United States
Environmental Protection
Agency
Atmospheric Sciences Research ^\v A '/
Laboratory _ ' ^—*
Research Triangle Park NC 27711 - ^
Research and Development
EPA/600/S3-85/063 Sept. 1985 <
&ERA Project Summary
Atmospheric Fates of Organic
Chemicals:
Prediction of Ozone and
Hydroxyl Radical Reaction
Rates and Mechanisms
Roger Atkinson, William P. L. Carter, Sara M. Aschmann, James N. Pitts, Jr.,
and Arthur M. Winer
During this three-year cooperative
agreement, the kinetic, mechanistic,
and product data available in the liter-
ature for the gas phase reactions of OH
radicals and O3 with organic compounds
have been evaluated and critically re-
viewed. Two review articles, one on O3
reactions, the other on OH radical
reactions, have resulted from this work.
The review dealing with O3 reactions
has been published in Chemical Re-
views. 84. 437-470 (1984). The OH
reaction review has been accepted for
publication in Chemical Reviews.
In addition to these extensive reviews,
an experimental program was conduct-
ed to obtain needed kinetic data for
selected OH radical and O3 reactions.
These data and the experimental tech-
niques used are summarized in this
summary report together with a discus-
sion of a priori predictive techniques for
the estimation of OH radical and O3
reaction rate constants for reactions
with organics for which experimental
data are not available.
This Project Summary was developed
by EPA's Atmospheric 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).
Introduction
Thousands of industrial chemicals,
many of which are critical to our economy,
are in use today. Recently, however, there
has been growing recognition of the need
to understand the environmental fates of
these chemicals, including their distribu-
tion, potential by-products, toxicity and
ecological effects, as well as their envi-
ronmental sinks and lifetimes. Unfortun-
ately, these factors are often poorly
characterized or, in many cases, com-
pletely undetermined. In order to provide
such data, which are essential for risk
assessments for existing and new chem-
icals, the national Toxic Substances Con-
trol Act (TSCA) was passed by Congress
and became effective January 1, 1977.
The provisions of the Act deal with four
principal areas: information gathering,
regulation, premanufacture screening,
and interagency cooperation.
One major goal of TSCA is to develop a
sufficiently large data base concerning
the environmental fates of chemicals so
that accurate a priori predictions can be
made concerning newly developed chem-
icals or chemicals presently in use for
which little or no experimental data exist.
Ready access to such an extensive and
reliable data base would then permit a
cost-effective, rapid assessment of the
environmental impact of both existing
and newly developed chemicals.
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The Atmospheric Sciences Research
Laboratory of the U.S. Environmental
Protection Agency (EPA) contracted, via
this cooperative agreement, the State-
wide Air Pollution Research Center
(SAPRC), University of California, River-
side, to establish a critically evaluated
data base for the rate constants and
atmospheric reaction mechanisms of the
reactions of 03 and OH radicals with
organic chemicals. Furthermore, this
cooperative agreement called for the
development of predictive relationships
for the estimation of 03 and OH radical
rate constants for reactions with organics
for which no experimental data are
available.
The overall objectives of this three-year
program were as follows:
1. To critically evaluate the literature
and develop a data base for the rate
constants for the gas phase reac-
tions of 03 and the OH radical with
organics.
2. To investigate, by using this data
base, predictive relationships based
on the molecular structures of
organics with the goal of developing
accurate means of predicting 03
and OH radical rate constants for
reactions with compounds for
which experimental data are not
available.
3. To critically evaluate, concurrently
with these objectives, the literature
dealing with the mechanistic as-
pects of these 03 and OH radical
reactions with organics under at-
mospheric conditions.
4. Since it was apparent that signif-
icant gaps in our knowledge of rate
constants for the reactions of O3
and OH radicals with organic com-
pounds existed, a modest, but high-
ly focussed, experimental program
was carried out to determine 03
and/or OH radical rate constants
for reactions with selected organic
compounds for which experimental
data were not available. The organic
compounds studied included, at the
request of the EPA, the three mono-
chlorobiphenyl isomers. These ex-
perimental studies were carried out
by using the protocols previously
developed and tested at SAPRC for
the EPA.
Experimental Program
In the experimental program, room
temperature OH radical and 03 rate con-
stants were determined for reactions with
approximately 60 organic compounds.
These OH radical and 03 rate constants
were chosen for study based on an initial
review of the data in the literature in
order to fill in much needed gaps in the
then available data base.
The experimental data obtained from
these kinetic studies provided a large
amount of important new data and al-
lowed the development of a priori pre-
dictive techniques for a large number of
classes of organic compounds. These data
were incorporated into the major tasks of
this cooperative agreement, namely the
critical evaluation and review of OH
radical and 03 reactions.
Review and Evaluation of OH
Radical and Oa Reaction Rate
Constants and Mechanisms
Under Atmospheric
Conditions and
Development of A Priori
Predictive Techniques
In two review articles1 the available
kinetic product and mechanistic data for
the gas phase reactions of 03 and OH
radicals with organic compounds were
compiled, evaluated, and reviewed. Em-
phasis was placed on the kinetics and
mechanisms of these reactions under
atmospheric conditions. Thus, kinetic
data obtained at low total pressures at
which the reactions of OH radicals with
certain of the alkenes, haloalkenes, al-
kynes, and aromatic hydrocarbons are in
the fall-off regime between second- and
third-order kinetics were not considered.
In a similar manner, high temperature (>
500 K) data were not included in these
reviews unless these data had been
obtained in investigations carried out over
temperature ranges extending to < 500 K.
The highlights of these two review articles
and their major conclusions are sum-
marized below.
Ozone Reactions
The major classes of organic com-
pounds that react with 03 at atmospher-
ically significant reaction rate constants
are the alkenes (including the monoter-
penes) and certain nitrogen-containing
compounds (such as the amines, hydra-
'Atkmson, R., and Carter, W P. L, Kinetics and
mechanisms of the gas phase reactions of ozone
with organic compounds under atmospheric condi-
tions, Chem. Rev.. 84,437-470 (1984); Atkinson, R.,
Kinetics and mechanisms of the gas phase reactions
of the hydroxyl radical with organic compounds
under atmospheric conditions, Chem. Rev , in press
(1985)
zines, diazo compounds, and hydrazones).
Only for the alkenes are sufficient data
available to allow any meaningful discus-
sion of rate constant correlations and
trends. For cycloalkenes, the available
kinetic data show that the existence of
ring strain leads to an enhancement, by
over an order of magnitude for bicyclo
[2.2.1]-2-heptene, of the room-temper-
ature rate constants over those for the
unstrained alkenes.
The data for the alkenes and non-
strained cycloalkenes show that the room
temperature rate constants can be ap-
proximately predicted from the configura-
tion and degree of substitution around
the double bond(s). The limited data
available for organics with more than one
type of sttbstrtuent on trie carbon-carbon
double bond suggest that the use of
multiplicative factors per substituent
(these factors are derived from the effect
that addition of this substituent to ethene
has on the rate constant) allows estima-
tion of the room-temperature rate con-
stants often to within a factor of ~3.
However, it is clear that further data are
needed before accurate a priori predictive
schemes can be derived for O3 reactions
with organic compounds.
OH Radical Reactions
Hydroxyl radicals react at atmospher-
ically significant rates with essentially all
organic compounds. These reactions pro-
ceed via two general types of mechan-
isms: those leading to overall H-atom
abstraction from C-H, 0-H, and N-H bonds
and those involving OH radical addition to
unsaturated carbon-carbon bonds (this
process includes addition to aromatic
rings).
Based on previous literature data, an a
priori predictive scheme has been 'de-
veloped that deals with (1) H-atom aJb-.
straction from C-H and O-H bonds, (2) OH
radical addition to >C=C< and -C^V-
bonds (including conjugated >C=C-C=C<
bond systems), and (3) OH radical addition
to aromatic rings.
For H-atom abstraction from C-H bonds,
-CH3, -CH2-, and >CH- group rate con-
stants are given by the following:
k(CH3-X) = kSnm F(X),
k(Y-CH2-X) = k°sec F(X) F(Y),
and
F(X) F(Y) F(Z),
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|where kpV,m, ksee, and k°ert are the rate
"constants per -CH3, -CH2-, and >CH-
groupsfor a given "standard" substituent;
X, Y, and Z are the substituent groups;
andF(X), F(Y), and F(Z)arethe correspond-
ing group factors. While the values of
kp,,m, k?ec, and k°tert can be adjusted for any
given substituent group X (=Y=Z), the
most appropriate standard substituents
are H- or CH3- groups. For practical use,
X = -CH3 is clearly the most useful,
leading to F(-CH3) = 1 .00 by definition.
Using our recommended rate constants
at 298 K with the other available room-
temperature rate constants given in the
relevant data tabulations, we have carried
out non-linear least-squares analyses of
these kinetic data, minimizing the sum of
the percentage errors, to derive values of
F(X) for a variety of substituent groups,
e.g., X = -CH2-, >CH-, >C<, -F, -Cl, -Br,
-CH2F, -CH2CI, -CH2Br, -CHF2, -CHCI2,
0
-CF3, -CF2CI, -CCI3, =0, -CHO, -C6H5, -C-,
O 00
-CHzL, -O-, -O(!i-, -Ho-, -OH, -ONO2,
and-CN.
For OH radical addition to unsaturated
>C=C< and -C=C- bonds, no significant
effects of ring strain have been observed,
and the approach used is based on the
(number of unconjugated double bonds or
conjugated double bond systems and the
degree, identity, and configuration of
substitution around these double bonds.
As an example, 2-methyl-1 ,4-pentadiene
(CH2=C-CH2-CH=CH2) contains a 1,1-di-
CH3
alkylsubstituted double bond (CH2=C<)
plus a mono alkyl-substituted double bond
(CH2=CH-), and the overall rate constant
is given by the sum of the rate constants
for 2-methylpropene (for CH2=C<) and
propene (for CH2=CH-).
Based on the data in the literature, the
optimum approach to the a priori predic-
tion of room-temperature rate constants
for OH radical addition to the aromatic
ring utilizes the excellent correlation
between the OH radical rate constants
kadd for addition to the aromatic ring and
the sum of the electrophilic substituent
constants, Z<7+. A unit-weighted least-
squares analysis of our recommended
room-temperature OH radical rate con-
stants yields the expression
log kadd(cm3 molecule"1 s~1) = -11.64 -
1.38 la+.
H-atom abstraction from C-H (and to a
lesser extent from O-H) bonds, and OH
radical addition to double and triple
carbon-carbon bonds and to aromatic
rings enable OH radical reaction rate
constants to be estimated with apparent-
ly reasonable reliability. It should, how-
ever, be noted that the available kinetic
data base for sulfur-, nitrogen- and
phosphorus-containing organics, and for
organometallics, is presently insufficient
for the extension of our predictive tech-
niques to these important classes of
organic compounds. Hopefully, this de-
ficiency will be reduced in future years by
the development of the necessary data
base. However, the present predictive
technique appears to be able to predict,
solely from the chemical structure of the
organic compound, room-temperature
rate constants to within a factor of <5
(and often to within a factor of 2 or better)
for reactions with a number of classes of
organic compounds.
The use of the above a priori predictive
(techniques, namely those applicable to
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Roger Atkinson, William P. L. Carter, Sara M. Aschmann, James N. Pitts, Jr., and
Arthur M. Winer are with Statewide Air Pollution Research Center, University
of California, Riverside, CA 92521.
Bruce W. Gay, Jr.is the EPA Project Officer (see below).
The complete report, entitled "Atmospheric Fates of Organic Chemicals:
Prediction of Ozone and Hydroxyl Radical Reaction Rates and Mechanisms,"
(Order No. PB 85-241 529; Cost: $11.50, 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 Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-85/063
00003-29 PS
AGENCT
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