Environ. Sci. Technol. 1904, 18, 142-148
Isoprene: A Photochemical Kinetic Mechanism J. P. Kilius and
G. 2. Whltten"
Systems Applications, Inc., San Rafael, California 94903
A computer-modeling study has produced a photochemical kinetic mechanism for the atmospheric chemistry of isoprene, a naturally occurring common constituent of the troposphere. The kinetic mechanism is ready for use in atmospheric models because the reactions described are shown to adequately reproduce the results of a series of outdoor smog chamber experiments which encompass a wide range of precursor conditions of isoprene and NO,. Isoprene is a very reactive molecule that can contribute as much as 50% of the overall reactivity of rural air even though isoprene might be only 6% of the ambient hydrocarbon level. The major intermediate products of the atmospheric oxidation of isoprene, methyl vinyl ketone, methacrolein, methylglyoxal, and formaldehyde are also highly reactive. Isoprene is one of the most reactive compounds yet to be studied in smog chamber experiments, with a rate of reaction with hydroxyl radicals that is roughly 3 times that of propene. Furthermore, the reaction products of isoprene are themselves highly reactive. These products include methyl vinyl ketone (MVK), methacrolein (MACR), methylglyoxal (MGLY), and formaldehyde. In addition to their rapid reaction with hydroxyl, all these product compounds apparently photolyze in natural sunlight to yield radical products and thus increase the reaction rate of the overall smog system. The high reactivity of isoprene relative to other compounds makes its effects on the photochemical process difficult to analyze. Arnts and Meeks (I) recently presented data indicating that the ambient concentrations of isoprene are low, even near areas of heavy vegetation that might be expected to emit significant quantities of biogenic hydrocarbons (including isoprene). Since the ambient isoprene in their data never exceeded 6 ppb of C and never constituted more than 6% ambient hydrocarbon, Arnts and Meeks concluded that isoprene was photochemically unimportant. However, if the fractional contributions of isoprene to total reactivity in the data of Arnts and Meeks are calculated, we find that isoprene can yield as much as 50% of the peroxy radicals formed in ambient rural air. Isoprene is nearly 50 times as reactive to OH as urban hydrocarbon blends; a concentration of 6 ppb of C may thus be equivalent in instantaneous OH reactivity to 0.3 ppm of C of urban hydrocarbon. Furthermore, the rapid loss of isoprene due to its reactivity implies a rather high emission rate; on the order of 0.01 0.1 g/(m2.day) is needed to maintain observed concentrations of isoprene within the forest canopy. This emission rate is consistent with the work of Zimmerman (2),who estimated biogenic isoprene emissions in the United States to be 1.5 X 1013 g/year, or over 50% of the annual emissions of hydrocarbons from anthropogenic sources. The development of photochemical kinetic mechanisms for isoprene and other biogenic hydrocarbons offers a way to resolve the question of biogenic impact on atmospheric photochemistry. By use of such a mechanism, hypothetical emissions of biogenics could be included in atmospheric modeling efforts, allowing the comparison of predicted and observed hydrocarbon concentrations. Studies of the sensitivity of urban and rural photochemistry to biogenics would then be straightforward.
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142 Environ. Sci. Technol., Voi. 18, No. 3, 1984
This paper describes a I;..otochemical inetic mechanism for isoprene that was developed and validated for smog chamber experiments under natural sunlight conditions at the University of North Carolina smog chamber facility. The results indicate that the mechanism describes the isoprene experiments closely enough to be used in atmospheric simulation studies. Primary Reactions of Isoprene: Addition of OH, 0, and NOs Hydroxyl radical addition to isoprene in the presence of NO, yields methyl vinyl ketone (MVK) and methacrolein (MACR), as indicated both by the UNC smog chamber experiments and by low-pressure flow tube data (3). Reaction product information implies a similarity to the reaction kinetics of simple olefins (see Figure 1). Addition reactions to olefinic bonds (for species OH, 0, and NO3)occur at rates that increase with greater alkyl substitution into the double-bonded carbon (e.g., trans2-butene > 1-olefins > ethylene). Rate constants for monosubstituted olefins are similar (propene l-butene l-pentene) (see ref 4 and 5). Addition reaction rates for OH, 0, and NO3 are similar to those for dialkyl-substituted olefins (cis-Bbutene, isobutene, and cis-2-pentene). Hydroxyl addition to dialkyl-substituted olefins occurs at a rate that is roughly twice that of l-olefins (6). We use nearly the OH rate constant for propene (7) (37 000 ppm-l min-l) as the rate of OH addition to the 3,4 double bond and twice that rate (similar to isobutene, the most comparable dialkyl-substituted olefin) for OH addition to the 1,2 double bond. The sum of these two addition reactions equals the reported OH rate constant for isoprene (8, 9). Product yields in the presence of nitric oxide (NO) for the OH isoprene reactions are 29% MACR, 58% MVK, and 13% nitrates. The nitrates come from the R02 and NO reaction by analogy with nitrate formation in alkanes (10). Nitrate formation in olefin systems by this pathway is still speculative; however, this reaction represents only a minor perturbation in the system at the 13% level. Oxygen atom (0)addition to the double bonds is also treated analogously to that of propene and isobutene (Figure 1). Radical production channels from these reactions are assumed to be 30%. Further reactions of the epoxide and aldehyde products are ignored; the simulations show a yield of these products of less than 10 ppb in all cases. Products at these concentration levels would have little effect on the experiments unless strongly involved in radical chemistry, e.g., through photolysis. Addition of NO,. to the 1,2 double bond (165 ppm-l min for isobutene) (5) represents a significant sink for NO, in the isoprene system, especially since the expected product is a dinitrate (11). Our simulations indicate that as much as 40% of total NO, may end up as the dinitrate. The detailed mechanism leading to dinitrate formation, starting with NO3addition to olefinic bonds, is currently uncertain (7), but the yield appears to be near unity, at least for propene (12). Figure 1and Table I show one of the two dinitrates that might form.
-
-
Ozone-Olefin Reactions Ozoneolefin reactions yield a complex series of radical
0013-936X/84/0918-0142$01.50/0
0 1984 Amerlcan Chemical Society
Table I. Isoprene Kinetic Mechanisma reaction
rate constantb 8.100E-03 ISOP t 0,= HCHO + MACR 73 3.600E-03 ISOP + 0, = HCHO t MVK 74 1.080E-03 75 ISOP t 0. = HCHO + ozonide t CO 5.2203-03 ISOP t 0 ; = HCHO t ozonide 76 9.600Et04 ISOP t OH = CH,=CHC(CH,)(O,*)CH,OH 77 4.800E t 04 78 ISOP t OH = CH,(OH)CH(O,)C(CH,)=CH, 1.116Et 04 ISOP t 0 = epoxide 79 1.080Et03 ISOP + 0 = CH,=C(CH,)CH,(O,~) t CO t HO, 80 6.750Et03 81 ISOP t 0 = CH,=CHCHICH,)O,* t CO t HO,* 5.400Et 02 82 ISOP + 0 = CHi=C(CH,)C(O)O;. t CH,O, 9.0203+03 83 ISOP t 0 = CH,=C(CH,)CH,CHO 2.100Et02 84 ISOP t NO, = CH,=CHC(CH,)(ONO,)CH,O,* 8.000E-04 85 MACR + 0, = MGLY + HCHO 8.000E-04 MACR t O ; = 86 3.490Et 04 MACR t OH = CH,=C(CH,)CO,. 87 1.160E+04 MACR t OH = CH,(OH)C(CH,)(O,*)CHO 88 1.000E-02 MACR t hu = HO, t CH,=C(CH,)O,. t CO 89 3.0 OOE- 03 MVK t 0, = MGLY t HCHO 90 3.000E-03 MVK + O ; = 91 8.800E t 03 MVK t OH = CH,(O,~)CH(OH)C(O)CH, 92 1.770Et 04 MVK t OH = CH,(OH)CH(O,~)C(O)CH, 93 6.000E-03 MVK t hu = CH,CO,* t CH,=CHO,* 94 9.000Et 03 MGLY t O H = CO t CH,CO; 95 2.5 00E-02 MGLY t hu = HO, t C O - t C-H,CO,. 96 1.000Et 04 97 CH,=CHC(CH,)(O,*)CH,OH t NO = NO, + HO, t MVK t HCHO 1.500Et 03 CH,=CHC(CH,)(O,~)CH,OH t NO = CH,=CHC(CH,)(ONO,)CH,OH 98 1.000Et04 CH,(OH)CH(O,~)C(CH,)=CH,t NO = NO, t HO, t MACR t HCHO 99 1.500EtO3 100 CH,( OH)CH( O,.)C( CH,)=CH, t NO = CH,( OH)CH( ONO,)C( CH,)=CH, 1.200Et 04 101 CIil,=C(CH,)CH,(O,~) t NO = NO, t HO, t MACR 1.200Et04 102 CH,=CHCH(CH,)O,* t NO = NO, t HO, t MVK 1.000E+ 04 103 CH,=CHC(CH,)(ONO,)CH,O,~ t NO = CH,=CHC(CH,)(ONO,)CH,(ONO,) 1.600Et04 104 CH,=C(CH,)CO,* t NO = CH,=C(CH,)O,. t NO, t CO, 1.000Et04 105 CH,=C(CH,)O,* t NO = CH,(O,.)C(O)CH, t NO, 3.000Et 02 106 CH,=C(CH,)O,. + NO = CH,=C(CH,)ONO, 1.000Et04 107 CH,(O,*)C(O)CH, t NO = HCHO + NO, t CH,CO,. 1.200Et04 108 CH,(OH)C(CH,)(O,*)CHO t NO = NO, t HCHO t MGLY t HO, 1.200E+ 04 109 CH,(O,~)CH(OH)C(O)CH, + NO = NO, t HCHO t MGLY t HO, 1.200Et03 110 CH,( O,.)CH( OH)C( O)CH, t NO = CH,( ONO,)CH( OH)C(O)CH, 1.000Et 04 111 CH,=CHO,. t NO = CH,(O,.)CHO t NO, 1.000Et 04 CH.(O:lCHO + N O = HCHO t NO. + HO, t CO 112 1.200Et 04 CH:{OH)CH(O,.)C(O)CH, t NO = go, t ~H,co,. t GLYA 113 1.000Et 04 CH,=CHC(CH,)(O,~)CH,OH t HO, = product 114 1.000Et 04 115 CH,(OH)CH(O,~)C(CH,)=CH, t HO, = product 1.000Et 04 116 CH,=C(CH,)C(O)O,. t HO, = product 1.000Et 04 117 CH,=C(CH,)O,. t HO, = product 1.000Et 04 CH,(O,.)C(O)CH, t HO, = product 118 1.000Et 04 119 CH,(OH)C(CH,)(O,~)CHO t HO, = product 1.000Et 04 CH,(O,~)CH(OH)C(O)CH, t HO, = product 120 121 CH,(OH)CH(O,~)C(O)CH, + HO, = product 1.000Et 04 122 7.000Et 03 CH,=C(CH,)C(O)O,. t NO, = CH,=C(CH,)C(O)O,NO, 123 2.000E-02 CH,=C(CH,)C(O)O,NO, = NO, + CH,=C(CH,)C(O)O,. CCl,=CHCl t O H = HO, 3.400Et 03 124 71 0, t wall= 2.200E-04 NO, t wall = 72 7.000E-05 a First 70 reactions are the standard mechanism for formaldehyde and inor anic chemistry as given in supplementary material (see paragraph a t end of paper regarding supplementary material). $Appropriate units to give each reaction a rate of ppm min-I. Activation energy of 1 4 000 K used for this reaction.
and nonradical products. Kamens et al. (13)have prepared a reaction scheme for isoprene and its olefinic products (MVK and MACR) based on the approach used by Dodge and Arnts (14)to model propene O3 systems. Kamens et al. have used their reaction scheme to simulate nighttime experiments of isoprene, MVK, and MACR plus ozone in the UNC smog chamber. The mechanism of Kamens et al. contains over 50 reaction steps for the reactions of Os with isoprene and subsequent products. Moreover, their mechanism is incomplete for daytime use, since reactions involving NO are neglected. For simplicity, therefore, we have retained the yields of stable products indicated in the mechanism of Kamens et al. (the stable product yields are empirically based). Net radical production from ozone-isoprene
chemistry was assumed to be low, which is consistent with mechanisms for propene and ethene that use Criegee intermediate chemistry based on the work of Dodge and Arnts (14). One interesting feature of ozone-isoprene chemistry is the fairly low rate of reaction that occurs. The measured rate constant for the overall Os-plus-isoprene reaction is approximately equal to that of propene (only half of what might be expected given the availability of two active olefinic bond sites). The anomalous rate for the reaction of O3with isoprene is probably related to the similar low rate seen for the reaction of O3 with isobutene. As noted previously, dialkylated olefinic bonds tend to have similar reaction rates for OH, 0, and NO3addition. Isobutene has rates for these Envlron. Scl. Technol., Vol. 18, No. 3, 1984
143
PdOTOLYSIS
FH3 ,1 CH2-C CO,
-
I
CHj
y 3
.3 Cri2-CHCH.
Flgure 1. Reactions of Isoprene.
+
+ CHO.
addition processes that are similar to those of cis- and trans-2-butene. However, the reaction rate for isobutene and O3 is similar to that of propene, which is a monoalkylated olefin. It is also worth noting that whereas MVK is the favored product for OH addition to isoprene, MACR is the major product of the ozone reaction (13). Apparently, some process deactivates the 1,2 double bond to O3 addition in isoprene, and this process is probably responsible for the low rate of O3 reaction for isobutene as well. Reaction Products: Methyl Vinyl Ketone and Methacrolein Methyl vinyl ketone and methacrolein are the major oxidation products of isoprene formed from reactions with hydroxyl radicals and ozone. Both products are highly reactive. In addition to reactions with ozone and OH, both products probably photolyze to radicals. Photolysis is indicated for both compounds to explain the decay rates seen in the UNC experiments. In our simulations, photolysis is comparable to other loss processes. Reactions with OH and O3 by themselves cannot be responsible for the rapid loss of MVK and MACR. The measured O3 concentration is too low to produce such a high loss rate, and the decay rates of other compounds that react with OH(e.g., formaldehyde and C2HC13)indicate that the OH concentration is both consistent with our simulations and too low to explain the loss of MVK andd MACR. Although little informationhas been published regarding MVK and MACR per se, the photolytic behavior of related compounds makes a photolysis mechanism plausible. Methyl ethyl ketone photolyzes to form CzH6- CH3C0.,
+
144
Envlron. Scl. Technol., Voi. 18, No. 3, 1984
with an absorption peak at 280 nm and a cutoff at -320 nm (15); the olefinic bond would be expected to red shift this absorption band. Both acrolein and crotonaldehyde have a high absorption at natural sunlight wavelengths (acrolein, A- 333 nm, E = 18; crotonaldehyde, A ,, 328 nm, t = 16). Both compounds follow an excited molecule mechanism leading to polymer formation or deactivation in the absence of oxygen. In the presence of oxygen and atmospheric pressures, radical formation for methacrolein is plausible, as is the case for biacetyl and methylglyoxal(15,16). Also, the smog chamber data suggest a rapid buildup of radicals in the isoprene system-another indication that the primary products, MVK and MACR, photolyze. Other secondary photolytic products (such as methylglyoxal) take longer to form and would not be as efficient for generating radicals early in the experiment. We have examined the alternate hypothesis, in which methylglyoxal is generated at high yield from MVK and MACR oxidation. The results are as expected: an underestimation of initial reactivity, followed by an overestimation of reactivity later in the simulation (with a rapid decay of ozone as a consequence of radical/03 reactions). Our mechanism still slightly underpredicts the decay of MVK and MACR, indicating the possibility of additional photolytic channels to nonradical products. Further experiments with these compounds should yield useful information concerning these processes. Preliminary experiments with MVK/NO, and MACR/NO, systems in the UNC chamber (17)indicate a very high reactivity for both compounds that is consistent with our radical photolysis estimates. Their decay rates are also consistent with the photolytic hypothesis. As in the case of isoprene, we used the ozone-olefin product yield of Kamens et al. (13) for MVK and MACR. We ignored possible radical products from these ozone-olefin reactions, since the generation of radicals from this source would be very small compared to the photolysis reactions of MVK and MACR. Reaction pathways for photolysis, OH addition, and OH abstraction for MVK and MACR are given in Figures 2 and 3. Rate constants for OH reactions with MVK, MACR, and MGLY are taken from Kleindienst et al. (9).
Table 11. Experimental Conditions and Maximum Ozone Predictions for Isoprene Smog Chamber Runs exptl predicted 0, max- 0, maxpreinitial HCINO,, isoprene, imum, dicted/ imum, HONO, (ppmC/ temp range, sky NO,, PPm PPm) K conditions ppm observed date chamber of C PPm PPb PPm 0.17 1.89 4 1.05 287-305 cloudy 0.09 0.50 0.477 6/20/80 red 0.63 0.78 clear 2.13 294-311 0.81 10 0.466 red 0.98 7/17/80 0.43 1.23 4 2.60 291-303 cloudy 0.35 0.50 0.192 6/17/80 red 1.04 0.81 clear 5.54 294-311 1.28 10 2.58 0.460 7/17/80 blue 0.66 1.16 cloudy 0.57 4 6.02 286-306 1.27 6/23/80 blue 0.211 0.57Q 1.14 0.5a clear 7.64 284-306 2 0.205 6/22/80 blue 1.56 0.67b 1.22 0.55b 0.58a 1.16 292-307 clear 0. 5a 6 8.93 0.243 2.17 7/14/80 blue 0.6gb 1.10 0.63b 0.49' 0.89 0.55' 293-310 clear 6 10.48 0.188 1.97 7/16/80 blue 0.63b 0.76 0.83 0.33' 1.14 0.39' 291-303 cloudy 0.098 2 14.29 1.40 6/17/80 blue 0.3gb 0.95 0.41b 0.46' 1.35 0.34' 284-306 clear 0.203 2 18.89 3.83 6/22/80 red 0.46b 1.18 0.3gb 0.42c 1.08 292-307 clear 0.39' 6 19.57 0.253 red 4.95 7/14/80 0.43b 1.00 0.43b 0.34' 0.92 293-310 clear 0.37' 0.186 10 21.56 4.01 7/16/80 red 0.38b 0.58 0.65b a
Plateau.
' Primary peak.
Secondary peak.
PHOTOLYSIS hi
CH3. + C H 2 = C ( C d 3 ) 0 i
CHZ=C(CH~)CHO
+02 CH2=C(CH3)0i CHz-C(CH3);
+
NO-NO2
+
CHz=C(C1I3)0
2CHi(02)C(CH3)0
C H 2 ( 6 ) C ( C H 3 ) 0 +D2 -1iCHO
+ CH3COj
OH ABSTRACTION CH?=C(CH3)CHO
+
OH-CH2=C(CH3)C03
+n2
CH2=C(CH3)C03 + N O CH2=C(CH3)COj
+
NO2 + GO2 + Cri2
