Environ. Sci. Technol. 1996, 30, 301-306
System To Measure Relative Rate Constants of Semivolatile Organic Compounds with Hydroxyl Radicals PHILIP N. ANDERSON AND RONALD A. HITES* School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
A system to determine rate constants for the reaction of the hydroxyl radical with semivolatile organic compounds under tropospheric conditions is described. The system incorporates on-line mass spectrometric detection to improve experimental time resolution and to minimize sampling volume. The reaction apparatus is heated to provide detectable vapor phase levels of semivolatile compounds without sample preconcentration. Temperature control of the system also allows investigation of the temperature dependence of OH-organic reactions. OH radicals are generated by the photolysis of O3 in the presence of water vapor to avoid the wall effects associated with the more commonly used alkyl nitrite photolysis. The temperature dependence of the OH-toluene reaction is investigated from 276 to 397 K in 1 atm of He and from 296 to 363 K in 1 atm of N2/O2. In He, the OH-toluene reaction rate constant at 298 K is (5.9 ( 1.5) × 10-12 cm3 s-1, and in N2/O2, the rate constant at 298 K is (5.8 ( 1.5) × 10-12 cm3 s-1. Rate constants for the reaction of OH with 4,4′-dichlorobiphenyl from 329 to 366 K are also reported. Over this temperature range, the 4,4′-dichlorobiphenyl reaction with OH is only weakly temperature dependent. From the experimentally determined temperature dependence, the OH rate constant at 298 K of 4,4′dichlorobiphenyl is (2.0 ( 0.5) × 10-12 cm3 s-1.
Introduction Because most organic compounds are primarily removed from the troposphere by reactions with the OH radical (1), substantial effort has been spent in determining rate constants for reactions of OH with numerous organic compounds (1, 2). Nearly all of this research has focused on volatile organic compounds (those with vapor pressures greater than about 10-2 Torr), but semivolatile organic compounds (SOCs), such as the polychlorinated biphenyls, dibenzo-p-dioxins, and dibenzofurans, are also important atmospheric pollutants (3-12). Despite their low vapor pressures (from 10-8 to 10-2 Torr), many SOCs are present * Corresponding author e-mail address:
[email protected].
0013-936X/96/0930-0301$12.00/0
1995 American Chemical Society
in the atmosphere primarily in the vapor phase (13, 14), and as a result, they can react with OH. Nevertheless, these reactions have received much less attention than those with volatile organic compounds, mostly because the lower volatility of SOCs significantly complicates the kinetic experiments. The technology that has been developed for measuring OH reaction rate constants typically uses large Teflon chambers operating at room temperature (1). These chambers have been used for two reasons. First, for many studies, the organic compound concentrations have been followed by periodic withdrawal of chamber aliquots and subsequent gas chromatographic (GC) analysis. With large chambers, the dilution factor associated with sampling is minimized. Second, the problem of “wall effects” is blunted by using a relatively nonreactive chamber surface such as Teflon. Increasing the ratio of chamber volume to wall surface area, achieved by using large chambers, also seems to help reduce observed wall effects. The issue of wall effects deserves further comment: In many of these chamber studies, the source of OH has been the photolysis of mixtures of synthetic air containing alkyl nitrites and NO. The wall effects seen in these studies appear to be associated with the complicated NOx chemistry associated with these OH production schemes (15). Although Teflon chambers seem to minimize these wall effects, the hydrophobic Teflon surface can cause problems when studying SOCs. For example, preliminary work in our laboratory showed that SOCs injected into a Teflon chamber partitioned almost completely to the chamber walls. Clearly, a method of generating OH radicals that was less sensitive to the chamber’s wall surface was necessary; we decided to use the photolysis of ozone in the presence of H2 or H2O to generate OH (16-18):
O3 + hν (λ < 320 nm) f O(1D) + O2
(1)
O(1D) + H2O f 2OH
(2)
This system mimics the natural atmospheric source of OH and avoids the complicated chemistry (and presumably wall effects) associated with the alkyl nitrite system. We report here a method for OH-SOC reaction rate constant determination in which OH is generated by ozone photolysis and subsequent reaction with water in a quartz reactor. Different diluent gases, at varying pressures, can be used in the system to mimic natural atmospheric conditions or to probe reaction details. To raise SOC vapor pressures to detectable concentrations, temperatures above ambient are required, and thus, the system is temperaturecontrolled to permit experiments over a wide range of temperatures. Our system incorporates continuous online mass spectrometric detection to minimize sampling perturbations to the reaction system and to improve the time resolution of the rate experiments, which in the past has frequently been limited by analytical throughput. Using on-line mass spectrometry, the time resolution is adequate to permit the OH rate constant to be determined even if the organic compound reacts with OH and ozone at comparable rates. Several features of our system are advantageous for OH-SOC relative rate experiments. SOC
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FIGURE 1. Reaction system schematic. Symbols: N2, ultra-highpurity nitrogen cylinder (in He fill gas experiments, this cylinder was replaced with one containing UHP He); O2, ozone supply oxygen cylinder; F, calibrated flow meter; GW, gas washing bottle containing HPLC grade H2O; IP, capillary injection port; OG, ozone generator; MS, Hewlett-Packard 5995A mass spectrometer; FS, 100-µm i.d. fused silica capillaries; CF, vacuum UV cutoff filter; UV, low-pressure Pen-ray mercury vapor lamp.
wall adsorption should be minimal with the elevated temperatures and the less hydrophobic wall surface. The increased time resolution of on-line MS sampling coupled with high [OH] generated by O3 photolysis allows rate experiments to be completed in a few minutes rather than a few hours for experiments using GC analysis. Even if wall adsorption of the SOC does take place, re-equilibration with the gas-phase SOCs, which interferes in the slower GC-based experiments, will not happen in the short time scale of our experiments. To characterize our system and to compare results with previously reported values, we have investigated the temperature dependence of the toluene-OH reaction rate in 1 atm of helium from 275 to 397 K and in 1 atm of nitrogen/oxygen mixtures from 296 to 363 K. We have also determined rate constants for the OH reaction with 4,4′dichlorobiphenyl from 329 to 366 K to demonstrate the utility of the technique for semivolatile compounds. These kinetic experiments all used the relative rate method (1) with cyclohexane as the reference compound.
Experimental Section Ultra-high-purity (UHP) helium (99.999%, Liquid Carbonic) and UHP nitrogen (99.999%, Liquid Carbonic) were taken directly from cylinders. Research-grade oxygen (99.998%, Air Products), used to produce ozone, was also taken directly from a cylinder. The solvent and hydrocarbons were 99.9% pure and were from the following sources: carbon tetrachloride (Aldrich), toluene (EM Science), cyclohexane (EM Science), and 4,4'-dichlorobiphenyl (Lancaster). HPLCgrade water (EM Science) was used to saturate the diluent gas with water. Figure 1 is a schematic diagram of the reaction system. All flows were measured by calibrated flowmeters. The reaction chamber is a cylindrical quartz chamber, approximately 5 cm in diameter and 9 cm long, with a volume of 195 mL. The chamber has ports at either end for throughchamber flow of the diluent gas and ozonized oxygen. A central port accommodates organic compound introduc-
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tion and sampling. The center of the chamber is continuously sampled through a 100-µm i.d. fused silica capillary, approximately 1.9 m long, into the ion source of a HewlettPackard (HP) 5995A mass spectrometer. The flow rate from the reaction chamber (at about 740 Torr pressure) into the electron impact ion source (at about 10-5 Torr) is approximately 1 mL/min. To allow temperature control of the reaction chamber, it is located in a gas chromatographic (GC) oven attached to the mass spectrometer. Compounds are introduced into the chamber from the GC’s splitless capillary injector through a short length (∼35 cm) of 100µm i.d. fused silica tubing. About 250 µL/min diluent gas flows through this injection system. Both the compound introduction and MS sampling capillaries are heated to minimize adsorption of the organic compounds on the tubing walls. Ozone is produced in a metered flow of oxygen, which is passed through a Pyrex high-voltage discharge tube. A 12-kV potential is generated by a neon-sign transformer; this ozonizer voltage is adjustable by a rheostat placed in the transformer line supply. The O3/O2 mixture is carried to the reaction chamber through 0.8-mm i.d. Teflon tubing. The ozone yield at various O2 flow rates and at various voltage settings was calibrated by measurement of the ozone concentration by neutral buffer KI colorimetry (19). The nitrogen/oxygen or helium diluent gas is saturated with water vapor at room temperature by passing it through a gas washing bottle containing HPLC-grade H2O prior to introduction into the chamber. OH radicals are generated by irradiation of the reaction mixture with a Pen-ray lowpressure Hg lamp (UVP Products) through a quartz window and vacuum UV cutoff filter (Melles-Griot) in the GC oven door. The UV lamp produces 300 mW/cm2 at 8 cm, approximately the distance to the center of the chamber. The UV cutoff filter, which completely absorbs the weak 185-nm line in the mercury spectrum, ensures that no Cl atoms are produced from photolysis of CCl4, the OH-inert solvent used for the introduction of organic compounds. This lack of photolysis was confirmed by monitoring m/z 117 (CCl3+) during photolysis control experiments; no change was observed during irradiation of the chamber. Prior to each experiment, diluent gas and ozonized oxygen were flowed through the chamber for at least 45 min. At the onset of an experiment, the chamber was isolated from all gas flows, and the mass spectrometer was set to monitor several masses to establish a signal baseline. After 1-5 min, 1-4 µL of a toluene, cyclohexane, or 4,4′dichlorobiphenyl solution in carbon tetrachloride was injected. The solution concentration was typically 6 µg/ µL; thus, an injection gave an organic compound concentration in the chamber from 2 × 1014 to 9 × 1014 cm-3. O3 concentrations varied from 5 × 1012 to 8 × 1014 cm-3. After the signal for each compound stabilized, the UV lamp was turned on for 1-5 min. After the lamp was turned off, the signal stabilized at a lower level, and after another period, the chamber was flushed with diluent gas. The baseline signal was again measured to correct for instrument drift. Normally, the instrumental drift on the time scale of a typical experiment was less than the instrumental noise. The mass spectrometer was operated in the selected ion monitoring mode (SIM), and it measured the two or three most abundant ions for each compound and other masses for blank corrections. Monitoring times for each mass were adjusted to provide between 10 and 20 SIM cycles per minute.
Assuming that reaction with OH is the only significant loss process for both the analyte (toluene or 4,4′-dichlorobiphenyl) and cyclohexane while the UV lamp is lit:
-d[analyte] ) k1[analyte][OH] dt
(3)
-d[cyclohexane] ) k2[cyclohexane][OH] dt
(4)
Because both organics are subject to OH attack simultaneously, eqs 3 and 4 can be combined and integrated to give
ln
(
)
[analyte]0 [analyte]t
)
(
)
k1 [cyclohexane]0 ln k2 [cyclohexane]t
(5)
where the concentrations of the organics in the equation are measured at time t ) 0 and at succeeding times t. Clearly, a plot of ln ([analyte]0/[analyte]t) versus ln ([cyclohexane]0/ [cyclohexane]t) will have a slope with the ratio of rate constants, k1/k2. Data files for each experiment were uploaded from the mass spectrometer Chemstation computer to the laboratory’s PC network and converted to HP Environquant data file format. The SIM cycle times, masses, and raw signal values for each mass were extracted and stored in an ASCII file. The text file was converted to a spreadsheet in Microsoft Excel. In Excel, after the background signal was subtracted for the masses corresponding to the analyte and reference compounds, the regression line of the signal values in the region immediately preceding the OH reaction was calculated. To correct for any reaction with ozone as well as the small dilution due to flow into the chamber from the injection port and removal of compounds by mass spectrometer sampling, the concentration value for the initial time (numerator in the logarithm terms of eq 5) for each time t was calculated from the regression for each compound. The results were plotted according to eq 5, and a linear regression was used to obtain the ratio of the rate constants from the plot’s slope. The rate constant ratio was converted to an analyte’s rate constant by multiplying the ratio by k2, the known cyclohexane rate constant. In our study, we calculated k2 for each experimental temperature from the temperature-dependent OH-cyclohexane rate expression recommended by Atkinson (1):
k(cyclohexane) ) +0.85 (2.66-0.65 ) × 10-17 T2 e(344(95)/T cm3 s-1 (6)
Results and Discussion Toluene. A plot of a typical experiment’s raw data is shown in Figure 2; the two traces show the natural logarithm of the background corrected signals for m/z 56 and 91, the most abundant ions in the mass spectra of cyclohexane and toluene, respectively. After compound injection at 1.2 min, each ion signal stabilizes in about 3 min, and as expected, minimal reaction of the organics with O3 is observed (see region A). When the UV lamp is turned on at 10.2 min, immediate production and subsequent attack of OH on the organics is evident (see region B). The reactions are clearly first order; note the straight line behavior in region B. After the lamp is turned off at 11.6 min, OH production halts and loss of the organic compounds ceases (see region C).
FIGURE 2. Plot of natural logarithm of signal intensity versus time for toluene/cyclohexane relative rate experiment.
Toluene absorbs radiation in the mid-UV range, but the lamp flux was sufficiently low such that direct photolysis was negligible. To confirm this expectation, we performed control experiments in which all reagents except ozone were present. In the control experiments, no change in the measured level of either organic was detected when the UV lamp was turned on. The reactions of other species with the organic compounds are unimportant compared to the OH-organic reactions. No rates have been reported for the reaction of O(1D) with alkanes or aromatics having more than three carbons, but these reactions clearly will be fast, as is the reaction of O(1D) with H2O (2.2 × 10-10 cm3/s) (20). However, even if the O(1D)-organic reactions occur at the collision rate (i.e., rate constants about 4 × 10-10 cm3 s-1), the much larger concentration of H2O (1019 cm-3) vs the organic concentrations (1015 cm-3) ensures that reaction of O(1D) with the organic compounds is minimal. O(1D) also reacts rapidly with CCl4, the solvent used in our experiments, to produce CCl3 and ClO (k ) 3 × 10-10 cm3 s-1) (21). However, at our typical concentrations of CCl4 of 6 × 1016 cm-3, the ratio of [OH] to [ClO] exceeds 20. Thus, reactions with CCl3 and ClO are clearly minor pathways, and we do not expect either radical to react at appreciable rates with the organics of interest. Secondary products of the O(1D)-CCl4 reaction would be present at even lower concentrations, and if these products reacted significantly, curvature in the OH reaction portion of the experiments (region B of Figure 2) would be present. Since no curvature was observed in any of the experiments, we are not concerned about these reactions either. In N2/O2 diluent gas, a significant fraction of O(1D) will be relaxed to the ground state O(3P), which will also react with toluene and cyclohexane. O(3P) reacts with O2 about 10 times slower than with cyclohexane, toluene, or by analogy, 4,4′dichlorobiphenyl (22, 23). However, the concentration of O2 present in the system was always more than 100-fold greater than the highest organic concentration present. Thus, the predominant fate of O(3P) atoms in the chamber is reactions with molecular oxygen to reform ozone. This assumption was confirmed by varying the level of O2 present in the chamber in the N2/O2 experiments from 0.4% to 95%; the measured rate constants varied randomly (RSD < 5%), not systematically, with the concentration of O2. The OH radical concentrations in the chamber at the beginning of the O3 photolysis region can be estimated
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TABLE 1
Experimental Rate Constant Ratios and Rate Constants for Reaction of Toluene with OH Radicals in Helium
FIGURE 3. Typical plot of ln ([toluene]0/[toluene]t) versus ln ([cyclohexane]0/[cyclohexane]t). The slope of this plot is k1/k2, and the intercept is equal to 0, within experimental error.
from the known concentration of cyclohexane (or toluene) in the chamber, the known rate constant of cyclohexane (or toluene), and the decay rate of the organic at the beginning of the OH-generating region. In experiments at 293 and 303 K with He diluent gas and O3 concentrations of approximately 2.5 × 1013 cm-3, we calculate [OH] to be about 1.6 × 109 cm-3. With N2/O2 diluent gas, [OH] is lower. This is as expected since N2 and O2 will collisionally relax a significant fraction of the O(1D) to O(3P). At 297 K and [O3] of about 5.0 × 1013 cm-3, the calculated [OH] is about 3.5 × 108 cm-3 in N2/O2. In relative rate studies, which generate OH by photolysis of alkyl nitrites, [OH] has been typically lower, ranging from 107 to 3 × 108 cm-3 (24, 25). Since we expect SOC-OH reaction rates to decrease as molecular weights increase, the higher [OH] provides an advantage for performing rate determinations of less reactive compounds in relatively short experiments. The short experimental times preclude SOC re-equilibration from the chamber walls, as noted above. Additionally, slower OH reaction rates are accessible in our system because OH reacts much more rapidly with alkyl nitrites than with O3; the most commonly used methyl nitrite reacts with OH twice as fast as O3. The products of OH-initiated oxidation of O3 (O2 and H2O2) are also less reactive than those from OH attack on methyl nitrite (HCHO and NO) (17). Because OH is consumed by its precursor and products at a lower rate in the O3/H2O system than in alkyl nitrite photolysis, much slower rate constants can be measured by the O3/H2O scheme. A typical plot of the application of eq 5 to the toluene/ cyclohexane competitive reaction with OH is shown in Figure 3. Note that the regression is very well-defined (r ) 0.999). Table 1 summarizes the rate constant ratios measured in our experiments from 276 to 383 K in He diluent gas. The stated uncertainties in the ratios are 2 standard deviations of the slopes derived from eq 5 plots for each experiment. These uncertainties average about 2.5% of the rate ratios. Unfortunately, the uncertainty in the calculated cyclohexane rate constant is estimated to be (25% (2). Clearly, this uncertainty is greater than the uncertainty in the experimental rate constant ratio; therefore, the uncertainties in the toluene rates themselves reflect the underlying uncertainty in the cyclohexane rate constant used as a reference. The average value of the He diluent rate constants at 293 and 303 K is (5.9 ( 1.5) × 10-12 cm3 s-1, which is in excellent agreement with the 298 K value
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temp (K)
k(toluene)/ k(cyclohexane)a
rate constant k(toluene) (×10-12 cm3 s-1)b
276 285 285 293 303 310 323 323 323 324 324 344 363 371 383
0.84 ( 0.01 0.88 ( 0.01 0.84 ( 0.02 0.78 ( 0.01 0.79 ( 0.01 0.75 ( 0.01 0.71 ( 0.02 0.67 ( 0.02 0.66 ( 0.01 0.68 ( 0.02 0.66 ( 0.01 0.60 ( 0.02 0.54 ( 0.01 0.37 ( 0.02 0.31 ( 0.02
5.9 ( 1.5 6.3 ( 1.6 6.1 ( 1.5 5.8 ( 1.4 6.0 ( 1.5 5.8 ( 1.5 5.7 ( 1.4 5.4 ( 1.3 5.3 ( 1.3 5.5 ( 1.4 5.3 ( 1.3 5.1 ( 1.3 4.9 ( 1.2 3.4 ( 0.9 3.0 ( 0.7
a The error limits correspond to two least-square standard deviations of the rate constant ratio as calculated by eq 5. b Toluene rate constants calculated by multiplication of the rate constant ratio by the cyclohexane-OH rate constant calculated from the temperature-dependent rate expression recommended by Atkinson (2).
TABLE 2
Experimental Rate Constant Ratios and Rate Constants for Reaction of Toluene with OH Radicals in Nitrogen/Oxygen temp (K)
k(toluene)/ k(cyclohexane)a
rate constant k(toluene) (×10-12 cm3 s-1)b
296 297 297 297 323 323 323 323 324 363 363 363 363 363 363
0.81 ( 0.08 0.77 ( 0.03 0.76 ( 0.06 0.75 ( 0.03 0.63 ( 0.01 0.68 ( 0.02 0.61 ( 0.02 0.64 ( 0.01 0.60 ( 0.02 0.52 ( 0.01 0.51 ( 0.01 0.52 ( 0.03 0.51 ( 0.01 0.50 ( 0.11 0.50 ( 0.01
6.0 ( 1.5 5.7 ( 1.4 5.7 ( 1.4 5.6 ( 1.2 5.0 ( 1.2 5.4 ( 1.4 4.9 ( 1.2 5.1 ( 1.2 4.9 ( 1.2 4.7 ( 1.2 4.6 ( 1.2 4.7 ( 1.2 4.6 ( 1.2 4.5 ( 1.1 4.5 ( 1.1
a The error limits correspond to two least-square standard deviations of the rate constant ratio as calculated by eq 5. b Toluene rate constants calculated by multiplication of the rate constant ratio by the cyclohexane-OH rate constant calculated from the temperature-dependent rate expression recommended by Atkinson (2).
of 5.96 × 10-12 cm3 s-1 recommended by Atkinson (1). Table 2 lists the rate constant ratios measured in our experiments from 297 to 363 K in N2/O2. The average value of the N2/O2 diluent rate constant at 296 and 297 K is (5.8 ( 1.5) × 10-12 cm3 s-1, which is also in excellent agreement with the 298 K value of 5.96 × 10-12 cm3 s-1 recommended by Atkinson (1). Five studies that have investigated the temperature dependence of the OH-toluene reaction rate constant have been published (26-30). Four of the studies used absolute rate methods in which the decay of OH as a function of the concentration of toluene was followed to obtain the rate constant of interest. These studies were carried out under low-pressure conditions (typically less than 120 Torr),
TABLE 3
Experimental Rate Constant Ratios and Rate Constants for Reaction of 4,4′-Dichlorobiphenyl with OH Radicals in Nitrogen/Oxygen
FIGURE 4. Arrhenius plot of the OH-toluene reaction. Rate constants from this work: (b) nitrogen/oxygen diluent gas, (9) helium diluent gas. Rate constants reported in the literature: (O) rate constants determined using techniques similar to ours (1, 2, 30). (0) rate constants determined with an absolute rate technique in He or Ar at low pressures (
