United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-82-092 Apr. 1983
&ER& Project Summary
Atmospheric Chemistry of
Several Toxic Compounds
Edward Edney, Steven Mitchell, and Joseph Bufalini
The hydroxyl radical initiated gas
phase oxidation of several toxic com-
pounds in nitrous acid, oxides of nitro-
gen, in air mixtures were investigated.
The chemical species studied were:
formaldehyde, acrylonitrile, vinylidine
chloride, trichloroethylene, ally! chlo-
ride, acetaldehyde, and acrolein. Pro-
pylene and ethylene were also studied
for comparative purposes. Experimen-
tal protocols were established for mea-
suring hydroxyl rate constants. Pro-
duct studies were also conducted.
The results and their atmospheric im-
plications are discussed.
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 doc-
umented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Large amounts of toxic chemicals are
emitted into the atmosphere. Because of
their possible carcinogenic/mutagenic pro-
perties, these chemicals are of concern to
man's ecosystem. EPA's Environmental
Sciences Research Laboratory is address-
ing the problem of hazardous chemicals
by conducting research programs to in-
vestigate the emissions rates, ambient
concentration levels, and atmospheric fate
of these species. This report examines the
atmospheric lifetimes of several hazardous
pollutants and determines their degrada-
tion pathways.
The atmospheric lifetimes of gaseous
pollutants are usually determined by their
photolysis La, the stability to solar radiation,
reactions with ozone, reactions with hy-
droxyl radicals, and reactions with other
free radicals such as RO, NOs, HOa, etc.
Photolysis is usually unimportant unless
the molecules absorb energy in the solar
radiation region (2900 A- 8000 A). The
organic compounds that photodissociate
most readily are the carbonyls and the
nitrites. For non-alkenes, the 03 reaction
is too slow to be important Other free
radicals (RO, NOs, etc.) are usually not
present in very high concentrations or they
react too slowly with organics to make
these reactions important For most haz-
ardous chemicals, therefore, the main
degradation pathway is reaction with OH
radicals. An examination of such reactions
is the subject of this paper.
In this study, the OH reaction rate con-
stants for some selected organic com-
pounds were measured. Reaction products
from OH-organic compounds reactions in
the presence of NOx were also studied.
The hazardous chemicals studied were:
vinylidene chloride, trichloroethylene, for-
maldehyde, acetaldehyde, acrylonitrile,
acrolein, and allyl chloride. In addition,
propylene was studied in order to obtain
the photolysis rate for HONO while ethylene
was studied to test the technique employed.
The choice of the particular toxic com-
pounds investigated in this study was
based on two factors: their high volume
production and their suspected carcino-
genic/mutagenic activities.
Procedure
All experiments were performed in a
700 liter cell. This cell consisted of 6
cylinders each 1.5 m in length and 0.31 m
in diameter. Each end of the cell contained
eight mirrors with multiple reflection optics.
This system gave a 216 m path length for
the experiments.
Each cylinder section of the cell was
surrounded by a bank of 16 40 W ultra-
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violet fluorescent lamps. Twelve lamps
were blacklights with an energy maximum
of 365 nm while four were sunlamps with
an intensity maximum at 310 nm. The
photolysis constant for N02 in nitrogen
was 0.60 min-1 .
A Digilab FTIR spectrometer coupled to
a computer controlled scanning Michelson
interferometer was used for the detection
system. Mercury-cadmium-telluride and
indium-antimonide were the two signal
detectors employed. The spectra were
taken at 1.0 crrr1 resolution.
HONO was used as the OH radical
source. When HONO is exposed to radia-
tion between 3000-4000 A, it decom-
poses to produce OH and NO. The HONO
was prepared by adding a solution of
sulfuric acid to sodium nitrite. The gaseous
HONO was flushed into the cell with
nitrogen gas.
The organic material was introduced
into the cell by liquid syringes or by a gas
dilution system. Formaldehyde was pro-
duced by slowly heating paraformaldehyde
in a mixing bulb and then using the gas
dilution system.
Data Analyses
The reaction mechanism for the photol-
ysis of HONO in the presence of an
organic compound and NOX in air can be
expressed as:
HONO + hv - OH + NO
OH + HONO - H20 + NO2
OH + NO - HONO
OH + N02 -g HONO2
OH + HC - R02
R02 + NO - NO + RO
RO + 02 -3 HO2 + CARS!
RO ~ CARB2 + CARBa + H02
H02 + NO - OH + N02
(1-1)
(1-2)
(1-3)
(1-4)
(1-5)
(1-6)
(1-7)
(1-8)
(1-9)
If steady state approximations are made
for OH, HO2, and RO2 radicals, we obtain
the following result for the steady state
concentration of OH radicals:
[OH]-
k, [HONO]
(1-4)
k2[HONO] + k3 [NO] + k4[N02]
The OH concentration is independent of
the HC concentration. The reason for this
is that the OH radical destroyed by the HC
is regenerated when the H02 radical is
converted back to OH by reaction (1-9).
The time derivative for the reaction of
the organic compound with OH radicals is
given by:
(d/dt)[HC(t)] = - kg [OH (t)] [HC (t)] (1-5)
and if we assume that the OH concentration
is constant, then
[HC(t>]
[HC(o)]
= exp (-kg[OH] t)
d-6)
This equation was used to determine all
the OH rate constants reported in this
study.
Although the first order dissociation
constant for N02 was measured, the dis-
sociation constant for HONO (reaction 1-
1) in our system was not determined. In
order to.derive this value, propylene was
employed as the reference compound.
Equations I-4 and I-6 can be combined
since k3 = K, = 1.7 k2. Then, the rate of
disappearance for propylene can be written
as:
[HC(o)]
[HONO]
k2 [HONO] -I- 1.7 [NO + N02] (l"10>
A value for the k-|/k2 ratio is found by
substituting in the observed concentra-
tions for the propylene experiment Once
the ^/k2 ratio is found, then a general
relationship can be developed:
k=5.47 x 10-9 —
cmj
molecule
1 [HC(t)J
where « = 1 n
t [HC(o>]
[HONO]0
and fi =
[HONO] + 1.7([NO]0 + [NO]0)
Results
Propylene was used as the reference
compound in order to obtain the photolysis
constant for HONO. Therefore, no OH rate
constant was obtained for this compound.
Products observed with the photooxidation
of propylene with HONO and NO in air
were acetaldehyde, formaldehyde, and nitric
acid. Ozone and PAN were not observed
since these experiments were conducted
over short irradiation times with NO still
present at the termination of the experi-
ments. Most of the carbon was accounted
for with the two aldehydes. However,
there were residual absorbances in the
spectra suggesting the presence of some
other compound or compounds. Perhaps
some propylene glycol 1,2-dinitrate was
present but we did not have any reference
spectra for this compound. The ANO/
Apropylene ratio was observed to be
2.17.
To make certain that the technique em-
ployed in this study was satisfactory,
ethylene was also tested. The products
observed with this hydrocarbon were for-
maldehyde, carbon monoxide and nitric
acid. The ANO/Aethylene ratio was 1.77
and an OH rate constant value of 8.0 x 10-12
cm3/sec was obtained.
The OH reaction of formaldehyde in the
presence of NOX and air resulted in the
formation of carbon monoxide and nitric
acid. The ANO/Aformaldehyde ratio was
1.03 and the OH-formaldehyde rate con-
stant was 14.1 x 10'12 cm3/sec.
The OH initiated reaction with acrylonitrile
resulted in formaldehyde, CO, and HNOa
as products. Some unknown bands were
also observed suggesting that a nitrile
group was present This was tentatively
identified as formyl cyanide. HCN was
also observed as a product The ANO/
Aacrylonitrile ratio was 2.13 while the
OH-acrylonitrile rate constant was 3.2 x
10'12 cm3/sec.
Vinylidine chloride reacted with OH and
NO in air to produce chloroacetyl chloride,
phosgene, formaldehyde, carbon monoxide
and nitric acid. The number of NO oxidized
per vinylidine chloride reacted was 1.68
and the OH-vinylidine chloride rate constant
was determined to be 12 x 10"12 cm3/sec.
In the OH reaction with trichloroethylene,
dichloroacetyl chloride, phosgene, formyl
chloride, carbon monoxide and nitric acid
were the observed products. The ANO/
Atrichloroethylene ratio was 2.25 and the
OH rate constant was 3.6 x 10"12cm3/sec.
The OH-allyl chloride rate constant was
observed to be 19.8 x 10'12 cm3/sec and
the ANO/Aallyl chloride was 1.80. For-
maldehyde and nitric acid were the only
identifiable products although a number
of unidentified absorption bands were
observed.
In the OH reaction with acetaldehyde/
N0x/air system, PAN, methyl nitrate,
methyl nitrite, and nitric acid were observed.
The ANO/Aacetaldehyde ratio was 1.60
and the OH rate constant was 27.7 x 10'12
cm3/sec.
The last compound studied was acrolein.
A peroxynitrate, glycoaldehyde, formalde-
hyde, carbon monoxide, and nitric acid
were the observed products. The OH rate
constant with this aldehyde was found to
be 34.3 x 10'12 with a ANO/Aacrolein
value of 1.71.
Discussion
The FTIR technique employed in these
studies has been shown to yield both
kinetic and product formation information.
Products as well as reactants have been
monitored with ease. The OH rate constant
determinations were in reasonable agree-
ment with the previously published values.
The lifetimes of the compounds studied.
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assuming an OH concentration of 106 cm'3
are shown below:
Atmospheric Lifetimes
Compound r(h)
Propylene 7.7
Ethylene 2.6
Acetaldehyde 9.0
Formaldehyde 13.6
Acrylonitrile 60.3
Trichloroethylene 53.6
Vinylidene chloride 16.1
Allyl chloride 9.7
Acrolein 5.6
The lifetimes shown for the compounds
studied are relatively short indicating that
these pollutants will not build up in the
troposphere. They will however be impor-
tant on an urban and regional scale.
The EPA authors Edward Edney, Steven Mitchell, and Joseph J. Bufalini (also
the EPA Project Officer, see below) are with the Environmental Sciences
Research Laboratory, Research Triangle Park, NC 27711.
The complete report, entitled "Atmospheric Chemistry of Several Toxic Com-
pounds," (Order No. PB 83-146 340; Cost: $13.00, 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
Government Printing Office: 1983-659-017/7054
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