United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-87/004 Mar. 1987
-/1
&ER& Project Summary
Atmospheric Persistence of
Eight Air Toxics
Larry T. Cupitt
The concept of the "atmospheric life-
time" of an air toxic chemical was
defined, and methods were described
for estimating the lifetimes of such
chemicals in the atmosphere. For many
air toxics, the primary removal mecha-
nism in the air is its reaction with
hydroxyl radicals. Because hydroxyl
radical chemistry is so important in
determining the atmospheric lifetime
of many chemicals, recommendations
are made for the "average" conditions
to use in estimating the lifetime of air
toxics over the continental U.S. These
methods and conditions are applied to,
and estimates of the "average" atmo-
spheric lifetimes are derived for eight
volatile air toxic chemicals which EPA
had identified in "Intent-to-List" notifi-
cations during 1985. The eight chemi-
cals for which lifetimes are derived are
methylene chloride, chloroform, carbon
tetrachloride, ethylene dichloride, tri-
chloroethylene. perchloroethylene,
1,3-butadiene, and ethylene oxide.
Seven of the eight chemicals are re-
moved from the atmosphere primarily
by reaction with OH radicals. The life-
times of the seven chemicals primarily
removed by OH reaction ranged from
around 4 hours to around 18 months.
The eighth chemical, carbon tetrachlo-
ride, has such a long atmospheric life-
time (ca. 50 years) that the primary
removal mechanism is not identifiable.
This Project Summary was developed
by EPA's Atmospheric Sciences Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings ot
the research project that Is fully docu-
mented In a separate report of the same
title (see Project Report ordering In-
formation at back).
Introduction
During 1985, the Environmental Pro-
tection Agency (EPA) published intent to
list decisions for eight organic chemicals
(i.e., methylene chloride, chloroform,
carbon tetrachloride, ethylene dichloride,
trichloroethylene, perchloroethylene, and
ethylene oxide). The atmospheric lifetime,
distribution, and ultimate fate of these air
toxics is determined by a variety of factors
which act to transport, remove, or redis-
tribute the chemicals. These factors con-
trol the magnitude and nature of any
potential exposure to the air toxic chemi-
cal. A wide range of processes may be
involved: they may be chemical in nature,
like reactions with ozone or hydroxyl
radicals, or physicochemical processes
like photolytic decomposition, or physical
mechanisms like washout, dry deposition,
transport to the stratosphere, etc.
Factors Affecting the
Atmospheric Lifetime
Atmospheric Lifetimes
Once a potentially hazardous air pol-
lutant, like those named above, is released
into the atmosphere, its concentration
and fate is determined by a variety of
chemical or physical processes. The
emitted material immediately begins to
mix in the atmosphere and to dilute,
reducing the exposure levels and the
associated risk. The total mass of the
chemical in the environment is not re-
duced, however, until some process acts
to remove or transform the chemical. The
rate at which physical or chemical pro-
cesses remove or transform the target
compound determines the atmospheric
lifetime of the chemical.
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The persistence of the air toxic chemi-
cals in the atmosphere can be described
by the characteristic decay time, TO. If the
decay rate in the air can be expressed as
a linear function of Q, one finds that r0 is
the time at which an initial injection of a
chemical has decayed until Q = 0,,/e,
where e is the natural number 2.718....
After one lifetime, about 37% of the
starting concentration is still present.
After a second lifetime, only 13.5% of the
chemical remains. After three lifetimes,
less than 5% of the starting material
remains. Similarly, the half-life, t1/2, then
is given by t1/2 = In 2/kD <= 0.693/kD «
0.693 r.
Chemical Reaction Processes
A wide range of reactive species are
known to be present in both the tropo-
sphere and the stratosphere which may
react with the emitted air toxics to trans-
form them into other compounds. For
many air pollutants, however, the pre-
dominant tropospheric reaction is with
the hydroxyl radical, OH, despite the fact
that its concentration is relatively low.
Reactions With Hydroxyl Radicals
Hydroxyl radicals are ubiquitously pre-
sent throughout the troposphere, being
formed from the normal photochemical
processes which occur even in "clean"
air. The radicals are so reactive that their
own atmospheric lifetime is very short,
and their concentration never becomes
very large. Because of their importance
to atmospheric and combustion chemis-
try, the reaction rates for OH and many
organic species have been measured. A
large data base of kinetic information is
already available which may be applied
to the problem of estimating atmospheric
lifetimes.
The choice of values to assign to kOH
and to the hydroxyl radical concentration,
[OH], is complicated by the fact that the
rate constant is a function of temperature,
and the [OH] depends upon a balance of
competing chemical reactions involving
temperature, light intensity and the con-
centrations of many other pollutants.
Atmospheric Mixing and
OH Removal
The eight air toxics under consideration
in this paper are man-made pollutants.
Their release, therefore, is most likely to
occur where man inhabits the earth,
namely near the surface, in temperate
zones, and into polluted air over the
continental land masses. These factors
suggest that such a released pollutant
will initially encounter an environment
which is likely to be warmer, and more
reactive, than the average troposphere.
As the pollutant is mixed, first within the
boundary layer, then vertically in the
troposphere, and finally horizontally
throughout the troposphere, it encounters
continually changing "average" temper-
atures and reactant species. If it persists
long enough, it becomes well-mixed
throughout the atmosphere, and the
"average" conditions no longer change.
Clearly, average tropospheric values for
temperature and [OH] are actually only
applicable for species which are suf-
ficiently long-lived to become evenly
dispersed throughout the troposphere. It
may be necessary, therefore, to consider
the temperature and [OH] in each of
several tropospheric layers as the chemi-
cal diffuses.
Estimation of Atmospheric
Lifetimes and Removal by OH
In order to estimate the removal rate of
air toxics due to reaction with OH radicals,
it is convenient to divide the troposphere
into three compartments: the boundary
layer, the vertically-mixed troposphere at
a specific latitude, and the horizontally-
mixed global or hemispheric troposphere.
The average temperature and [OH] in the
appropriate compartments can then be
used to estimate the lifetime of an air
toxic. Average values for [OH] and tem-
perature in each compartment (for con-
tinental air at about 37.5° N latitude) are
estimated, based upon current modeling
and measurement efforts. The recom-
mended values are shown in Table 1. The
actual choice of the compartment to use
depends upon the estimated lifetime of
the chemical and the mixing times be-
tween reservoirs. For species with life-
times of less than 2 or 3 days, the
estimates for the boundary layer should
be used. For chemicals with lifetimes of
about 3 weeks to 5 months, the values
for the vertically-mixed troposphere
should be used. For very stable chemicals
with lifetimes in excess of 3 years, the
global values should be used. For species
with lifetimes intermediate to the times
described above, lifetimes should be
estimated for the two bracketing regimes,
and the lifetime expressed as a range.
Atmospheric Persistence of
Eight Air Toxics
Methylene Chloride
Methylene chloride (dichloromethane)
is a high volume, commonly used solvent.
Its background concentration at 40° N is I
about 50 parts per trillion (ppt). Its con-
centration in urban areas of the U. S. is
highly variable, probably due in part to
the many sources resulting from its
frequent use. Average urban concentra-
tions are often ten to one hundred times
as large as the geochemical background
concentration.
Methylene chloride reacts with OH
radicals in the atmosphere at a moderate
rate. Reactions are not expected to occur
with ozone or other common atmospheric
pollutants. Dry deposition and rainout are
also expected to be very slow. Since
other removal mechanisms are not likely
to be effective, they can be ignored in
estimating the total removal rate. The
atmospheric lifetime of methylene chlo-
ride is, therefore, simply that predicted
from hydroxyl radical removal, around
130 days. The estimated half life is about
69% of the lifetime, or about 91 days. The
estimate of 130 days agrees reasonably
well with values derived by other re-
searchers using a two-box model to
estimate the global lifetime of methylene
chloride from its distribution throughout
the atmosphere.
Chloroform
Chloroform (trichloromethane) is
another chemical which is ubiquitously
present in the atmosphere. The chemical
is manufactured for use as a solvent, a
cleaning agent, and as an intermediate in
the manufacture of other chemicals.
Geochemical background concentrations
are around 16 ppt.
The reaction rate constant for chloro-
form with OH has been reported by at
least three investigators, and the data are
in excellent agreement. For chloroform,
reaction with other tropospheric pol-
lutants and removal by pnysicochemical
processes is not expected to be very
large. The atmospheric lifetime is equated
with the lifetime due to hydroxyl radical
reaction. It is appropriate to assign chloro-
form an atmospheric lifetime of between
181 and 378 days (or 0.5 to 1.0 years),
Carbon Tetrachloride
Carbon tetrachlonde (tetrachlorome-
thane) is nearly uniformly distributed
around the globe. This uniform distribu-
tion means that the chemical is well
mixed in the troposphere and suggests
that it has a very long lifetime compared
to the atmospheric mixing processes.
The measurement of carbon tetrachlo-
ride in the ambient atmosphere has
proven difficult over the years, with in-
vestigators reporting significantly dif-
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Table 1. Selected Average Values of Temperature and [OH] In Three Regimes of the
Troposphere at 37.5° N Latitude
Regime
Boundary Layer
Vertically-Mixed Troposphere
Hemispheric/Global Troposphere
Temperature
K
288
263
260
. IOH\
We molec cm3
1.0
1.0
0.5
Applicable
Lifetimes
< 3 days
3 wktoS mo
>3yrs
ferent values. Recently, however, a con-
sistent set of data on carbon tetrachloride
has begun to emerge. The geochemical
background concentration in 1978-1981
was found to be around 118 ppt. Con-
centrations of the chemical were found
to be increasing at about 2 ppt per year.
The reaction of carbon tetrachloride
with OH radical is so slow that the rate
constant has not been measured. Upper
limits for the rate constant have been
determined, and they suggest that the
atmospheric lifetime due to hydroxyl
radical reaction is longer than 50 years.
In addition, a number of removal pro-
cesses have been postulated including
chemical reactions and photolysis in the
stratosphere, hydrolysis in the ocean,
ion-molecule reactions, and photolysis
on sand. None of these processes has a
very large effect, however. Most of them
result in lifetime estimates of around 50
years.
Two modeling approaches have been
applied to estimate the atmospheric life-
time of carbon tetrachloride. A trend
analysis technique and an inventory
analysis method yielded most probable
lifetimes of 50 to 57 years, respectively.
While these modeling approaches did
produce lifetime estimates with substan-
tial uncertainties, they are consistent with
the known chemical and physical pro-
cesses involving carbon tetrachloride. A
lifetime estimate of around 50 years is
the best that can currently be done.
Ethylene Dichloride
Ethylene dichloride (1,2-dichloroethane)
is another high-volume man-made pol-
lutant which has become ubiquitous in
the atmosphere. A global background
concentration of ethylene dichloride of
25 ppt was reported for December 1981.
The reaction rate of ethylene dichloride
with OH radicals has not been so
thoroughly investigated as the reactions
of many other chlorinated alkanes. A
critical review of the available OH kinetics
data for ethylene dichloride suggests that
an estimated lifetime of 92 days is rea-
sonable. Other removal processes are,
once again, not likely to be effective in
removing this air toxic. Given uncertain-
ties in the kinetic estimates, however.
the value of 92 days should only be
viewed as the most probable value. Al-
lowing a factor of two as a margin of
safety, the estimated ethylene dichloride
residence time is between 0.13 and 0.50
years. The upper limit value is close to
the residence time of 0.6 ± 0.2 years
recently estimated by other researchers
from ambient concentration measure-
ments and a global budget model.
Trichloroethylene
Trichloroethylene (trichloroethene) is a
chlorinated alkene which has found use
as a degreaser and solvent. The Northern
Hemispheric background concentrations
in the Eastern Pacific Ocean were re-
ported as 12 ppt. A critical review of the
literature data reported median urban
concentrations of 150 ppt.
The available'rate data for reactions
with OH radicals are in reasonable agree-
ment. The activation energy for the re-
action with OH is negative: the rate
constant for this chemical will increase
as the temperature cools. Although tri-
chloroethylene is an alkene, with a double
bond which may be subject to attack by
ozone, the only reported rate constant for
reaction with ozone is very low. Ozonoly-
sis, therefore, plays no significant role in
the removal of this compound. Other
removal mechanisms are also expected
to be ineffective. The lifetime computed
for the vertically mixed troposphere is
even shorter than the lifetime in the
boundary layer, due to the negative
activation energy exhibited by this com-
pound. The lifetime of trichloroethylene
is estimated as slightly over 4 days.
Perchloroethylene
Perchloroethylene (tetrachloroethene)
is a volatile compound commonly used as
a degreaser and solvent. It is emitted in
significant amounts from dry cleaning
operations. The chemical is consistently
found in urban areas of the U.S. The
recent data compilation reported a median
concentration of 340 ppt in urban and
suburban areas of the U.S. During a
stagnant period in San Jose in December
1985, a two-hour integrated sample
yielded a value of 6639 ppt.
The only reported value for the rate
constant of the reaction of perchloroethy-
lene and ozone is very small, implying
that ozone will not effectively remove the
chemical from the atmosphere. Dry
deposition rates of perchloroethylene to
some common surface materials found
in urban areas were recently measured.
The rates were so slow that they were
often indistinguishable from zero. Dry
deposition, therefore, does not appear to
be a significant removal pathway. The
lifetime estimated from hydroxyl removal
rates is estimated to range from 119 to
251 days.
1,3-Butadlene
1,3-butadiene is a very reactive alkene
with two double bonds. It is commonly
used as a component in the synthesis of
rubber and many other diverse
compounds.
Butadiene reacts quickly with both OH
and ozone. The hydroxyl radical reaction
has a negative activation energy, indi-
cating that the rate constant will increase
as the temperature decreases. The es-
timated lifetime, based on reaction with
OH radicals, is 240 minutes. Assuming
an ozone concentration of around 40 ppt,
the lifetime computed for removal by
ozone is around 2800 minutes.
Obviously, the actual lifetime of such a
reactive compound will depend upon the
specific conditions at the time of release.
The quoted lifetimes were calculated for
"average" conditions. Because the es-
timated lifetime is so short, the actual
degradation of any real emissions is very
dependent upon time of day, sunlight
intensity, actual temperature, etc. While
the lifetime during the middle of tfie day
in the summer under polluted conditions
could be much shorter than the estimated
4 hours, the lifetime of emissions at
night could be essentially infinite. After
sunset, there will be no hydroxyl radicals
generated and the small amounts of
residual ozone present in the evening
will have little effect on the butadiene
concentrations. On average then, 1,3-
butadiene has an estimated lifetime of
around 4 hours.
Ethylene Oxide
Ethylene oxide (oxirane) is the smallest
possible organic epoxide. The nature of
the chemical structure induces a high
strain energy in the three-membered ring,
and this strain energy influences the
reaction kinetics and products.
Two investigators have recently mea-
sured the reaction rate constant for
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ethylene oxide with OH radicals. The
experimentally determined room tem-
perature rate constants were in very good
agreement, and they suggest an atmo-
spheric lifetime of from 217 to 578 days.
Of the eight chemicals named in "Intent
to List" notifications, ethylene oxide is
the most soluble in water. The solubility
of ethylene oxide in water has recently
been quantified at ambient temperatures:
the data suggest that neither rain out nor
adsorption into aqueous aerosol particles
in the air should remove very much of the
compound.
No other removal processes are known
which can rapidly deplete the ethylene
oxide from the air. Results from smog
chamber irradiations in both natural sun-
light and artificial illumination are also
consistent with a slowly reacting organic
chemical. The estimated lifetime, there-
fore, can be calculated simply from the
OH radical removal rate.
It is interesting that ethylene oxide,
with an estimated lifetime as long as 1.5
years, has not been observed in the
ambient atmosphere. A variety of other
methods have been reported for use in
analyzing for ethylene oxide, but these
methods all have reported sensitivities
from 0.05 parts per million to greater
than 3 parts per million. Using the 1982
estimate for production of ethylene oxide
in the U.S., and making reasonable es-
timates for other production and release
rates, one estimates a background con-
centration in the Northern Hemisphere to
be around 240 ppt. This value is a factor
of 200 to 12,000 below the quoted
analytical detection limits. It is not sur-
prising then, that no ambient data on
ethylene oxide have been reported. Nor
does the lack of ambient data argue that
there must be some rapid, but unknown,
removal mechanism. Until additional data
arrive to modify these conclusions, it is
appropriate to assign ethylene oxide an
atmospheric lifetime of from 0.6 to 1.5
years.
Conclusions
Relationships have been developed in
the full report which describe the atmo-
spheric lifetimes of potentially hazardous
chemicals in terms of their probable
removal mechanisms. These relationships
have been applied to eight air toxic
chemicals identified by EPA in Intent to
List notifications. The eight chemicals
and their estimated atmospheric lifetimes
are tabulated below.
For all the chemicals except carbon
tetrachloride, the dominant removal
mechanism from the atmosphere was
reaction with hydroxyl (OH) radicals.
Removal rates for carbon tetrachloride
were so slow that the dominant removal
mechanism could not be determined, and
the lifetime given is one, reported else-
where, based upon modeling. Average
tropospheric conditions for OH reactions
were defined and utilized as the basis of
the predicted atmospheric lifetimes for
the other chemicals. In the case of ethy-
lene oxide, questions regarding the at-
mospheric stability of the chemical and
the lack of ambient data were addressed
and resolved.
TaWe 2. Estimated Atmospheric Lifetimes
of Eight Air Toxics
Chemical
Name
Methylene chloride
Chloroform
Carbon tetrachloride
Ethylene dichloride
Trichloroethylene
Perchloroethylene
1 ,3-butadiene
Ethylene oxide
Atmospheric
Lifetime
131 days
181 to 378 days
50 years
46 to 184 days
4 days
1 19 to 251 days
4 hours
21 7 to 578 days
The EPA author L. T. Cupitt is with the Atmospheric Sciences Research
Laboratory, Research Triangle Park, NC 27711.
The complete report, entitled "Atmospheric Persistence of Eight Air Toxics,"
(Order No, PB 87-145 306/AS; Cost: $13.95, 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 author can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
U.3.OFROiALF/.A;j
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
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