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Experimental study of hydroxyalkyl peroxy radicals from OH-initiated reactions of isoprene
27 July 2001
Chemical Physics Letters 343 (2001) 49±54
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Experimental study of hydroxyalkyl peroxy radicals from OH-initiated reactions of isoprene Dan Zhang a, Renyi Zhang a,*, Clark Church b, Simon W. North b b
a Department of Atmospheric Sciences, Texas A&M University, College Station, TX 77843, USA Department of Chemistry, Texas A&M University, P.O. Box 300012, College Station, TX 77842, USA
Received 2 March 2001; in ®nal form 30 April 2001
Abstract Despite the importance of hydroxy±isoprene peroxy radicals in cycling of NO to NO2 , in partitioning of the ®nal products, and in propagation of the oxidation reactions, very few experimental studies involving these radicals as reactants have been carried out. Using a fast-¯ow reactor coupled to chemical ionization mass spectrometry (CIMS) detection, we have generated and detected the OH±O2 ±isoprene peroxy radical. By directly monitoring the peroxy radical we have obtained an overall rate constant of 7 3 10 13 cm3 molecule 1 s 1 for the reaction of the OH± isoprene adduct with O2 . Ó 2001 Published by Elsevier Science B.V.
1. Introduction Photochemical oxidation of atmospheric volatile organic compounds (VOCs) results in ozone and secondary organic aerosol formation, with major implications for local and regional air quality and global environmental changes [1,2]. Isoprene (2-methyl-1,3-butadiene, CH2 @C CH3 CH@CH2 ) is one of the most abundant hydrocarbons naturally emitted by the terrestrial biosphere, with a global averaged production rate of about 450 Tg yr 1 [3]. Due to its high chemical reactivity and proliferation in the generation of organic peroxy radicals, isoprene plays an important role in ozone formation in local and regional atmosphere [4,5]. Atmospheric oxidation of isoprene is initiated by reactions with a variety of oxidant species. Since isoprene is mainly produced *
Corresponding author. Fax: +1-979-862-4466. E-mail address: [email protected] (R. Zhang).
in the daytime, the reaction with hydroxyl radial OH is the dominant tropospheric removal pathway. The initial reaction between isoprene and OH proceeds mainly by OH addition to the C@C double bond, forming four OH±isoprene adduct isomers OH C5 H8 ! C5 H8 OH
1
Under atmospheric conditions, the hydroxyalkyl radical reacts primarily with oxygen molecules to form the hydroxyalkyl peroxy radicals C5 H8 OH O2 ! C5 H8 OHO2
2
Addition of O2 occurs only at the carbons b to the OH position for the OH±isoprene adducts of internal additions, but takes places at two centers (b or d to the OH position) for the OH±isoprene adducts of terminal additions. Hence O2 addition to the OH±isoprene adduct leads to the formation of four b- and two d-hydroxyalkyl peroxy radicals. The hydroxyalkyl peroxy radicals can further react
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 6 5 4 - 6
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D. Zhang et al. / Chemical Physics Letters 343 (2001) 49±54
with NO to form six (b or d) hydroxyalkyl alkoxy radicals. Hence the organic intermediate radicals play a key role in propagation or termination of the oxidation process, and it is of great importance to study the organic intermediate radicals. Numerous laboratory studies have been conducted to investigate the kinetics and mechanism of the oxidation reactions of isoprene initiated by OH [6]. A majority of the previous studies either investigated the initial step of the OH±isoprene reactions or analyzed the ®nal reaction products. Direct experimental data concerning intermediate processes of the OH±isoprene oxidation reactions are very limited. Jenkin et al. investigated the permutation reactions of the OH±isoprene peroxy radicals using the laser ¯ash photolysis/UV absorption spectrometry technique [7]. In this Letter, we present an experimental study of the hydroxyalkyl peroxy radicals arising from the OH-initiated reaction of isoprene. The OH± O2 ±isoprene peroxy radical has been generated and detected using a fast-¯ow reactor coupled to chemical ionization mass spectrometry (CIMS) detection. We also report kinetic measurements of the reaction of OH±isoprene adduct with O2 based on direct observation of the peroxy radicals. 2. Experimental A fast-¯ow reactor, in conjunction with CIMS detection, was used in the present experiments. The experimental setup is similar to that used by us previously, and a detailed description of the experimental apparatus has been given elsewhere [6,8]. Brie¯y, it consists of a ¯ow reactor of 1.23 cm i.d. and 80 cm in length. All surfaces exposed to the reactants and products were coated with a halocarbon wax. A ¯ow of He carrier gas (in the range of 1 to 3 STP l min 1 ) was injected to the ¯ow reactor through an entrance port in the rear of the ¯ow reactor. The pressure of the ¯ow reactor was regulated between 1 and 2 Torr and the temperature of the ¯ow reactor was maintained at 298 2 K. Typical ¯ow velocity in the ¯ow reactor ranged from 1300 to 2500 cm s 1 . Isoprene was added to the ¯ow reactor through a movable injector. The ¯ow reactor was operated under the
laminar ¯ow condition with Reynolds number Re 2auq=l typically in the range 10±40, where a is the internal radius of the ¯ow reactor, q the density of the gas, u the ¯ow velocity, and l the viscosity coecient of the gas. Under our experimental conditions, a laminar ¯ow was fully developed and homogeneous mixing of both OH and isoprene was eectively achieved [6,8]. Reactants and products of the OH±isoprene reaction were detected by CIMS using either positive or negative reagent ions. The CIMS employed a new approach, involving an electrostatic ion guide recently developed [9,10]. Positive or negative reagent ions were initiated using corona discharge at a high voltage ()5 kV). The SF6 reagent ions were generated by adding a small amount of SF6 to a He carrier ¯ow (about 1±2 slpm at STP) through the discharge. The positive reagent ions O 2 were produced by passing the He carrier ¯ow through the discharge and then adding a small amount of O2 downstream. An electrostatic ion guide was used to transport ions to the quadrupole mass analyzer. The use of the ion guide allows for ion transportation with a high eciency and preferential separation and removal of neutral molecules in a dierential pumping system [9,10]. OH radicals were generated in situ according to the reaction H NO2 ! NO OH k2 1:3 10 10 cm3 molecule 1 s 1 [11]. Hydrogen atoms were generated by passing a small ¯ow of 1% H2 / He mixture (about 0:5 cm3 min 1 at STP) through a microwave discharge followed by addition of an excess of 3% NO2 =N2 mixture. OH was detected in the negative ion mode using SF6 as the reagent ions, according to the ion±molecule reaction OH SF6 ! OH SF6 . Two methods were employed to calibrate OH concentrations in the ¯ow reactor. First, the OH concentration was obtained by comparing the relative signal intensities between OH and NO2 and subsequent calibration of NO2 in the CIMS, using the ion± molecule reaction rate constants of SF6 with OH and NO2 [6]. Alternatively, OH was calibrated by converting all of the OH into HNO3 followed by HNO3 calibration in the mass spectrometer. Typically, the initial concentrations of OH in the ¯ow reactor were in the range of 1 109 to 6 109 molecule cm 3 .
D. Zhang et al. / Chemical Physics Letters 343 (2001) 49±54
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Commercially available isoprene (Aldrich 99.5%) was used to volumetrically prepare 2 l glass bulb of a 1% isoprene/He mixture and introduced into the ¯ow reactor using a 10 sccm ¯ow meter. The concentrations of isoprene in the ¯ow reactor were regulated in the range 5 1011 ±5 1012 molecule cm 3 , so that the isoprene=OH ratio was at least a factor of 100 to ensure the pseudo®rst-order kinetic assumption. Isoprene was detected by the CIMS using positive reagent ions (O 2 ) at its parent ion peak. For both OH and isoprene the mass spectrometer signals were linear over the range of the concentrations used. To generate the OH±isoprene peroxy radical, OH, isoprene, and O2 were simultaneously introduced into the ¯ow reactor. O2 was added into the ¯ow reactor along with the He carrier gas. The production of the peroxy radicals was regulated by the initial concentrations of all three reactants, as well as the reaction distance. The peroxy radical was detected using the SF6 negative reagent ions according to the ion±molecular reaction: SF6 C5 H8 OHO2 ! SF6 C5 H8 OHO2
3
3. Results and discussion Fig. 1 shows time evolution of the 117 signal (corresponding to C5 H8 OHO2 for three experiments performed at 298 K and 1 Torr. Several procedures were taken to positively verify that the ions detected at m=e 117 were indeed attributable to the isoprene±OH±O2 radical, rather than from secondary ion±molecule reactions. We monitored the signal at mass 117 when either the isoprene or O2 ¯ow was stopped or when the microwave discharge for dissociating hydrogen molecules ceased. In Fig. 1a, the isoprene ¯ow was terminated at about 30 s, resulting in a disappearance of the signal at 117. The signal resumed at 140 s when isoprene was re-admitted to the ¯ow tube. In Fig. 1b, termination of the O2 ¯ow at about 70 s also led to a sharp drop in the mass 117 signal, until the O2 ¯ow was re-established at 170 s. At about 250 s, the mass 117 signal disappeared when the microwave discharge was switched o, and the signal recovered at 320 s when the discharge was turned
Fig. 1. Temporal variation of the C5 H8 OHO2 (m/e 117) and OH (m/e 17) signals (see text). Experimental conditions are: P 1:0 Torr, U 1799 cm s 1 , C5 H8 o 7:0 1011 molecule cm 3 , O2 o 5:0 1014 molecule cm 3 , and OHo 5:0 109 molecule cm 3 .
on. Fig. 1c shows time evolution of the signals at both 117 and 17 (corresponding to OH ) masses. The movable injector was pulled 10 in. upstream at 70 s and returned to its original position at 170 s. It is evident that the OH signal dropped from its initial steady-state value when reaction distance was increased, and the OH decrease was accompanied by an increase in the 117 signal. Hence those above procedures clearly illustrate that the mass peak at 117 is related to the presence of all three reactants in the ¯ow reactor. In addition, it is unlikely that the mass peak at 117 arose from the ion±molecule reaction of O2 with the OH±isoprene adduct, which also presented in the ¯ow reactor [12]. We did not observe any O2 signal in the presence of high concentrations of O2 in the ¯ow reactor, because O2 has a smaller electron
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D. Zhang et al. / Chemical Physics Letters 343 (2001) 49±54
anity (0.44 eV) than that of SF6 . Note that the mass spectrometer is sensitive only to the mass of ions that are detected and does not discriminate between isomers. The kinetics of the O2 addition to the four OH± isoprene adduct isomers is complex and the peroxy radical appearance curves will exhibit multi-exponential behavior. Each of the four OH±isoprene adduct precursors is formed with a single time constant given by the sum of the individual rate constants. The isomeric branching ratios are given by relative rate constants which have been calculated in our previous work [13]. The four OH± isoprene adduct isomers react with O2 , each with its own characteristic rate constant. Two of the OH±isoprene adducts, corresponding to terminal OH addition, each can form two distinct peroxy isomers resulting in two pairs of curves with exhibit time constants which re¯ect the sum of two O2 addition rate constants. As a consequence, the appearance curves for the O2 ±OH±isoprene adducts will consist of the sum of six appearance curves, four of which are unique, each weighted by the branching ratio determined by both the OH and O2 addition rate constants. Although the kinetics appear intractable, simulations suggest that the overall appearance curves of the peroxy radicals can be modeled using a single eective rate constant. Fig. 2 shows the results of kinetic simulations based on rate constants previously calculated for both the addition of OH to isoprene and the subsequent formation of the six distinct peroxy radical isomers [13]. The upper panel shows the appearance curves for each peroxy isomer, which could be measured if the detection scheme permitted one to distinguish between isomers. As mentioned previously, the CIMS technique does not permit selective detection, and the experimental peroxy appearance curve represents a weighted sum of these curves. The lower panel of Fig. 2 illustrates that this sum can be modeled using a single eective rate constant, with a value of 1:8 10 12 cm3 molecule 1 s 1 . We studied the evolution of the signal at mass 117, when the injector was successively pulled upstream. Fig. 3 shows that the mass 117 signal rises in accordance with OH disappearance. Under our experimental conditions regeneration of OH
Fig. 2. Simulations of peroxy radical isomer formation as a function of reaction time. The upper panel shows simulated time-dependent curves for the six distinct peroxy isomers (A±F) based on the rate constants calculated in [13]. The lower panel shows the sum of the six curves individual curves (circles) and the best eective rate constant for O2 addition to the OH±isoprene adducts (straight line).
Fig. 3. Variation of OH±O2 ±isoprene peroxy radical and OH signals as a function of reaction time. The curves are ®tting through the experimental data using a kinetic program (see text). Experimental conditions are: P 1:9 Torr, U 2287 cm s 1 , C5 H8 o 1:6 1012 molecule cm 3 , O2 o 2:7 1014 molecule cm 3 , and OHo 5:0 109 molecule cm 3 .
D. Zhang et al. / Chemical Physics Letters 343 (2001) 49±54
from subsequent reactions of the peroxy radicals was not important, due to the relatively low NO2 concentration used in our experiments (<1011 molecule cm 3 ). This was veri®ed as the OH signal followed the pseudo-®rst-order exponential decay. The OH decay data (solid curve) led to an eective bimolecular rate of 9:1 10 11 cm3 molecule 1 s 1 for reaction (1). The observed formation pro®le of the OH±O2 ±isoprene peroxy radical was simulated with a computer kinetic model that included reactions (1) and (2), along with other likely secondary reactions [6]. The model input included the initial concentrations of OH, isoprene, O2 , and all other precursors. The rate constant for reaction (1) was obtained from the pseudo-®rst-order decay of OH at various isoprene concentrations, and the value is consistent with the previous measurements [6]. The rate constant for reaction (2) was varied to best ®t the experimental data. Other related rate constants used in the simulations were taken from the recommendations of Atkinson [2] and DeMore et al. [11]. We did not include the decomposition of the peroxy radicals, since the decomposition rates were negligible compared to the formation rates of the peroxy radicals according to our theoretical calculations [13]. The dashed line in Fig. 3 represents the best ®t to the observed production of the peroxy radical. An eective bimolecular rate constant of 7 10 13 cm3 molecule 1 s 1 was inferred for reaction (2). For the data shown in Fig. 3, ®tting to experimental data was sensitive to the rate constant of reaction (2) within 10% for a 95% con®dence level. We did not perform kinetic measurements at very high concentrations of isoprene to drive reaction (1) into completion, since at high isoprene concentrations several anomalous mass peaks appeared in the negative spectra which could interfere with the detection scheme for the peroxy radicals. Fig. 4 shows formation of the peroxy radicals at O2 concentrations from 1:1 1014 to 8:5 1014 molecule cm 3 . The initial concentrations of OH and isoprene were 5:0 109 and 5:0 1012 molecule cm 3 , respectively. Fig. 4 illustrates that the measured signal of the peroxy radical intensi®es at a higher O2 concentration, re¯ecting a larger production of the peroxy radical
53
Fig. 4. Variation of OH±O2 ±isoprene peroxy radical signals as a function of reaction time. The curves are ®tting through the experimental data using a kinetic program (see text). The different symbols correspond to dierent O2 concentrations (1014 molecule cm 3 ): solid circles, 1.1; solid diamonds, 2.3; open circles, 3.3; down-triangles, 4.3; open squares, 6.4; open diamonds, 8.5. Experimental conditions are: P 1:7 Torr, U 2274 cm s 1 , C5 H8 o 1:0 1012 molecule cm 3 , and OHo 5:0 109 molecule cm 3 .
at such a condition. At high O2 concentrations the formation pro®les of the peroxy radicals were essentially non-distinguishable, as they approached an asymptotic curve de®ned uniquely by the isoprene and OH concentrations and the reaction time. Hence ®tting of the experimental data was not very sensitive at high O2 concentration. The bimolecular rate constant of the reaction of the OH±isoprene adduct with O2 used in ®tting all the data was 7 10 13 cm3 molecule 1 s 1 . Similar experiments were performed under dierent conditions (i.e., dierent pressures and initial reactant concentrations, etc.). The average value is 7 3 10 13 cm3 molecule 1 s 1 for reaction (2), derived from the kinetic data of the peroxy radical formation. The error is indicative of the scatter in the data at the one standard deviation level. We estimated a systematic error of about 50% for this reaction in the present data. The source of errors included uncertainty associated with detection and modeling of the OH±O2 ±isoprene peroxy radical, in addition to experimental
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D. Zhang et al. / Chemical Physics Letters 343 (2001) 49±54
uncertainties such as in the measurements of gas ¯ows, temperature, and pressure and in the ¯ow considerations. There are several con¯icting rate constants for O2 addition to the OH±isoprene adduct previously reported. Zetzsch and co-workers reported a value of 2 10 12 cm3 molecule 1 s 1 , based on OH cycling experiments [14]. A recent measurement of this reaction using CIMS determined a rate constant of 2:8 0:7 10 15 cm3 molecule 1 s 1 [6]. In the latter study, the kinetic data was obtained on the basis of the formation pro®le of the OH± isoprene adduct in the presence of O2 molecules, with the OH±isoprene adduct detected at its positive parent ion peak using the O 2 reagent ions. Another recent laboratory study also observed the OH±isoprene adduct using electron impact mass spectrometry at its parent mass peak (m=e 85) [15]; interestingly, there is evidence that the 85 peak did not deplete signi®cantly in the presence of high O2 concentrations [16]. It is possible that the observation of the OH±isoprene adduct signal using positive ions could be interfered by decomposition of the peroxy radical cations, which has been observed previously for other peroxy radical cations [17,18]. Our present results by directly monitoring the peroxy radical yield a rate constant of 7 3 10 13 cm3 molecule 1 s 1 . This rate is consistent with that reported in [14] and the effective value derived based on our previous theoretical prediction as shown in Fig. 3 [13], considering the respective uncertainties. Acknowledgements This work was partially supported by the Robert A. Welch Foundation (A-1417) and
by the Texas Advanced Research Program (ARP).
References [1] J.H. Seinfeld, S.N. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, Wiley, New York, 1997. [2] R. Akinson, J. Phys. Chem. Ref. Data Monogr. 2 (1994) 1. [3] R.A. Rasmussen, M.A. Khalil, J. Geophys. Res. 93 (1988) 1417. [4] M. Trainer, E.J. Williams, D.D. Parrish, M.P. Buhr, E.J. Allwine, H.H. Westberg, F.C. Fehsenfeld, S.C. Liu, Nature 329 (1987) 705. [5] W.L. Chameides, R.W. Lindsay, J. Richardson, C.S. Kiang, Science 241 (1988) 1473. [6] R. Zhang, I. Suh, A. Clinkenbeard, W. Lei, S.W. North, J. Geophys. Res. 105 (2000) 24627. [7] M.E. Jenkin, A.A. Boyd, R. Lesclaux, J. Atmos. Chem. 29 (1998) 267. [8] I. Suh, R. Zhang, J. Phys. Chem. 104 (2000) 6594. [9] R. Zhang, L.T. Molina, M.J. Molina, Rev. Sci. Instrum. 69 (1998) 4002. [10] R. Zhang, W. Lei, L.T. Molina, M.J. Molina, Int. J. Mass Spectrom. Ion Proc. 194 (2000) 41. [11] W.B. DeMore, S.P. Sander, C.J. Howard, A.R. Ravishankara, D.M. Golden, C.E. Kolb, R.F. Hampson, M.J. Kurylo, M.J. Molina, Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling, JPL Publication 97-4, NASA, 1997. [12] R. Zhang, W. Lei, J. Chem. Phys. 113 (2000) 8574. [13] W. Lei, R. Zhang, W.S. McGivern, A. Derecskei-Kovacs, S.W. North, J. Phys. Chem. 105 (2001) 471. [14] R. Koch, M. Siese, C. Fittschen, C. Zetzsch, in: A Contribution to the EUROTRAC Subproject LACTOZ, 1995, p. 268. [15] P.S. Stevens, E. Seymour, Z. Li, J. Phys. Chem. 104 (2000) 5989. [16] Z. Li, private communications, 2001. [17] P.W. Villalta, L.G. Huey, C.J. Howard, J. Phys. Chem. 99 (1995) 12829. [18] K.W. Scholtens, B.M. Messer, C.D. Cappa, M.J. Elrod, J. Phys. Chem. 103 (1999) 4378.
Chemical Physics Letters 343 (2001) 49±54
www.elsevier.com/locate/cplett
Experimental study of hydroxyalkyl peroxy radicals from OH-initiated reactions of isoprene Dan Zhang a, Renyi Zhang a,*, Clark Church b, Simon W. North b b
a Department of Atmospheric Sciences, Texas A&M University, College Station, TX 77843, USA Department of Chemistry, Texas A&M University, P.O. Box 300012, College Station, TX 77842, USA
Received 2 March 2001; in ®nal form 30 April 2001
Abstract Despite the importance of hydroxy±isoprene peroxy radicals in cycling of NO to NO2 , in partitioning of the ®nal products, and in propagation of the oxidation reactions, very few experimental studies involving these radicals as reactants have been carried out. Using a fast-¯ow reactor coupled to chemical ionization mass spectrometry (CIMS) detection, we have generated and detected the OH±O2 ±isoprene peroxy radical. By directly monitoring the peroxy radical we have obtained an overall rate constant of 7 3 10 13 cm3 molecule 1 s 1 for the reaction of the OH± isoprene adduct with O2 . Ó 2001 Published by Elsevier Science B.V.
1. Introduction Photochemical oxidation of atmospheric volatile organic compounds (VOCs) results in ozone and secondary organic aerosol formation, with major implications for local and regional air quality and global environmental changes [1,2]. Isoprene (2-methyl-1,3-butadiene, CH2 @C CH3 CH@CH2 ) is one of the most abundant hydrocarbons naturally emitted by the terrestrial biosphere, with a global averaged production rate of about 450 Tg yr 1 [3]. Due to its high chemical reactivity and proliferation in the generation of organic peroxy radicals, isoprene plays an important role in ozone formation in local and regional atmosphere [4,5]. Atmospheric oxidation of isoprene is initiated by reactions with a variety of oxidant species. Since isoprene is mainly produced *
Corresponding author. Fax: +1-979-862-4466. E-mail address: [email protected] (R. Zhang).
in the daytime, the reaction with hydroxyl radial OH is the dominant tropospheric removal pathway. The initial reaction between isoprene and OH proceeds mainly by OH addition to the C@C double bond, forming four OH±isoprene adduct isomers OH C5 H8 ! C5 H8 OH
1
Under atmospheric conditions, the hydroxyalkyl radical reacts primarily with oxygen molecules to form the hydroxyalkyl peroxy radicals C5 H8 OH O2 ! C5 H8 OHO2
2
Addition of O2 occurs only at the carbons b to the OH position for the OH±isoprene adducts of internal additions, but takes places at two centers (b or d to the OH position) for the OH±isoprene adducts of terminal additions. Hence O2 addition to the OH±isoprene adduct leads to the formation of four b- and two d-hydroxyalkyl peroxy radicals. The hydroxyalkyl peroxy radicals can further react
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 6 5 4 - 6
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D. Zhang et al. / Chemical Physics Letters 343 (2001) 49±54
with NO to form six (b or d) hydroxyalkyl alkoxy radicals. Hence the organic intermediate radicals play a key role in propagation or termination of the oxidation process, and it is of great importance to study the organic intermediate radicals. Numerous laboratory studies have been conducted to investigate the kinetics and mechanism of the oxidation reactions of isoprene initiated by OH [6]. A majority of the previous studies either investigated the initial step of the OH±isoprene reactions or analyzed the ®nal reaction products. Direct experimental data concerning intermediate processes of the OH±isoprene oxidation reactions are very limited. Jenkin et al. investigated the permutation reactions of the OH±isoprene peroxy radicals using the laser ¯ash photolysis/UV absorption spectrometry technique [7]. In this Letter, we present an experimental study of the hydroxyalkyl peroxy radicals arising from the OH-initiated reaction of isoprene. The OH± O2 ±isoprene peroxy radical has been generated and detected using a fast-¯ow reactor coupled to chemical ionization mass spectrometry (CIMS) detection. We also report kinetic measurements of the reaction of OH±isoprene adduct with O2 based on direct observation of the peroxy radicals. 2. Experimental A fast-¯ow reactor, in conjunction with CIMS detection, was used in the present experiments. The experimental setup is similar to that used by us previously, and a detailed description of the experimental apparatus has been given elsewhere [6,8]. Brie¯y, it consists of a ¯ow reactor of 1.23 cm i.d. and 80 cm in length. All surfaces exposed to the reactants and products were coated with a halocarbon wax. A ¯ow of He carrier gas (in the range of 1 to 3 STP l min 1 ) was injected to the ¯ow reactor through an entrance port in the rear of the ¯ow reactor. The pressure of the ¯ow reactor was regulated between 1 and 2 Torr and the temperature of the ¯ow reactor was maintained at 298 2 K. Typical ¯ow velocity in the ¯ow reactor ranged from 1300 to 2500 cm s 1 . Isoprene was added to the ¯ow reactor through a movable injector. The ¯ow reactor was operated under the
laminar ¯ow condition with Reynolds number Re 2auq=l typically in the range 10±40, where a is the internal radius of the ¯ow reactor, q the density of the gas, u the ¯ow velocity, and l the viscosity coecient of the gas. Under our experimental conditions, a laminar ¯ow was fully developed and homogeneous mixing of both OH and isoprene was eectively achieved [6,8]. Reactants and products of the OH±isoprene reaction were detected by CIMS using either positive or negative reagent ions. The CIMS employed a new approach, involving an electrostatic ion guide recently developed [9,10]. Positive or negative reagent ions were initiated using corona discharge at a high voltage ()5 kV). The SF6 reagent ions were generated by adding a small amount of SF6 to a He carrier ¯ow (about 1±2 slpm at STP) through the discharge. The positive reagent ions O 2 were produced by passing the He carrier ¯ow through the discharge and then adding a small amount of O2 downstream. An electrostatic ion guide was used to transport ions to the quadrupole mass analyzer. The use of the ion guide allows for ion transportation with a high eciency and preferential separation and removal of neutral molecules in a dierential pumping system [9,10]. OH radicals were generated in situ according to the reaction H NO2 ! NO OH k2 1:3 10 10 cm3 molecule 1 s 1 [11]. Hydrogen atoms were generated by passing a small ¯ow of 1% H2 / He mixture (about 0:5 cm3 min 1 at STP) through a microwave discharge followed by addition of an excess of 3% NO2 =N2 mixture. OH was detected in the negative ion mode using SF6 as the reagent ions, according to the ion±molecule reaction OH SF6 ! OH SF6 . Two methods were employed to calibrate OH concentrations in the ¯ow reactor. First, the OH concentration was obtained by comparing the relative signal intensities between OH and NO2 and subsequent calibration of NO2 in the CIMS, using the ion± molecule reaction rate constants of SF6 with OH and NO2 [6]. Alternatively, OH was calibrated by converting all of the OH into HNO3 followed by HNO3 calibration in the mass spectrometer. Typically, the initial concentrations of OH in the ¯ow reactor were in the range of 1 109 to 6 109 molecule cm 3 .
D. Zhang et al. / Chemical Physics Letters 343 (2001) 49±54
51
Commercially available isoprene (Aldrich 99.5%) was used to volumetrically prepare 2 l glass bulb of a 1% isoprene/He mixture and introduced into the ¯ow reactor using a 10 sccm ¯ow meter. The concentrations of isoprene in the ¯ow reactor were regulated in the range 5 1011 ±5 1012 molecule cm 3 , so that the isoprene=OH ratio was at least a factor of 100 to ensure the pseudo®rst-order kinetic assumption. Isoprene was detected by the CIMS using positive reagent ions (O 2 ) at its parent ion peak. For both OH and isoprene the mass spectrometer signals were linear over the range of the concentrations used. To generate the OH±isoprene peroxy radical, OH, isoprene, and O2 were simultaneously introduced into the ¯ow reactor. O2 was added into the ¯ow reactor along with the He carrier gas. The production of the peroxy radicals was regulated by the initial concentrations of all three reactants, as well as the reaction distance. The peroxy radical was detected using the SF6 negative reagent ions according to the ion±molecular reaction: SF6 C5 H8 OHO2 ! SF6 C5 H8 OHO2
3
3. Results and discussion Fig. 1 shows time evolution of the 117 signal (corresponding to C5 H8 OHO2 for three experiments performed at 298 K and 1 Torr. Several procedures were taken to positively verify that the ions detected at m=e 117 were indeed attributable to the isoprene±OH±O2 radical, rather than from secondary ion±molecule reactions. We monitored the signal at mass 117 when either the isoprene or O2 ¯ow was stopped or when the microwave discharge for dissociating hydrogen molecules ceased. In Fig. 1a, the isoprene ¯ow was terminated at about 30 s, resulting in a disappearance of the signal at 117. The signal resumed at 140 s when isoprene was re-admitted to the ¯ow tube. In Fig. 1b, termination of the O2 ¯ow at about 70 s also led to a sharp drop in the mass 117 signal, until the O2 ¯ow was re-established at 170 s. At about 250 s, the mass 117 signal disappeared when the microwave discharge was switched o, and the signal recovered at 320 s when the discharge was turned
Fig. 1. Temporal variation of the C5 H8 OHO2 (m/e 117) and OH (m/e 17) signals (see text). Experimental conditions are: P 1:0 Torr, U 1799 cm s 1 , C5 H8 o 7:0 1011 molecule cm 3 , O2 o 5:0 1014 molecule cm 3 , and OHo 5:0 109 molecule cm 3 .
on. Fig. 1c shows time evolution of the signals at both 117 and 17 (corresponding to OH ) masses. The movable injector was pulled 10 in. upstream at 70 s and returned to its original position at 170 s. It is evident that the OH signal dropped from its initial steady-state value when reaction distance was increased, and the OH decrease was accompanied by an increase in the 117 signal. Hence those above procedures clearly illustrate that the mass peak at 117 is related to the presence of all three reactants in the ¯ow reactor. In addition, it is unlikely that the mass peak at 117 arose from the ion±molecule reaction of O2 with the OH±isoprene adduct, which also presented in the ¯ow reactor [12]. We did not observe any O2 signal in the presence of high concentrations of O2 in the ¯ow reactor, because O2 has a smaller electron
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D. Zhang et al. / Chemical Physics Letters 343 (2001) 49±54
anity (0.44 eV) than that of SF6 . Note that the mass spectrometer is sensitive only to the mass of ions that are detected and does not discriminate between isomers. The kinetics of the O2 addition to the four OH± isoprene adduct isomers is complex and the peroxy radical appearance curves will exhibit multi-exponential behavior. Each of the four OH±isoprene adduct precursors is formed with a single time constant given by the sum of the individual rate constants. The isomeric branching ratios are given by relative rate constants which have been calculated in our previous work [13]. The four OH± isoprene adduct isomers react with O2 , each with its own characteristic rate constant. Two of the OH±isoprene adducts, corresponding to terminal OH addition, each can form two distinct peroxy isomers resulting in two pairs of curves with exhibit time constants which re¯ect the sum of two O2 addition rate constants. As a consequence, the appearance curves for the O2 ±OH±isoprene adducts will consist of the sum of six appearance curves, four of which are unique, each weighted by the branching ratio determined by both the OH and O2 addition rate constants. Although the kinetics appear intractable, simulations suggest that the overall appearance curves of the peroxy radicals can be modeled using a single eective rate constant. Fig. 2 shows the results of kinetic simulations based on rate constants previously calculated for both the addition of OH to isoprene and the subsequent formation of the six distinct peroxy radical isomers [13]. The upper panel shows the appearance curves for each peroxy isomer, which could be measured if the detection scheme permitted one to distinguish between isomers. As mentioned previously, the CIMS technique does not permit selective detection, and the experimental peroxy appearance curve represents a weighted sum of these curves. The lower panel of Fig. 2 illustrates that this sum can be modeled using a single eective rate constant, with a value of 1:8 10 12 cm3 molecule 1 s 1 . We studied the evolution of the signal at mass 117, when the injector was successively pulled upstream. Fig. 3 shows that the mass 117 signal rises in accordance with OH disappearance. Under our experimental conditions regeneration of OH
Fig. 2. Simulations of peroxy radical isomer formation as a function of reaction time. The upper panel shows simulated time-dependent curves for the six distinct peroxy isomers (A±F) based on the rate constants calculated in [13]. The lower panel shows the sum of the six curves individual curves (circles) and the best eective rate constant for O2 addition to the OH±isoprene adducts (straight line).
Fig. 3. Variation of OH±O2 ±isoprene peroxy radical and OH signals as a function of reaction time. The curves are ®tting through the experimental data using a kinetic program (see text). Experimental conditions are: P 1:9 Torr, U 2287 cm s 1 , C5 H8 o 1:6 1012 molecule cm 3 , O2 o 2:7 1014 molecule cm 3 , and OHo 5:0 109 molecule cm 3 .
D. Zhang et al. / Chemical Physics Letters 343 (2001) 49±54
from subsequent reactions of the peroxy radicals was not important, due to the relatively low NO2 concentration used in our experiments (<1011 molecule cm 3 ). This was veri®ed as the OH signal followed the pseudo-®rst-order exponential decay. The OH decay data (solid curve) led to an eective bimolecular rate of 9:1 10 11 cm3 molecule 1 s 1 for reaction (1). The observed formation pro®le of the OH±O2 ±isoprene peroxy radical was simulated with a computer kinetic model that included reactions (1) and (2), along with other likely secondary reactions [6]. The model input included the initial concentrations of OH, isoprene, O2 , and all other precursors. The rate constant for reaction (1) was obtained from the pseudo-®rst-order decay of OH at various isoprene concentrations, and the value is consistent with the previous measurements [6]. The rate constant for reaction (2) was varied to best ®t the experimental data. Other related rate constants used in the simulations were taken from the recommendations of Atkinson [2] and DeMore et al. [11]. We did not include the decomposition of the peroxy radicals, since the decomposition rates were negligible compared to the formation rates of the peroxy radicals according to our theoretical calculations [13]. The dashed line in Fig. 3 represents the best ®t to the observed production of the peroxy radical. An eective bimolecular rate constant of 7 10 13 cm3 molecule 1 s 1 was inferred for reaction (2). For the data shown in Fig. 3, ®tting to experimental data was sensitive to the rate constant of reaction (2) within 10% for a 95% con®dence level. We did not perform kinetic measurements at very high concentrations of isoprene to drive reaction (1) into completion, since at high isoprene concentrations several anomalous mass peaks appeared in the negative spectra which could interfere with the detection scheme for the peroxy radicals. Fig. 4 shows formation of the peroxy radicals at O2 concentrations from 1:1 1014 to 8:5 1014 molecule cm 3 . The initial concentrations of OH and isoprene were 5:0 109 and 5:0 1012 molecule cm 3 , respectively. Fig. 4 illustrates that the measured signal of the peroxy radical intensi®es at a higher O2 concentration, re¯ecting a larger production of the peroxy radical
53
Fig. 4. Variation of OH±O2 ±isoprene peroxy radical signals as a function of reaction time. The curves are ®tting through the experimental data using a kinetic program (see text). The different symbols correspond to dierent O2 concentrations (1014 molecule cm 3 ): solid circles, 1.1; solid diamonds, 2.3; open circles, 3.3; down-triangles, 4.3; open squares, 6.4; open diamonds, 8.5. Experimental conditions are: P 1:7 Torr, U 2274 cm s 1 , C5 H8 o 1:0 1012 molecule cm 3 , and OHo 5:0 109 molecule cm 3 .
at such a condition. At high O2 concentrations the formation pro®les of the peroxy radicals were essentially non-distinguishable, as they approached an asymptotic curve de®ned uniquely by the isoprene and OH concentrations and the reaction time. Hence ®tting of the experimental data was not very sensitive at high O2 concentration. The bimolecular rate constant of the reaction of the OH±isoprene adduct with O2 used in ®tting all the data was 7 10 13 cm3 molecule 1 s 1 . Similar experiments were performed under dierent conditions (i.e., dierent pressures and initial reactant concentrations, etc.). The average value is 7 3 10 13 cm3 molecule 1 s 1 for reaction (2), derived from the kinetic data of the peroxy radical formation. The error is indicative of the scatter in the data at the one standard deviation level. We estimated a systematic error of about 50% for this reaction in the present data. The source of errors included uncertainty associated with detection and modeling of the OH±O2 ±isoprene peroxy radical, in addition to experimental
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D. Zhang et al. / Chemical Physics Letters 343 (2001) 49±54
uncertainties such as in the measurements of gas ¯ows, temperature, and pressure and in the ¯ow considerations. There are several con¯icting rate constants for O2 addition to the OH±isoprene adduct previously reported. Zetzsch and co-workers reported a value of 2 10 12 cm3 molecule 1 s 1 , based on OH cycling experiments [14]. A recent measurement of this reaction using CIMS determined a rate constant of 2:8 0:7 10 15 cm3 molecule 1 s 1 [6]. In the latter study, the kinetic data was obtained on the basis of the formation pro®le of the OH± isoprene adduct in the presence of O2 molecules, with the OH±isoprene adduct detected at its positive parent ion peak using the O 2 reagent ions. Another recent laboratory study also observed the OH±isoprene adduct using electron impact mass spectrometry at its parent mass peak (m=e 85) [15]; interestingly, there is evidence that the 85 peak did not deplete signi®cantly in the presence of high O2 concentrations [16]. It is possible that the observation of the OH±isoprene adduct signal using positive ions could be interfered by decomposition of the peroxy radical cations, which has been observed previously for other peroxy radical cations [17,18]. Our present results by directly monitoring the peroxy radical yield a rate constant of 7 3 10 13 cm3 molecule 1 s 1 . This rate is consistent with that reported in [14] and the effective value derived based on our previous theoretical prediction as shown in Fig. 3 [13], considering the respective uncertainties. Acknowledgements This work was partially supported by the Robert A. Welch Foundation (A-1417) and
by the Texas Advanced Research Program (ARP).
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