United States
                     Environmental Protection
Environmental Sciences Research
Research Triangle Park NC 27711
                     Research and Development
EPA-600/S3-83-001  Mar. 1983
&EPA          Project  Summary

                     Reactivity of
                     Basil Dimitriades, Bruce W. Gay, Jr., Robert R. Arnts, and Robert L.Seila
                      Perchloroethylene  (PCE), a solvent
                     used in dry cleaning, has been suspected
                     of contributing significantly to photo-
                     chemical ozone/oxidant (O3/Ox) prob-
                     lems in urban atmospheres.  Past evi-
                     dence, however, was neither complete
                     nor consistent. To interpret more con-
                     clusively the past evidence, and to
                     understand further the role of PCE in
                     the O3/OX problem,  a smog  chamber
                     testing program was conducted.  The
                     program's objectives were (a) to gener-
                     ate additional evidence on  the mecha-
                     nism  of the PCE reaction  in smog
                     chamber atmospheres, and (b) to ex-
                     trapolate the smog chamber findings
                     regarding PCE reactivity  to the  real
                     atmosphere. Results showed that (a) in
                     smog chambers, PCE reacts and forms
                     O3/Ox following a Cl-instigated photo-
                     oxidation mechanism rather  than the
                     OH-initiated mechanism accepted in
                     current smog chemistry, and (b) in the
                     real  atmosphere  neither the Cl-insti-
                     gated nor the OH-instigated photooxi-
                     dations of PCE can generate substantial
                     concentrations of Os/Ox. In fact, PCE
                     contributes less to the ambient O3/0»
                     problem than ethane.
                      This Project Summary was developed
                     by EPA's Environmental Sciences Re-
                     search Laboratory, Research Triangle
                     Park, NC, to announce key findings of
                     the research project that is fully docu-
                     mented in a separate report of the same
                     title (see Project Report ordering infor-
                     mation at back).

                      The approach adopted by  the U.S.
                     Environmental Protection Agency (EPA)
                     to reducing photochemical ozone (O3) and
other oxidants (Ox) has been based on
unilateral control of the volatile organic
compound (VOC) precursors. However,
since the individual VOCs differ widely in
03- or 0,-forming potential, EPA accepted
the concept of discriminate control of
organic emissions, and from time to time
produced  lists of  VOCs that could be
exempted from control by virtue of their
negligibly low photochemical reactivity.
  Perchloroethylene (PCE), a VOC used
as a solvent and emitted in significant
amounts from dry-cleaning operations,
was first thought to be unreactive  and
was exempted from control. That early
judgment was based on Los Angeles Air
Pollution Control District (LAAPCD) smog
chamber studies of solvent reactivities in
the 1960s, which are now considered to
be in error.  In the years  following  the
LAAPCD studies, there were several other
PCE reactivity studies upon which EPA
judged that PCE's photochemical reactiv-
ity is not sufficiently low to justify total
exemption from control.
  EPA's latest judgment on PCE reactivity
was based mainly on findings from EPA
and EPA-sponsored studies in the mid-
1970s.  Other studies during that period
produced  results that agreed with  the
EPA policy on  PCE in  some respects,
disagreed in others, or were not compa-
rable. More  recently, there have been
additional studies, some  directly  ad-
dressed to PCE's Os-forming reactivity
and some to various mechanisms of PCE
photooxidation.  The new  evidence  ap-
peared again to partly agree and partly
disagree with the latest EPA judgment on
PCE reactivity. However, it became also
apparent that  most of the conflicting
evidence could be reconciled, which led

to reexamining  the question of PCE
reactivity in the  light of the reconciled
evidence. We concluded that more exper-
imental testing was needed to completely
reconcile conflicting evidence  and to
support a more  definitive and reliable
judgment regarding the reactivity of PCE.
  This report describes our latest exper-
imental study. We  also reanalyze  and
reinterpret all evidence now available on
the atmospheric  chemistry of PCE,  and
offer a new judgment of PCE's photo-
chemical reactivity with respect to the
ambient oxidant problem. To facilitate an
understanding of the rationale behind the
objectives and design of our new study,
and the reanalysis and interpretation of
the experimental  evidence,  we discuss
the concept of reactivity,  the  past data
and  other  relevant  literature on PCE
reactivity, and the current specific issues
and information gaps.

The Concept of Reactivity
  The reactivity aspect of primary interest
in this report is the ability of the VOC to
participate in atmospheric reactions and
form Oa/Ox. The  prevalent method for
measuring such reactivity has been the
smog chamber method, in which the test
VOC is exposed  in a smog  chamber to
reactant mixture and radiation conditions
similar to those  in real urban atmos-
pheres, and the yield in 03/0X is meas-
ured directly.
  In the past three decades there have
been numerous smog chamber studies of
VOC reactivity, and O3/Ox-yield reactivity
data are now available for a large number
of VOCs. There are problems, however, in
using  the  existing  data  base for  this
particular reactivity index. These prob-
lems forced researchers and regulators to
(a) turn to  simple,  two-class reactivity
classification schemes ("reactive," "un-
reactive") in preference to individual VOC
reactivity ranking schemes,  and (b) con-
sider other less direct but more usable
reactivity indices.  The  reactivity index
used here in addition to the "O3/Oxyield"
was the "VOC consumption rate" reac-
tivity. The significance of the latter index
lies mainly  in the fact that a VOC cannot
produce 63/0, unless  it  participates—
and, hence, is consumed—in the atmos-
pheric reaction.  The VOC consumption
rate, therefore, offers the most reliable
basis for recognizing the totally unreac-
tive VOCs. Thus,  VOCs whose consump-
tion rates are negligibly small are classi-
fied as unreactive. By extension, it was
also assumed, at first, that VOCs mani-
festing significant consumption rates in
smog chambers are reactive. This latter
assumption, however, has been question-
ed  by  some recent studies,  and,  as
discussed later, by this study also.

Analysis and Interpretation of
Existing Evidence on
Perchloroethylene Reactivity
  Existing evidence considered here con-
sists mainly of data on the O3/0, yield
and  PCE consumption  rate  reactivities
obtained in the laboratory. Since crude
measures of consumption rate reactivity
can also be derived from ambient con-
centration and emission rate data, these
were included in this analysis. Each piece
of reactivity data was critiqued to the
extent allowed by the reported informa-
tion, and labeled as supporting a "negli-
gibly reactive" or "reactive" classifica-
tion  for PCE. "Negligibly reactive"was
defined  here as equal  or  less reactive
than ethane (the latter organic taken by
the authors to be a "boundary" species
separating  the reactive VOCs from the
unreactive ones).
  Existing data on Oa/Ox yield and con-
sumption rate reactivities were extracted
from 13 formal and informal reports on
laboratory studies. Estimates of consump-
tion  rate  reactivity of PCE  were also
derived from PCE emission and aeromet-
ric data.
  Examination of  those data  revealed
considerable inconsistencies. Thus, of the
13 studies, six supported high or border-
line O3/Ox yield reactivity, six supported
negligible 03/0xyield reactivity, and all of
the studies supported high  but widely
varying  consumption rate  reactivities.
The aerometric data supported negligible
  In searching for clues to the causes of
these inconsistencies, the smog chamber
studies'  results were  compared with
reactivities deduced from smog chemistry
considerations. To  explain, currently ac-
cepted  smog  chemistry (derived from
atmospheric chemistry studies of hydro-
carbons  and aldehydes) explains VOC
reactivity in terms of the VOC's ability to
participate in a chemical process, a key
step of which  invariably  is the initial
reaction of the VOC with OH radicals.
Reactions subsequent to the OH attack
also have a role in the overall 03-forming
process, but obviously, only if the initial
reaction with OH occurs at a significant
rate. Therefore,  the value of the  rate
constant for the PCE + OH reaction can
provide a useful first check on the reli-
ability  of the existing laboratory data.
Such a value is known, and based on that
and  on the OH  concentration in the
atmosphere, the rate of PCE consumption
in ambient  air  was  computed to be
0.36%/h. Such a rate is 1 to 1 Vi orders of
magnitude lower than the rates observed
in smog chambers, which means either
that the performance of the smog cham-
bers was not consistent with smog chem-
istry predictions or that current smog
chemistry, for some reason, is not appli-
cable in the PCE case. To further evaluate
these two alternate explanations, con-
sumption rates were computed from k0H
data for several VOCs (hydrocarbons and
halocarbons), and compared with those
observed in the smog chambers. Results
showed generally good agreement be-
tween observed and predicted consump-
tion rate reactivities for hydrocarbons but
extremely  poor agreement for halocar-
bons. The halocarbon data disagreement
is invariably in the direction  of higher
observed than computed  reactivities.
Therefore, the inconsistencies of the
existing PCE reactivity data base cannot
be attributed directly to smog  chamber-
related factors alone.  The  more logical
conclusion is that current smog chemistry
cannot  be  applied  to polychlorinated
ethylenes, or,  more specifically, the re-
action with OH is not the key step in the
halocarbon consumption process. Some
species, more  potent than  OH, must be
responsible for the rapid PCE consump-
tion in smog chambers. This raises four
1.  What  is the chemical species that
    causes rapid PCE reaction in smog
2.  What is the chemistry of the process
    following that species attack? Does it
    result in O3/OX production?
3.  Does this  chemistry explain the in-
    consistencies  among  the  various
    studies data?
4.  Is this species and associated chem-
    istry operative  in  the  real atmos-

With  respect  to the  first and  second
questions, past basic and smog chamber
studies suggested that the PCE consump-
tion observed in smog chambers is caused
by a Cl-instigated chain photooxidation
process which, apparently, is also capable
of generating O3. The evidence available,
however, on the origin of the Cl atoms in
such  systems, and on the effects of
experimental factors on the PCE reaction
with Cl, was lacking. In the absence of
such evidence, the support of  the Cl-
instigated mechanism was weak, and the
question of whether this mechanism can
explain  the inconsistent results of past

studies could not be answered. Further-
more, there was no evidence that the Cl-
instigated  photooxi.dation  mechanism,
presumably operative in smog chamber
systems, is necessarily operative in the
real atmosphere also. These deficiencies
of the past evidence  led these  investi-
gators to conduct a new PCE study, and to
reexamine the question of PCE reactivity
in the  light of  the  new experimental

Procedures and  Results of
New Perchloroethylene
Reactivity Study
  The analysis of existing  evidence on
PCE reactivity  indicated that this experi-
mental effort's objectives should be to (a)
confirm that in the smog chamber meas-
urements of PCE reactivity the operative
chemistry in PCE degradation is Cl- rather
than OH-instigated  photochemistry, (b)
understand such  Cl  chemistry  well
enough to reconcile  some seemingly
conflicting evidence, and (c) extrapolate
the laboratory findings on PCE reactivity
to the real atmosphere.
  To achieve  the first objective,  tests
were conducted in which PCE was re-
acted with Cl atoms, and tests in which
PCE was reacted both in the presence and
the absence of Cl scavengers. Also, direct
photolysis  of  PCE  and of  some of its
photooxidation products was investigated
to determine the source of the Cl atoms
that instigate the rapid disappearance of
PCE observed  in some smog chambers.
To achieve the  other objectives, smog
chamber irradiation tests were conducted
to determine the effects on  PCE  con-
sumption of the following factors: initial
PCE concentration, presence/concentra-
tion of  organic co-reactant, radiation
spectrum, chamber  wall  material,  and
chamber use before the test.
  The irradiation tests used the absorp-
tion cell of a long path Fourier Transform
Infrared Spectrophotometer (FTIR)  and
several  bags  made  of Teflon as smog
chambers. Irradiation in both cases  was
provided by fluorescent lamps simulating
natural sunlight.
  Thirty-seven smog chamber tests were
conducted. Results showed that injection
of CI2 accelerated strongly  the PCE re-
action, and that presence of a Cl-scaveng-
ing co-reactant inhibited the PCE reaction
drastically. These results  indicated  that
PCE consumption was instigated by Cl
atoms. To identify the sources of Cl atoms,
tests  were conducted to determine
whether PCE  or its reaction products
photolyze to Cl atoms. Results suggested
that direct photolysis of PCE is the most
likely source of Cl atoms, although other
sources (e.g.,  the PCE +  OH reaction)
could not be completely ruled out.
  In an effort  to reconcile some incon-
sistencies in the data from past  studies,
tests were included to study some exper-
imental  factors that  were known or
suspected to have significant effects on
the PCE reaction and to  differ in the
various investigations. Most  important
among such factors were: radiation spec-
trum, chamber wall, and initial reactant
concentration. Of these factors, radiation
spectrum probably varied the most with
investigation, especially within the 2800-
3300A wavelength region.  Several tests,
therefore, were conducted  in which this
factor was varied either by changing the
lamp composition or by using glass to
screen out selected wavelengths. Results
showed that the PCE reaction was ex-
tremely sensitive to radiation within the
2800-3300A wavelength range.
  Two smog chamber wall factors were
investigated: one related to the radiation
transmitted by the window material, and
the other related to the scavenging of
radicals—especially of Cl atoms—by the
inside wall surface. As already discussed,
the radiation transmission  factor in this
study was a strong one. In regards to the
radical scavenging role of walls, tests in
this study were merely suggestive of such
an effect.
  Of the initial  reactant concentration
factors in photochemical reactivity stud-
ies, those relevant here are the concen-
tration of the organic reactant, the con-
centration of NO,, and the  interaction of
these two factors or the organic-to-NOx
ratio. In the case of PCE, the data from
this study indicated that the NO, does not
influence the  PCE reaction rate appre-
ciably. The PCE concentration, however,
was found to have a strong effect.
  The final objective in this experimental
effort was to obtain evidence that would
permit  extrapolation  of the laboratory
findings  on  PCE reactivity to the  real
atmosphere. The requisite evidence con-
sidered  the effects of various reactant
and radiation conditions on PCE reactivity,
especially the radiation intensity  and
spectrum, the  presence of co-reactant
organics, and  the PCE concentration.
Data on the co-reactant, PCE concentra-
tion, and radiation  factors have been
obtained in this study,  including data
from tests  in  which  PCE-air mixtures
were  irradiated  in  parallel,  with  the
laboratory radiation system and with
natural sunlight. Results from the latter,
natural sunlight irradiation tests showed
the PCE rates to be lower than those
observed in the  artificial sunlight tests
when all lamps were on, and comparable
to those observed when the 2800-3300A
component was reduced. Thus, reactivity
data taken with Teflon film smog cham-
bers and near-UV-rich radiation would
tend to be erroneously high.
  The evidence obtained in this and the
previous studies on  the  effects  of the
initial reactant concentrations, CI2, and
organic co-reactants upon PCE activity is
consistent  with a Cl-instigated chain
photooxidation mechanism analogous to
the OH-initiated mechanism accepted in
current smog chemistry:
CCI3CCI2 + 02 - CCI3CCI202


NO + 0
     I	03

In the absence  of  NO, main  reaction
products should beCCI3CCI(O) and COCI2.
In the presence of moderate concentra-
tions of NO, 03 also should form through
photolysis of  NO2, as well as PAN-type
products arising from the CCI3CO radical
through reactions similar to those in smog
chemistry for hydrocarbons/aldehydes.
Finally, as with  all  VOC/NOX systems,
excess NO should suppress production of
03, PAN and other oxidants.
  Of the various conceivable origins of Cl
atoms, direct photolysis of PCE appears to
be the one most consistent with existing
data,  although other sources (e.g.,  re-
action of PCE  with  OH)  cannot  be
completely ruled out. The above  mecha-
nism seems to provide reasonable expla-
nations of the  wide  diversity  of PCE
reactivity results obtained in the various

  studies. Thus, the higher consumption
  rate reactivity results observed  in most
  studies are probably due to the higher
  intensities of 2800-3300A radiation used
  in those studies.  The high 03/0X yield
  reactivity results are probably due to the
  same factor and also to optimum PCE-to-
  NOX ratio conditions.
    The final and most important question
  addressed was whether this Cl-instigated
  photooxidation chemistry observed  in
  smog chambers is effective in producing
  O3 in the real atmosphere also. An answer
  was provided by the evidence from this
  study related to the effects  of the PCE
  concentration and  organic  co-reactant
  factors on PCE reactivity. Based on that
       evidence, the presence of much higher
       concentrations of Cl-scavengingco-react-
       ants in the real atmosphere would cause
       the PCE to have an extremely low reactiv-
       ity, lower than that of ethane.

       Conclusions and
         Currently  accepted  smog  chemistry,
       with OH attack as the key initial reaction
       step, does not explain the rapid disappear-
       ance  of  PCE observed  in some smog
       chambers. A Cl-instigated photooxidation
       mechanism appears to be more consist-
       ent with the smog chamber data available.
       This should be verified through quantita-
       tive comparisons of existing smog cham-
ber data with  mechanistic  predictions
using modeling techniques.
  Based  on the proposed Cl-instigated
photooxidation  mechanism, it is derived
that in  the real  atmosphere  the PCE
consumption rate is extremely  small
because relatively high concentrations of
Cl-scavenging organics  are  present in
ambient air. In fact, the PCE consumption
rate, and, hence, its 03/0xyield reactivity
also, are estimated to be lower than those
of ethane.
     The EPA authors Basil Dimitriades (also the EPA Project Officer, see below),
      Bruce W.  Gay, Jr., Robert R.  Arms, and Robert L. Seila are with the
      Environmental Sciences Research Laboratory, Research Triangle Park, NC
     The complete report, entitled "Photochemical Reactivity of Perchloroethylene,"
      (Order No. PB 83-163 014; Cost: $8.50, subject to change) will be available only
            National Technical Information Service
            5285 Port Royal Road
            Springfield, VA 22161
            Telephone: 703-487-4650
     The EPA Project Officer can be  contacted at:
            Environmental Sciences Research Laboratory
            U.S. Environmental Protection Agency
            Research Triangle Park, NC 27711
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