Environ. Sci. Technol. 1997, 31, 2949-2953
Kinetics of the Reactions of Propionylperoxy Radicals with NO and NO2: Peroxypropionyl Nitrate Formation under Laboratory Conditions Related to the Troposphere STEPHAN SEEFELD AND J. ALISTAIR KERR* EAWAG, Swiss Federal Institute for Environmental Science and Technology, ETH Zu ¨ rich, CH-8600 Du ¨ bendorf, Switzerland
Peroxyacyl nitrates [RC(O)OONO2] are formed in the atmosphere in the oxidative degradation of many organic compounds of both anthropogenic and biogenic origin. They are important oxidant components of photochemical smog and can cause eye irritation and plant damage. Moreover, peroxyacyl nitrates act as temporary reservoirs for reactive intermediates involved in photochemical smog formation, and they play an important role in the transport of NOx in the troposphere. It is therefore essential to establish reliable data on the kinetics and mechanisms of their formation and removal for inclusion in models of atmospheric chemistry. The kinetic data base for the most atmospherically abundant of the series, acetylperoxy nitrate [CH3C(O)OONO2, PAN], is quite well established, but data are lacking for the higher homologues, e.g., peroxypropionyl nitrate [CH3CH2C(O)O2NO2, PPN], which has also been detected in the troposphere. Here we report data on a relative rate study of the reactions CH3CH2C(O)O2‚ + NO2 + M f CH3CH2C(O)O2NO2 + M (1) and CH3CH2C(O)O2‚ + NO f CH3CH2‚ + CO2 + NO2 (2) carried out in an atmospheric flow system in which the relative yields of CH3CH2C(O)O2NO2 (PPN) have been measured as a function of the ratio of reactants [NO]/[NO2]. Over the temperature range 249302 K, at a total pressure of ∼1 atm, the ratio was independent of temperature with a mean value of k1/k2 ) 0.43 ( 0.07, where the error limits are 2σ. This ratio is 1.05 times higher than the corresponding ratio for PAN. In a second type of experiment involving the relative rates of formation of PPN and PAN in the flow system, as a function of [NO]/ [NO2], the value (k1/k2)PPN/(k1/k2)PAN ) 0.89 ( 0.13 was determined. The results are discussed with reference to literature data of k1/k2 for PPN and PAN.
Introduction Peroxyacyl nitrates (RC(O)OONO2) are well-known to be important components of tropospheric oxidant generated in photochemical smog (1, 2). They are formed in the NOx polluted atmosphere as a product of the oxidative degradation of many organic compounds. Peroxyacetyl nitrate [PAN, CH3C(O)OONO2], is the most abundant of the peroxyacyl nitrates found in the troposphere. * Corresponding author fax:
[email protected].
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1997 American Chemical Society
It has been extensively monitored, and its chemistry, toxicology, and role in NOy transport have been investigated in many studies (1-5). Recent studies show that higher peroxyacyl nitrates, especially peroxypropionyl nitrate [PPN, CH3CH2C(O)OONO2], are also often present in ambient air. [PPN]/[PAN] ratios in the range 0.1-0.2 have commonly been reported (6-10), but ratios as high as 0.28 have been observed (7). There have been indications that PPN could be several times more phytotoxic than PAN (11) and that PPN may also have biogenic precursors (12). In addition tropospheric measurements of PAN and PPN and of the [PPN]/[PAN] ratio have been interpreted to provide information about the history of air masses (10). In computer modeling studies of the troposphere, to save memory and processor time, all peroxyacyl nitrates are frequently grouped into one model species. Owing to the limited kinetic dataset for higher peroxyacyl nitrates and the large scatter of the data, the kinetic parameters for PAN are often taken to represent those of all the relevant peroxyacyl nitrates. This assumption needs to be tested by further kinetic studies of the higher peroxyacyl nitrates. The precursors of peroxyacyl nitrates in the atmosphere are acyl radicals (RCO‚), which are formed from the hydroxyl radical attack on an aldehyde, but also from photolysis of a ketone or from the thermal decomposition of an β-ketoalkoxy radical. The initial acyl radical is rapidly oxidized to the corresponding acylperoxy radical under atmospheric conditions:
RCO‚ + O2 + M f RC(O)OO‚ + M In the NOx polluted atmosphere, there are two main competing channels for the acylperoxy radicals: k1
} RC(O)OONO2 + M RC(O)OO‚ + NO2 + M {\ k
(1, -1)
-1
k2
RC(O)OO‚ + NO 98 RC(O)O‚ + NO2
(2)
where reaction 2 is followed by the fast reaction fast
RC(O)O‚ 98 R‚ + CO2
(3)
Note that since peroxyacyl nitrates are thermally unstable, their decomposition via reaction -1 becomes significant at higher ambient temperatures. In addition to the decomposition rate constant (k-1), the ratio R ) k1/k2 plays an important role in formation and decomposition of peroxyacyl nitrate in the atmosphere. Computer modeling studies of the troposphere have shown a high sensitivity of the calculated ozone formation to the rate constants of the PAN chemistry (13). For PAN, the ratio RPAN has been studied in detail (14-19) and a limited number of analogous studies of higher peroxyacyl nitrates have been reported (20-24). For PPN, Kerr and Stocker measured k1/k2 ) RPPN at a single temperature (302 K) from a kinetic analysis of the rates of formation of NO, NO2, and PPN in a flow system (21) while Becker and Kirchner reported a value RPPN also at a single temperature (313 K), by measuring the rate of decomposition PPN as a function of the [NO]/[NO2] ratio (24). Here we report measurements of the ratio k1/k2 ) RPPN over a range of tropospherically relevant temperatures by a method involving the production of propionylperoxy radicals from the photolysis of propionyl chloride in the presence of
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air containing NO and NO2:
CH3CH2COCl + hν f CH3CH2CO‚ + Cl‚
(4)
CH3CH2CO‚ + O2 + M f CH3CH2C(O)OO‚ + M (5) By measuring the resulting PPN concentrations on a relative scale as a function of the [NO]/[NO2] ratio, we have been able to measure RPPN at atmospheric pressure and at lower temperatures than hitherto reported. The chlorine atoms generated in reaction 4 seem likely to react with O2 and possibly with the substrate molecule CH3CH2COCl, which has the largest mixing ratio of the trace gases in the reaction mixtures. The resulting CH2CH2COCl and CH3CHCOCl radicals are not expected to generate PPN and consequently do not interfere with the kinetic analysis. In a second set of experiments, we have measured RPPN relative to RPAN, with the same flow system involving a reactant mixture of acetyl and propionyl chlorides. The relative measurement reduces the possibility of systematic error when comparing RPPN to RPAN from the literature data.
Experimental Section Materials. Most of the chemicals have been described in a previous publication (19). Propionyl chloride (Fluka, >98%) and 3,4-hexadione (Merck, >99%) were used without further purification other than bulb-to-bulb distillation. PPN was synthesized by the liquid phase nitration of perpropionic acid in n-tridecane solution (25). Flow System. The experimental system has been described in detail previously (19) and will only be discussed briefly here. The experiments involved flowing an airreactant mixture through an irradiated reactor with a residence time of about 50 s, and analyzing the reactants and PPN product either with an NOx-chemiluminescence analyzer or by gas chromatography (GC-ECD). The experiments were carried out at atmospheric pressure (965 ( 5 hPa) and over the temperature range 249-346 K. A steady flow of synthetic air containing propionyl chloride, NO, and NO2 in the parts per million range was established through the reaction vessel. Mass flow controllers allowed the separate control of all the concentrations without affecting the total flow, which was constant over the time scale of the experiments and usually at the rate of 400 cm3 min-1. For the reactant propionyl chloride, a mixture in synthetic air (6500 ppm) was prepared and stored in a 25 dm3 glass bulb at a pressure of about 1250 hPa, by first evacuating the bulb and filling with about 10 hPa of the propionyl chloride using a precision pressure gauge and then adding synthetic air. The combined gas streams from the MFCs were passed through a gas-mixing device prior to entering the flow reactor. The reaction vessel consisted of a Pyrex tube with an internal diameter of 2.9 cm and a length of about 56 cm, which allowed precise control of temperature and reaction time. The reaction vessel was illuminated by a 1 kW Xe arc lamp. The light passed to the flow reactor through a set of collimating lenses, a water filter, and an NO2-filter to minimize photolysis of NO2 in the reactor. The resulting spectrum has a main peak from 400 nm to 1000 nm with a maximum at 700 nm and a small tail down to 250 nm. Analysis. The PPN was measured by GC operated at 35 °C and fitted with an electron capture detector (ECD) at 100 °C. Gas samples were drawn from the exit of the reactor and transferred via an automatic gas sampling valve using a 100 µL stainless steel loop into a cryotrap. The compounds were separated on a 16 m capillary column (4% PS-255) (26) with H2 as the carrier gas at a flow rate of 10 cm3 min-1. The retention times of PPN and PAN were 4.0 and 2.0 min under these conditions. Linearity checks of the ECD were carried out by measuring parts per billion levels of PPN simulta-
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FIGURE 1. Plot of the measured peak area of PPN (b) as a function of [NO]/[NO2] (0) for an experiment at T ) 302 K. The dashed line shows th PPN[NO])0 value used in the calculations. The x-axis shows the time scale of the experiment, which is not required in the kinetic treatment of the data. neously with the GC-ECD and the NOx-analyzer. NO and NO2 were measured with a commercial chemiluminescence NO-NO2-NOx analyzer fitted with a stainless steel NO2 to NO converter operating at a temperature of 625 °C. Under these conditions, PPN is also converted to NO, and this adds to the NO2 signal, but since the PPN concentration was always more than a order of magnitude smaller than that of the NO2, no correction was applied. Measurement of the Rate Data. During each experiment, only the concentrations of NO and NO2 were varied. All other parameters such as temperature, total flow, reaction time, light intensity, and the propionyl chloride concentration were kept constant. Starting with no NO in the mixture, the [NO]/ [NO2] ratio was increased to a value of about 4 and subsequently decreased stepwise back to 0, and at each selected value of the [NO]/[NO2] ratio the relative amount of PPN formation was measured. This procedure confirmed that the system was stable over the time scale of the experiments. Typical levels of reactants and products, in units of parts per million, were [NO], 0-4; [NO2], 1-5; [propionyl chloride], 24; and [PPN], 0.01-0.1. A typical set of results under the conditions described above is shown in Figure 1. An attempt to produce propionyl radicals from the photolysis of 3,4-hexadione failed, because it was not possible to establish a stable flow of 3,4-hexadione, and consequently no stable PPN signal was obtained. The photolysis of Cl2 in the presence of CH3CH2CHO was not used, because this system is sensitive to HO production as explained by Seefeld et al. (19). Combined PAN and PPN Measurements. In a second set of experiments, the propionyl chloride was replaced by a mixture of propionyl and acetyl chlorides obtained by filling the 25 dm3 glass bulb with about 10 hPa of each compound and then adding synthetic air until a total pressure of about 1250 hPa was reached. In these competive experiments, the increased number of components in the reactant mixture together with the increased number of products made the accurate measurement of PAN and PPN difficult. This was alleviated by removing the NO2 filter, which lead to larger PAN and PPN peaks, but also to an increased photolysis of NO2. Additionally, the changes in NO and NO2 arising from reactions 1 and 2 were increased, and hence the NO to NO2 ratio was no longer as constant over the reaction time (see Discussion below).
Results k1/k2 for PPN. Under the present experimental conditions, with mixing ratios of NOx (NO + NO2) in the parts per million range, the rates of formation of PPN and CO2 are governed by reactions 1 and 2 above. Furthermore, the occurrence of
TABLE 1. Rate Constant Ratios, k1/k2, for PPN as a Function of Temperature at Atmospheric Pressure (965 hPa) T/K
k1/k2a
T/K
k1/k2a
249 258 268 273 279
0.454 ( 0.038 0.438 ( 0.028 0.415 ( 0.018 0.494 ( 0.016 0.400 ( 0.016
290 302 310 322 346
0.421 ( 0.053 0.398 ( 0.018 0.345 ( 0.035b 0.284 ( 0.040b 0.026 ( 0.002b
a Errors limits are the sum of 2σ from linear least-squares fit and 2σ of PPN[NO])0 determination. b Not used to calculate the average because thermal decomposition of PPN cannot be neglected at these temperatures.
FIGURE 2. Kinetic plot of the ratio of PPN measured as a function of [NO]/[NO2] according to eq 10 for T ) 302 K, based on measurements shown in Figure 1. the self reactions of CH3CH2C(O)O2 radicals can be neglected owing to the relatively low mixing ratios of these species in the system. Assuming that reaction -1 is negligible, which holds at low temperatures (see later Discussion), leads to the rate expressions
d[PPN] ) [CH3CH2C(O)O2‚][NO2]k1 and dt
(6)
d[CO2] ) [CH3CH2C(O)O2‚][NO]k2 dt
(7)
Combining eqs 6 and 7 and integrating the result over time, assuming that [NO] and [NO2] are constant, gives the following equation:
k1[NO2]0
[PPN]t [CO2]t
)
(8)
k2[NO]0
where the subscripts 0 and t indicate concentrations at the beginning of the reaction and at time t, respectively. The ratio k1/k2 is derived from a kinetic analysis involving solely the measurement of the PPN formation as a function of the [NO]/[NO2] ratio. This approach is based on the fact that the C2H5CO radicals will react exclusively with O2 in the presence of the high [O2]/[NOx] ratios and that the resulting C2H5C(O)O2 radicals will form either PPN or CO2 in reactions 1 and 2. Furthermore, the sum ([PPN]t + [CO2]t) will be independent of the [NO]/[NO2] ratio for fixed experimental conditions of temperature, total flow rate, light intensity, and mixing ratio of propionyl chloride. This sum can be determined by measuring [PPN][NO])0 , which is [PPN]t when t [NO]/[NO2] ) 0 at which point [CO2]t ) 0. Thus, independent of the [NO]/[NO2] ratio, [CO2]t can be expressed as
[CO2]t ) [PPN][NO])0 - [PPN]t t
FIGURE 3. Plot of the ratio of PPN measurements as a function of the ratio of PAN measurements with varying [NO]/[NO2] values, according to eq 12 at T ) 295 K. deviation. To allow for unknown systematic errors in our measurements, we suggest that the error limits should be increased by ∼50%, i.e., we recommend the value k1/k2 ) 0.43 ( 0.11 over the temperature range 249-302 K. Above 302 K the decomposition reaction -1 cannot be neglected, and these measurements yield a ratio which is underestimated. This effect can be explained by the onset of the decomposition of PPN, reaction -1, in relation to the time scale of the flow experiments (see Discussion). Relative Measurements k1/k2 for PPN vs k1/k2 for PAN. The peroxyacetyl radical shows the same reactivity toward NO and NO2 as the peroxypropionyl radical, and therefore the ratio R ) k1/k2 is important in both cases. Although the simultaneous measurement of RPPN and RPAN is theoretically possible with the present experimental system, the experiments were not carried in this way owing to the change in the NO and NO2 concentrations, which are described in the Experimental Section. However, the system is suitable for measuring the ratio RPPN/RPAN. By combining eq 10 and the corresponding equation for PAN,
[PAN][NO])0 t
(9)
[PAN]t
Combining eqs 8 and 9 and replacing k1/k2 by RPPN yields
[PPN][NO])0 t [PPN]t
[NO]0 )
1
[NO2]0 RPPN
[NO]0
)
1 +1 [NO2]0 RPAN
(11)
we get
+1
(10)
On the basis of eq 10, the ratio of the rate constants RPPN ) k1/k2 was derived by a linear least-squares fit from the plots of [PPN][NO])0 /[PPN]t as a function of [NO]/[NO2], as shown t in Figure 2. The rate coefficient ratios found in this study are listed in Table 1, where the errors quoted are 2σ. No significant temperature dependence is found over the temperature range 249-302 K, and the average of these measurements is k1/k2 ) 0.43 ( 0.07, where the error limits are twice the standard
/[PPN]t) - 1 ([PPN][NO])0 t ([PAN][NO])0 /[PAN]t) t
) -1
RPAN RPPN
(12)
It follows that the concentrations of NO and NO2 do not have to be constant over the reaction time for eq 12 to be valid. On the basis of eq 12, the ratio of the RPPN/RPAN was derived by a linear least-squares fit from plots of [PPN][NO])0 / t [PPN]t as a function of [PAN][NO])0 /[PAN]t as shown in Figure t 3. The ratios found in this study at three temperatures are listed in Table 2, where the errors quoted are 2σ. No
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TABLE 2. Relative Rate Constant Ratios, rPPN/rPAN, as a Function of Temperature at Atmospheric Pressure (965 hPa) T/K
rPPN/rPANa
262 284 295
0.92 ( 0.09 0.85 ( 0.16 0.91 ( 0.13
a Errors are the sum of 2σ from linear least-squares fits, 2σ for PAN[NO])0, and 2σ for PPN[NO])0 determination.
FIGURE 5. Comparison of present data on k1/k2 for PPN with literature values: (b) this study; (0) Kerr and Stocker (21); (O) Becker and Kirchner (24). The dotted line represents the mean value of present results over the temperature range 249-302 K.
FIGURE 4. Comparison of present data on k1/k2 for PPN with a kinetic model of the reaction scheme: (b) measured ratios, (solid line) modeled ratios. systematic trend with temperature is found, and the average of these measurements is RPPN/RPAN ) 0.89 ( 0.13, where the error limits are twice the standard deviation.
Discussion Checks on the Proposed Mechanism. As noted in the Results and in Table 1, the measurements of k1/k2 at 310, 322, and 346 K show distinctly lower values than those measured over the temperature range 249-302 K. This effect can be explained by the onset of the decomposition of PPN, reaction -1, in relation to the time scale of the flow experiments. Using k-1 ) 2 × 1015 exp(-12800/T) (27), at 346 K we calculate the half-life of PPN at high [NO]/[NO2] ratios to be 4 s compared to the residence time in the flow system of ∼50 s for the standard flow rate of 400 cm3 min-1. As further evidence of this situation, we have compared the experimental results to those from a kinetic model of the reaction scheme (reactions 1-5) using the rate coefficients recommended by Atkinson et al. (27). Details of the modeling calculations are presented elsewhere (28), and the results are shown in Figure 4, where the agreement is reasonably satisfactory. The present kinetic treatment assumes that [NO] and [NO2] do not change significantly during the course of the reaction. Of course, reactions 1 and 2 affect the mixing ratios of these two compounds and every ethyl radical produced leads to further NO to NO2 conversion. In our system, however, the changes in the mixing ratios of NO2 and NO brought about by these reactions are relatively small since the sum of the mixing ratios of NO and NO2 was at least an order of magnitude higher than the mixing ratios of the products, PPN and CO2. In addition, the photolysis of NO2 was minimized by the presence of the NO2-filter in the light train. Direct experimental confirmation of the essential stability of the ratio [NO]/ [NO2] during the course of a reaction was obtained by switching on and off the light or the flow of substrate while monitoring NO and NO2. The NO produced by the photolysis of NO2 was always found to be smaller than 1% of the NO2 concentration. Comparison with Literature Data. Figure 5 shows a plot of the present results in relation to previous determinations
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of RPPN as a function of temperature and at pressures close to 1 atm. Our temperature-independent value of RPPN ) 0.43 ( 0.07 is in agreement with the value RPPN ) 0.53 ( 0.11 recommended by Atkinson (3), which is based on the measurements of Kerr and Stocker (21) but is significantly smaller than the value RPPN ) 0.70 ( 0.06 of Becker and Kirchner (24). Unfortunately, there appear to be no absolute determinations of k1 or k2 in the literature to compare with our relative rate measurements of k1/k2 for the propionylperoxy radical. The present measured value of ratio RPPN/RPAN ) 0.89 ( 0.13 is consistent with the ratio calculated from RPPN measured in this study and RPAN reported by Seefeld et al. (19), RPPN/ RPAN ) 1.05 ( 0.19. It is also within the error limits of the ratio RPPN/RPAN ) 0.93 ( 0.27, where RPPN is taken from Kerr and Stocker (21) and RPAN is taken from Cox et al. (14) where both ratios were determined by comparing the NO to NO2 conversion to the PAN and PPN production rates, respectively. A further comparison taking RPPN from Becker and Kirchner (24) and RPAN measured by the same group and by the same technique (17) yields the ratio RPPN/RPAN ) 1.67 ( 0.31, which is significantly higher. The somewhat higher values of the measured ratios k1/k2 at atmospheric pressure for PPN (0.43) compared to PAN (0.41) are in line with the observation that the values of k2 appear to be independent of the size of the alkyl group in the RO2‚ radical (29) while the value of k1 is expected to be closer to the high-pressure limit for PPN than for PAN. On the other hand, within the relatively large error limits to be assigned to the measured values of RPPN and RPAN, the ratios could be regarded as being the same for both PPN and PAN over the temperature range 250-300 K at a pressure of 1 atm of air.
Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
Stephens, E. R. Adv. Environ. Sci. Technol. 1969, 1, 119-147. Roberts, J. M. Atmos. Environ. 1990, 24A, 243-287. Atkinson, R. J. Phys. Chem. Ref. Data 1994, Monograph 2, 1-216. Gaffney, J. S.; Marley, N. A.; Prestbo, E. W. In The Handbook of Environmental Chemistry; Hutzinger, O., Ed.; Springer: Berlin, 1989; Vol. 4 (B), pp 1-38. Kleindienst, T. E. Res. Chem. Intermed. 1994, 20, 335-384. Williams, E. L., II; Grosjean, E.; Grosjean, D. J. Air Waste Manage. Assoc. 1993, 43, 873-879. Grosjean, D.; Williams, E. L., II; Grosjean, E. Environ. Sci. Technol. 1993, 27, 110-121. Kourtidis, K. A.; Fabian, P.; Zerefos, C.; Rappenglu ¨ ck, B. Tellus 1993, 45B, 442-457. Shepson, P. B.; Hastie, D. R.; So, K. W.; Schiff, H. I. Atmos. Environ. 1992, 26A, 1259-1270. Grosjean, E.; Grosjean, D.; Fraser, M. P.; Grass, G. R. Environ. Sci. Technol. 1996, 30, 2704-2714.
(11) Taylor, O. C. J. Air Poll. Contr. Assoc. 1969, 19, 347-351. (12) Grosjean, D.; Williams, E. L., II; Grosjean, E. Environ. Sci. Technol. 1993, 27, 979-981. (13) Yang, Y.-J.; Stockwell, W. R.; Milford, J. B. Environ. Sci. Technol. 1995, 29, 1336-1345. (14) Cox, R. A.; Derwent, R. G.; Holt, P. M.; Kerr, J. A. J. Chem. Soc., Faraday Trans. I 1976, 72, 2061-2075. (15) Cox, R. A.; Roffey, M. J. Environ. Sci. Technol. 1977, 11, 900-906. (16) Hendry, D. G.; Kenley, R. A. J. Am. Chem. Soc. 1977, 99, 31983199. (17) Kirchner, F.; Zabel, F.; Becker, K. H. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 1379-1382. (18) Tuazon, E. C.; Carter, W. P. L.; Atkinson, R. J. Phys. Chem. 1991, 95, 2434-2437. (19) Seefeld, S.; Kinnison, D. J.; Kerr, J. A. J. Phys. Chem. 1997, 101, 55-59. (20) Zabel, F.; Kirchner, F.; Becker, K. H. Int. J. Chem. Kinet. 1994, 26, 827-845. (21) Kerr, J. A.; Stocker, D. W. J. Photochem. 1985, 28, 475-489. (22) Kirchner, F.; Zabel, F.; Becker, K. H. Chem. Phys. Lett. 1992, 191, 169-174. (23) Kenley, R. A.; Hendry, D. G. J. Am. Chem. Soc. 1982, 104, 220224.
(24) Becker, K. H.; Kirchner, F. Bergische Universita¨t Gesamthochschule Wuppertal, Fachbereich 9 Physikalische Chemie, Kinetische Untersuchungen an Peroxynitraten und Peroxy-Radikalen, 1994. (25) Gaffney, J. S.; Fajer, R.; Senum, G. I. Atmos. Environ. 1984, 18, 215-218. (26) Grob, K.; Grob, G. J. High Resolut. Chromatogr. Chromatogr. Commun. 1983, 6, 133-139. (27) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr, J. A.; Rossi, M. J.; Troe, J. J. Phys. Chem. Ref. Data 1997, 26, 521-1009. (28) Seefeld, S. Ph.D. Thesis, Swiss Federal Institute of Technology Zurich (ETH), 1997. (29) Eberhard, J.; Howard, C. J. J. Phys. Chem. 1997, 101, 3360-3366.
Received for review February 21, 1997. Revised manuscript received May 27, 1997. Accepted June 3, 1997.X ES970150I X
Abstract published in Advance ACS Abstracts, August 1, 1997.
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