United States
Environmental Protection
Agency
Environmental Sciences Research"
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-81-024 May 1981
Project Summary
Experimental Protocol for
Determining Ozone
Reaction Rate Constants
James N. Pitts, Jr., Arthur M. Winer, Dennis R. Fitz, Sara M. Aschmann, and
Roger Atkinson
An experimental protocol for the
determination of room temperature
rate constants for the reactions of
ozone with chemicals in the gas phase
has been developed and validated.
This protocol provides a basis for
evaluating the importance of one
atmospheric reaction pathway, attach
by ozone, for organic substances
which may be emitted into the environ-
ment.
The experimental technique is based
upon monitoring the pseudo-first
order decay of ozone (initially present
at ~1 ppm) in the presence (and ab-
sence) of known excess concentrations
of the test compound, using pure air as
a diluent gas. The ozone reaction rate
constants are then calculated from the
dependence of the observed ozone
decay rates on the concentration of
the test compound.
Experimentally, the reactions are
carried out in —150-180 liter Teflon
reaction bags. The reaction bag is
initially divided into two approximately
equal sub-chambers. Known concen-
trations of ozone and the test com-
pound are then introduced into the
two sub-chambers, ozone into one,
and the test compound into the other.
The barrier between the two sub-
chambers is then removed, the con-
tents of the reaction bag mixed, and
the ozone concentration monitored as
a function of time.
Using this technique, ozone rate
constants in the range ~10~20 cm3
molecule'1 sec"1 to ~10~16 cm3 mole-
cule"1 sec"1 can be readily measured
for test compound concentrations of
~0.1 torr (i.e., 100 ppm). This range of
rate constants generally encompasses
the reactivities of interest from an
atmospheric point of view.
This Project Summary was develop-
ed by EPA's Environmental Sciences
Research Laboratory, Research Tri-
angle Park. NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back}.
Introduction
Under the sponsorship of the U.S.
Environmental Protection Agency, the
Statewide Air Pollution Research Center
at the University of California, Riverside
is developing and validating experimen-
tal protocols to assess the atmospheric
fates and lifetimes of organic compounds.
Chemical compounds emitted into the
atmosphere are removed or degraded by
pathways involving gas phase reactions
or wet or dry deposition. Laboratory and
environmental chamber studies have
shown that for the ambient atmosphere
the following homogeneous gas phase
removal routes are likely to be important:
• Photolysis, which involves absorp-
tion of light followed by decomposi-
tion or isomerization.
-------
• Reaction with ozone.
• Reaction with the hydroxyl radical.
• For aromatic compounds contain-
ing an -OH substituent group,
reaction with the nitrate (N03)
radical.
In order to assess the atmospheric
lifetime of compounds with respect to
these gas-phase removal processes and
the relative importance of each of these
reaction pathways, rate constants for
photolysis and/or chemical reaction
must be experimentally determined for
individual compounds.
The experimental procedures detailed
in this protocol are designed to enable
rate constants for reaction of organics
and certain inorganics with 03 to be
determined at room temperature. With
a knowledge of the rate constant for
reaction with ozone, the atmospheric
lifetime may be estimated as shown
below. In the atmosphere, where the
reaction
03 + chemical — products
occurs, the decay of the chemical via this
reaction is given by
-d[chemical]/dt = k°3[03Ichemical] (1)
where [ ] denotes concentration and
k°3 is the rate constant for the reaction
of 03 with the particular chemical. Since
the Os concentration remains constant
or approximately so in the ambient at-
mosphere, equation (1) may be rearranged.
to yield
ofk°
-10"16 cm3 molecule
"1 sec'1, the
-dln[chemical]/dt = k°3[03]
and
ln([chemical]to/[chemical]t) =
k03[03](t-t0}
(2)
(3)
where [chemical]to and [chemical]t are
the concentrations of the chemical at
times t0 and t, and In is the logarithm to
the base e. Under atmospheric condi-
tions, the l/e lifetime, r (i.e., the time for
the concentration of the chemical due to
reaction with 03 to decrease by a factor of
e = 2.7), of any chemical with respect to
reaction with 03 is given by
T = (k°aIOsjr
(4)
where [O3] is the ambient atmospheric
concentration.
For tropospheric purposes, where the
03 concentration in the lower tropo-
sphere is approximately 40 ppb, rate
constants k 3 > 10"20 cm3 molecule"1
sec'1 are of importance, corresponding
to lifetimes > 3 years. For a rate constant
lifetime is similarly calculated to be ~3
hours.
Experimental Approach
The experimental approach described
below is based upon observing the
increased rate of ozone decay in the
presence of a large excess of the test
compound in the dark. Thus the two
processes removing 03 in the presence
of the test compound are:
03 + test compound — products
with a rate constant k03
03 + wall — loss of ozone
with a rate constant kw
-185± 10cm-
and hence
-d[03]/dt = (kw
[test compound]) [03]
k°3
(5)
With the concentration of the test
compound being much in excess of the
initial ozone concentration ([test com-
pound]/[03]initiai> 10), the test compound
concentration remains essentially con-
stant throughout the reaction, and
equation (5) may be rearranged to yield:
-dln[03]/dt = kw -f k°3
[test compound] (6)
k°3 may be readily determined from a
knowledge of the background ozone
decay rate in the absence of added
compound, kw/ and the ozone decay
rates, -dln[O3]/dt, at known concentra-
tions of the test compound. Since it is
difficult to detect incremental changes
in the ozone decay rates that are much
smaller than k«, lower limits of the rate
constants that can be determined occurs
when k°3[test compound] « kw, while
the upper limit is determined by the
response time of the ozone monitoring
device.
Experimental
Reactions are carried out in a ~175-
liter volume Teflon bag, constructed out
of a 2 mil thick, 180x140cmFEPTeflon
sheet, heat-sealed around the edges,
and fitted with Teflon injection and
sampling ports at each end of the bag, as
shown in Figure 1. For the determination
of 03 decay rates in the presence of a
reactant, the Teflon bag is initially
divided into two subchambers of approxi-
mately equal volume by metal rods. One
of these two subchambers is filled with
a known volume of ultra-high purity air
and —20 cm3 of ~1 % 03 in 02 (produced
by a Welsbach T-408 ozone generator)
Centerline
/
/ Q
*V\ ^J>
Ports
p
14C
±10cn
\
C
B
(c) Heat Seal AB to DC. BF to CF. AE
toDE.
Figure 1. Construction of the
Teflon reaction bag.
is then injected using an all-glass, gas
tight syringe. This amount of ozone i
sufficient to yield an O3 concentration (
—1 ppm (1 ppm = 2.40 x 1013 molecul
cm at 735 torr total pressure and 29
K) in the entire reaction bag. The reai
tant organic is introduced into the othi
subchamber, again using ultra-hie
purity air as the diluent gas. If tr
organic is gaseous, then this subchar
ber is filled with a known volume of tl
diluent gas, and a known volume of tl
organic is injected using an all-gla
gas-tight syringe. If the organic is
liquid, then a known volume of the liqi
is introduced into a —1 -liter Pyrex bi
and the contents flushed into the si
chamber by a known flow of the dilu<
gas.
The reaction is commenced by remi
ing the metal barriers and mixing 1
contents of the two subchambers
pushing down on alternate sides of i
entire reaction bag for 1 to 2 mins.
concentrations are monitored ai
function of time after the mixing b
chemiluminescence ozone analyz
The organic reactant concentrations
the entire bag are calculated from
amount of organic introduced and
total volume of air used to fill the '
subchambers.
Background ozone decay rates, in
absence of the reactants, are deterr
ed periodically during the rate cons
determinations. In these cases,
entire reaction bag is filled with u
high purity air, and the O3 then injec
-------
Rate constants k°3 are then derived
from the slopes of plots of the ozone
decay rates, -dln[03]/dt, against the
organic concentration, in accordance
with equation (6). Such a plot is shown
in Figure 2 for the case of o-cresol, from
which a rate constant of 3.67 x 10"*
pprrf1 min"1, or 2.55 x 10"19cm3 mole-
cule"1 sec"1, is obtained.
James N. Pitts. Jr., Arthur M. Winer, Dennis R. Fitz, Sara M. Aschmann, and
Roger Atkinson are with the Statewide Air Pollution Research Center,
University of California, Riverside, CA 92521.
B. W. Gay, Jr. is the EPA Project Officer (see below).
The complete report, entitled "Experimental Protocol for Determining Ozone
Reaction Rate Constants," (Order No. PB 81-171 647; Cost: $6.50, 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:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
0.00
0 20 40 60 80 WO 120
[o-Cresof] ppm
Figure 2. Plot of ozone decay rate
against test compound
concentration for the
case of ortho-cresol.
This experimental technique has
been validated by demonstrating excel-
lent agreement between the rate con-
stants determined for the reaction of
ozone with ethene, propene and 1-
hexene and the corresponding litera-
ture values.
U.S. OOVEflNMENT PRINTING OFFICE 1M1 -757-012/7108
-------
United States Center for Environmental Research Fees Paid
Environmental Protection Information Environmental
Agency Cincinnati OH 45268 Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
SSS ECIIUN AGKKCJ
CHICAGO IL 60604
------- |