&EFA
United States
Environmental Protection
Agency
Environmental Sciences ResearcnJP^W >•
Laboratory " -'- ^*-*"
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-81-051 Dec. 1981
Project Summary
Experimental Protocol for
Determining Absorption Cross
Sections of Organic
Compounds
James N. Pitts, Jr., Arthur M. Winer, Dennis R. Fitz, Arne K. Knudsen,
and Roger Atkinson
An experimental protocol for the
determination of gas phase absorption
cross sections and the calculation of
maximum photolysis rates is described
in detail. Utilization of this protocol
will provide a basis for evaluating the
possible relative importance of one
atmospheric reaction pathway (pho-
tolysis) for organic substances emit-
ted into the environment.
The experimental technique
involves measuring the absorption
spectrum over the wavelength region
285-825 nm at various known gas
phase concentrations of the test com-
pound in one atmosphere of ultra-pure
air. From the measured absorbances
(averaged over 10 nm wavelength
regions), absorption cross sections
(again averaged over 10 nm wave-
length increments) can be calculated.
These absorption cross sections,
together with solar flux data from the
literature, permit calculation of the
photolysis rates under atmospheric
conditions. Since a photolysis
quantum yield of unity is assumed in
these calculations, the resulting
photolysis rates are upper limits.
Calculating the maximum photoly-
sis rate permits an assessment of the
importance of photolysis (in compari-
son to reaction with ozone and with
the hydroxyl radical) as an atmospheric
reaction pathway. If the photolysis
rate is shown to be of importance,
further experimental data on the quan-
tum yield for photolysis under atmos-'
pheric conditions are required to pre-
cisely determine the actual photolysis
rate.
This Project Summary was develop-
ed by EPA's Environmental Sciences
Research Laboratory, Research Triangle
Park, NC. to announce key findings of
the research project that is fully doc-
umented in a separate report of the
same title (see Project Report order-
ing 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
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 atmos-
phere, the following homogeneous gas
phase removal routes are likely to be
important:
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• photolysis, which involves absorp-
tion of light followed by decom-
position or isomerization,
• reaction with ozone,
• reaction with hydroxyl radical, and
• for aromatic compounds contain-
ing an -OH substituent group,
reaction with the nitrate (NOs)
radical.
In order to assess (a) a given
compound's atmospheric lifetime with
respect to these gas phase removal
processes, and (b) the relative
importance for that compound of each
of these reaction pathways, rate con-
stants for photolysis and/or chemical
reaction must be experimentally deter-
mined. The protocol described here
permits the determination, at room
temperature of absorption cross
sections of organics. These absorption
cross sections can be used, in conjunc-
tion with available solar flux data, to
calculate maximum photolysis rates.
Rationale
For a chemical to be removed from the
atmosphere by photolysis, it must (a) ab-
sorb light in the wavelength region
applicable to the troposphere (A > 290
nm), and (b) having absorbed light,
undergo isomerization or decomposi-
tion.
For tropospheric purposes, the wave-
length region of interest is that between
~290 and 800 nm. The short-wave-
length cutoff is determined by the
transmission properties of the atmos-
phere. The long-wavelength limit is
determined by thermochemistry, since
light of wavelength >800 nm has
insufficient energy to break chemical
bonds of ground state molecules.
Under atmospheric conditions, the
photolysis rate constant, kp, for the
reaction
compound + hv— products
is given by
(Eq. D
where CT\ is the absorption cross section
at wavelength, ^, 0X is the correspond-
ing quantum yield (the fraction of the
molecules that absorb light and subse-
quently isomerize or decompose;
always <1.00 for atmospheric condi-
tions), and JA is the solar flux at
wavelength A. Since the solar flux is
conveniently tabulated as average
values'of JAA over 10 nm intervals the
integral in Equation 1 can be replaced by
a summation to yield
800
(Eq.2)
where CTA* and #A\ also represent the
absorption cross section and quantum
yield averaged over 10 nm intervals.
Since \ <1.00, an upper limit to the
atmospheric photolysis rate is given by
. max -r 800
Kp ^-A
(Eq-3)
and, since values of JA\ are readily
available (Peterson 1976), an upper
limit to kp can be calculated from a
knowledge of the absorption cross
section, a*, which can be averaged over
10 nm intervals to yield CTA*. Should the
compound fail to exhibit a sufficient
absorption cross section or should it
have a vapor pressure too low to yield an
accurately determinable absorbance,
an order of magnitude approximation to
its vapor phase spectrum may be
acquired from solution phase data.
The actinic flux, J\, is also a function
of elevation and zenith angle, which are,
in turn, functions of the time of day,
season, and various light attenuating
factors (cloud cover, dust, etc.). A com-
pilation of JAA values, averaged over the
entire day for the summer and winter
solstices and the spring and fall equi-
noxes at 10°, 30°, and 50°N latitude, is
included in the complete report. From
these data and values of (TAX determined
using the protocol, upper limits to kp
(i.e., kpmax) may be derived, yielding
lower limits to the photolysis lifetime,
rp, since
= l/kp
(Eq. 4)
Comparison of this calculated photol-
ysis lifetime with the lifetimes calcu-
lated for reaction with ozone (Pitts et al.
1981 a) and with the hydroxyl radical
(Pitts et al. 1981b) allows an assess-
ment of the possible importance of
photolysis as an atmospheric removal
process. If photolysis is of possible
importance, further experimental test-
ing is necessary to determine the actual
photolysis rate under atmospheric
conditions.
Description of the Protocol
Obtaining Absorption
Cross Section
The gas phase absorption spectra are
obtained using a cylindrical Pyrex cell
10 cm in 'length fitted with quartz end
windows and a commercially available
spectrophotometer with an absorbance
sensitivity of 0.001 at a signal-to-noise
ratio of unity.
The test compound, if a liquid, is
degassed using a greaseless, high-
vacuum gas-handling rack. Known
pressures of the compound are intro-
duced into the gas absorption cell,
which is then filled to atmospheric
pressure with dried ultra high purity air.
The ultraviolet (UV)-visible spectrum
is measured relative to a matched cell
filled with dried ultra high purity air
from the same cylinder. The spectrum is
measured from 285 to 825 nm using
minimum slit openings. This procedure
is repeated with at least two other pres-
sures of differing factors of between 2
and 10 (e.g., 1, 5, and 20 torr), depend-
ing on the strength of the absorption
and the sensitivity and stability of the
spectrophotometer. Since many com-j
pounds do not absorb at the longer!
wavelengths, it is acceptable to begin
the scan at ~50 nm above the onset of
the lowest energy absorption.
Immediately prior to and after each
set of runs, a blank spectrum is ob-
tained. The UV cell is evacuated to 10"5
torr and filled with dried ultra high pur-
ity air. The cell is then placed in the
spectrophotometer and a spectrum ob-
tained relative to the reference cell,
using the same sources, scale, and slit
widths that will be used for sample
spectra.
If the properties of the compound
(e.g., width of absorption cross sections',
vapor pressure) are such that the
maximum absorbance obtainable is
one-tenth of the most sensitive spectro-
photometer scale or less (i.e., <0.001
absorbance), a solution phase study
should be undertaken. While not
capable of yielding an accurate value for
ox solution phase results are reproduc-
ible and yield an order-of-magnitude
approximation.
Calculating the Maximum
Photolysis Rate Constant
The blank spectrum, with both the
reference and sample cells filled wit[j|
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either dried ultra high purity air or
solvent, is used to correct the sample
spectra for absorbance due to small
spectral differences between the cells.
A matched pair of cells should require
minimal correction.
The average absorbances over each
of the 10 nm intervals for which JA\
values are listed (e.g., 285-295, 295-
305 nm, etc.) are tabulated. Photochem-
ical cross sections are determined from
the Beer-Lambert Law
0AX =
where &&>. is the wavelength-averaged
cross section (in cm2 molecule"1), 1 is
the ceir pathlength (in cm), C is the con-
centration of absorbing species (in
molecules cm"3), and AAX is the wave-
length-averaged absorbance (in loga-
rithm to the base e units). An absorb-
ance given in base 10 units must be
multiplied by 2.3026 to convert to base e
units.
The cross sections,
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Environmental Protection
Agency
Center for Environmental Research
Information
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