Ozone Formation Potential of Organic Compounds Joseph J. Bufalini', Theodore A. Walter, and Marijon M. Bufalini Gas Kinetics and Photochemistry Branch, Environmental Sciences Research Laboratory, Research Triangle Park, N.C. 2771 1
1 A reactivity scale for organic compounds based on ozone
production is developed. I t is based on the concept that ozone can be considered as the intermediate ( B )of two consecutive reactions, A B C. The organic compound, A, is assumed to react only with OH radicals that are present in the atmosphere. It is also assumed that the organic compound is completely oxidized to either COz and/or formic acid. A reactivity scale based on these premises heavily weighs the number of carbon and hydrogen atoms present in the compound. The scale, developed only on theoretical grounds, predicts that all hydrocarbons lead to ozone formation and that the larger molecules lead to larger quantities of ozone. The reactivity scale predicts that high concentrations of ozone would be expected in rural downwind areas away from high emissions sources. This is in agreement with recent findings of high ozone levels in rural areas.
--
proach is appropriate, i.e., all hydrocarbon emissions regardless of reactivity must be considered as candidates for control since they all lead to ozone formation. We have derived a reactivity scale for 0 3 formation based on theoretical considerations. The basis of this scale is the differences in rates between 03-forming reactions and 03degradation. No experimental values other than known rate constants are employed.
Assumptions The basic assumption of the method is that hydrocarbons are degraded principally by their reaction with OH radicals. In the presence of small quantities of oxides of nitrogen, ozone is ultimately formed. The extent to which 0 3 is produced will depend upon the rate of production of O3 as compared to the rate of destruction. Ozone can be considered as an intermediate, B, of two consecutive reactions ko
Organic matter including hydrocarbons is essential for the formation of photochemical smog. However, not all hydrocarbons manifest themselves equally in the smog symptoms such as eye irritation, plant damage, visibility reduction, and oxidant formation. The literature suggests that several of these symptoms of reactivity must be considered when planning control strategies (1-3). However, for the present, oxidant/ozone is the only photochemical product for which there is an Air Quality Standard. Achievement of the oxidant standard is based on control of organic emissions. The role of the oxidant precursors, Le., the hydrocarbons, has been extensively studied in many laboratories over the past several years. Various parameters such as types of hydrocarbons, HC/NO, ratios, light intensity, water vapor, and temperature have all been investigated for possible effects on oxidant formation (1).From these studies, definitions of reactive organics such as those given in Rule 66 ( 4 ) have been developed by the Los Angeles County. The Rule 66 approach is to limit the emissions of hydrocarbons that are relatively high in reactivity with the expectation that this will result in a decrease in the amount of oxidant produced in the atmosphere of Los Angeles County. Appendix B in the Federal Register (August 1971) tends to overlook the relative reactivities of different hydrocarbons and puts forth the concept that all but a few hydrocarbons should be controlled. This approach severely limits emissions of all but the most unreactive hydrocarbons. More recently, a linear summation model for the control of hydrocarbons has been proposed ( 5 ) .With this model, hydrocarbons are classified according to five reactivity classes. Any combination of these five classes could be emitted into the atmosphere as long as the linear summation does not exceed the reactivity of the class of least reactive hydrocarbons. Some recent findings in the Midwest by EPA (6), as well as recent investigations ( 7 , 8 )of high oxidant in rural areas, have led us to look into the formation of high oxidant concentrations from the so-called unreactive hydrocarbons. Smog chamber studies have shown that unreactive or moderately reactive hydrocarbons, i.e., the paraffins, can produce high ozone concentrations when irradiated over a prolonged period (9). This is the condition that a hydrocarbon is subjected to when irradiated for several days during transport. These findings led us to believe that a modified Appendix B ap908
Environmental Science & Technology
A+B+C
kb
where A is the hydrocarbon,md C is the degradation product of ozone, 0 2 . The rate constants k , and kb may be a composite of several rate constants. In the case of a fast reacting hydrocarbon, such as an internally double-bonded olefin, ozone will be observed because h, >> kb, Le., the rate(s) at which O3 is produced is much greater than the rate(s) a t which O3 reacts. In the case of a slow reacting compound such as methane, the reactions involving degradation are greater than those involving ozone formation ( h , < hb). As a consequence, little ozone is observed in the photooxidation of methane. The internal double-bonded olefins and methane are the two extremes of hydrocarbon reactivity. When a moderately reactive hydrocarbon is considered so that h, = kb, then some intermediate concentration of ozone can be expected. Construction of the reactivity scale is based upon the theoretical average O3 that one can expect to be produced during the time interval for 99% of the hydrocarbon to react. Synergistic effects are not relevant in this proposed reactivity scale. The reason for this is that only the maximum oxidant-forming potential of the hydrocarbon is considered. Synergistic effects would not significantly affect this potential. In the construction of this scale, the following premises are used: The degradation of the hydrocarbon under consideration is initiated by its reaction with OH. The hydrocarbon is eventually completely degraded to CO2 and/or formic acid. The oxidation of NO to NO2 by ROz, RC03, and HO2 radicals, which are formed in the hydrocarbon degradation process, always leads to ozone formation. The photochemical decomposition of NO2 will produce one molecule of 0 3 . The radicals R02, RC03, and HOs are never taken as reactants in a chain terminating reaction. The OH radical concentration is assumed to be constant, ppm (V/V) was used in the calculaand a value of 1.7 X tions. This is the approximate concentration given by a computer model of the atmospheric oxidation of 1ppm of CO in the presence of 50 ppb of NO and 10 ppb of NO2. The concentration of OH radicals is in the same general level as that calculated by others (10, 1 1 ) . The OH concentration is expected to be zero at nighttime hours. This was not considered in the model. A unimolecular decay constant for O3 was derived. This decay constant is a composite rate for all the decay paths for
O3 except for its reaction with NO, its reaction with olefins, and its deposition reactions. The reaction of O3 with NO is not a degradation path since NO2 is formed which will photodecompose and result in reforming the 03.The reaction of NO with O3 was also considered with concentration of NO kept a t a constant value of 5 ppb (V/V). This will be discussed later. The reaction of O3 with olefin was not considered because its contribution was minimal. It was found to be most important with a fast reacting olefin like trans- 2-butene. However, even in this case, its contribution was only 0.3% to the average ozone provided. The depletion of O3 by dry deposition was not considered since the problem is extremely complex. We do not know, for example, the atmospheric stability or the ground terrain in which the calculations should be riade. Also, even if the calculation for a specific diffusion rate were made, the rate would change during the course of the day as the temperature inversion lifts. Therefore, it was decided that this parameter, ground deposition, is important but cannot be included until better meteorological models become available. To obtain the composite decay constants, O3 degradation was simulated by a computer model. The model chosen was the reactions of 0.2 ppm 03,1.4 ppm CH4, 0.1 ppm CO, a t 50% relative humidity, and a 40' solar zenith angle. These concentrations of methane and CO were chosen since they are the geophysical concentration levels of these compounds. The relative humidity and solar zenith angle were arbitrarily chosen. The mechanism involved 31 reactions which include the photodissociation of O3 by sunlight and the subsequent reaction of O('D) with water to produce OH radicals. The OH radicals react with CHI and CO that are present. A plot of log ( 0 3 ) vs. time gave a straight line from which a unimolecular min-' was derived. This constant decay constant of 1.6 X is used in the calculations. As with the change in OH concentration, the decay rate for ozone is expected to be altered during the nighttime hours. This was not considered in the model. Both the OH concentration and the O3 decay rate could be made time dependent. However, this would add to the complexity of the calculations and would not significantly alter the conclusions. Existing reactivity scales are based on ozone formation data obtained from smog chamber irradiations. These data are usually obtained by irradiating for several hours a mixture of hydrocarbon, oxides of nitrogen, and water vapor. Generally, the irradiation period is for only 3-6 h, and the maximum O3 concentration produced is used to construct a reactivity scale. If the reactivity of the organic compound is low, little of the hydrocarbon reacts, and as a result little ozone is produced. In our proposed reactivity scale, the reactivity parameter is the average O3 produced during the time required for 99% of the hydrocarbon to react. For comparison purposes, the O3 concentration a t the peak O3 value, ( 0 3max, ) is calculated. In these calculations, the time required for 99% reaction of the hydrocarbon and the time to the ozone peak value refer to daylight time, time during which the light intensity corresponds to a solar zenith angle of 40'. The OH concentration can be expected to approach zero during the nighttime hours. Method of Calculation
Maximum Possible Ozone. The maximum possible number of O3 molecules that can be produced from one hydrocarbon molecule is calculated on the basis of the first two premises. A mechanism is written for the oxidation of the organic compound. The oxidation is initiated by reaction with OH, and the hydrocarbon is completely degraded to CO2 and/or formic acid. Only those reactions which are thermodynamically possible are considered in the degradation process. The maximum possible number of 0 3 molecules that can be produced from one hydrocarbon molecule can then be calculated from the number of times R03, Ron, and HOz
radicals react with NO to form a NO2 molecule. T o illustrate, using CHI as an example, CH4 + OH CH3
+0 2
--
+ NO CH30 + O2 HOz + NO
CH3
-
+
+ HzO
CH302
+ CH30 I HCHO + H 0 2 HO + NO2 I1 HCHO + OH HzO + HCO HCO + 0 2 HOz + CO I11 CO + OH HOz + CO2 IV
CH3O2
NO2
--*
-
+
-+
(1) (2)
(3)
(4) (5)
(6) (7) (8)
Reactions 3,5,7, and 8 can form a molecule of NOz, Le., from one molecule of CH4, four molecules of NO2 are formed, and hence four molecules of O3 will be formed. For each class of organic compounds, a formula can be derived for the maximum number of O3 molecules that can be formed as shown in Table I. Because the number of ozone molecules produced by OH abstraction and OH addition are not the same with respect to olefin and acetylene, only the abstraction is used in the remainder of the calculation, as a first approximation. Since a thorough product study has not been made for aromatic compounds, a mechanism could not be written for the aromatics without considerable speculation. We assumed that the number of 0 3 molecules produced from the aromatics is equal to those produced by olefins with OH abstractions. There is a question as to the validity of using the values from Table I for the aromatics. These hydrocarbons produce aerosols when photooxidized in the presence of oxides of nitrogen. The carbon balance in these systems is generally poor, and a large quantity of the hydrocarbon fragment is presumed to go to the walls of the reaction vessel (I2,13).Therefore, the values shown in Table I for the amount of ozone produced for a unit value of aromatic hydrocarbon are probably not valid since the ultimate decay product is not CO2 for these hydrocarbons in the atmosphere. A more reasonable fate for the bulk of the aromatics is probably aerosol formation and deposition. Time for 99% Reaction. To derive the time for 99% of the organic compound to react, a simple second order reaction rate is used:
--d(A) - k,(OH)(A) dt
(9)
4.6 k, (OH) By use of this expression and assuming that the (OH) concentration is 1.7 X ppm, the values in Table I1 were calculated. The rate constants, ha, used in the calculation are also given in Table 11. t99%
=
~
Table 1. Maximum Possible Ozone Produced from Various Classes of Organic Compounds Max poso ozonea
Compound
Monoaldehydes Alkanes Monoalkenes (OH abstraction) Monoalkenes (OH addition) Monoalkynes (OH abstraction) Monoalkynes (OH addition) Aromatics
nC-knH-1 nC-tnH-1 nC+nH nCSnH-1 nC+nH+l nC+nH nC+nH
a n C and n H are the number of carbon atoms and number of hydrogen atoms, respectively, in t h e specific compound.
Volume IO, Number 9, September 1976 909
Average 0 3 Produced During 99% Reaction. The rate of formation of ozone can be expressed as:
where n’is the maximum possible number of ozone molecules that can be produced from one molecule of hydrocarbon A (column 2 in Table I). The rate of decay of 0 3 is
(F) decay
= 1.6 X
(03)
(12)
Since the average value of any quantity is given by
,
d,,
then
If we substitute t for 99% reaction (Equation 10) and ignore e-ka(oH)tsince this term is small, we obtain
Equations 11 and 12 are combined to give the net rate of production of 03:
-d(03) - nka(A)(OH) - 1.6 X
x
(03)
dt
The first term of the above can be rewritten as The solution to Equation 13 is then
- e-l.6X10-3 t 1.6 x 10-3 - OH)
1 - e-7.36X10-3/ka(OH)
1.6 x 10-3
)
(17)
By assuming that (OH) = 1.7 X ppm and using the rate constants, ha, given in Table I1 for the HC OH reactions, Table I11 was constructed. The time for 99% reaction (column 2, Table 11)for the hydrocarbons is probably short for actual atmospheric conditions. The OH concentration is expected to be very low or zero during the nighttime hour.
+
n’k,(OH)e-ko(OH)t since A = e - k ( O H ) t e-ko(OH)t
O3 = n’h, (OH)
~-1 ( k a (OH)
(14)
Table II. Reactivity Parameters for Various Organic Compounds Compound
1,2,4-Trimethyl benzene m-Xylene Toluene Benzene trans-2-Butene Propene Ethene Acetylene n-Butane Ethane Methane Butyraldehyde Propionaldehyde Acetaldehyde Formaldehyde Carbon monoxide
Time for 9 9 % reaction, min
Tlme l o peak ozone value, min
61.6 87.4 492 529 25.5 128.8 602.6 1.22 104 768.2 6026 2.25 105 123 123 123 128.8 1.28 104
48.5 63.9 224.4 225.4 26.7 91.2 259.5 1178.9 303.4 886.2 2777.1 88.0 88.0 88.0 91.2 1209.7
x
x
x
(OH) rate constant, ppm-’ min-1
4.9 x 3.4 1.7 x 5.6 10.5 2.1 4.5 2.2 3.5 4.5 1.2 2.2 2.2 x 2.2 2.1 2.1
x
x x x x x x x x x x x x
104 104 104 103 104 104 103 102 103 102 10’ 104 104 104 104 102
Table 111. Ozone Produced by Various Organic Compoundsa Compound
1,2,4-Trimethyl benzene
mX yIe ne Toluene Benzene trans-2-Butene Propene Ethene Acetylene n-Butane Ethane Methane Butyraldehyde Propionaldehyde Acetaldehyde Formaldehyde Carbon monoxide a
Max poss ozone, ppm
Av ozone for 99% reactlon, ppm
Ozone peak value, ppm
21 18 15 12 12 9 6 5 13 7 4 11 8
16.06 13.25 8.60 6.72 9.24 6.54 3.30 0.27 6.46 0.74 0.01 7.95 5.78 3.61 1.44 0.05
19.42 16.25 11.04 8.35 11.49 7.77 3.97 0.75 8.02 1.70
5 2 1
Based upon 1 ppm compound.
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Environmental Science & Technology
0.05 9.56 6.97 4.34 1.73 0.14
Av 0 3 , NO = 5 ppb
3.26 1.94 0.28 0.21 3.57 0.66 0.093 0.004 0.18 0.91 1 0.0002 0.83 0.60 0.38 0.15 0.0007
Cpd left at 0 3 max, ppm
0.020 0.027 0.123 0.129 0.009 0.039 0.142 0.634 0.167 0.503 0.946 0.038 0.038 0.038 0.039 0.638
Time to O3 Peak Value and O3Peak Value. The time to the O3 peak value was derived by taking the derivative of Equation 14, setting the resultant expression equal to zero, and solving for t . The following expression was obtained and was used to calculate the time to O3peak value. These values are tabulated in Table 11. to3 peak =
- In 1.6 X OH) - 1.6 x 10-3
In k,(OH)
(18)
The O3 peak value was then calculated by substituting the time to O3 peak into Equation 14. These O3 peak values are tabulated in Table 111. These values are extremely high. Since no such O3 concentrations have ever been observed either in the atmosphere or in smog chambers, it was decided to note what effect a small but measurable quantity of NO would have on O3 production. With NO a t a constant value of 5 ppb, Equation 19 was derived. n’[k,(OWI2
‘ 0 3 )= 4.6[0.107 - &(OH)] 1 (kQ(OH)
x ~-
1 - e-0.49/ka(OH)
0.107
)
(19)
With this equation, column 5 of Table I11 was obtained.
Ozone Forming Potential The ozone concentration produced by the reaction of 1ppm of organic compound was significant in each case except for methane. In each case (except for methane), the air quality standard for ozone is exceeded. P u t on this basis, all hydrocarbons (except methane) or organic compounds need to be controlled. However, the degree of control is obviously not only dependent upon the parameters used to derive this ozone-forming potential but also on the meteorology that would affect the rate destruction of ozone and the diffusion of the HC. As discussed earlier, the dry deposition rate for ozone can be significant depending upon the particular diffusion rate that is considered for different atmospheric conditions. The 0 3 values given in columns 3 and 4 of Table 111 are upper limit values, Le., if no deposition is occurring and all of the ozone produced remains in a box type atmosphere. I n actuality, as shown in column 5 of Table 111,in the presence of NO, these O3 concentrations are never realized either in the atmosphere or in smog chambers. Ozone produced from the photolysis of NO2 is governed by the expression 0 3 = K(NO-J/NO. We chose to ignore the reaction of ozone with NO largely because the reactivity scale was derived to be independent of the hydrocarbon/NO, ratio. If a large quantity of NO, were employed, for example, then little 0 3 would be produced even though the organic compound would eventually (but more slowly) be consumed. However, if too little NO, were used, then the chain terminating steps involving NO, eventually would remove all the NO, from the system, and no further production of O3 or organic compound degradation would occur. Therefore, how does one establish a reactivity scale that is applicable in the real world where the HC/NO, ratio is vari-
able? Obviously, one can write a model for various organic compounds and a t various cpd/NO, ratios and arrive at a list of O3 concentrations that one can expect at different HC/NO, ratios for every compound. Such a table would probably be valuable, but again, since new inputs (emissions) are always being introduced, the basic question remains, which cpd/NO, ratio does one employ when the reactivity or O3 forming potential of a compound is desired? Also, the presence of aerosols and dry deposition in the atmosphere and wall reactions in smog chambers are expected to reduce the ozone levels. The importance of wall reactions in a chamber is more important than the photolysis of O3 (16). The O3 values given in Table I11 are not meant to show ozone levels that are to be expected by the photooxidation of a particular organic compound with NO,. Instead, the table shows the O3 formation potential of the compound. Exact experimental verification for the proposed reactivity scale is not possible since most chamber studies have been done with short irradiation times (3-6 h) and usually employ rather high NO, concentrations. Some recent studies were performed to qualitatively test the hypotheses that a saturated hydrocarbon (n-butane) could produce significant concentrations of ozone comparable to a moderately reactive olefin (ethylene). After 30 h of irradiations, HC/NO, ratios of 20 resulted in higher average ozone concentration for n-butane than for the ethylene. Table IV shows the ozone-forming potential of compounds when put into five arbitrary classes. This table shows some interesting points when compared with the Dimitriades reactivity scale (Table v). Class I has essentially the same group of unreactive compounds, methane, ethane, and acetylene in both scales. However, the Dimitriades scale also contains benzene, whereas in the proposed theoretical scale, this compound appears in Class IV. Again, it must be emphasized that the aromatics cannot be modeled properly since the degradation products are known. The most reactive hydrocarbons, the internally double-bonded olefin, and the trialkylbenzene are in the most reactive class in both scales. A reactivity scale could also be based on peak ozone concentration. Depending upon where the group divisions are made, there is not a significant change in the order to reactivities. The only compounds that change order are propene and n-butane. Of the 15 compounds investigated, the n-butane is ninth in reactivity, and propene is tenth when the 99% HC reaction is used. When the ozone peak is used, these two compounds shift positions. We believe that a reactivity scale based on 99% reaction of the organic compounds is a better indication for basing reactivities. In Table 111, for example, are given the concentrations of the various organic compounds at the point where the O3 maximum has been reached (column 6). Although the fast reacting organic compounds are essentially gone at to, max, some of the slower reacting ones are not. At the toumax, only approximately 84% of the n-butane has reacted. For ethane, only 50% has reacted. Carbon monoxide has reacted only 36%. Therefore, these organic compounds can still have considerable potential in producing ozone if they have not been dissipated. For this reason, a reactivity scale based on 99% compound reaction is more useful.
Table IV. Reactivity Classification Based on Ozone Formation Potential Class I, < 1 ppm
Class II, 1-4 ppm
Class 111. 4-6 ppm
Class IV. 6-8 ppm
Carbon monoxide Methane Acetylene Ethane
Formaldehyde Acetaldehyde Ethylene
Propionaldehyde
n-Butane Propene Butyraldehyde Benzenea
a
Class V 8-16 ppm
trans-2-Butene mXylenea 1,2,4-Trimethyl benzenea Toluene
The aromatics could not be modeled since the degradation products are unknown. They may not be in the particular classification shown for this reason,
Volume 10, Number 9, September 1976
911
Table V. Reactivity Classificationof Organics a Class I, nonreactive
C1-CB paraffins Acetylene Benzene Benzaldehyde
Class II, reactive
Mono- fed-alkyl benzenes Cyclic ketones fert-Alkyl acetates 2-Nitropropane
Acetone Methanol fed-Alkyl alcohols Phenyl acetate Methyl benzoate Ethyl amines Dimethyl formamide Perhalogenated hydrocarbons a
Class ill, reactlve
C4+-paraffins Cycloparaffins Styrene &Alkyl ketones Primary and secondary alkyl acetates Kmethyl pyrrolidone N,N-dimethyl acetamide
Class V, reactive
Primary and secondary alkyl benzenes Dialkyl benzenes Branched alkyl ketones Primary and secondary alkyl alcohols Cellosolve acetate
Aliphatic olefins
Partially halogenatedolefins
Ethers Ceilosoives
a-Methyl styrene Aliphatic aldehydes Unsaturated ketones Diacetone alcohol
Partially halogenated paraffins
Reactivity scale proposed by Dimitriades (4).
Although this reactivity scale is not perfect, we believe that it could be used to complement that proposed by the use of smog chamber data. The proposed scale is valid not only in the center city but also in rural areas. If control strategies are planned based on the reactivities similar to those given in Table 111,the goal to achieve significant reductions in oxidant levels both in center cities and rural areas will be met but may be unnecessary for center cities.
Literature Cited (1) Altshuller, A. P., Bufalini, J . J., Enuiron. Sci. Technol., 5, 39 (1971). (2) Heuss, J. M., Glasson, W. A., ibid., 2,1109 (1968). (3) .Altshuller, A. P., Cohen, I. R., Znt. J . Air Water Pollut., 7, 787 (1963). (4) Rules and Regulations, Air Pollution Control District, County of Los Angeles, Los Angeles, Calif., 1971. (5) Dimitriades, B., “Proceedings of the Solvent Reactivity Conference”, EPA-650/3-74-010, November 1974. (6)EPA Report No.: 450/3-74-034, “Investigation of Ozone and Ozone Precursor Concentrations at Nonurban Locations in the Eastern United States,” Research Triangle Park, N.C., May 1974.
912
Class IV, reactive
Environmental Science 8, Technology
(7) Stasiuk, W. N., Coffey, P. E., J . Air Pollut. Control Assoc., 24, 564 (1974). (8) Cleveland, W. S.,Kleiner, B., “The Transport of Photochemical Air Pollution from the Camden-Philadelphia Urban Area”, Bell Laboratories, Murray Hill, N.J. (9) Altshuller, A. P., Kopczynski, S. L., Wilson, D., Lonneman, W., Sutterfield, F. D., J . Air Pollut. Control Assoc., 19, 787 (1969). (10) Calvert, J. G., McQuigg, R. D., Znt. J. Chem. Kinetics, submitted for publication. (11) Doyle, G. J., Lloyd, A. C., Darnall, K. R., Winer, A . M . , Pitts, J. N.,Jr., Enuiron. Sci. Technol., 9, 237 (1975). (12) Gay, B. W.. Jr.. Bufalini, J. J.. ibid.. 5.422 (1971). (13) Kopczynski, S. L., Znt. J Air Water Pollut., 8,107 (1964). (14) Demerjian, K. L., Kerr, J. A,, Calvert, J. G., “The Mechanism of Photochemical Smog Formation”, in “Advances in Environmental Sciences and Technology”, Vol4, pp 1-262, J. N. Pitts, Jr., and R. L. Metcalf, Eds., Wiley-Interscience, New York, N.Y., 1974. (15) Garvin, D., Hampson, R. F., Eds., “Chemical Kinetics Data Survey. VII. Tables of Rate and Photochemical Data for Modeling the Stratosphere”, Report NBSIR 74-430, National Bureau of Standards, Washington, D.C., January 1974. (16) Dodge, M. C., Hecht, T. A., private communication, 1975. Receiued for reuieu July 16, 1975. Accepted March 29, 1976.
