-------
accelerated the PCE reaction by factors of 1.6 to 6.0. This effect is graphi-
cally illustrated in Figure 3.
Additional evidence on the role of Cl chemistry was provided by the FTIR
tests 1, 3, 4, 5, 6, 8, and 11 and Teflon bag tests 12, 13, 14, and 23-30, in
which PCE was irradiated in the presence and the absence of another organic
compound capable of scavenging Cl atoms. The scavenger organic compounds used
were trichloroethylene (FTIR tests) and n-butane (Teflon bag tests). Results
from the FTIR tests showed that trichloroethylene, present at a concentration
equal to that of PCE, inhibited the PCE reaction by approximately a factor of
2. The Teflon bag test results showed that n-butane, at concentrations as low
as four times that of PCE, reduced the PCE reaction rate from about 100%/h to
about 4%/h.
To determine whether direct PCE photolysis is the source of Cl atoms in
smog-chamber-irradiated PCE/air systems, Teflon bag tests 18, 20, 23, 24, and
25 were conducted, in which PCE was irradiated alone or in the presence of
high n-butane concentrations (1000-10,000 ppm). The rationale for these tests
was that in the presence of high concentrations of a radical scavenger (butane)
the only cause of PCE disappearance would be direct photolysis. Results
suggested that PCE may indeed be photolyzing, albeit at an extremely low rate.
Specifically, in tests 23, 24, and 25, (in which PCE was irradiated in the
presence of butane) PCE disappeared at 0.4, 0.7, 0.3, 2.4, and 0.4%/h, whereas
in tests 18 and 20 (in which the PCE mixtures were allowed to stand in the
dark) the PCE disappearance rates were 0.4 and 0.2%/h. At the 70% probability
level, the dark and irradiated rates (and their confidence intervals) are
calculated to be 0.3 (±0.3)%/h and 0.84 (±l.l)%/h, respectively. This is
interpreted to mean that, if PCE photolyzes, its photolysis rate cannot exceed
approximately 1.9%/h.
The appearance of an induction period in tests 12, 13, 29, and 34, in
which the PCE reacted rapidly, suggested that a PCE reaction product may be a
source of Cl atoms and thus exert an autocatalysis-type effect on the PCE
reaction. To explore this possibility, we conducted FTIR tests 7 and 10, in
which PCE was irradiated before and after injecting trichloroacetyl chloride
(TCAC). TCAC was selected because of evidence that it is a PCE photooxidation
product (10), and because it was considered the most likely reaction product
34
-------
INJECT 50 ppbCI2
0
345
IRRADIATION TIME, hours
6
8
Figure 3. Effect of C\2 on photooxidation of PCE (FTIR Chamber).
35
-------
that could be photolyzed by solar radiation. Although the reaction system in
the FTIR system receives a smaller amount of short wavelength radiation (2800-
3300A) than in the bag chambers (see Figure 1), the FTIR chamber walls still
transmit enough such radiation for the purposes of these tests. Similar tests
were also conducted using Teflon bags (tests 36 and 37). Results showed no
perceptible effect of TCAC on the PCE reaction. Also, consistent with this
findino was the result of test 9, in which TCAC disappeared rather slowly
(10.3%/h) when irradiated in the FTIR chamber. These tests showed that
photolysis of TCAC could not be a significant source of Cl atoms in smog
chamber photooxidation of PCF.
In an effort to reconcile some conflicting data on PCE reactivity, studies
were included of some experimental factors that were known or suspected to
have significant effects on the PCE reaction and to differ Tn the various
investigations. Most important among these factors were judged to be:
radiation, chamber wall, and initial reactant concentration.
Radiation intensity did vary somewhat with investigation but this variation
did not seem to correlate with the variation of the PCE reactivity measurement
results. Therefore, this factor was judged to have little relevance to this
study's objectives. Radiation spectrum also varied with investigation,
especially within the 2800-3300A wavelength region, and there was no reason to
discard this factor as irrelevant. Therefore, several tests were conducted in
which the radiation spectrum was varied either by changing the lamp composition
or by using glass to filter out selected wavelengths. Specifically, the
amount of 2800-3300^ radiation was disproportionately reduced either by shutting
off the sunlamps or by placing the reactor bag inside a Pyrex flask. The
effects of these modifications on the wavelength spectrum are shown in Figure 1.
Teflon bag tests 12, 13, 15, 16, 17, 19, 29, 30, 34, and 35 specifically
studied this factor. Results showed the PCE reaction to be extremely sensitive
to radiation within the 2800-3300$ wavelength range. With all lamps on and
without the glass screen, PCE reacted at rates from 110%/h (four tests) to
20%/h (one test). But with the sunlamps off or when the bag was placed inside
the Pyrex flask the PCE reaction rate was from 0.4 to 11.0%/h. While a part
of this effect is undoubtedly due to the total light intensity loss caused by
the glass screen or by shutting off the sunlamps, this loss is obviously far
36
-------
too small (see Figure 1) to account for the drastic reduction in the PCE
reaction rate. Based on these results, and on the differences in radiation
intensity and radiation spectrum between the FTIR chamber and the Teflon bag
(see respective kQ values and Figure 1), we concluded that the lower PCE rates
observed in the FTIR chamber are mainly due to lower 2800-3300A* radiation
intensity.
The smog chamber wall could have two distinct effects on the PCE reaction:
one related to the radiation transmitted by the window material, and one
related to the scavenging of radicals (especially of Cl atoms) by the inside
wall surface. As discussed above, the radiation transmission was found in
this study to be a strong factor, suggesting that using sunlamps in smog
chambers and Teflon film as window material should cause relatively higher PCE
reactivity measurement results. There is an abundance of evidence published
on the radical scavenging role of walls. In fact, evidence has been reported
for the specific case of Cl atom scavenging by Teflon surfaces. Specifically,
NASA investigators reported that the strong inhibitive effect of Cl atoms on
CL formation from (L photolysis was considerably reduced when the Teflon
surface of their reactor was exposed to prolonged CFC1-, photolysis reaction (36)
The investigators attributed this effect to a reduction in the Cl-scavenging
ability of the Teflon walls, caused by excessive deposition of CFCl^ reaction
products. Tests 15, 16, 17, and 19 in this study also suggested such an
effect. Specifically, when Teflon bag #1 was used four times in succession to
irradiate 2.5-2.6 ppm PCE (with the blacklight lamps only) for several hours,
the PCE loss rate appeared to increase with bag usage, from 0.4 to 11.0%/h.
Considering that bag #1 had been used (and perhaps already been conditioned)
before these four tests, and that some of the rate value variation is due to
error, these results may or may not reflect a significant surface effect.
Of the initial reactant concentration factors in photochemical reactivity
studies, those relevant here are the organic reactant concentration, the NO
/\
concentration, and the interaction of these two factors or the organic-to-NO
s\
ratio. In the case of PCE, the data from this study indicated that the NO
A
does not appreciably influence the PCE reaction rate. Thus, the reaction
rates in the absence of N0x (tests 6 and 8) are well within the range of
values obtained in the presence of NO (tests 1,4, and 5). Teflon bag tests
X
37
-------
12, 13, 21, 22, 29, 30, 34, and 35, provide evidence on the effect of PCE
concentration. Irradiation of 2.5-3.0 ppm PCE resulted in PCE rates of 20 to
110%/h» whereas for initial PCE concentrations of 0.6-55 ppb, the rates were
0.5 to 1.5%/h. These results show that the initial PCE concentration is a
strong factor, and that variation of this factor should be reflected in the
various PCE studies' results.
The final objective in this experimental effort was to obtain evidence
that would permit extrapolation of the laboratory data on PCE reactivity to
the "real atmosphere". As already discussed, such evidence is on the effects
on PCE reactivity of radiation intensity and spectrum, co-reactant organic
compounds, and PCE concentration. Data on the co-reactant, PCE concentration,
and some radiation factors were described above. There were also additional
data on the radiation factor obtained in this study, from parallel tests in
which PCE-air mixtures were irradiated with the laboratory radiation system
and with natural sunlight. Results from the natural sunlight irradiation
tests (tests 31, 32 and 33) showed the PCE rates to be 2 to 5%/h. These rates
are considerably lower than those observed in the artificial sunlight tests
(tests 12, 13, 29, 30, and 35), and comparable to those observed when the
2800-3300^ component was reduced (i.e., tests 15, 16, 17, 29, 30, 34, and 35).
This means that use of Teflon film smog chambers and of radiation with a
strong near-UV component would tend to cause unrealistically high PCE reactivity
results.
38
-------
SECTION 5
DISCUSSION
The evidence obtained in this and the previous studies on the effects
upon PCE reactivity of the initial reactant concentrations, C12, and organic
co-reactants, is consistent with a Cl- instigated chain photooxidation mechanism
analogous to the OH-initiated mechanism accepted in current smog chemistry:
CC12 = CC12 + Cl * CC13CC12
cci3cci2 + o2 -* cci3cci2o2
CC1QCC1900 + NO -> CC1,CC190 + NQ9
J hv
- + 0 n
U
cci3cci2o -> cci3c(o)ci + ci
-* COC10 + CCU
°
+ cio
In the absence of NO, main reaction products should be CC1.,CC1(0) and COC19.
o c.
In the presence of moderate concentrations of NO, 0^ also should form through
photolysis of N02, as well as PAN-type products arising from the CCUCO radical
through reactions similar to those in smog chemistry for hydrocarbons/aldehydes.
Finally, as with all VOC/NO systems, excess NO should suppress production of
X
03, PAN and other oxidants.
Conceivable sources of Cl atoms in the irradiated PCE/NO system are
/\
several, for example:
OH + CC12 = CC12 + -v Cl (or + C12 5V 2C1 ) or
0(3P) + CC12 = CC12 -> -> Cl (or C12 iv 2C1) or
39
-------
cci2 = cci2 + ci + cci2 = cci
Additionally, photolysis of the CC1.,CC1(0) product could produce Cl atoms,
which would tend to ccaise an "autocatalysis"-i'ype acceleration of PCE consump-
tion.
The rate constants for the PCE reactions with 0( P) and OH are, respec-
tively, 0.8 X 10"13 cm3 molecule"1 sec"1 (29) and 2 X 10"13 cm3 molecule"1
sec" (27), whereas the concentrations of 0( P) and OH in photochemical
_Q _y
VOC/NO systems are roughly 10" ppm and 10" ppm, respectively (37). These
3
data show that the reaction of PCE with 0( P) may be relatively less important.
Furthermore, Appleby found that PCE disappeared rapidly when irradiated in a
Teflon bag in mixture with pure N9 (15). This finding, if indeed valid, would
3
suggest that neither 0( P) nor OH are necessarily the main instigators of the
Cl-releasing process. It is conceivable, however, that some atmospheric 02
permeated the Teflon bag, in which case OH could not be ruled out as an insti-
gator. We concluded from tests 7, 10, 36, and 37 of the current study that
photolysis of CC13CC1(0) also should be ruled out as a source of Cl atoms.
These findings and the results of tests 18, 20, 23, 24, and 25 of this study
tend to support direct photolysis of PCE as the most likelyalthough weak
source of Cl atoms in smog chamber irradiated PCE/air or PCE/NO /air systems.
X
Consistent with this deduction is the absorption spectrum in Figure 4, showing
that liquid PCE--and conceivably gaseous PCE alsoabsorbs at wavelengths near
3000$ (38). Other sources, e.g., reaction with OH, are not likely but cannot
be completely ruled out either.
The next question of interest is whether Cl-instigated PCE photooxidation
explains the wide diversity of PCE reactivity results in the various studies.
With respect to the consumption rate reactivity, the present study and
two others (5,6) resulted in the highest values, ranging from 20 to 110%/h,
whereas all the other studies except Dilling et al. (9) resulted in values of
2.3 to 9.5%/h. Factors our current study identified as having strong effects
on the PCE consumption rate are the radiation intensity within the 2800-3300$
wavelength region, the presence of organic co-reactants, and the PCE concen-
tration. Since widely diverse data were obtained for comparable PCE concen-
trations, and there were no organic co-reactants except the background
40
-------
2.0
uj 1-5
O
z
^f
ABSORB/
_>
b
0.5
0
27
' \
-
\
1 1 1 1 1 1 1 1 1
00 3000 3500
WAVELENGTH, angstroms
Figure 4. UV absorption spectrum of liquid perchloroethylene.
-------
contaminants (see Table 3), the radiation factor could conceivably explain the
diversity. PCE investigators have not reported complete quantitative data on
this radiation factor but the little qualitative information available is
usually consistent with this explanation. Thus, of the chambers reporting
low consumption rate data, the ones of Brummelle et al. (3), Wilson (7),
Kopczynski (16), Gay et al. (10), Yanagihara et al. (18), and the FTIR chamber
of the present study used as window material either glass or Tedlar film both
of which are less transparent to 2800-3300$ radiation than Teflon (39), the
window material used in our smog chamber and the Lillian et al. (6) studies.
Sickles et al. (22), who also reported low consumption rate results, used
Teflon chambers but they also used natural sunlight radiation whicn, relative
to the radiation used in the present study, was found to cause lower consump-
tion rates (tests 31, 32, and 33 versus tests 12, 13, 15-17, 29, 30, 34, and
35). The Dimitriades/Joshi chamber (5), associated with high consumption rate
data, was made of glass but because of its relatively small volume and cylin-
drical shape, the 2800-3300$ radiation generated by the chamber's six sunlamps
was more intense than in all the larger chambers except the FTIR chamber of
this study.
Differences in 2800-3300$ radiation apparently do not explain the differ-
ence in consumption rate results observed between the Dimitriades and Joshi
chamber and (5) the FTIR chamber of this study. Thus, considering these two
chambers' diameters, the numbers of sunlamps used, and the fact that the
chambers are made of the same material, the 2800-3300$ radiation intensity
should be higher in the FTIR chamber. Yet the PCE consumption rates observed
for comparable initial PCE concentrations were higher for the Dimitriades and
Joshi chamber by a factor of approximately 2. The disagreement in this case
may be related to chamber wall effects. Thus, the pattern of PCE disappearance
in the Dimitriades and Joshi chamber (rapid acceleration of the reaction
following an induction period) is consistent with little or no scavenging of
Cl atoms by the chamber walls. In contrast, the uniform PCE disappearance
observed in the FTIR chamber and the tendency for the effect of injected Cl?
to diminish with time (see Figure 3) are consistent with strong Cl scavenging
by the chamber walls. Therefore, a difference in the walls' Cl-scavenging
ability (evidently due to different usage histories) most likely caused the
disagreement in PCE consumption rate results between the two chambers.
42
-------
Finally, no attempts were made to reconcile the reactivity data obtained
by Schuck and Doyle (12) and by Billing et al, (9) with those of the other
studies. The effects of the Schuck and Doyle chamber's unorthodox design
(lamps were inside the chamber) and of the unrealistic radiation and reactant
concentration conditions used by Dill ing et al. are not well enough understood
to permit meaningful comparisons with the other studies.
Hith respect to the (L/0 yield reactivity, the proposed Cl-photooxidation
o X
mechanism dictates that such reactivity should be influenced by the same
factors that influence the consumption rate reactivity and, additionally, by
the PCE-to-NO ratio factor. This means that of the reported studies, those
X
that resulted in the highest consumption rate reactivity data, and those that
used optimum PCE-to-NO ratio conditions in the smog chamber tests, should
X
also have resulted in the highest CU/0 yield reactivity data. This is
O X
indeed the case, as the data in Tables 3 and 7 show. Thus, the highest Oo/0v
j X
yield reactivity data were obtained by Lillian et al. (6), Dinritriades and
Joshi (5), Sickles et al. (22), Gay (20), and the present study. Of these,
Lillian et al., Dimitriades and Joshi, and the present study also gave high
consumption rate reactivity results. Gay, Sickles et al. and some tests of
this study used a PCE-to-NO ratio of 20, which is much more conducive to
X
Q.,/0 formation (22) than the ratios of 2-B or >100 used in the other studies.
O X
To summarize the discussion and conclusions thus far, the evidence obtained
in this study is consistent with the deduction derived from past studies that
in smog chamber testing of PCE, current smog chemistry, with the OH attack as
its key reaction step, is not operative; instead, a Cl-instigated chain photo-
oxidation chemistry is operative. Furthermore, in the light of this new
evidence, the Cl-instigated photooxidation mechanism seems to provide reasonable
explanations of the reactivity differences observed in the various smog chambers.
The most significant feature, however, of this mechanism is that, relative to
the OH-initiated mechanism of current smog chemistry, it results in substantially
higher levels of PCE reactivity. This raises then the final and most important
question, which is whether the Cl-instigated photooxidation chemistry observed
in smog chambers can also produce 03 in the real atmosphere.
Analysis of atmospheric data indicated a negligibly reactive nature for
PCE which suggests that the Cl chemistry is not effective in the real atmosphere.
43
-------
But such data are usually too limited to support reliable conclusions. One
must rely, therefore, on existing laboratory evidence and theoretical deduc-
tions. The relevant laboratory evidence is the smog chamber data on the
effects of various factors on PCE consumption in irradiated PCE/NO systems.
A
These factors, their effects, and their implications regarding the question in
hand are discussed below.
First, the PCE concentration factor was found in our present study to
have a strong co-directional effect on PCE consumption rate. Thus, the PCE
consumption rates for initial PCE concentrations of 2.5-3.0 ppm were 20 to
110%/h (tests 12, 13, 34, and 35), whereas for initial PCE concentrations of
0.6-55 ppb, the rates were 0.5 to 1.5%/h (tests 21, 22 and 36). Tnat is,
reducing initial PCE concentration by 2 to 3 orders of magnitude reduced the
normalized PCE consumption rate by about 2 orders of magnitude. Extrapolation
of these results to the real atmosphere, where the PCE concentration is only a
few ppt, can be made only very roughly. Since the normalized PCE consumption
rate is a function of PCE concentration, it follows that the order of the PCE
consumption reaction with respect to the PCE reactant must be greater than 1.
Lacking detailed mechanism and complete kinetic data, the assumption is made
that the kinetic rate equation is R = k[PCE]n (where n > 1) and the normalized
rate (R1, %/h) equation is
R1 = rW X 100 = kCPCE]"'1.
Using these authors' data (tests 12, 13, 21, 22, 29, 30, and 34-36), k and n
are estimated to be 31.6 and 1.77, respectively. From these results it follows
that for the ambient hemispheric PCE concentration of 40 ppt (25), the PCE
consumption rate should be 0.01%/h. In urban atmospheres, where the PCE
concentration can reach 1-7 ppb (23), the PCE consumption rate can be as high
as 1.75%/h. For comparison, the atmospheric reaction rate of ethane, based on
its reaction with OH (kQH = 0.3 X 10~12cm3 molecules sec"1, [OH] = 3-5 X 105
molecules/cm ) is 0.03 to 0.04%/h. Similar results are also derived from the
Dilling et al. data (9). Dilling et al. reported that reduction of the
initial PCE concentration in their smog chamber from 100 ppm to 10 ppm caused
the normalized PCE consumption rate to be reduced by a factor of 2.9, down to
44
-------
a half-life of 11.2 h. Again using again the rate equation R = k[PCE]
(where n > 1), and the Dilling et al. data, k and n are calculated to be 33.1
and 1.47, respectively, values that lead to a PCE consumption rate in the real
atmosphere from 0 004%/h (hemispheric average) to 0.7%/h (urban maximum). The
highest rates are higher than those of the ethane reaction with OH in the real
atmosphere or of the PCE reaction with OH (Table 5). Therefore, based on the
effect of the PCE concentration factor alone, the PCE consumption rate reactivity,
and by inference the 0^/0 yield reactivity also, could be significant in the
J A
real atmosphere.
Co-reactant VOCs capable of competing with PCE for Cl atoms were found in
our present study and by others (9,15,40) to inhibit the PCE reaction in smog
chambers extremely strongly (see, for example, results from tests 12-14, and
23-30 of our study). This finding is consistent with available kinetic data
-13 3 -1
showing the rate constant for the Cl + PCE reaction (5.3 X 10 cm molecule
sec" ), to be 2 to 3 orders of magnitude lower than those for the reactions of
Cl with non-methane hydrocarbons (31). Another role of the co-reactants, also
resulting in inhibition of the PCE reaction, could be to scavenge the radicals
(e.g. OH) that instigate the Cl-releasing process. Applying these results to
the real atmosphere, where PCE is invariably accompanied by hydrocarbons at
concentrations several hundred times its own, the PCE reaction with Cl should
proceed at a rate 4 to 5 orders of magnitude lower than the rates of the non-
methane hydrocarbon reactions with Cl. Since the ambient hydrocarbons are
known to have lifetimes of hours or longer (41), and their disappearance is
due mainly to their reaction with OH rather than with Cl, the PCE reaction
with Cl in the real atmosphere seems too slow to have any significance.
Therefore, based on the effect of the co-reactant factor, the PCE reactivity
in the real atmosphere should be negligible.
In conclusion, although the laboratory evidence currently available is
not absolutely consistent and complete, it does permit collective interpretation
of the diverse smog chamber data available and extrapolation of such data and
findings to the real atmosphere. Furthermore, notwithstanding its limitations,
the evidence clearly points to the conclusion that Cl-instigated photooxidation
of PCE cannot occur in the real atmosphere at a high enough rate for substantial
Q.,/0 production. The main bases of this conclusion is that in ordinary urban
O X
45
-------
atmospheres, Cl atoms are effectively scavenged by the unavoidably present
hydrocarbon pollutants. In rural atmospheres, it is, additionally, the
extremely low concentrations at which PCE typically occurs. Thus, contrary to
the smog chamber data, and to the viewpoint held thus far within EPA, PCE is
now judged to contribute less to the ambient photochemical CU/0 problem than
*5 A
equal concentrations of ethane.
46
-------
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29. Heicklen, J. P., E. Sanhueza, I. C. Hisatune, R. K. M. J?.yanty, R.
Simonaitis, L. A. Hull, C. W. Blume, and E. Mathias. 1975. Oxidation
of Halocarbons. EPA-650/3-75-008. Research Triangle Park, NC: U.S.
Environmental Protection Agency.
30. Sanhueza, E., I. C. Hisatsune, and J. Heicklen. 1976. Oxidation of
Haloethylenes. Chem. Rev. 76:801.
31. Davis, D. D., W. Brann, and A. M. Bass. 1970. Reactions of Cl: Absolute
Rate Constants for Reaction with H2, CH4, C2H6, CHeCl2, C2C14, and C-CsH^-
International J. Chem. Kinetics, 2:101.
32. Joshi, S. B., and J. J. Bufalini. 1978. Halocarbon interferences in
chemiluminescent measurements of NOX. Environ. Sci. Technol. 12:597-599.
33. Cicerone, R. J., R. S. Stolarski, and S. Walters. 1974. Stratospheric
ozone destruction by man-made chlorofluoromethanes. Science 185:1165-
1166.
34. Tuesday, C. S. 1961. The atmospheric photooxidation of nitric oxide and
trans-butene-2. In: Chemical Reactions in Upper and Lower Atmosphere.
(C. Tuesday, ed.) pp. 15-49, New York: Interscience Press.
35. Arnts, R. R., and B. W. Gay, Jr. 1979. Photochemistry of Some Naturally
Emitted Hydrocarbons. EPA-600/3-79-081, Research Triangle Park, NC:
U.S. Environmental Protection Agency.
36. Bittker, D. A., and E. L. Wong. 1977. Effect of Trichlorofluoromethane
and Molecular Chlorine on Ozone Formation by Simulated Solar Radiation.
NASA Technical Paper 1093. Cleveland, OH: NASA, Lewis Research Center.
37. Demerjian, K. L., J. A. Kerr, and J. G. Calvert. 1974. The mechanism of
photochemical smog formation. Adv. Environ. Sci. Technol. 4:1-262.
38. Texas A&M University. 1969. Ultraviolet Spectral Data. Thermodynamic
Research Center Data Project. College Station, TX: Texas A&M University.
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39. Dimitriades, B. 1967. Methodology in air pollution studies using irradia-
tion chambers. Journal of Air Poll. Contr. Assoc. 17:460-466.
40. Gay, B. W. 1982. U.S. Environmental Protection Agency, Environmental
Sciences Research Laboratory, Research Triangle Park, NC 27711. Private
communication.
41. Altshuller, A. P. 1981. Lifetimes of Organic Molecules in the Tropo-
sphere and Lower Stratosphere. Advances in Environ. Sci. Techno!. 10:181.
(J. Pitts and R. Metcalf, editors) Mew York: Wiley & Sons, Inc.
50
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APPENDIX
DESCRIPTION OF EXPERIMENTAL APPARATUS AND PROCEDURES
The experimental apparatus used in this study included (a) a long path
Fourier Transform Infrared Spectrometer (FTIR), the absorption cell of which
was used as a smog chamber, (b) an aluminum box inside which plastic film bags
could be irradiated by artificial sunlight, (c) an air purification system,
and (d) analytical equipment. The experimental apparatus and procedures are
described below, except where such information has been reported elsewhere.
THE FTIR SPECTROMETER AND SMOG CHAMBER
Construction design and optics-related information on the FTIR spectro-
meter and smog chamber have been given elsewhere (1). Figure A-l shows a
schematic of the system. The infrared absorption coefficients of reactants
and products were determined in most cases for conditions similar to those of
the irradiation experiments. Thus, known small amounts of gaseous samples
were injected in the absorption cell, diluted to the concentration level
desired with zero grade air, and spectra were taken at 23°C and 1 atmosphere
(760 torr) using 0.5 or 1 wave number spectral resolution and 360 meters
optical path length. PCE was measured from its infrared absorption bands at
>th
-1
780, 805, and 918 cm"1, trichloroethylene at 848 and 940 cm"1, and trichloro-
acetyl chloride at 745 and 1250 cm
The requisite concentrations of the reactant vapors were obtained for
irradiation by diluting torr amounts of the chemical compound in a glass
manifold and glass gas handling system. The dilution system consisted of a
multiport manifold and a 12.4-1 dilution flask. Two MKS Baratron pressure
gauges were used to measure accurately pressure in the 0-10 and 0-1000 torr
ranges. Once the appropriate reactant concentration was established in the
dilution flask, the flask contents were transferred to the partially evacuated
long path infrared cell via the glass manifold to injection ports located in
the five spacers separating the cylindrical sections of the cell.
Linde zero grade compressed tank air containing less than 0.1 ppmC total
organic impurities was used as a diluent gas in the FTIR chamber experiments.
Filling the 690-liter chamber with tank air resulted in a low water vapor
51
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ULTRAVIOLET
LAMPS
INFRARED
SOURCE
INTERFEROMETER
VACUUM
PUMP
PATH OF INFRARED RAp]ATION
MANIFOLD | [_
GAS-HANDLING SYSTEM
--0
DETECTOR
Figure A-1. Schematic of long path infrared photochemical reaction chamber system.
52
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content (approximately 100 ppm H^O). To increase the water vapor content for
those experiments requiring higher relative humidity the tank air was bubbled
through 30 ml of distilled water, using a course frit bubbler.
The PCE used in these experiments was research grade Eastman tetrachloro-
ethylene freshly purified by redistillation from which only the middle fraction,
in the 119-119.5°C range, was used. The trichloroethylene used was ACS-
certified research grade and was used without further purification. The
trichloroacetyl chloride was Kodak research grade and was also used without
further purification. Gases such as nitrogen dioxide (NO^), nitric oxide
(MO), and chlorine (Cl,,) were of high purity research grade, obtained in
lecture bottles and used without any further purification.
THE BAG CHAMBER FACILITY
Irradiation of PCE/air blends in Teflon bags was carried out in an air-
conditioned aluminum box 0.78 m wide, 1.35 m long, and 1.12 m high. Two banks
of 40-watt ultraviolet fluorescent lamps, evenly distributed along two lengths
inside the chamber, provided the necessary irradiation. In these experiments
22 General Electric F40 BLB filtered blacklamps with energy maxima at 3660$
and four Westinghouse GS40 sunlamps with energy maxima at 3160$ were used to
simulate lower atmospheric solar radiation between 2900 and 3800$. The light
intensity was measured by irradiating M09 in N9 (2). Resultant first order
-1
rate constant, kd, for N02 disappearance was 0.45 min , representing total
N02 disappearance due to photolysis and secondary reaction of 0 atoms with
NOp. Since the photolysis rate constant, k,, is 0.64 k, (3), k, for this
irradiation system is 0.29 min~ . The air-conditioned irradiation box main-
tained a temperature of 25°C (± 2°C) around the bags during irradiation. In
this study one or two bags were suspended in the chamber. Air samples for
analytical instruments were withdrawn via FEP Teflon tubing. When two bags
were irradiated simultaneously, sampling was alternated between the bags using
a microprocessor-controlled (Chrontrol, Lindburgh Enterprises) three-way
Teflon solenoid valve (Model DV3 122A1, The Fluorocarbon Company).
The bags were made of DuPont Type A (heat seal able) 2-mil FEP Teflon film
heat-sealed with a thermal impulse heat sealer (Vertrod Corporation, Brooklyn,
NY). FEP Teflon was chosen for its chemical inertness, flexibility, and
53
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transparency to near-UV radiation. Before use the bags were flushed with
clean air several times and evacuated, filled with clean atr and placed in the
irradiation box for twenty-four hours, and finally evacuated.
A laboratory purification system supplied clean air for preparing reaction
mixtures. This system consisted of a Thomas Teflon-lined diaphragm pump
(Model 917CA18TFE), which draws air from the laboratory roof and pressurizes
it into a 1.8 liter stainless steel sphere that serves as a ballast. From
there the air is forced through a bowl moisture trap and pressure gauge, and
through a catalytic combustor consisting of a 13/16" i.d. X 14" stainless
steel pipe packed with rhodium-on-aluninuni pellet catalyst (Engelhard) held at
600°C in a Lindbergh tube furnace. After combustion to remove hydrocarbons
the air stream is cooled by passing through 12 ft. of 1/4" o.d. stainless
steel tubing submerged in water at 20°C, and further purified by passing
through three Wilkerson (Model X03-02-600) filters. These three filters are
filled with silica gel (for moisture control), Purafil (Borg Warner) odor-
oxidant (for NO and NCL removal), and activated charcoal (for hydrocarbon
removal), respectively. The chemically filtered air finally passes through a
particle filter to remove particles larger than 0.2 micron (Matheson Model
6184P4). A Brooks needle valve and Rotameter (Model 1355CB1C1AAA) control and
monitor the flow. All system tubing connections are either Teflon or stainless
steel.
Contaminants in the air supplied by this system are: NO < 2 ppb,
/\
DO < 0.5 ppb, CO - 0.1 ppm, CH» 0.7 ppm, and total nonmethane vapor organics
50 ppbC (mostly ethane and propane). Water vapor within the bags is generally
near ambient levels due to rapid permeation of ambient moisture through the
Teflon film.
ANALYTICAL METHODS
As previously discussed, the FTIR spectrometer made almost all necessary
measurements of the FTIR smog chamber program. Analytical needs of the Teflon
bag program were limited to measuring PCE and butane and are discussed here.
Gas chromatography with flame ionization detection (GC-FID) was the PCE
analysis method for all irradiation tests in which initial PCE concentration
was 2-3 ppm; the same method was also used for the butane measurements. For
-------
the irradiation tests in which the initial PCE concentration was below 100 ppb,
gas chromatography with electron capture detection was preferred.
The gas chromatograph was positioned on a bench one meter from the irra-
diation chamber. Sample was drawn from Lhe irradiation bags through the GC
gas sampling valve using the laboratory's vacuum line or a pump. The sample
flow was turned off seven seconds before each GC injection to allow the sampling
loop pressure to come to atmospheric pressure, thereby eliminating a source of
measurement variability. An electronic timer was programmed to turn on and
off the sample flow, injection valve, and peak area integrator at the desired
time intervals. When dual bag irradiations were conducted, a Teflon 3-way
solenoid valve was used to control the sampling. The GC analysis parameters
are listed in Table A-l and Figure A-2 shows a typical chromatogram.
For quality assurance, tests were conducted to determine the linearity of
detector response and the precision and reproducibility of the method. To
test the response-linearity of the GC-FID method, samples were prepared using
a 1000-ml syringe to dilute a sample of PCE four times. Resultant PCE concen-
trations in the samples ranged from 12 to 0.3 ppm. Results gave a linear
2
regression correlation coefficient r = 0.998, which was interpreted to mean
that the FID response to PCE was linear.
The initial reactant mixtures prepared for irradiation were also used as
calibration standards. These mixtures were prepared by syringe injection of
known amounts of liquid PCE into Teflon bags filled with known volumes of air.
Data from repeated GC/FID measurements upon several such mixtures are listed
in Table A-2. These data were statistically treated by an Analysis-Of-
Variance method to determine standard deviations (S) associated (a) with
sampling and GC/FID measurements upon a single calibration mixture (S ),
(b) with preparation of calibration mixtures (S ) and (c) with day-to-day
variation in instrument response (S-). Resultant standard deviations,
expressed in terms of Coefficients of Variation (CV = S X 100/mean) are
included in Table A-2.
The GC-ECD method was the same as the GC-FID method, except that a
tritium-scandium electron capture detector was used in lieu of FID, and 5%-
methane-in-argon was used as carrier instead of He. The GC-ECD analysis
parameters are listed in Table A-3. Figure A-3 shows a typical chromatogram.
55
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TABLE A-l. PCE-FID ANALYSIS PARAMETERS
Gas Chromatograph
Integrator
Gas Sampling Valve
Detector
Column
Temperatures
Column
Detector
Sample Valve
Carrier Gas
Flow Rates
Carrier
Detector Air
Detector Hydrogen
Sample
Sample Size
Detector Voltage
Chart Speed
Total Cycle Time
Perkin Elmer Model 900
Hewlett-Packard Model 3390A
Seiscor (Tulsa, Oklahoma)
Flame lonization
6-ft X 1/8-inch o.d. ss packed with
10% SF-96 on 60/80 mesh Chromosorb
W-AW
100°C
250°C
Room Temperature, 24°C
Helium
25 ml/min
536 ml/min
39 ml/min
160 ml/min
2 and 4 ml loops used
1 volt, 0.1 volt
0.9 cm/min
5 min
56
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PCE, 2.6 ppm
0123
RETENTION TIME, min
Figure A-2. Chromatogram of GC-FID analysis of PCE.
57
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TABLE A-2. PRECISION OF GC/FID ANALYSIS OF PCE CALIBRATION MIXTURES
GC/FID Data for 4-cc Samples (Arbitrary Units)
Date
Calibration Mixture
(2.592 ppm PCE)
#1
Calibration Mixture #2
(2.592 ppm PCE)
4/20
4/21
4/26
4/27
5/11
5/12
6/8
6/9
3103 3121 3113
3208 3251 3221 3189
3085 3266 3310
3005 3030 3022
3009 3083 3095
3204 3231 3213
2921 2968 2955 2966
2904 2906
2991
3189
2862
2968
2991
3198
2934
2940
2903
3192 3167
2857 2874
GC/FID Data for 2-cc Samples (Arbitrary Units)
6/16
6/17
6/21
6/24
6/29
6/30
7/6
7/7
7/8
7/9
1299
1342
1274
1301
1036
1232
1168
1151
1227
1274
1302
1326
1286
1313
1039
1244
1192
1146
1243
1292
1314
1343
1292
1073
1237
1203
1314
1333
1299
1062
1241
1302
1343
1353
1042
1224
1332
1311 1306
1317
1305
1277
1220
1264
1258
1251
1237
1298
1323
1297
1266
1206
1267
1207
1232
1229
1312
1336
1285
1239
1241
1192
1293 1281 1296
1241
1240
1198
1267
Analysis of Variance
CV a/ CVr^/
CVi
Data for 4-cc, Mix #1
Data for 4-cc, Mix #1, #2
Data for 2-cc, Mix #1, #2
3%
11%
6%
12%
23%
- Coefficient of variation associated with sampling and GC/FID measurements upon a
single calibration mixture.
-/ Coefficient of variation associated with preparation of calibration mixture.
c/
- Coefficient of variation associated with day-to-day variation in calibration
mixture preparation and instrument response.
58
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TABLE A-3. PCE-ECD ANALYSIS PARAMETERS
Gas Chromatograph
Integrator
Gas Sampling Valve
Detector
Column
Carrier Gas
Temperatures
Column
Detector
Sample Valve
Flow Rates
Carrier
Sample
Sample Size
Detector Voltage
Chart Speed
Total Cycle Time
Perkin Elmer Model 900
Hewlett-Packard Model 3390A
Seiscor 6-port
Electron Capture, Analog Technology
Corporation, Model 140
6-ft X 1/8-inch o.d. ss packed with
10% SF-96 on 60/80 mesh Chromosorb
W-AW
5% methane in argon
100°C
160°C
Room Temperature, 24°C
25 ml/min
160 ml/min
0.63 ml
1 mv
0.9 cm/min
5 min
59
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XI024 I
PCE. 50ppb
0123
Figure A-3. Chromatogram of GC-ECD analysis of PCE.
60
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For calibration of the GC-ECD system, standard PCE mixtures were prepared
using a 163-ml exponential dilution flask. First, 4 pi of PCE was injected
into a 1000-ml heated glass bulb. Then 0.25 ml of the glass bulb contents was
injected into the dilution flask. The oi'tlet of the flask was connected to
the GC gas sampling valve, and tank air was used as the diluent gas. A sample
from the exponential dilution flask was analyzed every five minutes. The
response-versus-concentration curve (Figure A-4) was not linear, but it did
2
fit a power curve function with a regression correlation coefficient R of
0.994. The precision of the method was determined from 14 repetitive analyses
of a 267-ppb PCE/air sample. The relative standard deviation was 2.3% and the
95% confidence interval was ±4.8%. The method's reproducibil4ty was estimated
from the calibration input variables (syringe error, volume error, and flow
rate error) to be ±18% (95% confidence interval).
REFERENCES
1. Gay, B. W., Jr., P. L. Hanst, J. J. Bufalini and R. C. Noonan. 1976.
Atmospheric oxidation of chlorinated ethylenes. Environ. Sci. Technol.
10:58-67.
2. Tuesday, C. S. 1961. The atmospheric photooxidation of nitric oxide and
trans-butene-2. In: Chemical Reactions in Upper and Lower Atmosphere
(C. Tuesday, ed.) pp. 15-49. New York: Interscience Press.
3. Stedman, D. H. and H. Niki. 1973. Photolysis of N02 in air as measure-
ment method for light intensity. Environ. Sci. Technol. 7:37.
61
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5001
C/J
0
120
20 40 60 80 100
PCE CONCENTRATION, ppb
Figure A-4. PCE calibration curve for GC-ECD analysis.
52
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