Environ. Sci. Technol. 1983, 17, 308-311
NOTES Ozone-Forming Potential of Light Saturated Hydrocarbons Joseph J. Bufallnl* and Marcla C. Dodge US. Environmental Protection Agency, Environmental Sciences Research Laboratory, Research Triangle Park, North Carolina 2771 1
Computer simulations were conducted in order to establish the ozone-forming potential of saturated hydrocarbons such as n-butane when in the presence of more reactive hydrocarbons such as propene. One modeling exercise conducted in this study showed that the ozoneforming potential of n-butane relative to propene is variable and depends on the hydrocarbon-to-NO, ratio. At a low ratio of 4.3/1, the ozone-formingpotential of n-butane relative to propene was only 0.14. At an intermediate ratio of 13.9/1, n-butane was 0.29 times as reactive as propene in its ability to generate ozone. At a hydrocarbon-to-NO, ratio of 43/1, the reactivity of n-butane relative to propene increased to 0.34. In another modeling exercise, the reactivity of propane relative to trans-2butene was assessed. It was found that the relative reactivity of these two organics is highly variable and depends on the hydrocarbon-to-NO, ratio. Introduction It is well-known that the phootooxidation of hydrocarbons in the presence of oxides of nitrogen leads to the formation of ozone. Equally well-known is the fact that different hydrocarbon classes have different ozone-forming potential. It is generally agreed that olefins produce ozone very quickly, aromatics slightly less quickly, and paraffins very slowly. These differences in reactivity led to the introduction of “Rule 66” in Los Angeles County as a method for controlling ozone formation through hydrocarbon substitution. The rationale was to substitute a less reactive hydrocarbon (or other organic compound) for a more reactive one and thus decrease ozone production. It has been demonstrated in a number of studies that the light saturated hydrocarbons (LSHCs) can be reactive when exposed to lengthy irradiation or when irradiated at very high HC/NO, ratios (1-4). This finding has important implications when ozone formation downwind from a major city is considered. Urban HC/NO, ratios tend to be fairly low. On the basis of an analysis of air quality data, Trijonis (5) estimated NMHC/NO, ratios for Los Angeles that ranged from 5/1 to 15/1. Rural ratios tend to be higher. In one study conducted in suburban and rural Ohio (6),NMHC/NO, ratios that ranged from 13/1 to 53/1 were reported. Under rural conditions, therefore, when an air mass is exposed to multiday irradiations at high HC/NO, ratios, significant levels of O3may be generated. In a recent article, Singh et al. (7) reported that the LSHCs could produce from 25% to 125% as much oxidant as the alkenes. These estimates were based upon mechanistic, experimental, and computer simulation approaches. The rather high contribution of the LSHCs to the formation of oxidant was attributed to their large 308
Environ. Sci. Technoi., Vol. 17, No. 5, 1983
atmospheric abundance since, on a weight basis, the alkanes are as much as 5.5 times more abundant than the alkenes. The authors further stated that, in rural areas, the alkanes are expected to be even more important to oxidant production because their lifetimes are much greater than the lifetimes of the alkenes. In their modeling study, Singh et al. (7) used a detailed chemical kinetics mechanism to estimate the effect of variations in initial propene and n-butane concentrations on ozone formation. Using a NO, concentration of 0.24 ppm, simulations were run in which either n-butane or propene was reduced from its base concentration of 0.80 and 0.24 ppmC, respectively. These concentrations were chosen because the resulting HC-to-NO, ratio of 4.3/1 was thought to be typical of early morning conditions in Los Angeles. When the authors varied the initial hydrocarbon concentration in their simulations, they found that the same percent reduction in either n-butane or propene led to similar reductions in the ozone dosage. Although we do not dispute these findings, we do believe that Singh et al. (7) ignored one very vital aspect of the reactivity of LSHCs. The HC/NO, ratio at which the LSCHs are reacting must be considered. The reactivity of paraffins has been reported to be quite large at high HC/NO, ratios (1-4). Their importance to ozone/oxidant production, therefore, may be even greater than reported by Singh et al. (7). In the work presented here, we repeated the modeling study of Singh et al. (7) at the HC/NO, ratio of 4.3/1 and then extended the modeling exercise to HC/NO, ratios of 13.9/1 and 43/1. We also used modeling techniques to investigate the effect of the HC/NO, ratio on the reactivity of trans-2-butene relative to propane. Modeling Exercise The chemical model used in this study was an updated version of the 1977 mechanism used in the Singh et al. study. Our mechanism incorporated recent data, obtained between 1977 and 1981, on rate constants and reaction products for a number of key reactions. These new data were obtained from the recent review of Atkinson and Lloyd (8). The initial conditions employed in our study were similar to those of Singh et al. (7). Butane was taken to be 0.80 ppmC, propene 0.24 ppmC, and CO 2.0 ppm. The concentrations of NO, used were 0.24,0.075, and 0.024 ppm with an NO/N02 ratio of 2.0. These three NO, concentrations resulted in HC/NO, ratios of 4.3/1,13.9/1, and 43/1, respectively. Also included in the calculations were the Singh et al. initial concentrations of 0.01 ppm formaldehyde, 0.01 ppm acetaldehyde, 0.01 ppm H20z,0.01 ppm peroxyacetyl nitrate (PAN), 0.01 ppm nitrous acid, 0.005 ppm 03, and 20000 ppm H20(56% relative humidity min-’ was assumed. at 298 K). A dilution rate of 1.2 X
Not subject to U S . Copyright. Published 1983 by the American Chemical Society
-
LIGHT INTENSITY
-----SO%
,.----__
I
10.24 ppmC PROPENEt0.80 ppmC *BUTANE1
----BASE
................-SOYo BUTANE
0.16
-
I
BOTH
1 -
1 -
(il
OS00
i
I
I
I
I
lo
0800
1000
1200
1400
le00
1800
If 0.02
I
----BASE
.
'
.--.--.-5O% ---..-5O%
TIME IPSTI. hr
Figure 1. Modeling results showing the variations In ozone levels and light intensity at a HC/NO, ratio of 4.311.
0600
OS00
1000
1200
.
-
50% BUTANE
PROPENE BOTH
1400
1600
1800
TIME IPSTI, hr
Figure 3. Modeling results showing the variations in ozone levels at a HCINO, ratio of 4311.
TIME (PSTI. hr
Figure 2. Modeling results showing the varlations in ozone levels at a HCINO, ratio of 13.911.
This dilution is equivalent to doubling the mixing height over a 10-h period. Ultraviolet light intensity corresponding to Los Angeles summer solstice was also used in the calculations.
Results and Discussion The simulated O3 profiles obtained when propene and n-butane were reduced individually or together are shown
in Figures 1-3. The data in the first figure were obtained at the 4.311 HC-to-NO, ratio. Those in the second figure were obtained at the 13.9 ratio while the third figure shows the 4311 ratio. At the low HC/NO, ratio of 4.311, ozone does not maximize until about 1600 PST, when the solar radiation is rapidly decreasing. The maximum ozone produced at the 4.311 ratio, therefore, is controlled by the amount of light energy available. If the system were irradiated under constant light intensity for a longer period of time, more ozone would be produced. At the 13.911 hydrocarbon-to-NO, ratio, shown in Figure 2, the maximum ozone for all scenarios is observed between 1200 and 1500. The 50% reduction in both butane and propene scenario may be light limited since the light intensity at 1500 is decreasing rapidly. At the 4311 hydrocarbon-to-NO, ratio, as shown in Figure 3, the ozone maximum for all cases appears at about 1 ~ 1 1 0 PST, 0 long before the maximum (noonday)light intensity is reached. At this ratio, the maximum ozone is not light limited. The various hydrocarbon control scenarios depicted in Figures 1-3 show vast differences at the three HC/NO, ratios. In Figure 1, a 50% reduction in n-butane produced a 23% reduction in maximum ozone. The same reduction in n-butane at the 13.9/1 ratio produced an 8.2% decrease in ozone while a 5.3% decrease was observed at the 4311 ratio. A 50% reduction in propene produced a 52% reduction in ozone at the 4.311 ratio but only a 14.1% decrease at the 13.911 ratio and even less (5%)at the 43/1 ratio. The results obtained when both n-butane and propene were reduced by 50% are even more striking. At the low ratio of 4.311, a 65% reduction is observed. At a 13.911 ratio, a 25% reduction is observed while at 4311 only a 11% reduction is observed. The reactivity of n-butane relative to propene can be compared by considering the change in peak O3 that occurred when the carbon content of the simulations was reduced. At the 4.311 ratio, a 50% reduction in n-butane lowered peak O3 by 0.029 ppm (from 0.127 to 0.098 ppm). Environ. Sci. Technol., Vol. 17, No. 5, 1983
309
Table I. Comparison of the Reactivity of trans-2-Butene Relative t o Propane mix/ NO, 2 3 3 4 4 5 5 5 10 10 20 40 60
mix 0.5 0.30 0.80 0.30
1.o 0.10 0.50 1.o 0.10 1.o 1.o 0.8 1.8
RtIRp
RtIRp
NO,
(90/10)
(75/25)
0.25 0.10 0.266 67 0.075 0.25 0.02 0.10 0.20 0.01 0.10 0.05 0.02 0.03
33.0 21.7 26.0 16.5 20.9 6.8 16.0 18.3 4.5 7.1 5.8 5.1 1.2
22.5 14.3 16.8 9.6 12.7 5.1 7.7 8.3 4.7 6.5 4.3 2.4 a
a The addition of trans-2-butene caused a decrease in 0, production,
This reduction in O3 was accomplished by reducing the butane concentration by 0.4 ppmC (from 0.8 to 0.4 ppmC). Thus A03 = -0.029 - 0.072 AC(n-butane) 0.4
where A 0 3 is the change in peak O3and AC(n-butane) is the reduction in n-butane that led to this decrease in ozone. Similarly, O3 obtained when propene was reduced from 0.24 to 0.12 ppmC was 0.061 ppm and A03
AC(propene)
= -o‘061 0.12
- 0.508.
The reactivity of n-butane relative to propene can be expressed as the ratio of these two numbers, 0.072/0.508, or 0.14. In a similar fashion, the reactivity of n-butane relative to propene at the 13.9/1 ratio is calculated to be 0.29. At the 4311 ratio, n-butane is 0.34 times as reactive as propene. Thus, if the LSHCs are 5.5 times more plentiful than the olefins, under rural conditions at high HC/NO, ratios, they could be about 200% more important than the “high reactivity” hydrocarbons. Another approach to test the reactivities of hydrocarbons a t different HC/NO, ratios was undertaken. In this approach, the reactivities of propane and trans-2butene were measured relative to an “urban mix” composed of n-butane and propene at 75/25 and 90/10 carbon ratios. In each of these simulations, either propane or trans-2-butene, equivalent to 10% of the carbon content of the “urban mix”, was added and then the amount of urban mix that had to be subtracted in order to restore O3to its original level was determined. The reactivity of trans-2-butene relative to propane, RJR,, was then defined as the ratio of the amount of mix that had to be subtracted in the presence of trans-2-butene in order to restore 0, to its former value to the amount of mix that had to be subtracted in the presence of propane. The results obtained for both the 90110 and the 75/25 n-butanelpropene mix are shown in Table I. The greatest differences between the reactivities of propane and trans-2-butene appear with the 90/10 composition and the low HC/NO, ratios. Note that at a HC/NO, ratio of 6011, in the 90/10 combination, the trans-2-butene was only 1.2 times more reactive than propane. Furthermore, in the 75/25 combination, the addition of trans-2-butene in fact caused a decrease in peak 0, at the HC/NO, ratio of 60/1. There is a tendency to classify all olefins as highly reactive hydrocarbons that produce copious quantities of 310
Environ. Sci. Technol., Vol. 17, No. 5, 1983
Table 11. Oxidant Dosage (ppm.min) at Various Levels of Initial NO, Concentrationa 1 ppm ‘/z P P ‘Ilo ~ ppm NO, NO, NO,
a
10 pprn C in atmospheric mix 10 ppm C in paraffin mix 5 ppm C in atmospheric mix 5 ppm C in paraffin mix From Kopczynski et al. (1 ).
138
138
58
2
19
24 82
92 54
0
5
66
ozone. As shown in Table I, however, the 75/25 n-butane/propene combination at a HC/NO, ratio of 60/1 produced less ozone when trans-2-butene was added to the mixture. The addition of olefin at this HC/NO, ratio destroyed ozone because of the rapid reaction between trans-2-butene and ozone. Also, more ozone could not be produced from the ozonolysis products of the trans-2butene/ozone reaction because there was insufficient NO, to regenerate ozone. Most of the NO, is consumed in the chain-terminating reactions involving the formation of nitric acid and peroxyacetyl nitrate (PAN). An experimental study qualitatively agreeing with the above modeling results was conducted by Arnts and Gay (9). In this study, the extremely reactive natural olefinic hydrocarbonsproduced less ozone than propene. This was observed at all HC/NO, ratios studied but was more pronounced at high HC/NO, ratios. The explanation offered by Arnta and Gay is that the natural hydrocarbons act as a sink for O3 because of their very rapid rate of reaction with ozone. The authors tested this hypothesis by modeling a propene/NO, system in which they arbitrarily increased the 03/propene rate constant by a factor of 10. This model showed little ozone production at high HC/NO, ratios. Arnts and Gay suggested that the terpenes may act more as a sink for O3than a source because of the low NO, concentrations that have been observed in rural areas (IO). It is surprising that Singh et al. (7) did not note any O3 dependency on NO,. As stated above, the HC/NO, ratio has a large effect upon the amount of O3produced. The LSHCs have increased reactivity at low NO,. The olefins, however, appear to produce high O3 levels at some intermediate HC/NO, ratios (-5/1 to 9/1). At low HC/NO, ratios, ozone formation is limited because of NO inhibitive effects. At high HC/NO, ratios, the excess olefins react with ozone. Oxidant dosage dependence on NO, for mixtures were studied by Kopczynski et al. ( I ) , and the data are reproduced in Table 11. These chamber data show that a reactive hydrocarbon mix (five paraffins, five aromatics and six olefins) produced the greatest O3dosage at the two low HC/NO, ratios investigated in this study. The paraffinic mix, however, produced more ozone at the high HC/NO, ratios than did the atmospheric mix. The resulta of Kopczynski et al. also show that the ozone dosage appears to be somewhat insensitive to HC composition. A 30% olefin/70% paraffin mix produced a 100 ppmmin dosage for a 300-min irradiation. A 70% olefin/30% paraffin mix produced a 120 ppmmin dosage. The data suggest that there is a leveling effect on O3dosage and the system becomes insensitive to further substitution of olefins for paraffins. Conclusions The results obtained in this study indicate that it is meaningless to suggest that the LSHCs are some fixed
Environ. Sci. Technol. 1983, 17, 311-312
quantitative fraction as reactive as the alkenes. As noted earlier, the HC/NO, ratio is very important. The HC/NO, ratio also increases as an air mass moves downwind to rural areas. Thus, the ozone-forming potential of LSHCs increases as the air mass moves downwind. In rural areas, highly reactive olefins such as terpenes emitted from vegetation may even act as sinks for ozone. In these situations, the alkanes would be infinitely more reactive than the alkenes. In a complex mixture, synergistic effects occur, and normally slow reacting hydrocarbons at moderate HC/NO, ratios become more reactive. Our experimental and modeling results indicate that all hydrocarbons of molecular weight greater than ethane will generate ozone at significant levels if irradiated at an appropriate HC/NO, ratio. The selective control of hydrocarbons above ethane, therefore, may have little beneficial effect on an urban area if the downwind environment as well as the urban area is considered. Registry No. 03, 10028-15-6;NO, 10102-43-9;trans-2-butene, 624-64-6; propane, 74-98-6; butane, 106-97-8;propene, 115-07-1.
Literature Cited (1) Kopczynski, S. K.; Kuntz, R. L.; Bufalini; J. J. Enuiron. Sci. Technol. 1975, 9, 648-653.
Heuss, J. M. Research Publication GMR-1802, General Motors Corp., Warren, MI, Feb 1975. Altshuller,A. P.; Kopczynski,S. L.; Wilson, D.; Lonneman, W.; Sutterfield, F. D. J . Air Pollut. Control Assoc. 1969, 19, 787-790. Altshuller,A. P.; Kopczysnki, S. L.; Wilson, D.; Lonneman, W.: Sutterfield, F. D. J . Air Pollut. Control Assoc. 1969, 19,’791-794. . Trijonis, J.; Hunsaker, D. EPA Report 600/3-78-019; U.S. EnvironmentalProtectionAgency: Research Triangle Park, NC, Feb 1978. Research Triangle Institute, EPA Report 450/3-75-036;U.S. EnvironmentalProtectionAgency: Research Triangle Park, NC. Mar 1975. SinghrH. B.; Martinez, J. R.; Hendry, D. G.; Jaffe, R. J.; Johnson, W. B. Environ. Sci. Technol. 1981,15,113-119. Atkinson, R.; Lloyd, A. C. J. Phys. Chem. Ref. Data, in press. Arnts, R. R.; Gay, B. W., Jr. EPA Report 600/3-79-081;U.S. EnvironmentalProtectionAgency: Research Triangle Park, NC, Sept 1979. Martinez, J. R.; Singh, H. B. SRI Project 6780-8,SRI International, Menlo Park, CA, 1979. Received for review February 5, 1982. Revised manuscript received December 22, 1982. Accepted January 25, 1983.
CORRESPONDENCE Comment on “Calculation of Evaporative Emissions from Multicomponent Liquid Spills” SIR: The recent paper by Drivas (1)’presented a theoretical approach for estimating the time-varying rates of evaporation of individual components of a multicomponent liquid mixture from a spill. Model estimates were compared with experimental data from other investigators. This paper would have benefited from a short discussion of the limitations of the model as a result of the simplifying assumptions used in its formulation. The model assumes that the liquid solutions are ideal, following Raoult’s law. This assumption is likely to be correct only when the mixture’s components are quite similar in molecular structure (2). For mixtures of common classes of organic solvents, the activity coefficient at infinite dilution, a measure of nonideality, usually ranges from just less than 1to somewhat greater than 10, with some values as high as 300000 (3). Bishop et al. ( 4 ) found errors in estimating the ratios or organic vapor concentrations from several binary mixtures by Raoult’s law ranging from 10% to greater than 1000%. They showed that much better estimates of vapor ratios could be obtained by use of a computer program that estimates activity coefficients based on the contributions of functional groups within the molecules of mixture components. Thus, while the model proposed by Drivas may produce reasonable agreement for oil spills, considerable care should be used in application to other mixtures. A less significant source of error is the determination of the diffusion coefficient of vapor in air for the calculation of the mass transfer coefficient. Only a limited number of diffusion coefficients are available from ex0013-936X/83/0917-0311$01.50/0
perimental observations. Theoretical estimation is quite arduous by hand, but these calculations can be computerized. The range of diffusion coefficients of common volatile substances is less than 1order of magnitude (5), so that anticipated errors would be considerably less than a factor of 10 for volatile materials. Of course, the potential error from an incorrect diffusion coefficient value is less than directly proportional to the error in the diffusion coefficient itself. Nevertheless, some improvement in model accuracy would be expected by use of separate diffusion coefficient values for each of the evaporating compounds. For materials of low volatility, estimation of pure compound vapor pressures for many classes of materials is still problematic, and large errors in both experimental and correlative estimates are common (6). The agreement between Drivas’ model predictions and long-term crude oil experiments shown in his Figure 2 is quite good, but one must suspect that this is partially the result of the assumption that 50% of the total oil weight was evaporable. No justification of this assumption was given. If the percent evaporable material was selected on the basis of the fit between theoretical and experimental points in Figure 2, much less significance should be attached to the agreement obtained.
Literature Cited (1) Drivas, P. J. Environ. Sei. Technol. 1982, 16, 726. (2) Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. “The Properties of Gases and Liquids”;McGraw-Hill: New York, 1977; p 296. (3) Walsham, J. G.; Edwards, G. D. J . Paint Technol. 1971, 43, 64. (4) Bishop, E. C.; Popendorf, W.; Hanson, D.; Prausnitz, J. Am. Ind. Hyg. Assoc. J . 1982, 43, 656. 0 1983 American Chemical Society Environ. Sci. Technol., Vol. 17,No. 5, 1983 311
