12188
J. Phys. Chem. 1995, 99, 12188-12194
Reaction of Methylbutenol with Hydroxyl Radical: Mechanism and Atmospheric Implications Yinon Rudich," Ranajit Talukdar,' James B. Burkholder,' and A. R. Ravishankara**'$g Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80303 Received: January 20, 1995; In Final Form: June 2, 1 9 9 9 The tropospheric fate of 2-methyl-3-buten-2-01 (methylbutenol, MBO), a recently identified emission by vegetation, was investigated by measuring its UV absorption cross sections (210-300 nm) and the rate coefficient for its reaction with hydroxyl free radicals. UV absorption cross sections were found to be too small for photolysis to be an important removal pathway for MBO in the troposphere. The rate constant applicable under tropospheric conditions for the reaction of OH with MBO was determined to be k = (8.2 f 1.2) x e((610*50)'n cm3 molecule-' s-I. The OH reaction proceeds mainly via addition of the OH to the double bond in MBO. In the absence of 0 2 , about 15-20% of the adducts eliminate the alcohol-OH group. However, 0 2 can scavenge the adduct before it decomposes at T 4 300 K. This mechanism was confirmed by measuring the rate coefficients for the reactions of OD and Ix0H and determining the rate coefficient for the OH reaction in the presence of 7-13 Torr of 0 2 and in SF6 buffer gas. The elimination of the alcoholOH group was substantiated by observing OH production in the reactions of Ix0H and OD. The obtained OH reaction rate coefficient suggests that the primary daytime loss of MBO in the troposphere is via its reaction with OH.
Introduction Volatile organic compounds are emitted in large quantities by various anthropogenic and natural sources. Forests and agriculture fields' are major emission sources of chemically active natural volatile organic compounds (NVOCs). In some areas, such as in southeastem USA, the NVOC emissions can exceed those from anthropogenic source^.^-^ In the presence of mostly anthropogenic nitrogen oxides, NVOCs can act as the fuel for photochemical ozone production in the troposphere and thus have a substantial effect on regional air quality. This connection between biogenic hydrocarbon emissions, abundances, NO, and ozone concentrations in rural and forest environments4 as well as in cities3 has been substantiated by model calculation^.^.^ Because of the role played by NVOCs, control of anthropogenic hydrocarbon emissions may be ineffective in reducing ozone levels in both urban and rural areas.2 Thus, it is very important to understand and characterize the abundance and reaction pathways of NVOCs in the troposphere. In addition to their direct impact on ozone levels in the troposphere,NVOCs play other important roles in the chemistry of the lower troposphere6 and even influence some atmospheric measurement^.^ Reactions of NVOCs lead to organic peroxides (R02), which are long enough lived to be transported from rural areas to cities and influence the local ozone levels there.6 Longlived organic nitrates fonned by degradation of NVOCs may be transported from polluted to rural areas to fumish NO, needed for ozone production. NVOCs are also a source for aerosolsx which contribute to haze formation. The fast reaction of NVOCs with NO3 can have a major influence on the nighttime oxidation capacity of the atmosphere. Because of these reasons, studies of the chemistry of these compounds are needed. Such studies will also help understand the role of NVOCs in the preindustrial atmosphere before major human influences. NOAAINational Research Council Post-Doctoral Research Associate. Also affiliated with the Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309. 5 Also associated with the Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80303. * To whom correspondence should be addressed. Abstract published in Advance ACS Abstracrs, July 15, 1995. @
The presence of NVOCs in the atmosphere can influence the measured levels of constituents such as OH. Recent comparisons of model calculations with direct OH measurements show that the models overestimate the OH concentration^.^.^ The results presented in these papers imply that detailed NVOCs chemistry should be included in further models to account for the discrepancies between observations and models. Until recently, the most abundant NVOCs that have been identified as emitted by vegetation are isoprene and a few monoterpene hydrocarbons such as a-pinene and /3-pinene.Io In several recent field studies"-12 another organic compound of biogenic origin was found in large quantities. Changes in the preconcentration procedure for gas chromatography-flame ionization detection (GC-FID)'I used for hydrocarbon measurements enabled the identification of the alcohol 2-methyl-3-buten2-01 (also known as methylbutenol or MBO): OH
CH3
In one of the field campaigns, MBO's mixing ratio during the day was found to be between 2 and 3 ppbv, 5-8 times larger than that of isoprene." This study was carried out in Niwot Ridge, CO, a site which is located in a small clearing in a predominantly lodge pole pine tree forest with a significant mixture of aspen and some Colorado blue spruce. Although MBO was previously known as a pheromone of the spruce bark beetle (Ips typographus), its appearance in correlation with isoprene and at high concentrations suggests that it is emitted by a vegetative source rather than just by the insects in the barks of the trees. The rapid increase in the MBO mixing ratio with sunlight is very similar to that of isoprene, indicating that its emission is light mediated and that it may be produced via the same (or similar) biochemical process as isoprene. In the troposphere, NVOCs can be either removed by various processes such as their reactions with OH, NO3, and 0 3 or, possibly, photolyzed by solar radiation. It is assumed that direct wet or dry deposition of these compounds is negligibly slow to be important. During daytime, reactions with OH usually dominate, and reactions with ozone may also contribute to the
0022-365419512099-12188$09.00/0 0 1995 American Chemical Society
Reaction of Methylbutenol with OH Radical
J. Phys. Chem., Vol. 99, No. 32, I995 12189
removal of NVOCs from the troposphere. At night, reactions with NO3 radicals and 0 3 are possible reaction pathways. Therefore, the rate coefficients and mechanisms of the reactions of NVOCs with these species should be determined. In the absence of such kinetic information, the atmospheric loss rate of MBO was estimated to be 5-8 times slower than that of isoprene.' This estimation relied on analogous rate constants for the reaction of OH with isoprene13and with other compounds with similar functional groups.14 We have recently initiated a study of the reactions of MBO with various oxidants and of its photolysis. Here we present data on the two possible pathways for its removal: photochemical destruction and the reaction with OH. We report the UV absorption cross sections within the region of 210-300 nm and the rate coefficients for the following reaction:
OH
+ MBO -products kl
(R1)
To understand the mechanism of this reaction, we have also studied the following reactions,
OD + MBO
-
"OH + MBO
k2
k3
products products
(R2) (R3)
and the effect of higher bath gas pressure and addition of and of SF6 on the rate coefficient for these reactions.
0 2
Experiments and Results Absorption Cross Section Measurements. The U V absorption spectrum of MBO between 210 and 300 nm was measured using a 50 cm long absorption cell, a D2 lamp, and a diode array spectrometer. This apparatus and the data acquisition procedures have been described elsewhere. Initial measurements showed that MBO decomposed in the absorption cell to produce isoprene. The decomposition of MBO seemed to be surface catalyzed. We measured the absorption spectrum of isoprene and found its absorption cross section at 213.9 nm (Zn line) to be 7.6 x cm2. Further, isoprene exhibits very distinct structure with peaks near 215 and 222 nm. The isoprene absorption cross sections at 1 > 290 nm were less than cm2. To avoid interference by isoprene, the absorption cross sections of MBO were determined by flowing MBO through the absorption cell, at a pressure of 10 Torr, and recording the spectrum. Flowing MBO minimized interference from absorption due to isoprene. The presence of isoprene could be easily identified by its strong peaks at 215 and 225 nm. These features were not observed in the MBO spectra measured under flowing conditions. MBO absorbed very weakly in the wavelength range important for tropospheric photolysis; the absorption cross section, u, is less than 2 x cm2 at 1 > 290 nm. The spectrum at shorter wavelengths is continuous and relatively weak, with u =7 x cm2 near 210 nm. On the basis of the absorption cross sections of MBO and isoprene, we estimate isoprene concentration in the flowing MBO to be less than 0.01% and its contribution to absorption at 1 > 290 nm to be insignificant. The measured absorption spectrum is shown in Figure 1. We estimate the uncertainties in the cross sections to be 10% at 95% confidence limit. Using typical numbers for the daytime average solar flux at the earth's surface27(40" N, summer), also shown in Figure 1, and the absorption cross sections measured in this study, the tropospheric photolysis rate of MBO was calculated. In the wavelength range in which MBO showed reasonably strong absorption, 1 < 250 nm, the solar flux is essentially zero. Using l 5 3 l 6
250
ZOO
I v-
300
350
Wavelength (nm)
Figure 1. Absorption cross sections of MBO between 200 and 300 nm measured using the diode array system. The solar flux at the earth's surface is also shown. It is clear that MBO does not absorb at wavelengths where the solar flux is significant.
an exponential extrapolation of the measured absorption cross sections at wavelengths less than 300 nm to longer wavelengths (up to 350 nm), we obtain a lower limit of 5 years for the atmospheric lifetime of MBO. Therefore, the MBO photolysis loss rate is sufficiently slow compared to its reactive losses (see below) that it is insignificant. Hence, the cross sections at other temperatures were not measured. OH Rate Coefficient Measurements. The details of the pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) apparatus used for studying the OH MBO reaction have been described in previous publications from this l a b ~ r a t o r y . ' ~ ~ ' ~ Therefore, the description below is limited to the features necessary to understand the current experiments. A -150 cm3jacketed Pyrex cell was maintained at a constant temperature by flowing a liquid (methanol or ethylene glycol) from a thermostated source. The temperature in the reactor was constant to within f l K and known to better than 0.5 K. The temperature of the gas in the reaction zone (defined by the volume produced by the intersection of the photolysis and probe laser beams) was directly measured using a retractable thermocouple. The reactant OH and OD were generated by photolysis of HONO and DONO, respectively, at 355 nm from a frequency tripled Nd:YAG laser. This OWOD source was chosen because of the negligibly low absorption cross section of MBO at 355 nm (see above). The temporal profiles of OH and OD, following their photolytic production, were obtained by measuring their concentrations via laser-induced fluorescence (LIF) at various delays between the photolysis and probe lasers. The fluorescence was excited by pumping the Ql(1) line of the (A2X,v=1) (X2n,v=0)transition at 281.1 and 287.6 nm for OH and OD, respectively. The probe beam, which propagated perpendicular to the photolysis beam, was generated by doubling the output of a dye laser, which was pumped by the second harmonic of a Nd:YAG laser. Fluorescence was collected orthogonal to both laser beams and was focused onto a photomultiplier tube through a lens system and band-pass filter centered at 308.8 nm. MBO was premixed with He in a 12 L glass bulb to form a 1% mixture at a total pressure of -lo00 Torr. All gases flowed into the reactor through Teflon tubing. High-purity He carrying HONO flowed into a mixing volume, where it was mixed with the MBO/He mixture and other gases, such as 0 2 , SF6, and He, prior to entering the reaction cell. When DONO was used, it was added via an injector directly into the reactor, just above the reaction volume, to minimize contact between DONO and MBO and, thus, suppress any isotopic exchange.
+
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12190 J. Phys. Chem., Vol. 99, No. 32, 1995
+
TABLE 1: Rate CoeflTcients, &I, for OH MBO Reaction and the Associated Experimental Conditions Used To Acquire the Dataa k, lo-" cm3 diluent press., range of [MBO], T (K) molecule-' s-I gas Torr 1OI4cm-3
1
.
OD o O H
J
I
0
50
'
1
.
100
1
1
150
200
,
250
Reaction tlme (ps)
Figure 2. Temporal profiles of OH (0)and OD (m) due to reaction with MBO.The concentrations of MBO were 1.8 x lOI4 cm-3 and 2 x lOI4 cm-3 respectively for OD and OH decays. The decays are
7.9 f 0.8 7.2 f 0.7 6.9 f 0 . 6 5.7 f 0.5 5.4f0.4 5.0 f 0.5 5.0 f 0.4 4.9 f 0.4 4.6 f 0.4 4.4 f 0.4 4.3 f 0.4 4.2 f 0.4 3.7 f 0.4 4.0 f 0.4 3.4 f 0.3 2.1 f 0.3 2.9 f 0.3
254 267 269 29 1 299 300 315 318 334 348 354 363 366 372 388 40 1 410
He
He He He He
He He He He
He He He He/,!& He
He He/SF6 He
100 100 105 100 100 315 110 105 110 110 105 100 110 110 110 110 100
1.5-3.4 1.0-3.0 1.5-3.5 1.0-2.2 0.17-2 1.8-5.0 0.3-2.8 0.8-3.5 0.4-3.3 0.2-2.5 0.9-3.2 0.4- 1.6 0.3-2.5 0.2-2.4 0.3-2.7 0.3-2.0 0.3-1.4
exponential, as is expected from the pseudo-first-order conditions employed in the study.
The quoted error bars include systematic and precision errors at 95% confidence level.
The amounts of the MBOkIe mixture, 0 2 , He, and SF6 in the reactor were determined from the measured mass flow rates, using calibrated electronic mass flow meters, and the pressure in the reactor. The concentration of MBO in the reactor was calculated from the MBO fraction in the stock mixture and the amount of the mixture in the reactor. The fraction of MBO in the stock mixture was primarily derived from pressure measurements during its preparation. Pressure in the reaction cell was usually -100 Torr; a few experiments were performed at a total pressure of -300 Torr. The gas flow velocities under these conditions were -15 cm s-l, which replenished the reaction zone with a fresh gas mixture between laser shots (operating at 10 Hz). The initial concentration of OH, [OHIO,was approximated as follows: The first order loss rate coefficient for OH in the presence of HONO alone was measured. This rate coefficient was attributed entirely to the reaction of OH with HONO ( k = 4.5 x at 298 KI9) to estimate the concentration of HONO. [OHIO was estimated by using the known absorption cross section for HONO (UHONO = 3.6 x cm2 at 355 nm) and the fluence of the photolysis laser measured at the reactor. Obviously, this yields an upper limit to the concentration of OH in the reactor since some of the measured first order rate constant for OH loss is due to reaction.with the impurities in the bath gas, with NO2 from the HONO source and to diffusion from the reaction volume. Reaction R1 was studied under pseudo-first-order conditions in OH; Le., the OH concentration was much lower than that of MBO. MBO concentrations ranged from 1 x lOI3 to 1 x lOI5 molecules ~ m - while ~ , [OHIOwas always less than 5 x 10" molecules ~ m - ~Under . these conditions, the OH time profile followed the pseudo-first-order rate law,
of MBO did not affect the measured value of k. The reaction between OH radicals and the reaction products was estimated to contribute less than 2% to the measured loss rate, in the worst possible scenario. The bimolecular rate coefficients were measured at various temperatures between 230 and 410 K. High purity gases were used throughout the experiments. MBO from a commercial source was used. The purity of the sample was determined by gas chromatography-mass spectrometry (GC-MS) to be >99.9% pure; the main impurities were 2-methyl-2-butanol (less than 0.1 %) and isoprene (about 0.1%). Since the reaction of OH with MBO is so rapid, the presence of these impurities at such a low level has no effect on the measured rate constants. Both the stock sample and the prepared MBO/He gas mixtures were checked for impurities by GC after a few weeks; no changes in their purity were observed. HONO (DONO) was prepared, in situ, by dropwise addition of NaN02 solution (0.1 M) in H20 (D20) to 10% HzSO4 (D2S04) in H20 (D20). The reaction of MBO with OH appears to be complex with multiple reaction channels. To unravel this complexity, we studied the OH reaction with MBO as functions of temperature, added 0 2 , pressure of the bath gas, and different bath gases. Further, reaction R2 was studied by observing OD loss and OH formation. Similarly, reaction R3 was investigated by observing l80H loss and formation of 160H. For ease of presentation, the results of these studies are described separately. OH MBO Reaction. The measured rate coefficients for the reaction of OH with MBO, along with various experimental conditions used to make these measurements, are listed in Table 1. They are also shown in Figure 3 (filled squares) in the conventional Arrhenius form,
[OH,]/[O&] =
+
a
+
(1)
where K = k[MBO] /Q and /Q is the first-order rate coefficient for the loss of OH in the absence of MBO. Figure 2 shows a sample profile of OH and OD decays due to reaction with MBO. Both of these decays show pseudo-first-order behavior. The values of K were measured at various concentrations of MBO, and k was obtained from the plots of K versus [MBO]. In these experiments, /Q was usually 30-100 times smaller than the rate coefficient, K, measured in the presence of MBO. Therefore, any small fluctuations (which were less than 10%) in the HONO concentration between runs made with different concentrations
The straight lines in Figure 3 are fit of the data to eq 2. The estimated uncertainty for the rate coefficients, which includes both precision and estimated systematic errors, was calculated using the expression commonly used in rate data evaluations l 9
(3) In the above expression, which can be used for calculating the
Reaction of Methylbutenol with OH Radical 400
f
350
:or'
J. Phys. Chem., Vol. 99, No. 32, 1995 12191
+
250
' '
OD
+
MBO
OH t MBO 7
TABLE 2: Rate Coefficients, kz, for OD MBO Reaction and the Associated Experimental Conditions Used To Acquire the Dataa k, lo-" cm3 diluent press., range of [MBO], T(K) molecule-' s-I gas Torr 1OI4cm-3 235 11.2 f 1.0 He 95 0.4-2.6 254 8.9 f 0.8 He 100 0.4-2.7 266 8.4 f 0.7 He 100 0.3-2.5 299 6.8 f 0.5 He/?& 105 0.4-2.5 300 6.2 f 0.5 He 95 0.4-2.9 314 5.7 f 0.5 He 105 0.4-2.2 327 5.3 f 0.5 He/SF6 105 0.3-2.5 344 4.9 f 0.4 He 105 0.3-2.0 37 1 4.3 f 0.4 He 105 0.4-2.0 392 3.6 f 0.3 He 105 0.4-2.0 410 2.9 f 0.3 He 105 0.8-1.4 The quoted error bars include systematic and precision errors with 95% confidence level.
h
.-3 C
I 0
1000
2000 T OrJ)
3000
I 4000
Figure 4. Temporal profile of OH regenerated in the reaction of OD with MBO. The solid line is a simulation of the reaction mechanism where OD reacts with MBO to regenerate OH part of the time (see text). The loss of OD was exponential; an examples of OD loss are shown in Figure 2.
than those in the atmosphere may not be applicable to the atmosphere. Therefore, sF6, a very efficient vibrational quencher, was added to the reactor. Addition of about 60 Torr of SF6 to the reactor or working at a total pressure of 300 TOITof the He did not change (to within 10%)the observed rate constants (see Tables 1 and 2). Hence, we conclude that our measured rate constants are in the high-pressure limit even at the lowest pressure used here and are applicable to the atmosphere. Formation of OH in Reaction R2. It was suspected that the reason for the rate coefficient kl being smaller than k2 is that the OH group in MBO may be eliminated, part of the time, after the attack of OH and OD on the double bond in MBO. To check whether such an OH elimination is occurring, the temporal profile of OH in the reaction of OD MBO was measured. Very small concentrations of OH could be detected, since the OH background in the reaction cell was very low in the absence of MBO. When OD was generated in the presence of MBO, formation of OH was clearly observed, as shown in Figure 4. This figure shows the temporal profiles of OH in the reaction of OD with MBO, while OD decays are exponential, as can be seen in Figure 2. It is clear that OD exhibits a simple exponential decay, whose rate depends on [MBO], showing that it is merely removed via reaction with MBO. On the other hand, OH is first produced and lost via reactions involving MBO. The temporal profile of OH (Figure 4) is consistent with it being formed by the reaction of OD with MBO and lost via its reaction with MBO. In these experiments, OH was not observed in the absence of either MBO or DONO; Le., both DONO and MBO had to be in the reactor for OH formation, showing that OH was produced only by the reaction of OD with MBO. In principle, the temporal profile of OH can be analyzed to obtain
+
Rudich et al.
12192 J. Phys. Chem., Vol. 99, No. 32, 1995
+
TABLE 3: Rate Coefficients, kl, for OH MI30 in Presence of 0 2 and the Associated Experimental Conditions Used To Acquire the Dataa k, lo-'' cm3 T (K) molecule-' s-] 231 255 264 268 301 301 319 353 373 373 408
*
11.6 1.3 9.0 & 1.0 8.7 & 0.9 8.3 & 0.9 5.9 & 0.6 6.2 i.0.6 5.3 i 0.5 4.2 i.0.5 3.5 J! 0.6 3.9 i.0.4 3.0 J! 0.3
press., range of [MBO], Torr lOI4 cm-3
diluent gas He/6Torrof02 He/6Torrof 0 2 He/7 Torrof 0 2 He16Torrof 0 2 He/2 Torr of 0 2 He/7 Torrof 02 He/7 Torrof 0 2 He17 Torr of 0 2 He/13.5 Torr of 0 He/7 Torr of 0 2 He/6 Torr of 02
2
100 100 100 100 90 100 110 110 100 110 105
0.3-2.0 0.3-2.0 0.4-2.0 0.4-2.0 0.5-2.0 0.2-2.0 0.2- 1.6 0.2- 1.7 0.1-2.0 0.2- 1.6 0.3-1.6
a The quoted error bars include systematic and precision errors with 95% confidence level.
400
2.5
350
3.0
T (K) 300
3.5
250
4.0
4.5
1000 / T (K)
Figure 5. Arrhenius plot for the reaction of OD (0)and of OH in presence of 0 2 (0)with MBO. The solid line is the least-squares fit of the data to eq 2 over the temperature range 230-360 K. The obtained and E/R = 610 k 50. These parameters are A = (8.2 & 1.2) x parameters are applicable for the atmosphere (see text). The error bars are la precision only. kl and k2. However, accurate values of the rate coefficients for these two processes could not be derived from the observed temporal profiles because kl and k2 are so similar to each other. However, the temporal profile calculated using the above mechanism (solid line) is consistent with the measured [OH] variation with time. In these experiments, the gas flow lines and cell were exposed to a constant flow of D20 prior to photolysis to minimize exchange of D atoms in DON0 with H atoms in H20 that could be present on the walls. Formation of OH in reaction R2 was seen at other temperatures (254, 298, and 410 K) also. Reaction of OH and OD with MBO in the Presence of 0 2 . Molecular oxygen is a radical scavenger but does not attach to OH. Even in the atmosphere, one of the primary reactions of 0 2 is attaching to alkyl type radicals to make RO2. To check whether the adduct suspected to be formed in the reaction of a hydroxyl radical with MBO could be scavenged by 0 2 , we measured kl and k2 in the presence of 0 2 . The rate coefficient kl was measured in the presence of 0 2 , and the obtained rate coefficients are shown in Table 3 and also plotted in Figure 5 (open circles). It is clear that addition of 0 2 enhances kl by up to 15-20% such that it is essentially the same as k2, which is not affected by the presence of 0 2 (Figure 5). This enhancement varied with temperature. Also, the formation of OH in the reaction of OD with MBO was almost completely suppressed by -7 Torr of 0 2 .
Experiments with l80H.In addition to studying OH and OD reactions, we also investigated the reactions of IsOH (produced by the reaction of O(lD) with H2I80) with MBO. The rate coefficient for the loss of I80H was essentially the same as that for OD. In addition, we also observed the formation of I60H in this reaction. This experiment was performed only at room temperature. However, due to the slow background reaction of 0 3 with MBO and the extra difficulties in carrying out these reactions, they were not carried out at other temperatures. Yet, even the limited data that were obtained provided corroboration to the thesis that the alcohol OH group is eliminated after the attack of OH on the double bond. Discussion Various experimental checks were carried out to ensure that the rate coefficients measured in the present study were not affected by unwanted and undetected secondary reactions. The routine checks such as variations in the initial concentration of OH or OD by a factor of 5 , the linear gas flow rate through the reactor, variations in the photolyte concentration, and the photolysis fluence did not change the measured rate coefficients. As mentioned earlier, the pressure in the reactor also did not affect the rate coefficient. To check whether there are impurities in our sample of MBO, we analyzed it via GCMS, as mentioned earlier, and found no major impurities. To ensure that MBO was stable in the stock mixture, the mixture was analyzed every few days by GC to check the level of MBO and the formation of any impurities. No changes in MBO concentration or formation of impurities were observed. In addition, absorption measurements with a Zn lamp (213.9 nm) were used to check for isoprene over a period of several days. The level of isoprene in the MBO/He mixtures which were used for the kinetics experiments did not change. On the basis of the measurements with the Zn lamp and the GC-MS and GC measurements, we estimate the isoprene concentration to be less than 0.1% of MBO; hence, its contribution to the measured rate coefficients is expected to be less than 1%. In addition, rate coefficients measured using different MBO mixtures were the same. Therefore, we are confident that our measured MBO concentrations are accurate and that impurities were not influencing the measured rate constants. The majority of the experiments were carried out using HONO/DONO as the source of OWOD. However, other sources of OH were also tried. We produced OH via the photolysis of ozone in presence of water vapor and photolysis of H202. In the case of H202, even though the obtained results were in reasonable agreement (within 10%) with those from HONO photolysis, there were larger scatter in the data. This appeared to be due to the reaction of MBO with H202, very likely on the walls of the reactor or the gas lines. It is known that MBO reacts with 0 3 , and this reaction prevented routine use of this source. As shown in the case of the '*OH studies, we could indeed use this source. The observation that k2, with or without 0 2 , and kl in the presence of 0 2 are the same demonstrates that there was no significant exchange of D atoms with H atoms of the alcohol group in MBO while it was flowing in the tube. Had there been any exchange of this type, the rate coefficients would be different. On the basis of these checks, we conclude that our measured rate coefficients are devoid of any major undetected systematic errors. The primary source of systematic error in our rate coefficients is the knowledge of the concentration of the reactant MBO. As mentioned earlier, the concentrations were primarily
Reaction of Methylbutenol with OH Radical
J. Phys. Chem., Vol. 99,No. 32, 1995 12193
TABLE 4: Rate Coefficients, k, at 298 K, at High Pressure and Arrhenius Parameters (k = Ae-Bm)for the Reaction of OH Radical with Alkenes and with MBO in Presence of 02 compound 2 methyl-3-buten-2-01 (MBO) 3-methyl- 1-butene 2-methyl- 1-butene 2-methyl-2-butene 2,3-dimethyl-2-butene 3,3-dimethyl-1-butene 1,3-butadiene 2-methyl- 1,3-butadiene (isoprene)
101*kat 298 K, cm3 101*A, cm3 molecule-’ s-I molecule-’ s-I B, K
ref
8.2
-600 this work
31.8 61 86.9 110 28 66.6 101
5.32
-533 28 28 -450 28 28 28 -448 28 -410 28
14.8 25.4
I
(CH&CCH=CHz
+ OH
-
OH
I
(CH&CCH-CHzOH (CH&=CHCH20H
64.8
19.2
OH
obtained via flows and manometric measurements. We have estimated this error to be -5% (20) and added it to the precision in quoting the overall uncertainties. To our knowledge, we are the first to report the rate coefficient for the reactions of OH with MBO. Yet, we can compare our kl with the rate coefficients of the reactions with similar reactants. Table 4 lists the rate coefficients for the reactions of OH with olefins similar in complexity to MBO. It can be seen that except for isoprene (2-methyl-1,3-butadiene), which has a very high rate coefficient, the activation energies and rate coefficient for compounds with similar functional groups are very close to that of MBO. Since isoprene has a conjugated double bond system, with a methyl group adjacent to it, a higher rate constant is to be expected. MBO is most similar to 3-methyl-l-butene, whose reaction with OH exhibits a very similar activation energy; its k(298 K) is half that of reaction R1. This comparison also shows that abstraction from side methyl chains is probably very minor. For MBO, contribution to the overall measured rate coefficient due to abstraction from side methyl chains may account for not more than 5% of the total reaction at 410 K, where abstraction may be the most pronounced. Mechanism of the Reaction of OH with MBO. The reaction of hydroxyl radical with MBO is complex and involves the formation of an adduct. Insight into the mechanism for this reaction can be obtained from the data that are presented above. The observations relevant to the mechanism involved can be summarized as follows: 1, The rate coefficients for the reactions of OH and OD with MBO are large and exhibit negative activation energies (Le., rate coefficients increase with decreasing temperature). 2. k 2 is about 15-20% higher than kl in the absence of 0 2 at T 350 K, however, both kl and k 2 have the same activation energy. 3. Unlike kl, k2 is not affected by the presence of 0 2 . 4. kl measured in the presence of oxygen is the same as k2 at T < 300 K. 5. At higher temperatures, oxygen has little effect on the kl; kl is approximately equal to k 2 . The rate coefficients are not affected by the nature or pressure of the diluent gas and the presence of 0 2 . 6. Deviation from Arrhenius behavior at high temperatures is most pronounced for kl and less pronounced for k2. 7. OH is produced by the reaction of OD with MBO at all temperatures that were studied. 8. I60H is produced by reaction of I80H with MBO (measured only at room temperature). 9. I60H regeneration is suppressed by addition of 0 2 in the reaction of OD or I80H with MBO. These observations are consistent with the following mechanism, where the hydroxyl radical initially attacks the double bond in MBO.
+ OH
(The OH is shown in bold only for clarity.) The addition reaction is at the high-pressure limit in the pressure range studied here. A fraction (-15%) of the hydroxyl-MBO adduct decomposes to eliminate the alcohol-OH group and form a different butenol. A difference of about 15-20% between kl and k 2 in the absence of 02 at temperatures lower than 300 K is most likely caused by the elimination of an OH radical from the MBOOH adduct. The ejected OH radical is detected by the probe laser, leading to a measurement of an apparent smaller firstorder rate coefficient for the loss of OH when I60H is monitored; when either OD or l8OH is the reactant, I60H production is not observed, and its regeneration does not affect the measured rate coefficient. At elevated temperatures, kl and k 2 (with and without 0 2 ) are the same. As at lower temperatures, the pressure and nature of the bath gas had no effect on the measured rate coefficients. We believe that at these higher temperatures the adduct starts to undergo thermal decomposition. Studies of these rate coefficients at still higher temperatures could not be carried out. Such studies would be very enlightening because it is likely that one could observe the equilibrium between the adduct, hydroxyl radical, and MBO. The approach to equilibrium and the equilibrium itself have been observed in the reaction of OH with aromatic hydrocarbon^'^-^^^^' and olefins.I4 It is expected that the OH would preferentially add to the outer carbon atom. This is because addition to the terminal carbon atom, which is the least substituted carbon atom in the double bond, leads to the most substituted radical. According to the anti-Markownikoff mechanism, the more substituted radicals can be more easily stabilized.22 Such addition is also consistent with the observations in the reaction of OH with propene where OH adds to the terminal carbon atom -65% of the time.23 Addition to the inner carbon may also be sterically hindered in MBO. The adduct formed in this reaction may eliminate either the attacking OH or the alcohol-OH group. At low temperatures, the appearance of OH in the reaction of OD MBO and of I60H in reaction of ’*OH with MBO is a clear indication that the alcohol-OH group is being eliminated from the MBO-OD adduct 15-20% of the time. The other 80-85% can be attributed to cases where the adducts do not eject an OH radical. Elimination of the alcohol OH is likely when the incoming OH is attached to the outer carbon and leads to a new alcohol, 3-methyl-2-buten-1-01, If this were to happen every time OH attacked the terminal carbon, we should see a very large branching for the elimination of the alcohol-OH group and a larger difference between kl and k2 and in the value of kl with and without 0 2 , contrary to the observations. Therefore, we conclude that either this elimination occurs only part of the time or the initially formed adduct rearranges to transfer the OH group to the inner position. Identification of the products of the reaction and the intermediateswould be very enlightening in terms of elucidation of the full mechanism of the OH reaction with MBO. Atmospheric Implications. The rate coefficients for the OH MBO reaction in the presence of 0 2 at T = 300 K were similar to the rate coefficients for the reaction of OD with MBO, but larger than kl in the absence of 02,as can be seen when comparing Tables 1 and 2 with Table 3 and Figures 3 and 5. In the atmosphere, because of the presence of 0 2 , the OH loss rate coefficients due to reaction with MBO will be those measured for kl in the presence of 0 2 . Since we do not expect
+
+
Rudich et al.
12194 J. Phys. Chem., Vol. 99, No. 32, 1995 a significant isotope effect (to first order) in the addition process, the values of k2 are also applicable to the atmosphere. Therefore, we have used the values of k2 and kl in the presence of 0 2 to derive the rate coefficients applicable to the atmosphere. Since there are departures from Arrhenius behavior at temperatures much above 298 K and since the temperatures in the atmosphere are seldom above 305 K, we have combined the ' values of k2 and kl (with 0 2 ) in the temperatures between 230 and 300 K to derive the needed rate constants. They are A = 8.2 x 10-l2 cm3 molecule-' s-l, Ea = -1220 cal/mol, Ma = 100 cal/mol, and A298) = 1.1 in the format used in data evaluations. Even though the reaction of OH with MBO leads to the elimination of the alcohol OH group, such a process will not be important in the atmosphere because of the rapid scavenging of the adduct by 0 2 . However, release of OH due to thermal decomposition starts to become important at temperatures much above 300 K. The tropospheric lifetime of MBO would be less than 5 h due to reaction with OH (assuming OH concentration of lo6 ~ m - ~ )Since . this work was completed, the rate coefficient for the reaction of MBO with 0 3 at 291 K has been reported25to be (10.0 f 0.3) x cm3 molecule-' s-l. This value of the rate coefficient for the MBO 0 3 reaction and a reasonable ambient concentrations of O3 lead to an atmospheric lifetime of more than 15 h for the removal of MBO by 0 3 reaction (assuming ozone concentration of 50 ppb). Thus, the reaction of O3 does not compete favorably with reaction R1 during daytime for the removal of MBO. If the MBO emission continues until sunset, MBO that is the left over at night would react with NO3 and 0 3 and make these reactions important. However, if the MBO production is related to the photosynthesis, the nighttime MBO levels could be lower. Goldan et aLIi assumed that kl was -2 x lo-" cm3 molecule-' s-l in their first assessment of the impact of MBO on the troposphere. The higher rate constant measured here implies that the sources of MBO must be higher to sustain the observed atmospheric concentrations. In addition, MBO can scavenge OH faster and lead to lower OH values if the subsequent reactions do not regenerate HO, species. Such scavenging can change the concentration of measured OH, and inclusion of MBO-OH reaction in modeling of such field studies is essential. Since the rate constant for OH isoprene reaction is 1 x 1O-Io cm3 molecule-' s-l, approximately a factor of 2 larger than that with MBO, 2 ppbv of MBO is equivalent to 1 ppbv of isoprene. It has also been argued that different vertical distributions of isoprene and MBO might account for the differences between ground level versus long path (high above the tree line) OH detection scheme^.^ Although isoprene's vertical profile has been measured in the past,26that of MBO is yet unknown. More measurements of MBO should be carried out to better characterize its role in tropospheric chemistry and its exact origin. 19324
+
+
Conclusions The absorption cross sections of MBO in the UV were measured and found to be too small to account for MBO losses in the troposphere. The rate coefficients for the reactions of MBO with OH (with and without 0 2 ) and OD were measured. The reaction mechanism is mainly addition of OH to the double bond in MBO. At room temperature and below, about 1520% of the adduct eliminates the alcohol-OH group. The adduct can be stabilized by adding an 0 2 molecule, as evidenced by absence of OH regeneration. At high temperatures, thermal decomposition of the adduct is more dominant. The rate coefficient at 298 K was found to be about 3 times higher than
the value estimated previously, implying the existence of strong biogenic sources of MBO. Such a high reaction rate coefficient and high tropospheric concentration of MBO imply that MBO may be an important OH sink in rural and forest areas.
Acknowledgment. This work was performed while Yinon Rudich held a National Research Council-NOAA Research Associatship. The authors acknowledge the help of Scott Hemdon in carrying out some of the measurements and Steve Montzka of N O M C M D L for performing the GC-MS analysis of the MBO sample. This work was funded in part by N O M s Climate and Global Change program.
Supporting Information Available: Table of MBO absorption cross sections (1 page), Ordering information is given on any current masthead page. References and Notes (1) Geron, C. D.; Guenther, A. B.; Pierce, T. E. J. Geophys. Res. 1994, 99, 2773. (2) Chameides, W. L.; Lindsay, R. W.; Richardson, J.; Kiang, C. S. Science 1988, 241, 1473. (3) Cardelino, C. A.; Chameides, W. L. J. Geophys. Res. 1990, 95, 13971. (4) Johnson, C.; Jamson, R. W. J. Geophys. Res. 1993, 98, 5121. (5) Poppe, D.; Wallasch, M.; Zimmerman, J. J. Atmos. Chem. 1993, 16, 61. (6) Fehsenfeld, F.; et al. Global Biogeochem. Cycles 1992, 6, 389. (7) Eisele, F. L.; Mount, G. H.; Fehsenfeld, F. C.; Harder, J.; Marovich, E.; Parrish, D. D.; Roberts, J.; Trainer, M.; Tanner, D. J. Geophys. Res. 1994, 99, 18605. (8) Zhang, S.; Shaw, M.; Seinfeld, J. H.; Lagan, R. J. Geophys. Res. 1992, 97, 20717. (9) Poppe, D.; et al. J. Geophys. Res. 1994, 99, 16633. (IO) Lamb, B.; Guenther, A.; Gay, D.; Westberg, H. Atmos. Environ. 1987, 21, 1695. (1 1) Goldan, P. D.; Kuster, W. C.; Fehsenfeld, F. C. Geophys. Res. Lett. 1993, 20, 1039. (12) Goldan, P. Private communication, 1994. (13) Atkinson, R.; Aschmann, S. M.; Tuazon, E. C.; Arey, J.; Zielenska, B. Int. J. Chem. Kinet. 1989, 21, 593. (14) Atkinson, R. J . Phys. Chem. Re$ Data 1989 (Monogr. I), 18. (15) Burkholder, J. B.; Talukdar, R. K.; Ravishankara, A. R. Geophys. Res. Lett. 1994, 21, 585. (16) Burkholder, J. B.; Talukdar, R. K.; Ravishankara, A. R.; Solomon, S. J. Geophys. Res. 1993, 98, 22937. (17) Talukdar, R.; Mellouki, A.; Gierczak, T.; Burkholder, J. B.; McKean, S.A.; Ravishankara, A. R. J. Phys. Chem. 1991, 95, 5815. (18) Vaghjiani, G.L.; Ravishankara, A. R. J. Phys. Chem. 1989, 93, 1948. (19) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. Chemical Kinetics and Photochemical Data f o r use in Stratospheric Modeling; Jet Propulsion Laboratory, JPL Pub. No. 92-20, 1992. (20) Perry, R. A,; Atkinson, R.; Pitts, Jr., J. N. J. Phys. Chem. 1977, 81, 296. (21) Tully, F. P.; Ravishankara, A. R.; Thompson, R. L.; Nicovich, J. M.; Shah, R. C.; Kreutter, N. M.; Wine, P. H. J. Phys. Chem. 1981, 85, 2262. (22) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry, Part A ; Plenum Press: New York, 1984. (23) Cvetanovic, R. J. 12th. International Symposium on Free Radicals, Laguna Beach, CA, 1976. (24) Atkinson, R.; Baulch, D. L.; Cox, R. A,; Hampson, R. F.; Kerr, J. A.; Troe, J. J. Phys. Chem. Re$ Data 1992, 21, 1125. (25) Grosjean, E.; Grosjean, D. Inr. J. Chem. Kinet. 1994, 26, 1185. (26) Andronache, C.; Chameides, W. L.; Rodgers, M. 0.;Martinez, J.; Zimmerman, P.; Greenberg, J. J . Geophys. Res. 1994, 99, 16989. (27) Stammes, K.; Tsay, S.-C.; Wiscombe, W.; Jayaweere, K. Appl. Opt. 1988, 27, 2502. (28) Atkinson, R. J. Phys. Chem. Re$ Data 1989 (Monogr. 2), 1994. JP950207S
