry The smog chamber studies carried out under simulated atmospheric conditions have been mentioned earlier and adequately demonstrate the importance of aldehydes and other oxygenates in promoting photochemical smog formation.

Computer modeling studies have further emphasized the importance of these compounds. Computer results have been shown to be significantly impacted by uncertainties in initial concentrations and photo- chemical parameters such as quantum yields Niki et al. Thus Dodge and Hecht state that aldehyde photolysis is among the most critical reactions for quantitative photochemical smog modeling when one combines the sensitivity with the uncertainty in the rates and mechanism of the reactions.

The situation is more complex for modeling atmospheric conditions. Uncertainties in the photochemistry and ambient concentrations are compounded by ill-defined emission rates for aldehydes and other oxygenates. These combined uncertainties can have a significant impact upon model calculations. The photodissociation rates used in the model are those given by Peterson It is evident from the results shown in figure 9, that the two parameters which we varied in this calculation can have a significant impact on the results. Of course, each parameter must be can'ed separately to isolate individual effects.

Effect of aldehyde photolysis rates and initial concentrations on a trajectory model run for November 5, in the Los Angeles basin. Summary The above discussions have covered the sources, ambient concentrations, radical reactions and photochemistry of aldehydes, and to a lesser extent, ketones, alcohols, ethers and esters. In general, the photolysis and reactions with the OH radical appear to be the major sinks for oxygenat- ed hydrocarbons in the lower atmosphere.

The state of knowledge of OH radical reactions with these oxygenates is currently adequate for modeling purposes given the uncertainties in other areas. Consequently, a major thrust towards further refinement in these rate constants specifically for tropospheric modeling purposes has little merit and attention should be focused on other areas. These areas may be summarized under the general categories below. Ambient Concentrations. Much greater emphasis should be placed upon obtaining concentration-time profiles for aldehydes and ketones in the atmosphere as a function of location, season and time of day.

The increase in chemical sophistica- tion of atmospheric models, which have been validated using well characterized smog chamber data, places increased demands on the quality and extent of ambient air quality measurements. This is typified in the case of aldehydes and ketones for which chemical reactions may exist in the computer model mechanism, but for which no ambient air quality data are available either for initial conditions or to test the predicted concentration- time behavior of these pollutants.

A greater knowledge of aldehyde and other oxygenate emissions is also needed to form the basis of a good emission inventory for modeling purposes. It is realized, of course, that the above requirements have not been met in many locations for the common hydrocarbon classes of alkenes, alkenes and arenes and in some cases, not even for non-methane hydrocarbons.

The photodissociation of aldehydes and ketones appears to be the major depletion mechanism for these compounds in the lower atmosphere based on the calculations present- ed earlier. Although there has been significant advances in our knowledge of formaldehyde quantum yields, the ambient photolysis rates of other aldehydes, and particularly ketones, are poorly known. Consequently, further studies of the photolysis of aldehydes and ketones as a function of pressure up to atmospheric, and in the presence of 02 should be carried out.

Kinetics and Mechanism. As indicated above, the general status of knowledge for the most important free radical, OH, is satisfactory for modeling purposes. The mechanism of reaction should receive further attention in the areas of: HCO oxidation under ambient conditions; OH and H02 addition to formaldehyde as suggested by Horowitz, Su and Calvert ; the oxidation of aromatic aldehydes under ambient conditions, and the photooxidation of ketones and other oxygenates under ambient conditions.

Finally, the kinetics of the possible HSOi, radical reactions with aldehydes and ketones should be studied to test the suggestion of Benson Acknowledgments The trajectory modeling work is being performed under Coordinating Research Council funding. Calvert of the Ohio State University and Dr. Moortgat of the Max-Planck Institute, Mainz, for receipt of results prior to publication; and the Chemical Kinetics Data Center of the Nation- al Bureau of Standards for providing a bibliography on aldehyde photooxidation.

References Akimoto, H. Altshuller, A. Photo- chem. P j Air Poll. Control Assoc. Q, Archer, A. Roy Soc. Baldwin, A. Baldwin, R. I 68, Barker, J. Kinetics 9. Benson, S. Black, F.

Reaction Kinetics in MATLAB

Brunelle, M. Bufalini, J. Tuesday, ed. Cadle, R. Calvert, J. Campbell, I. I 71, Carter, W. Kinetics, Clark, J. Thesis, University of California, Berkeley Cleveland, W. Environment Cohen, I. Environ- ment 1 , ; Environmental Science and Technology 1, Cox, R. I 72, Cox, R. Cvetanovic, R. DeGraff, B. Demerjian, K.

Dimitriades, B. Air Pollu- tion Control Assoc. Elliot, M. Air Pollution Control Assoc. Faubel , C. Fracchio, M. Gaffney, J. Gordon, S. Gorin, E. Graedel, T. Grob, K. Haagen-Smith, A. Hanst, P. Hecht, T. Hendry, D. Herron, J. Data 2, Horowitz, A. Hunziker, H. Jaffe, S. Johnston, H. Katou, T. Katz, M. Press, New York, Kelly, N. Klein, R. Laity, J. Lande, S. Leighton, P. Levy, H.

Levy, A. Lewis, R. C,, J. Linell, R. Lloyd, A. Kinetics 6, MacCagken, M. II, October Mack, G. I 69, 1 I 69, T Mack, G. Physics Martinez, J. PW prepared for Coordinating Research Council McQuigg, R. Moortgat, G. Physics Lett. Morris, E. Zi, Niki, H. Combustion p. Institute Chem, in press Oberdorfer, P. Osif, T.

Photochem 4, Overend, R. Parmenter, C. Pate, C. T, Atkinson, R. Health, Environ. Peterson, J. Pitts, J. Air Poll. Purcell, T. Radford, H. Renzetti, N. Robinson, E. Auto Exhaust. Schuck, E. Scott, W. Progress Report.

Modeling of Chemical Reactions (Comprehensive Chemical Kinetics) Modeling of Chemical Reactions (Comprehensive Chemical Kinetics)
Modeling of Chemical Reactions (Comprehensive Chemical Kinetics) Modeling of Chemical Reactions (Comprehensive Chemical Kinetics)
Modeling of Chemical Reactions (Comprehensive Chemical Kinetics) Modeling of Chemical Reactions (Comprehensive Chemical Kinetics)
Modeling of Chemical Reactions (Comprehensive Chemical Kinetics) Modeling of Chemical Reactions (Comprehensive Chemical Kinetics)
Modeling of Chemical Reactions (Comprehensive Chemical Kinetics) Modeling of Chemical Reactions (Comprehensive Chemical Kinetics)

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