United States
 Environmental Protection
 Agency	
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park NC 27711
 Research and Development
EPA/600/S3-89/010 Aug. 1989
 Project  Summary

 Validation  Testing of  New
 Mechanisms With Outdoor
 Chamber Data

 H. E. Jeffries, K. G. Sexton, J. R. Arnold, and  J. L. Li
  The UNC smog chamber data base
was used to compare the perform-
ance of two state-of-the-sctence pho-
tochemical mechanisms: the Carbon
Bond Four Mechanism and the Carter,
Atkinson, Lurmann Mechanism. A
number of tasks had to be  performed
before the comparisons  could  be
conducted. These  included com-
paring and reconciling the thermal
rate constants used in the two mech-
anisms, re-assessing the  photolytic
rates for the  UNC chamber and pro-
ducing a  new radiation  transfer
model, performing new experiments
with new instrumentation for formal-
dehyde and  hydrogen peroxide to
confirm older data, and  analyzing
three years of ambient hydrocarbon
data to determine a default compos-
ition for use in EKMA comparisons. A
series of chamber experiments was
modeled with both mechanisms to
assess their performance for specific
chemistry. A  series of State Imple-
mentation  Plan EKMA calculations
was  also  performed with both
mechanisms  for a range  of condi-
tions.
  Although  there  were other
differences between the two photo-
chemical methods, both mechanisms
showed good agreement for ozone
and oxides of nitrogen chamber data.
The agreement for  other products
such as formaldehyde and peroxy-
acetylnitrate was not as good. Neither
mechanism was considered superior
to the other  either  in fitting smog
chamber data or in predicting VOC
control requirements.
  This Project Summary was devel-
oped by EPA's Atmospheric Research
and Exposure Assessment Laboratory,
Research  Triangle  Park,  NC, to
announce key findings of the research
project that is fully documented in
four separate reports of the same title
(see Project Report ordering informa-
tion at back).
Introduction
  The  U.S. Environmental  Protection
Agency (EPA) has a long-standing pro-
gram to develop accurate photochemical
mechanisms for incorporation into air-
quality  models. The EPA has sponsored
the development of two basic types  of
mechanisms: the Carbon Bond approach
with its emphasis on conservation  of
carbon mass, and the surrogate approach
with its  emphasis on  quasi-explicit
chemistry. Neither approach has  a clear
theoretical advantage;  both require that
choices be make  between  detail and
accuracy, and each may  have specific
advantages in particular applications.
  The  work described in this summary
compared two specific mechanisms: the
Carbon Bond Four mechanism (CB4)
developed by System Application Inc.
and the Carter, Atkinson, Lurmann mech-
anism (CAL) developed by the University
of California, Riverside (UCR) and Envi-
ronmental Research and Technology
(ERT).  During their development,  both
mechanisms had been tested using se-
lected  data from  the UNC and UCR
chambers and  were  found to agree
reasonably well with these data. Of
concern was  whether  the two mecha-
nisms would predict similar VOC control
requirements when used in EPA's EKMA
procedure, and,  if not,  to determine why
the mechanisms disagreed.

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  This comparison  task involved  six
activities: (1)  comparison of rate con-
stants for the  portion of the mechanisms
considered  to  be "well  known",  and a
reconciliation  of  the differences, (2) a
complete re-assessment of the photolytic
rates used to  simulate the UNC outdoor
chamber, (3)  conducting a  two-month
outdoor  chamber experimental  program
comparing five  methods  for  measuring
formaldehyde  and three methods  for
hydrogen peroxide -  products  that  the
mechanisms were not predicting well, (4)
simulating  a  selected  set  of  UNC
chamber experiments using  identical in-
puts for  both mechanisms, (5) analysis of
ambient NMOC data to determine default
hydrocarbon  compositions  for use  in
EKMA,  (6)  comparison of the  mechan-
isms'  performance in  a  series of State
Implementation Plan scenarios using  the
EKMA method. An overview  of each of
these  tasks is described below.

Rate Constant Comparison
  When  the  inorganic  and carbonyl
reaction  sets  of  both mechanisms were
compared in the early part of the  pro-
gram, significant differences were found.
The differences were reconciled with  the
mechanisms'  authors and the  resulting
versions of the  mechanisms  are  now
comparable  in  their  inorganic  and
carbonyl sections.

Photolytic Rate Calculation
  As part of the experimental portion of
this program, a  computerized,  portable,
spectroradiometer was used to collect a
large  body  of  spectral irradiance meas-
urements outside and inside the UNC
chamber. These data showed that neither
mechanism developer  had correctly cal-
culated  in-chamber photolytic rates,  nor
had the UNC  researchers supplied suf-
ficient information to  estimate the rates
properly. In addition, analysis  of these
spectral  data  showed  that the  method
that had been  used  for  more than 12
years,  a  look-up table based  upon
Peterson's  calculation of actinic flux,
provided a poor approximation  of  the
radiation field  at the UNC site because of
the effect of aerosols on the radiant flux.
In response to these findings,  a  new
radiation transfer model  was developed.
This model uses the  World Radiation
Center extraterrestrial  solar  flux  and a
one-layer approximation  of  atmospheric
transmission due to Rayleigh scattering,
total column ozone, total column water
vapor, and  aerosol absorption and scat-
tering to compute the surface spectral
irradiance and surface spherical spectral
irradiance. It incorporates three  aerosol
models  that are  sensitive  to relative
humidity and composition. In addition, the
model includes two broad-band models,
one for  the  total  solar radiation  (TSR)
sensor and one for the Eppley ultraviolet
(UV) radiometer.
  Studies were conducted to  determine
the solar spectrum optical properties of
the TFE Teflon film used to  enclose the
UNC chamber. Additional studies  were
conducted  to  determine the spectral
albedo of  the  reflective  chamber  floor.
Chamber geometry relative to the  sun's
path was re-assessed  and a full chamber
transmission model was developed. By
operating radiometers inside  and  outside
the chamber, sufficient information was
collected  to  develop  an in-chamber
actinic flux model.
  The TSR and UV radiometer data for
286 days from  1976  to  1986, were re-
assessed and modeled with  the broad-
band  radiation  models. Cloud  effects
were then introduced  into the model and
correction factors for  the theoretical mo-
del were derived  for  each day.  Finally,
the actinic  flux (spherical  spectral ir-
radiance) at 16-minute  intervals  for the
286 days was calculated.


New Formaldehyde and
Hydrogen Peroxide  Data
  In past tests  by the developers, the
mechanisms  have generally failed to
predict  the observed  formaldehyde
(HCHO) product  profiles in  the  UNC
chamber data. The  model  developers
suggested  that the  UNC HCHO  data
could  be too high by a factor  of  two.
Further,  in a previous  study conducted at
UNC, a laser system was used to  monitor
hydrogen peroxide (H202) and the results
were difficult to understand given today's
knowledge of H202 chemistry.  Therefore,
an intercomparison study of  five  HCHO
methods and three H202 methods was
undertaken. Research groups from EPA,
NCAR,  Texas Tech  University,  Uni-
Search, Inc., and UNC set up their HCHO
methods at the  UNC  smog chamber
laboratory  to collect  HCHO  data  during
chamber experiments over a two-week
period. The NCAR, Texas Tech, and Uni-
Search groups also made H202 measure-
ments.
  The results show that  HCHO monitor-
ing methods can  perform nearly  identi-
cally, if the  calibration sources are
reconciled.  The standard  UNC HCHO
method  was shown to be in  very  close
agreement  with the  other methods and.
not as  suggested  by  the  modeler
subject to a positive interference of up
a factor of two.
  The hydrogen  peroxide  data  from  <
three  measurement methods were
good agreement  in the range 0-800 pf
H202. The half-life for H202 in the UN
chamber, however, was extremely sho
ranging  from  1  hour in  the  dark to  '
minutes in  the  noon sun. Furthermor
the addition  of  350  ppm CO, whic
should have  increased the half-life  t
recycling  hydroxyl radicals  back
hydroperoxy  radicals,  actually accele
ated the lost  of H2O2 dramatically. Mo
confusing of  all,  during  the accelerate
decay of the  350 ppb H2O2 there was
loss of 55 ppm CO.
  Similarly, during experiments designe
to produce large  quantities of H202 fro
chemical reactions of VOC  and NOX, on
4-10  ppb  was  ever observed.  Tl-
assumption is that  the  insitu-produce
H2O2  also decays rapidly,  and therefoi
high levels are  never reached in  tr
chamber.
  Speculation as  to the mechanism  i
H202  loss  in  the UNC chamber include
direct absorption  on the walls,  absorptic
into a thin water film on the walls, or
catalytic process involving  the  absorbe
nitrogen species,  CO,  and  H2O2 th;
rapidly consumes H2O2 and,  by a cha
process, CO.
  The basic  conclusion  reached froi
these  results is that the chamber walls i
the UNC chamber are sufficiently reactiv
towards H2O2 that it is  not possible I
produce data to test  mechanism patf
ways  for generating H2O2. Based upc
the nitric  acid data  reported  by  th
UniSearch laser system, nitric acid also i
probably lost almost as rapidly to  th
walls.


Demonstration Modeling with
New Inputs
  To demonstrate the effect  of  differer
mechanism rate  constants and  reactio
products, 22  dual UNC chamber expei
iments were  modeled with both meet
anisms using the same  simulation cor
ditions. The 22 case  days included  1
single hydrocarbon component expei
iments and 12 mixture experiments. Th
following points were noted:

1.  Both mechanisms show exceller
    agreement with experimental data ft
    HCHO-NOX systems.
2.  In ethylene experiments, the mecr
    anisms show good fits to NO, NOJ
    03,  and  ethylene  decay  profile!

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     Both  mechanisms, however, under-
     predict the formation of HCHO  and
     CO.
 3.   Neither mechanism does well at sim-
     ulating 1-butene experiments on cool
     days; both mechanisms  are much
     too  reactive  in  producing  ozone.
     CAL, however, produces an excellent
     fit to t-2-butene. CB4's surrogate of
     two ALD2s for t-2-butene shows too
     little reactivity throughout the simula-
     tion, but  does begin  to converge
     towards the correct  ozone  concen-
     tration  near  the  end  of  the
     experiment.
 4.   CB4  does  better  than  CAL in
     simulating toluene experiments; CAL
     is better  than CB4 in  simulating
     xylene experiments.  Both mechan-
     isms, however, overpredict HCHO in
     these systems by nearly  a  factor of
     two  or three. Both mechanisms
     predict PAN fairly well  in the xylene
     experiments, but both  overpredict
     PAN in the toluene experiments  with
     CAL predicting nearly twice  as much
     PAN  as  CB4. Both mechanisms
     overestimate  the  reactivity of o-
     xylene compared to m-xylene  and
     both  underpredict  CO  formation in
     these systems.
 5.   In a HC mixture with no aromatics,
     both  mechanisms  provide excellent
     fits to NO, N02 and 03, and  correctly
     simulate the  decay of  propylene,
     ethylene,  n-pentane  and n-butane.
     HCHO is  underpredicted,  however
     (by 50% in the case of UNCMIX).
 6.   In the SynAuto HC  mixture experi-
     ments,  both mechanisms  provide
     excellent fits to NO, N02 and 03, and
     to propylene,  ethylene and  toluene
     decays. HCHO predictions tend to be
     much closer, suggesting  a compen-
     sation between an overproduction in
     the  aromatics chemistry  and  an
     underproduction  in  the  olefin
     chemistry.
 7.   Both  mechanisms are uniformly too
     reactive in generating ozone for the
     SynUrban mixture experiments, even
     though the propylene, ethylene,  and
     toluene decays are well fitted.

 Urban Hydrocarbon
 Composition
   The application of the two mechanisms
 in  EKMA control  strategy calculations
 requires the specification of an urban hy-
 drocarbon composition.  Previously,  SAI
 had supplied the so-called "default"
^hydrocarbon composition, but new data
 rom several EPA projects to  monitor
 hydrocarbons in ambient air   and to
speciate  the hydrocarbons  using  gas
chromatography  provided  a new data
base.  An  analysis of this data base  was
done  by  OAQPS and another was done
by ERT as part of the CAL mechanism
development. The  carbon fractions  ob-
tained  in  these two efforts were signif-
icantly different.  A new analysis of the
data was undertaken for both the CB4
and CAL  mechanisms using a single set
of assumptions and speciation guidance
from the  model developers. A total of 66
city-years of detailed HC data (compris-
ing 773  individual  analyses)  was avail-
able. A total of 56 HC samples collected
aloft by Washington State University  was
also added  to the data base. These data
were speciated into the carbon fractions
for hydrocarbon species used in  the two
mechanisms. Fractions for each mechan-
ism are reported for each  city-year,  for
five clusters of city years with  similar
internal composition,  and  for the total.
The five  clusters were determined using
two forms of cluster analysis. Multiple re-
gression  was also  used  to confirm the
overall average carbon fractions.  Four
analyses  of automobile exhaust from the
UNC  autoexhaust smog chamber study
were also speciated to assist modelers in
testing chamber data in the same  manner
as atmospheric data. Recommendations
for default carbon fractions for  both
mechanisms were developed and  the
fractions  have now been included in the
computer code for OZIPM4. Selected
simulations  using both mechanisms were
also  conducted  to  demonstrate  the
effects of  HC composition  on ozone
predictions.

EKMA Control Calculation
Comparison
  A number of EKMA calculations were
carried out  with both mechanisms using
the OZIPM4  program. The  conditions
selected  were derived from cases used
for State Implementation Plan develop-
ment. Fourteen cases were  simulated for
Nashville, Tulsa,  Puget, Washington  DC,
Phoenix,  Philadelphia, and New York.
Baseline ozone values range from 0.13  to
0.234 ppm and calculated VOC reduction
estimates ranged  from  13% to 57%.
Sometimes CB4  predicted the higher
control requirement and sometimes CAL
predicted the higher control requirement.
The two  mechanisms  gave control re-
quirements  that differed at most  by only
7%.
  In a series of sensitivity tests, the CB4
mechanism  showed more sensitivity to
external radical  sources (e.g. hydrocar-
bon aloft) than did CAL. CB4 calculated
controls also exhibited  a much larger
sensitivity  to temperature than  did CAL:
CB4 showed a 10% change in  control
requirements for a 5°F change in simu-
lation temperature while CAL  showed
only a 5% change for the same temper-
ature variation. This sensitivity is directly
related  to the  different temperature
dependencies used in the two mechan-
isms to describe  the formation of PAN.
CB4 uses  a much higher negative value
for the activation energy than does CAL.
The CB4 value is based upon fits needed
to simulate  winter experiments  in  the
UNC chamber, whereas the CAL value is
based upon the limited temperature
range of the kinetic data for this reaction
that are found in the literature. The large
value used in the CB4 must be regarded
as speculation  until experimental tests
confirm or  refute the value. This may limit
the application  of these mechanisms to
ambient temperatures above about 75
degrees F.

Conclusions
  In  this project, two state-of-the-science
photochemical  mechanisms were com-
pared. Many differences occurring during
the development of the mechanisms
were examined  and resolved  where
possible.  Other activities  needed to  ob-
tain common inputs,  such as photolysis
rates, were undertaken.
  The mechanisms  were  compared
against chamber experiments.  Both
mechanisms simulate HCHO/NOX  sys-
tems nearly  perfectly.  Both  mechanisms
simulate ethylene systems  reasonably
well, but  require  variable "wall radical"
sources to fit ozone,  NOX, and ethylene
data. Neither mechanism fits HCHO pro-
duct data in  these systems.  For organics
more complex  than  ethylene, the  two
mechanisms do diverge and the quality
of the fits varies. For example, CAL is
better for propylene, neither is very good
for 1-butene, but CAL is much  better for
trans-2-butene. CB4 is better for toluene
and CAL is  better for xylene. Neither is
very good at prediction PAN or HCHO
formation from aromatics. Both  are good
for simple urban-tike  mixtures,  and  both
are good for synthetic autoexhaust. Both,
however,  are too reactive for  synthetic
urban hydrocarbon mixtures. Both mech-
anisms give similar predictions of control
requirements in EKMA  applications,  but
CB4 is very temperature sensitive.
  Finally, neither mechanism is superior
to the other  in either fitting smog cham-
ber data or in  predicting VOC control
requirements. Thus the choice between
them must be made on other grounds.

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H. E. Jeffries, K. G. Sexton, J. R. Arnold, and  J. L Li  are with the University of
  North Carolina, Chapel Hill, NC 27514.
Mania C. Dodge is the EPA Project Officer (see below).
The complete report consists of four volumes,  entitled 'Validation Testing of New
  Mechanisms with Outdoor Chamber Data:" (4 volume set: Order No. PB 89-159
  024/AS; Cost: $81.00)
  'Volume 1. Comparison of CB4 and CAL Mechanisms," (Order No.  PB 89-159
   032/AS; Cost: $28.95)
  'Volume 2. Analysis of VOC Data  for  the CB4 and CAL Photochemical Mech-
   anisms," (Order No. PB 89-159 040/AS; Cost: $21.95)
  'Volume 3. Calculation of Photochemial Reaction Photolysis Rates in the  UNC
   Outdoor Chamber." (Order No. PB 89-159 057/AS; Cost: $21.95)
  'Volume 4. Appendixes to Photochemical Reaction Photolysis Rates in the  UNC
   Outdoor Chamber," (Order No. PB 89-159 065/AS; Cost: $21.95)
The above documents will te available only from: (Costs subject to change)
        National Technical Information Service
        5285 Port Royal Road
        Springfield, VA 22 1SJ
The EPA Prefect Officer caw tee 
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