.% *."/
United States
Environmental Protection
Agency
Environmental Sciences Research ~,
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA.-600/S3-83-100 Jan. 1984
&ER& Project Summary
Experimental Protocol for
Determining Photolysis
Reaction Rate Constants
William P. L Carter, Roger Atkinson, Arthur M. Winer, and James N. Pitts, Jr.
An experimental protocol for the
determination of photolysis rates of
chemicals which photolyze relatively
rapidly in the gas phase at room temper-
ature has been developed, and is de-
scribed in detail. This procedure pro-
vides a basis for evaluating the relative
importance of one atmospheric reaction
pathway (i.e., photolysis) for organic
substances which may be emitted into
the environment and which strongly
absorb actinic radiation and photode-
compose with high efficiencies. This
technique is based upon monitoring the
disappearance rates of the test com-
pound and of a reference organic, both
of whose OH radical reaction rate
constants are accurately known, in
irradiated NO-organic-air mixtures.
Irradiations employing blacklamps
emitting in the actinic region are carried
out in —75-1 volume cylindrical Teflon
bags. The concentrations of the reac-
tantsare: NO, ~5ppm; test compound,
~1 ppm; reference organic, ~1 ppm.
The test compound and reference or-
ganic are monitored by gas chromatog-
raphy; NO, NO, and O3 are monitored
by chemiluminescence. The light inten-
sity in the experimental system is moni-
tored by NOa actinometry, and correc-
tions for the differences in light intensity
between the experimental system and
the atmosphere are made by multiply-
ing the measured photolysis rate of the
test compound by the ratio of the
calculated atmospheric photolysis rate
to the measured laboratory NO2 photol-
ysis rate.
Ideally, the light source in the cham-
ber should have the same spectral
distribution and intensity as solar radia-
tion at the earth's surface at the appro-
priate latitude. Unfortunately, practical
limitations (i.e., cost and complexity)
dictate the use of blacklamps, the type
typically used in environmental cham-
ber studies of the photooxidations of
organics. The fact that the spectral
distribution provided by these lamps is
different from the solar spectral distribu-
tion leads to uncertainties when extra-
polating the chamber results to ambient
conditions.
Depending on the reproducibility of
the analysis technique, it is anticipated
that photolysis rates > 1 x 10"5 sec'1 in
the chamber can be determined using
the technique. This limits the procedure
to these organics which photolyze rela-
tively rapidly, such as nitrites, a-dicar-
bonyls and nitrosamines. However,
while the technique cannot be used for
simple aldehydes and ketones and other
relatively weakly absorbing organics,
for most species this may not be a
serious limitation because their atmos-
pheric lifetimes are determined mainly
by reaction with OH radicals.
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).
Introduction
Under the sponsorship of the U.S.
Environmental Protection Agency, the
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Statewide Air Pollution Research Center
(SAPRC) at the University of California,
Riverside has developed and validated
experimental 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 absorption
of light followed by decomposition or
isomerization.
• Reaction with ozone.
• Reaction with the hydroxyl radical.
• For aromatic compounds containing
an -OH substituent group, reaction
with the nitrate (N03) radical.
In order to assess (a) the atmospheric
lifetime of compounds with respect to
these gas-phase removal processes and
(b) the relative importance of each of
these reaction pathways, rate consta nts for
photolysis and/or chemical reaction must
be experimentally determined for individ-
ual compounds.
The experimental procedures detailed
in this protocol are designed to enable
rate constants for photolysis of organics
and certain inorganics to be determined
at room temperature. With a knowledge
of these rate constants, the atmospheric
lifetimes of these compounds may be
estimated with respect to photolysis as
shown below. Due to experimental limita-
• tions, this protocol is applicable only for
organics which photolyze relatively rap-
idly (lifetimes of ^ 30 hrs). These include
strongly absorbing species which photo-
dissociate rapidly, such as nitrites, a-
dicarbonylsand nitrosamines. Fortunate-
ly, most organics which photolyze also
react rapidly with the hydroxyl radical, so
for these compounds this shortcoming is
not overly serious in terms of estimating
atmospheric lifetimes. However, there
are a few organics (e.g., acetone, methyl
ethyl ketone and diethyl ketone) which
react only slowly with OH radicals and do
not react with O3, and which photolyze at
rates lower than the range for which this
protocol is useful; for these compounds
alternate techniques for determining
photolytic lifetimes are required.
Under atmospheric conditions, the pho-
tolysis rate constant, kp, for the reaction
test compound + hv — products (1)
2
iS9ivenby
_r800
~
(I)
where a^ is the absorption cross-section
at the wavelength ^, ^ is the correspond-
ing quantum yield (the fraction of mole-
cules absorbing light which subsequently
isomerize or decompose; always < 1.00
for elementary reactions), and J^ is the
solar flux at the wavelength ^.
The short wavelength cut-off of A =*
290 nm is determined by the transmis-
sion properties of the atmosphere, while
the long wavelength limit is set by
thermochemistry, since light of wave-
length > 800 nm is not of sufficient
energy to break chemical bonds of ground
state, thermally stable molecules. The
solar flux is conveniently tabulated as
average values of J^ over 10 nm
intervals, so the integral in equation (I)
can be replaced by a summation to yield
_ T800
~
nm
(ID
where 0^/1 ar)d ^A/l a'so rePresent the
•absorption cross-section and quantum
yield values averaged over 10 nm inter-
vals. Tabulated values of J^ for various
latitudes and various times are given
elsewhere (Peterson 1976, Hendry and
Kenley 1979, Pitts et al. 1981).
Thus from a knowledge of the absorp-
tion coefficients &AA °^ tne Or9an'c of
interest, the quantum yields ^^ for
photolysis reactions which lead to its
removal, and with the tabulated values of
JA/I- tne Photolysis rate constant kp can
in principle be calculated for any latitude,
season, and time of day without the
necessityfor experimental rate measures.
However, the accurate determination of
quantum yields for organic compounds in
air as a function of wavelength is a highly
complex and time-consuming proposi-
tion, and the routine determination of
absolute quantum yields is not practical
at the present time.
On the other hand, a relatively straight-
forward experimental protocol is available
for the measurement of absorption co-
efficients of organics (Pitts et al. 1981).
Since quantum yields for elementary
photochemical reactions cannot exceed
unity, then upper limits to the atmos-
pheric photolysis rate constants can be
estimated, on the assumption that ^ =
1.00
(III)
^=290
This approach, which is described in
detail elsewhere (Pitts et al. 1981), is
useful for determining the organics for
which atmospheric removal by photolysis
is not important. However, for those
organics whose maximum photolysis rate
is significant, a more quantitative indica-
tion of the actual tropospheric photolysis
rates must also be obtained experimen-
tally.
General Approach Employed
The approach described in this docu-
ment for obtaining estimates for atmos-
pheric photolysis rates for organics con-
sists of (1) experimentally measuring in
the laboratory the photolysis rate (or its
upper limit) of the organic in air, employ-
ing a light source whose intensity and
spectral distribution approximates as
closely as practical that of the tropo-
sphere; (2) measuring the rate of N02
photolysis upon irradiation with the same
light source and the same intensity as
used in the organic photolysis rate meas-
urements; and (3) estimating the photol-
ysis rate of the organic in the troposphere
from the known tropospheric N02photol-
ysis rates based on the assumption that
the NO2/organic photolysis rate ratios is
the same in the experimental system as
in the troposphere. This approach in-
volves a number of assumptions which '
may not always be valid, and the uncer-
tainties and limitations involved are dis-
cussed in detail in the report.
Laboratory Measurement of
Photolysis Rates
The experimental approach for the
laboratory measurement of the photolysis
rate of the organic test compound is
based on measuring the disappearance
rates of that compound and of a reference
organic in irradiated NO-test compound-
reference organic-air mixtures, where
conditions are such that the major loss
processes for the test compound should
only be photolysis and perhaps reaction
with the hydroxyl radical. The reference
organic is chosen to be a compound
which does not photolyze and whose
major loss process in such systems is
reaction with the hydroxyl radical, pref-
erably with a rate constant similar to that
of the test compound. The NO levels
employed are sufficiently high that the
concentrations of 03 and N03 are sup-
pressed, so that reaction of the organics
with these species is minimized.
Thus the reactions which must be
considered in the NO-organic-air system
used to determine photolysis rates are A
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those listed below:
kp
test compound + hv — products (1)
k2
test compound + OH — products (2)
k3
test compound + wall — loss of (3)
test compound
k4
reference organic + OH — (4)
products
Note that in practice, for a test compound
whose behavior in the gas phase may not
be well characterized, the possibility of
heterogeneous removal (reaction 3) must
be considered. Based on the above reac-
tions, we can derive:
-din [test compound]/dt = (IV)
and
-din [reference organic]/dt =
k4[OH]
(V)
and hence.
is
the effective quantum yield for the net
production of nitric oxide upon photolysis.
This effective quantum yield was calcu-
lated by Zafonte etal. (1977) to be 1.61 ±
0.07 under the range of reactant con-
centrations generally employed ([N02] =
1 -4 ppm; [N0]0 = 0-0.3 ppm). Zafonte et
al. (1977) also calculated that no correc-
tion for refraction of light through quartz
is necessary when a tubular reactor is
employed.
Estimation of the Tropospheric
Photolysis Rate of the Test
Compound
The tropospheric photolysis rate of N02
(kf^Qp) can be calculated as a function of
the solar zenith angle, Z, from tabulated
solar fluxes, J^rop(Z) and the known NO2
absorption coefficients, tr^ 2 and photo-
decomposition quantum yields, ^J^
using the equation
> _ (430 itrop,7, -rN02 ,
-
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la)
3 D
t
45 cm.
!
< Idn^m -Jf
(b)
M Heat Seal BC to DE: A to BD. F to CE
Figure 1. Construction of the Teflon
reaction bag.
GEF15T8—BL
Blacklights
Fan
Wire Mesh
Screen
Initial concentrations of the reactants
are typically: NO, ~5 ppm; test and
reference organics, ~1 ppm. The NO is
present to minimize Osformation and any
reaction with the organics. The organic
reactants are monitored by gas chroma-
tography prior to and during the irradia-
tions. With the protocol developed here,
the irradiation should be carried out for at
least one hour at full light intensity.
For organics which react with ozone
(i.e., the alkenes) care should be taken not
to obtain data when the reaction with O3
becomes important (i.e., reaction with O3
should contribute <10% of the organic
reaction rate with the OH radical).
Conclusion
The results are analyzed using equa-
tion (VI). The detailed protocol provides a
list of recommended reference organics
together with a detailed discussion of
potential problems associated with this
technique.
References
Hendry, D. G. and R. A. Kenley, 1979.
Atmospheric Reaction Products of Or-
ganic Compounds. EPA-560/12-79-
001, June.
Peterson, J. T., 1976. Calculated Actinic
Fluxes (290-700 nm) for Air Pollution
Photochemistry Applications. EPA-
600/4-76-025, June.
Pitts, J. N., Jr., A. M. Winer, D. R. Fitz, A.
K. Knudsen and R. Atkinson, 1981.
Experimental Protocol for Determining
Absorption Cross Sections of Organic
Compounds. EPA-600/3-81-051, De-
cember.
Zafonte, L, R. L Rieger and J. R. Holmes,
1977. Nitrogen Dioxide Photolysis in
the Los Angeles Atmosphere. Environ.
Sci. Technol., //, 483.
-SO cm.
1
Wire Mesh
Screen
V-
I Jl__j
Aluminum
Surround
Fan
Figure 2. Fluorescent lamp assembly.
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W. P. L Carter, R. Atkinson. A. M. Winer, and J. N. Pitts, Jr., are with the
University of California, Riverside, CA 92521.
Bruce W. Gay, Jr., is the EPA Project Officer (see below).
The complete report, entitled "Experimental Protocol for Determining Photolysis
Reaction Rate Constants."(Order No. PB 84-110 139; Cost: $8.50, 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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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