United States
Environmental Protection
Agency
Atmospheric Sciences "^ ,
Research Laboratory *.
Research Triangle Park NC 27711 '
Research and Development
EPA/600/S3-86/005 May 1986
AEPA Project Summary
The Primary Photochemical
Processes of Acrolein
Edward P. Gardner, Paul D. Sperry, and
Jack G. Calvert
Pollutants are removed from the
atmosphere by a number of processes:
reaction with OH and O,, dry and wet
deposition, photodissociation by solar
radiation, or biodegradation. This sum-
mary discusses the photodissociation
processes of acrolein. Quantum yields of
acrolein loss are given. The dominant
reactions in the lower troposphere are
the formation of C.H and CO. Also
produced are CH2CHCHO, H, CH2CH. and
HCO radicals, but at lower quantum yields.
Since the OH attack on acrolein is quite
large (1.9 x 1011 cm3 motec"1 s'1), ambient
levels of OH (~ 10* molec cm 3) will
remove acrolein very rapidly, usually in
~ 15 h. Thus, the major loss mechanism
for acrolein in the troposphere is OH
attack, and the photodissociation pro-
cess is of negligible importance.
Thlt Project Summary wat developed
by EPA't Atmospheric Sc/ences Re-
search Laboratory, Research Triangle
Park, NC, to announce key finding* of
the research project that It hilly docu-
mented In a separate report of the tame
title (tee Project Report ordering In-
formation at back).
Introduction
The atmospheric concentration of
hazardous pollutants is determined by a
number of factors: the release rate of
these pollutants, their rate of generation
if produced in situ (e.g., formaldehyde),
their rate of photodissociation by sunlight,
rates of OH and 0, reaction, rates of
dilution and dispersion, and rate of bio-
logical degradation.
This summary presents the results of a
quantitative investigation of the mecha-
nism of and quantum yields for the
trope-spheric photooxidation of acrolein,
the simplest unsaturated aldehyde and a
potent lachrymator.
Acrolein has been observed at con-
centrations as high as 13 ppb (v/v) in the
atmosphere and is often observed at
about 15% of the formaldehyde con-
centration. When present in polluted
atmospheres, acrolein reacts quickly in
the presence of NO, to produce ozone
and formaldehyde.
Experiments with acrolein were de-
signed to simulate closely the conditions
encountered in the troposphere. Small
concentrations of acrolein were photo-
oxidized by light at a wavelength of 313
mn at 25°C in the presence of synthetic
air.
Experimental
The experimental apparatus for study-
ing the mechanism and quantum yields
for acrolein photooxidation were (1) a
vacuum line, (2) a reaction cell, (3) a light
source, and (4) a sampling apparatus
consisting of a gas chromatograph and a
mass spectrometer. The vacuum line was
a multifunctional gas-handling system
consisting of a storage facility, a mea-
surement facility to monitor precise
volumes of gas, a distillation flask, and a
calibration/mixing system. With this sys-
tem, very precise and pure concentrations
of acrolein could be prepared. The re-
action cell is coupled to the vacuum line
and is designed to create a photochemical
system. The internal optical path of the
cell is 155.8 cm and it has Suprasil end
windows. A narrow-band interference
filter was used with the cell; this enabled
light at 313 nm to be transmitted. The
-------
light source was a high-pressure mercury
arc (Osram HBO 500 w/2).
A Varian gas chromatograph equipped
with flame ionization and thermal con-
ductivity detectors was employed to
monitor both the acrolein and the reaction
products (CH , CO,, C.H HCHO, HOH,
CH3OH, CH3CHO, and HCOCHO). The
mass spectrometer (CEC model 21-104)
was used to confirm the identity of
compounds identified by the gas chro-
matographic analyses and to quantify
the hydrogen product.
Results and Discussion
Because the primary objective of this
study was to determine the primary and
secondary decomposition paths of acrolein
in the troposphere, experiments were
conducted to determine the rate of photo-
decomposition with increasing atmo-
spheric pressure. Table 1 shows the data
obtained when acrolein (20 to 800 torr)
was irradiated at 25°C with 313 nm
wavelength light isolated by a Jarrell-
Ash grating nonochromator (runs 1M -
6M) or narrow band filter (runs 7F -11F).
All runs were conducted with acrolein at
a pressure of approximately 0.355 torr
with 20% 02 and 80% N2.
A complete list of product quantum
yields for all runs (1M - 11 F) is provided
in Table 2. From these data, the fol-
lowing observations were made:
(1) The dominant products are CO and
C2H4. The identified quantified pro-
ducts are listed in the following order
according to the amount produced.
CO > C H > HCHO (« H ) >
HCOCHO > CO > CH3OH (~ CH4)
Traces of acetaldehyde, acetylene, and
acetic acid were also observed.
(2) Much C2H4 was formed, in spite of
high 02 levels; this suggests that C2H4
is eliminated (as is CO) by a primary
dissociative pathway. The other pro-
ducts, HCHO and HCOCHO in particu-
lar, are secondary 02-associated pro-
ducts; their production involves CH2CH
and CH3CH free radicals.
(3) The small quantum yield of acrolein
loss suggests that the deactivation of
excited acrolein is very efficient; this
also suggests that energy is trans-
ferred very quickly to oxygen followed
by intersystem crossing of the singlet
to triplet state.
(4) The quantum yield of acrolein loss
and product formation decreases with
increasing air.
(5) The presence of CH4 as a product is
indicative of the primary formation of
the ethlidine (CH3CH) radical, which
oxidation would preface the reaction
CH COOH* -
the C02/CH4
C02
ratio
CH4. However,
is greater than
unity.
(6) The CO/C2H4 quantum yield ratio in-
creases with increasing pressure from
approximately 1.0 at 20 torr to ~ 3.0
at 700 torr and above.
(7) Both C2H4/CO2 and C0/C02 ratios de-
creased with increasing concentra-
tions of air. The C2H4/C02 ratio
showed an exponential decrease; the
CO/CO2 ratio was linear. The pro-
duction of CO was apparently the
result of a secondary process.
The quantum yields of acrolein loss
observed in this study can be represented
as a function of the concentration of air
[M], expressed as molecules per cubic
centimeter. The mathematical expression
of this is
1/($>A- 0.00400) =
0.086+1.613 X10"17[M]
This expression shows that a marked
increase in photolytic rate will occur with
increasing altitude, i.e., lower pressures
of air [M].
Tab/o 1. Summary of Photolysis Conditions
Kmax = 3130 A
Run
#
1M
2M
3M
4M
5M
6M
7F
8F
9F
10F
11F
Table 2.
Run
1M
2M
3M
4M
5M
6M
7F
8F
9F
10F
11 F
IA
Quanta
2.1911 x 10"
1.8694 x 10"
2.6468 x 10"
1.2858 x 10"
1.1724x10"
1. 1363 x 10"
1.3261 x 10*°
1.6889 x 10*°
1.5464 x JO20
1.9265 x 10*°
1.8918x10*°
Filling Total
Temp Pressure
°C torr
22.75 658.29
24.60 73.22 1
23.42 524.67
22.80 25.540
22.35 790.85
24.25 359.46
24.08 25.607
25.80 359.96
24.34 110.43
23.28 237.73
25.42 47.523
Summary of Quantum Yields
Total
Pressure *CH=CHCHO *C2H4
658.29
79.221
524.67
25.540
790.85
359.46
25.607
359.96
1 10.43
231.79
47.523
0.00693
0.0334
0.00734
0.0834
0.00649
0.00909
0.0786
0.00882
0.0257
0.0137
0.0681
0.00181
0.0121
0.00193
0.0523
0.00177
0.00234
0.0521
0.00244
0.00838
0.00346
0.0230
*co2
0.00179
0.00349
0.00175
0.0106
0.0019
0.00171
0.0101
0.00167
0.00273
0.00197
0.00566
Mole
Fraction
of
Acrolein
Mole Mole Total Number
Fraction Fraction Density
of of molec cm'3
Nitrogen Oxygen
5.3840x10'* 0.79983 0.19963 2.1483x10"
4.4795 x10~3 0.80071 0.19436 2.5689x10"
6. 7797 x 10'4 0.8O759 0.19173 1. 7O84 x 10"
1.3886x10'* 0.79737 0.18874 8.3333x10"
4.4792 x10'4 0.79958 0.19997 2.5843x10"
9.8466 x10'4 0.79907 0.19995 1.1671x10"
1.3914x10'* 0.78564 0.19905 8.3192x10"
9.3656x10'' 0.80947 0.18954 1.1627x10"
3.20415 x 10'3 0.79741 0.19906 3.6243x10"
1. 53407 xlO'3 0.79838 0.20008 7.5506x10"
7.44088 xlO'3 0.81311 0.17870 1.5370x10"
*CO
0.00533
0.0244
O.O0589
0.0714
0.00785
0.0674
0.00823
0.0188
0.0102
0.0420
* H
0.000341
0.00137
0.000388
0.00317
0.00036
0.000477
0.00418
O.OOO438
0.000625
0.000628
0.00376
*HCHO
0.000533
0.00583
0.000610
0.0149
0.0004 /
O.OO0897
0.0141
0.001OO
0.00447
0.00126
0.00788
*HCOCHO
—
0.00561
0.00238
0.00200
O.O0211
0.003O6
*CH
—
O.OOO1909
O.OO04522
O.O010300
O.OO013OO
Total Run
Time
min
2341
2100
2610
1200
1621
1620
1620
2770
2351
2340
2359
*«
j
0.00891
0.00625
-------
The data obtained from this study show
that at a solar zenith angle of 40° , the
photodissociation lifetime of acrolein in
the troposphere will be approximately 5
days. For comparison, at the same zenith
angle lifetimes for acetone, acetaldehyde,
and formaldehyde were calculated to be
14.8 days, 5.3 days, and 5.9 h, respec-
tively. If photodissociation were the only
removal process for acrolein, a reasonably
long residence time would be expected
and emissions would cause regional
problems. However, the rate constant for
the OH-acrolein reaction is quite high (k
= 1.9 X 1CP1 cm3 molec'1 s"1). Therefore,
at an atmospheric OH level of 106 molec
cm-3, the lifetime of acrolein will be only
14.6 h. Thus, the major loss mechanism
for acrolein is the reaction with OH
radicals, and the photochemical destruc-
tion mechanism is relatively unimportant
in the troposphere.
Edward P. Gardner, Paul D. S perry, and Jack G. Ca/vert are with National Center
for Atmospheric Research, Boulder, CO 80307.
Joseph J. Bufalini is the EPA Project Officer (see below).
The complete report, entitled "The Primary Photochemical Processes of A crolein,"
(Order No. PB 86-145 802/AS; Cost: $16.95, 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 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
MAYZO'86
U.S.POSfAGE .
i
• Q ;? p :: j
^
Official Business
Penalty for Private Use $300
EPA/600/S3-86/005
0000329 PS
U S ENVIR PROTECTION AGENCY
REGION 5 LIBRARY
230 S DEARBORN STREET
CHICAGO IL 60604
------- |