United States
Environmental Protection
Agency
Atmospheric Research and H N
Exposure Assessment Laboratory *£"' %
Research Triangle Park NC 27711 'i \
Research and Development
EPA/600/S3-88/041 Feb. 1989
&EPA Project Summary
Validation Data for
Photochemical Mechanisms:
Experimental Results
K. G. Sexton, J. R. Arnold, H. E. Jeffries, T. L. Kale, and R. M. Kamens
The smog chamber facility of the
University of North Carolina (Chapel
Hill) was used to collect experimental
data of various hydrocarbon and
oxides of nitrogen (NOX) systems.
These data are intended to be used
by atmospheric model developers for
testing and validating kinetic
mechanisms of photochemical smog
formation. The previous set of
experiments conducted in the UNC
outdoor smog chamber for initial
development and testing of chemical
kinetics was organized and analyzed.
New experiments were then
performed to: (1) provide "missing"
data; (2) resolve differences between
model predictions and the existing
experimental data; (3) extend the
range of urban-like conditions in the
data base; and (4) aid in model
construction and testing as re-
quested by model developers. In this
study, 71 dual experiments were
performed using NOX and various
individual hydrocarbons and hydro-
carbon mixtures. In addition, a
number of experiments were con-
ducted to better understand and
characterize: (1) the chamber when
operated dynamically to simulate
continuous emissions and meteor-
ological dilution, and (2) the light
inside the smog chamber.
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 docu-
mented in a separate report of the
same title (see Project Report
ordering information at back).
Introduction
The UNC smog chamber facility is
used to measure the reactants and
products that participate in the
photochemical smog formation process.
The smog chamber, located outdoors in
rural North Carolina, is an A-frame
structure covered with Teflon film. The
chamber is partitioned into two sections,
each with a volume of 156 m3, so that
two experiments can be conducted
simultaneously. The dual chamber is
operated under natural conditions of solar
radiation, temperature and relative
humidity. The smog chamber data
collected are used to test photochemical
mechanisms Earlier mechanism testing
against chamber data had shortcomings
in that the range of test data available
was not great enough to encompass the
range of conditions likely to occur in an
urban situation. This is in part because
previous testing had concentrated on the
immediate need to study basic chemistry
and reactivity issues. Modeling results
and analysis of the existing smog
chamber and support data indicated four
areas where additional research was
needed to test chemical mechanisms
more completely. An overview of the
chamber experiments conducted in each
of these areas is described in the
following four sections.
Provide Missing Data
Experiments were conducted to fill
missing gaps in the existing database.
The "missing data" fell into three broad
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categories: missing smog chamber
experiments of individual hydrocarbons,
missing information that would aid
modelers in simulating the urban
environment, and experiments to test
new developments (new species and
new chemistry) in the more recent
chemical mechanisms. In addition, some
previous experiments that were of limited
usefulness due to weather problems, lack
of product data, or poor supporting
calibration data were repeated.
In this portion of the study, a number
of toluene, m- and o-xylene and tri-
methylbenzene experiments were carried
out. Experiments were also conducted in
which these aromatic species were
added to simple and complex hydro-
carbon mixtures. Experiments were also
conducted utilizing urban-like hydro-
carbon mixtures to demonstrate the
effect of reduced total hydrocarbon
(hydrocarbon control) and hydrocarbon
substitution. Model simulations using the
Empirical Kinetics Modeling Approach
(EKMA) indicated that small amounts of
aldehydes could have significant effects
on reactivity. Therefore, smog chamber
experiments were carried out where
varying amounts of formaldehyde were
added to urban surrogate mixtures. The
results showed that the effect of the
added formaldehyde depended on the
general reactivity of the system and the
hydrocarbon-to-NOx ratio. Less reac-
tive systems could be made more
reactive by addition of formaldehyde
while reactive systems were less
effected.
Model/Data Differences
Some previous experiments conducted
with ethylene and aromatic hydrocarbons
proved to be difficult to simulate. The
observed changes in reactivity with
changes in the HC-to-NOx ratio could
not be simulated. To resolve these dif-
ferences, a number of ethylene experi-
ments were conducted with the aid of
special analytical techniques; these
experiments were carried out over a wide
range of ethylene-to-NOx ratios.
Several experiments were also carried
out using a six-component aromatics
mix to test the representation of aromatic
hydrocarbons that are used in some of
the newer mechanisms. Other experi-
ments were conducted in which the
same concentration of NOX and total
hydrocarbon was added to the two sides
of the chamber, but the ratio of toluene
to m-xylene in the two sides was varied.
Experiments were also conducted to test
the chemistry of new species and new
reactions that are included in the latest
state-of-the-science photochemical
mechanisms.
Urban-Like Conditions
New synthetic hydrocarbon mixtures
for auto exhaust and urban conditions
were designed. Several experiments
were conducted with these mixtures
where the same level of NOX but different
amounts of hydrocarbon were added to
the two sides of the chamber. These
experiments were designed to test the
models ability to simulate the effect on
ozone production of varying the
hydrocarbon-to-NOx ratio.
Assistance for Modelers
Many experiments were conducted as
requested by different modelers. The
Unisearch tunable laser system for
formaldehyde, hydrogen peroxide and
nitric acid and their high sensitivity
luminol NOa monitor were used in
several experiments to obtain data eithc
rarely obtained or that would validal
measurements made earlier with oth«
methods. Some experiments wer
conducted to investigate the reactivity <
several aromatic oxidation products. Th
use of isobutene as a surrogate fc
formaldehyde was also tested. Ozon
photolysis was studied with the aid <
nitrous oxide. Butane and biaceti
experiments were conducted to hei
characterize the smog chamber.
A number of dynamic experimenl
were also carried out to test the rea
world effects of continuous emission
and meteorological dilution. The dynam
experiments that were carried out in
previous UNC study had some problerr
in that modelers had difficulty dete
mining the emission and dilution rate
These problems were investigated an
resolved. Model simulations of a ke
dynamic experiment carried out in tr
present study are presented in tr
Project Report. The sun position ar
structure effects on the chamber wei
also studied to better understand ar
estimate the photolytic rates inside tt
chamber.
Conclusions
A database of the 346 experimen
conducted in past UNC chamber studii
and the 71 new experiments carried o
in the present study has been prepan
to aid modelers in evaluating chemic
mechanisms. This database has alreac
been supplied to modelers for analys
under EPA contracts and it is nc
available for use by others in tl
scientific community. Copies of th
database on floppy diskettes and oth
supporting information are available frc
the authors.
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K. G. Sexton, J. R. Arnold, H. E. Jeffries, T. L Kale, and R. M. Kamens are with the
University of North Carolina, Chapel Hill, NC 27514.
Mania C. Dodge is the EPA Project Officer (see below).
The complete report, entitled "Validation Data for Photochemical Mechanisms:
Experimental Results," (Order No. PB 89-124 6141 AS; Cost: $15.95, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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Official Business
Penalty for Private Use $300
EPA/600/S3-88/041
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