Environ. Sci. Technol. 1989, 23, 970-978
Investigation of Background Radical Sources in a Teflon-Film Irradiation Chamber Willlam A. Glasson and Alan M. Dunker"
Environmental Science Department, General Motors Research Laboratories, Warren, Michigan 48090-9055 The background radical sources in a 500-L Teflon-film chamber were studied by irradiating mixtures of NO, NOz, and CO in clean dry air. On the basis of the dependence of the NO concentration on time and previous work, the background radical sources were assumed to be an initial concentration of nitrous acid, [HONOIo, and a constant flux of OH radicals, kOH. Using simulations with a detailed chemical mechanism, [HONOIoand koH were determined from each experiment by a least-squares fitting procedure. The average [HONOIoand koH for all 34 experiments are 3.5 f 1.9 ppb and 0.016 f 0.008 ppb min-', respectively. The results indicate that [HONOIoincreases with NO and that kOH increases with light intensity and temperature. Interrupted irradiations were conducted in which the lights were turned off in the middle of the experiments. These experiments indicate that the processes yielding [HONOIo and koH do not occur in the dark a t appreciable rates after the chamber contents are mixed.
Introduction In attempts to model hydrocarbon/NO, irradiations carried out in smog chambers, workers have found it necessary to postulate background free-radical sources (1-5). The background radical sources appear to be specific to chambers and are not used when applying chemical mechanisms to simulate the atmosphere. Until recently, there were no experimental measurements of the radical sources, and as a result, assumptions on the nature and magnitude of the sources varied. Differences in these assumptions are responsible for some of the differences in the predictions of chemical mechanisms in atmospheric simulations (6-9). Experimental determinations of the background radical sources in different chambers are, therefore, imperative for the effective use of chamber experiments in developing and evaluating chemical mechanisms for smog formation. The magnitudes of background radical sources have been determined experimentally by two methods. In the first method, developed by Carter et al. (IO),very low concentrations (10 ppb) of propene and propane are irradiated in air in the presence of excess NO (400 ppb). After correcting for the loss of propene due to reaction with 0 atoms and 03,the remaining losses of the hydrocarbons are ascribed to reactions with the hydroxyl radical (OH), and the OH concentration is calculated from the decay rate of propene relative to propane. The net radical source in the system is assumed to be the background OH source and the net radical sink is assumed to be the reaction of OH with NO2. By equating the former to the latter and using the OH and NOz concentrations, the background OH source rate may then be calculated. This method has been used in studies of background radical sources in an evacuable chamber and in Teflon-film chambers (10-12). In the second method, proposed by Killus and Whitten (13),a high concentration of NO (500 ppb) is irradiated in air in the presence of excess CO (50 ppm). Any OH or HOz radicals present participate in the chain reactions
OH + CO ----+ H o p + COZ H02 + NO OH + NOp 0 2
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and convert NO to NO2. The OH concentration can be calculated from the NO loss rate, the CO concentration, and the rate constant for the OH CO reaction. The background OH source rate can be calculated as in the f i i t method. A limited application of this method has been made to determine the background radical sources in a large Teflon-film chamber (14). Both methods are chemical amplifiers, but the CO method is a more efficient amplifier because the chain length is longer in this method. The NO concentration and the changes in the NO concentration are greater than the hydrocarbon concentrations and the changes in the hydrocarbon concentrations. Of particular importance, the changes in the hydrocarbon concentrations become quite small and difficult to measure if the background radical source is very low. The hydrocarbon concentrations could be increased in such a situation to make the changes easier to measure, but this would also increase the accumulation of aldehyde products. Since aldehydes produce free radicals by photolysis, the OH concentration could be increased after several hours by such an effect, making accurate determination. of the background radical source more difficult. In contrast, the known chemistry involved in the CO method is simpler than the chemistry involved in the hydrocarbon method, and low background radical sources can presumably be detected by increasing the CO concentration, thereby further amplifying the NO changes. In the work reported herein, we have conducted a detailed study of the background radical sources in a small Teflon-film chamber, such as those previously employed in captive-air experiments (15).The purpose was to determine the usefulness of such chambers for quantitative studies of smog formation. Due to the previously mentioned difficulties associated with the hydrocarbon method, the CO method was employed. Values for the background radical sources were derived from the experimental data by simulations with a detailed chemical mechanism, and the uncertainties in these values were estimated as well. The effects of various parameters, such as light intensity and NO and NOz concentrations, on the radical sources were studied to provide the necessary information for taking the radical sources into account in modeling future chamber experiments.
+
Experimental Approach Teflon-Bag Irradiation Facility. A brief description of the facility is presented here; details are given elsewhere (16). The 500-L bag is made of 0.05-mm FEP 200A Teflon and contains six ports, accessed by Teflon fittings and tubing. Two ports are used for introducing chemicals and air into the bag and for evacuating the bag, one port is used for a thermocouple, and three ports are used for sampling. The bag is supported in a frame and is surrounded by four 6000-W xenon arc lamps, with the entire system being contained in a temperature-controlledenclosure. Chemical species were very stable in the reactor. Over the course of the experiments and over the temperature range studied, the half-life of O3 in the dark ranged from 34 to 77 h, and the half-life of NO2 in the dark ranged from 127 to 154 h. The same bag was used for all experiments reported
00 13-936X/89/0923-0970$0 1.50/0
0 1989 American Chemical Society
Table I. A Simplified Chemical Mechanism for the CO/NO, System rate constant, k, ppm min
reaction
--
3. 4. 5. 6. 7. 8. 9. 10. 11.
NOz + hv NO + 0 O3 + hv O2+ O(lD) OH + NO HONO + hv 0 + O2 M - O3 + M 0 + NO2 NO + 02 NO + O3 NOz + Oz NO + NO + 0 2 2N02 O('D) + M 0+M O('D) + H 2 0 20H OH + NO HONO OH + NOz HNO,
12. 13. 14. 15. 16. 17. 18. 19. 20.
OH CO HOz + COZ HOz + NO OH + NO2 HOzNOz HOP + NO2 HOz + NO2 HOZNOZ HzO + NO2 OH + HONO O3 wall wall NO NOz wall OH
1. 2
+
+
--4
4
+
02
--4
4
+
uncertainty factor for rate constant"
kl is light-intensity dependent 5.2 x 1 0 - ~ k-~7.1 x 10-~k, 0.18k1 - 0.20k, 10.9 T2.3 1.38 x 104 2.66 X 103e-1370/T 1.20 x 10-10e530/~ 4.30 x 104 3.30 x 105 7.10 x 103 1-60 x 104
1.05 1.8 1.2 1.1 1.1 1.2 1.25 1.2 1.2 1.4 1.2
3.55 x 102 5.50 x 1 0 3 ~ 2 4 0 / ~ 2.10 x 103 3.60 x 1016~-1M70/T 9.80 x 103 1.5 x 10-4 - 3.4 x 10-4 5.0 x 10-5 7.5 x 10-5 - 9.1 x 10-5 this work
1.3 1.2 1.25 1.25 2.0 1.3 1.8 1.4 this work
notes
b, c c, d c, d e
e e
f
e
e e
e e e e e
f g g g
h
Upper and lower uncertainty limits on the rate constant corresponding to one standard deviation are obtained, respectively, by multiplying and dividing the rate constant by the uncertainty factor. bThe rate constant was determined by the photostationary-state method (17). cThe uncertainty factor is for the ratio ki/ke. dThe ratio ki/kl was determined by using measurements of the light intensity distribution and the absorption cross sections and quantum yields for NOz and O3 or HONO (20-23). The range for the rate constant encompasses the values measured for different lamps or different combinations of lamps. e Reference 21. 'Reference 20. #Loss rate measured in'the dark. The loss rate varied with temperature and varied over the course of the study within the indicated range. hDerived by the least-squares procedure.
herein, and it is one of a group fabricated from the same roll of Teflon film. Based on limited tests on the other bags, the one used for these experiments has wall loss rates typical of others in the group. The light intensity was varied by controlling the power to the lamps and the number of lamps lit. The light intensity was monitored by an International Light, Inc., Model No. IL 540 radiometer equipped with a 380-nm cut-off filter. The output of the radiometer was calibrated against the NO2 photolysis rate measured by the photostationary-state method (17). The spectral distribution of the xenon arc lamps was measured from 280 to 760 nm by using an EG&G Model 555 spectroradiometer equipped with 5-nm slits. In general, the xenon arc lamps simulate solar radiation (18) reasonably well although they are somewhat deficient in the 300-350-nm region. Analysis. NO/NO, and O3 were determined by using chemiluminescent analyzers, a Monitor Labs 8840 and a McMillan Electronics 1100, respectively. The sample inlet of the Monitor Labs instrument is equipped with a nylon filter to remove HN03 (19). CO was determined with a Bendix Model 8501-53 nondispersive infrared analyzer. The NO/NO, and CO analyzers are calibrated with commercial gas mixtures in conjunction with a Dasibi Calibrator, Model 1005-2. The O3analyzer is calibrated against a dedicated Dasibi Model 1003-AH O3 analyzer using an O3 generator incorporated into the Dasibi calibrator. Chemicals. The hydrocarbon-free air was a product of Scott Specialty Gases with total hydrocarbons 200 ppb the amplification factor using 50 ppm CO is only -5, due to the increased rate of OH loss by the OH + NOz reaction. The CO concentration must be increased to 150-200 ppm to maintain an amplification factor 210 with 1200 ppb initial NOz. Most of our experiments were, in fact, done with -200 ppm CO to provide high amplification of any radicals. Interrupted Irradiations. In runs 17,18,25, and 26, the irradiation was carried out for 100 min followed by a 180-min dark period and a second 100-min irradiation.
500 1
I
I
I
10
RUN 2 5
3
c
'T
400k
i.Ol
1
L i g h t s - . l ( t - - - L i g h t '\, sl
0 0
200
I
I
I
I
200
400
600
800
1000
Initial NO (ppb) Flgure 4. Effect of initial NO on initial HONO. The runs were done over a period of 2 weeks with the initial NO, below 80 ppb and a NO, photolysis rate of 0.1 1 min-'. The line is a weighted least-squares fit to the data and the bars indicate f l u uncertainty limits.
RUN 17
0
1
L
I ~~
300
0
100
200
300
400
Time (min) Flgure 3. NO concentration as a function of time for runs 17 and 25. The solid curve is the simulation resutt using the best-fit values In Table 111 ([HONO], = 3.5 ppb, OH flux = 0.012 ppb/min for run 17; [HONO], = 1.4 ppb, OH flux = 0.021 ppb/min for run 25) with no HONO generation w OH flux during the dark period. The dashed curve is the same as the solM curve in each plot except that for the dashed curve HONO is assumed to be formed during the dark period at a rate equal to the OH flux during the irradiation period.
The initial NO, concentration in the runs was varied from 9 to 441 ppb, but the other experimental conditions were held constant. The purpose of these experiments was to establish whether the formation rate of radical precursors during the dark period would be high enough to explain the radical flux, kOH, observed during the irradiation periods. Figure 3 presents NO vs time plots for runs 17 and 25, runs with high (287 ppb) and low (9 ppb) initial NO, concentrations, respectively. In run 25, the usual burst of NO loss a t the start was followed by a slower rate of loss, a t which point the lights were turned off. The NO concentration then decreased slowly because of the thermal oxidation and wall loss of NO. After 180 min, the lights were turned on again. The NO concentration then decreased a t essentially the same rate as that prevailing immediately before the lights were turned off. This latter result means that no significant concentration of radical precursors was formed during the dark period. To illustrate the point, the dashed curve in Figure 3 gives the NO profile that would have resulted had HONO been formed during the dark period a t the same rate as the OH flux for the run (0.021 ppb min-'). Other simulations showed that a HONO formation rate of -0.004 ppb min-' during the dark period of run 25 would have been readily detectable when the lights were turned on the second time. The resulb for run 17 were similar to the results for run 25 except for the different NO curve shape resulting from the presence of a high initial NO, concentration. Again,
0
200
400
600
800
1000
Initial NO (ppb) Flgure 5. Effect of initial NO on initial HONO. The runs were done over a period of 10 months with the initial NO, below 80 ppb and the NOp photolysis rate in the range 0.11-0.61 min-'. The line is a weighted least-squares fit to the data and the bars indicate f 1u uncertainty limits.
there was no evidence for formation of radical precursors during the dark period, the minimum detectable HONO formation rate in the dark for this run being -0.005 ppb min-'. Runs 18 and 26 also showed NO loss rates immediately before and after the dark period that were essentially the same. Thus, these experiments indicate that whatever the nature of the radical sources in our Teflon chamber, the dark formation rate of radical precursors can explain a t most -0.004 ppb min-' of the radical flux during irradiation. Effect of the Experimental Variables on [HONO],. We have examined the results given in Table I11 to determine the effects of the light intensity (Itl), the initial temperature, [CO],, [NO],, and [NO,], on the calculated [HONO],,. The analysis showed no discernible effect of light intensity, initial temperature, or [CO], on the calculated [HONO],,. Runs 17, 18, and 21-26 suggest [HONO], may increase as [NO,], increases. However, the weighted coefficient of determination, wR2, is only 0.18 for these runs. (In calculating this and other weighted statistics, the data were weighted by the reciprocal of the variance of the [HONO], estimate.) Also, the weighted F test (30) of the hypothesis that [HONOIois independent of [NO,], gives a large significance probability, P(wF) = 0.30, indicating that the hypothesis cannot be rejected. Thus, there is no strong evidence in our data for an effect of [NO,], on [HONO],,. The effect of [NO], on [HONO], is shown in Figure 4, where results of runs 1-6 are plotted. A clear dependence of [HONO], on [NO], is evident with wR2 = 0.99 and P ( w 0 < 0.001. Assuming variables other than [NO], and [NO,], have no effect on [HONO],, a larger subset of runs Environ. Sci. Technol., Vol. 23, No. 8, 1989
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Table IV. Background Radical Sources i n Various Chambers"
[HONOlo, ppb -O%RH -50%RH
koH, ppb min-' -O%RH -50%RH
-4
7-10
0.21
10.2Ctd
20.8C*d
20.04'
-
2d
c5 e 55e
3.7 i 1.d
0.04 0.014 f 0.0088
0.29 20.17c 0.13 0.08-0.12 0.07-0.15
chamber v01,L
chamber materialb
ref
5800 6065 65000 50000 6400 500
TFE coated aluminum walls; quartz windows PFA coated steel walls; quartz and Pyrex windows FEP film FEP film FEP film FEP film
10, 11, 31 25, 33 14 12, 31 11, 12, 31, 32 this work
"For conditions as close as possible to the following: k l = 0.4 min-'; temperature = 300 K; [NO], = 400 ppb; [NO,],= 100 ppb; average [NO,] = 250 ppb after the first 60 min of irradiation. "FE, FEP, and PFA signify, respectively, polytetrafluoroethene Teflon, fluorinated ethene-propene copolymer Teflon, and tetrafluoroethene-perfluoroalkyl vinyl ether copolymer. Lower limit estimates assuming the hydrolysis of NO2 to HONO is the only source of radicals. dAssumes a 45-min time for hydrolysis of NO2 to HONO in the dark before irradiation begins. eUpper limit estimate based on measurements in another chamber (31). 'Average and standard deviation for runs with IN01, = 400-500 DDb. INO,l,, = 0-80 m b . #Average and standard deviation for runs with k , = 0.4 min-'.
can be examined, as shown in Figure 5, where the only selection criterion was [NO,], < 80 ppb. The dependence of [HONO], on [NO], is less clear in Figure 5 (wR2 = 0.07, P ( w F ) = 0.17). Apart from the narrower ranges of experimental variables considered in Figure 4 than in Figure 5 , it should also be noted that the experiments shown in Figure 4 were conducted over a shorter time span (2 weeks) than the experiments in Figure 5 (10 months). Taken together, Figures 4 and 5 indicate that [HONO]Oincreases with [NO],, but that one or more uncontrolled experimental variables play a role in determining [HONO],,. Effect of the Experimental Variables on the OH Flux. We investigated the effects of the light intensity, the average temperature, and the average [CO], [NO], and [NO,] on kOH. The dependence of kOH on the concentrations averaged from 60 min to the end of the run was examined because there are rapid changes in the NO and NO, concentrations in the first 60 min of some runs (Figures 1 and 3). However, no effect of average [NO], [NO,], or [CO] on kOH was found. The effect of the NO, photolysis rate on kOH is shown in Figure 6. In order to ascertain the effect of temperature on koH as well, results are plotted for the lowest temperatures employed in the experiments, 293-296 K, and the highest temperatures, 300-306 K. For both temperature ranges, the average NO, was restricted to 100-270 ppb. Figure 6 indicates that koH increases slowly with increasing NOz photolysis rate, the statistics being wR2 = 0.58, P(wF) = 0.02 for the lower temperature range and wR2 = 0.65, P(wF) = 0.01 for the higher temperature range. The values of kOH in the higher temperature range are generally greater than those in the lower temperature range a t the same NO, photolysis rate, and the weighted least-squares fit to the higher temperature values gives a uniformly greater koH than the fit to the lower temperature values, suggesting kOH increases with temperature. However, the differences in kOH are not great, and the effect of temperature on kOH should be verified in experiments over a wider temperature range.
Discussion The background radical sources in several other chambers have been investigated previously. Table IV presents a comparison of [HONO], and kOH values derived from the previous studies (1&12,14,25,31-33) with those obtained in the present study for similar experimental conditions. Where possible, results are given for both -0% and 50% relative humidity (RH). As Table IV shows, the initial HONO concentrations found in the present study at -0% RH are similar to those reported by Carter et al. (10, 31), Pitts et al. (11), and Leone et al. (14) at either -0% or -50% RH. Our initial
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0.04
1
I
.,
.=t 0.031 'E
1
U
I
0.00 0.0
I
L
I
0.2
1
0.4 NO2 Photolysis Rate (min-')
I
06
Figure 6. Effect of the NOp photolysis rate on the OH flux. The solid squares are runs with the average temperature in the range 300-306 K and the open squares are runs with the average temperatwe in the range 293-296 K. For all runs the average NO2 concentration is in the range 100-270 ppb. The s o l i and dashed lines are weighted least-squares fits to the solid and open squares, respectively, and the bars indicate f 1u uncertainty limits.
HONO concentrations, however, are significantly larger than the lower limit estimates for the NIES chamber derived from the dark formation rate of HONO (25) for low (100 ppb) NO, levels. This second observation is not surprising, considering the results of the interrupted irradiations described in the preceding section, which showed no detectable generation of radical precursors in the dark (
