PB80-221948
Pate of Toxic and Hazardous
Materials in the Air Environment
(U.S.) Environmental Sciences Research Lab.
Research Triangle Park, NC
L
Aug 80
U.S. DEPARTMENT OF COMMERCE
National Technical Information Service
NTIS
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/3-80-084
4. TITLE AND SUBTITLE
FATE OF TOXIC AND HAZARDOUS
THE AIR ENVIRONMENT
2
> MATERIALS IN
7. AUTHOR(S)
Larry T. Cupitt
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Environmental Sciences Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
12. SPONSORING AGENCY NAME AND ADC
Environmental Sciences Rest
Office of Research and Dev«
U.S. Environmental Protect"
Research Triangle Park, Noi
JRESS
=arch Laboratory - RTP, NC
jlopment
Lon Agency
th Carolina 27711
3.REI pnap-jai.ou j
8. REPORT DATE
August 1980 Issuing Date.
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
10. PROGRAM ELEMENT NO.
05A1A/07 - 0016 (FY-80)
11. CONTRACT/GRANT NO.
13. TYPE OF REPORT AND PERIOD COVERED
In-house
14. SPONSORING AGENCY CODE
EPA/600/09
15 SUPPLEMENTARY NOTES
16. ABSTRACT
The atmospheric fate of potentially toxic /hazardous chemicals currently
undergoing assessment by EPA was evaluated. Both chemical and physical removal
processes are discussed. Mathematical descriptions of physical removal mechanisms
were developed and applied to specific chemicals, i.e., acrylonitrile, ethylene
dichloride.-perchloroethylene, vinylidene chloride and benzo(a)pyrene. Estimates
of physical removal by rainfall suggest half-lives of 300 days or longer for these
compounds. Calculations based on reported photo-decomposition rates of halomethanes
in contact with silica surfaces (e.g., desert sands) suggest half-lives on the order
of 25 years for such halogenated chemicals, and dry deposition of the other compounds
of interest is probably equally slow. Adsorption on aerosol particles is a reasonable
removal mechanism only for benzo(a)pyrene, and all physical removal processes are
generally demonstrated to be rather inefficient. Forty-six individual materials
were evaluated relative to their probable fates and tropospheric lifetimes. Known
or theoretical rate constants are listed for reaction with hydroxyl radicals and
ozone. The probability of photolysis and of physical removal was assessed, and
residence lifetimes assigned. Probable products of tropospheric oxidation processes
were also tabulated.
17.
a. DESCRIPTORS
* Air pollution *
* Hazardous materials
* Photochemical reactions
* Reaction kinetics
* Scavenging
* Adsorption
* Dissolving
.* Deposition
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
KEY WORDS AND DOCUMENT ANALYSIS
b. IDENTIFIERS/OPEN ENDED TERMS c. COSATI Held/Group
Half-life ^
11G
07E
07D
13H
18H
19. SECURITY CLASS IThii Report) 21.
UNCLASSIFIED
20. SECURITY 'CLA&jj JThil pagt) 22. PRICE
EPA Form 2220-1 (R«y. 4-77) PREVIOUS EDITION is OBSOLETE
REPRODUCED BY
NATIONAL TECHNICAL
INFORMATION SERVICE
U.S. DEPARTMENT OF COMMERCE
SPRINGFIELD, VA. 22161
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NOTICE
THIS DOCUMENT HAS BEEN REPRODUCED
FROM THE BEST COPY FURNISHED US BY
THE SPONSORING AGENCY. ALTHOUGH IT
IS RECOGNIZED THAT CERTAIN PORTIONS
ARE ILLEGIBLE, IT IS BEING RELEASED
IN THE INTEREST OF MAKING AVAILABLE
AS MUCH INFORMATION AS POSSIBLE.
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EPA-600/3-80-084
August 1980
FATE OF TOXIC AND HAZARDOUS
MATERIALS IN THE AIR ENVIRONMENT
by
Larry T. Cupitt
Atmospheric Chemistry and Physics Division
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U. S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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u;
?£ncy
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DISCLAIMER
This report has been reviewed by the Environmental Sciences Research
Laboratory, U. S. Environmental Protection Agency, 'and approved for publi-
cation. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
11
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ABSTRACT
Potential toxic/hazardous chemicals are currently undergoing Type I
(preliminary) and Type II (detailed) assessment by the Office of Air
Quality Planning and Standards. This report evaluates the atmospheric
fate of these compounds, i.e., their probable lifetime in the
troposphere. Emphasis has primarily been given to the volatile chemicals
undergoing Type II Assessment, i.e..^acrylonitrile, ethylene dichloride,
perchloroethylene, vinylidene chloride and1-l)enzo(a)pyrene (as a repre-
sentative polycyclic organic material).
Chemical and physical removal processes are discussed. Various
parameters were considered that influence chemical removal processes
involving photolytic transformations and reactions with hydroxyl radicals,
ozone, and other tropospheric species. Mathematical descriptions of physical
removal mechanisms were developed and applied to the volatile Type II
Assessment chemicals. Estimates of physical removal by rainfall suggest
half-lives of 300 days or longer for all volatile Type II compounds.
Calculations based on reported photo-decomposition rates of halomethanes
in contact with silica surfaces (e.g., desert sands) suggest half-lives
on the order of 25 years for such halogenated chemicals, and dry deposition
of the other compounds of interest is probably equally slow. Adsorption
on aerosol particles is a reasonable removal mechanism only for benzo(a)
pyrene and all physical removal processes are generally demonstrated
to be rather inefficient.
Forty-six individual materials were evaluated relative to their
probable fates and tropospheric lifetimes. Known or theoretical rate
constants are listed for reaction with hydroxyl radicals and ozone.
The probability of photolysis and of physical removal was assessed, and
iii
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residence lifetimes assigned. Probable products of tropospheric oxidation
processes were also tabulated.
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CONTENTS
Abstract iii
Tables '' vi
1. Introduction 1
2. Conclusions 2
3. Chemical Removal Processes 3
Introduction ...... 3
Reaction with OH 3
Reaction with ozone 5
Photolytic transformations 6
4. Physical Removal Processes 7
Dissolution 7
Adsorption on aerosol particles 12
Dry deposition 17
5. Fate of Chemicals under Assessment . 19
References 26
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TABLES
Estimated Concentrations of Reactive Species-
in the Lower Troposphere
Physical and Calculated Parameters Influencing
Removal of Some Atmospheric Pollutants by
Rain Droplets 11
Parameters Influencing Adsorption of Some
Air Pollutants on Aerosol Particles 16
Fraction (0) of Material Adsorbed on Aerosols
for Various Values of Aerosol Surface Area (9)
and Chemical Saturation Vapor Pressure (p ) 17
Fates and Residence Times of Some Chemicals
under Assessment 22
vi
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SECTION 1
INTRODUCTION
The Office of Air Quality Planning and Standards (OAQPS) has recently
distributed lists of potentially toxic/hazardous chemicals being assessed
in regard to possible regulatory action. The list of "43 chemicals" under
Type I (preliminary) Assessment, and a second list of chemicals under Type
II (detailed) Assessment, contain a variety of species that may be found in
the air environment. Because of the high probability of harmful effects
resulting from exposure to these chemicals, this report attempted to
evaluate the atmospheric fate of many of these compounds. Particular
attention was paid to those chemicals currently under Type II Assessment
that may possibly be found as gaseous emissions, specifically, acrylonitrile,
ethylene dichloride, perchloroethylene, vinylidene chloride, and polycyclic
organic materials. Because the phrase "polycyclic organic materials"
includes an infinite variety of possible compounds, all of which cannot be
addressed in any document, this report considered benzo(a)pyrene (BaP) a
representative of the class.
Ambient concentrations of toxic/hazardous materials (T/H M's) represent
a dynamic balance between emission rates, dilution, and physico-chemical
removal rates. Sources may be either anthropogenic or natural, and sinks
may involve chemical/physical processes. This paper did not address source
strengths or localized dilution effects. Instead it surveys a variety of
chemical/physical removal processes and assesses the likely fate of in-
d ividual compound s.
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SECTION 2
CONCLUSIONS
By taking experimental and theoretical parameters into consideration,
reasonable values may be assigned to the rate of reaction of T/H M1s with
reactive species found in the lower troposphere. For the volatile chemicals
currently undergoing Type II Assessment (i.e., acrylonitrile, vinylidene
chloride, ethylene dichloride, perchloroethylene and benzo(a)pyrene),
chemical removal residence times, based upon reaction with hydroxyl radicals
and ozone, are estimated to range between approximately 3 to approximately
70 days.
Estimates can also be made of the physical removal rates for the volatile
Type II chemicals. Removal by dissolution into rain droplets yields
estimated half-lives of from 0.8 to greater than 100,000 years. Dry
deposition rates imply half-lives of approximately 25 years, and adsorption
on aerosols is demonstrated to be a reasonable removal mechanism only for
materials with saturation vapor pressures of 10 torr or less. Only in
the case of B(a)P, where adsorption on aerosol particles suggested a
lifetime of about 8 days, was a physical removal mechanism significant.
Quantitative description of the atmospheric degradation or photolytic
transformation of the T/H M must be on a chemical-by-chemical basis and
requires a great deal of experimental evidence that may not yet be
available. Nonetheless, estimates of reasonable residence times are
feasible on the basis of the normally predominant chemical removal mech-
anisms. Based upon current knowledge of atmospheric oxidation mechanisms,
possible tropospheric reaction products may be anticipated.
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SECTION 3
CHEMICAL REMOVAL PROCESSES
INTRODUCTION
For a wide variety of molecules, the most important chemical removal
process in the troposphere is reaction with hydroxyl (OH) radicals. For
those organic chemicals containing isolated double bonds, ozonolysis is
also available as a second reaction pathway. In addition, reactions with
minor tropospheric species or with species known to exist in the strato-
sphere (e.g. 0(3P), 0(1D), H02, R02, NOg, 02 (^ g), etc.) must be considered
for chemicals that are relatively long-lived. Table 1 lists average tro-
pospheric concentrations of some of these reactive species.
REACTION WITH HYDROXYL RADICALS
Despite the fact that the average concentration of OH radicals given
in Table 1 is much lower than for many other reactive species, reaction with
hydroxyl radicals is the predominant tropospheric removal mechanism for a
wide variety of organic molecules. The reaction pathway with OH may proceed
by abstraction, addition, or both.
Abstraction reactions occur when a hydrogen atom is available to be
removed by the OH to form H?0 and an organic radical. The rate at which
this reaction occurs depends upon the ease with which the organic molecule
gives up the hydrogen atom. Since this process is controlled by the number
of hydrogens and the nature of the molecular orbitals influencing both the
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original carbon-hydrogen bond and the resulting radicals, substituent
groups in the vicinity of the abstractable hydrogen influence the rate of
abstraction. While the rate constant for many abstraction reactions is
in the range of 1 to 10 x 10 cm molecule sec , the actual rate
constant is dependent upon the particular molecular composition.
TABLE 1. ESTIMATED CONCENTRATIONS OF REACTIVE
SPECIES IN THE LOWER TROPOSPHERE
Concentration
Specie
OH
°3
02(1Ag)
o (-"-i)
0(3P)
O^D)
H0?
RO.
N03
molecule cm
1 x
1 x
5 x
7 x
5 x
2 x
5 x
5 x
1 x
io6
io12
io8
io2
10A
io-1
io9
io9
io5
ppm
4 x
4 x
2 x
3 x
2 x
8 x
2 x
2 x
4 "x
io-8
io-2
ID'5
io-11
ID"9
io-16
io"4
io-4
io-9
Q
Concentrations are taken from References 1 to 3 and from sources therein.
Conversion factors between concentration units assume 1 atmosphere pressure
at 300°K.
In the addition reacLion pathway, the electrophilic OH radicals will
add to molecules having areas of high localized electron density. For alkenes,
alkynes and aromatics, the OH adds across a multiple bond to form a B-hydroxyl
organic radical. Substituents also affect the molecular orbitals of the
parent, transition state, and product species, thereby influencing the
reaction kinetics. Addition of OH to an organic molecule forms an adduct
which may fall apart and revert back to the reactant species unless stabilized
by collison or by energy transfer to other parts of the molecule. This
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reversion means that OH addition reactions are subject to pressure effects,
but the more complex the species the lower the "fall off" region between
second-order and third-order kinetics.
Atmospheric reactions are a combination of abstraction and addition
processes leading primarily to oxygenated organic compounds like aldehydes,
ketones, and dicarbonyls. Halogenated organics tend to lose halogen atoms
" 1
in the form of halo-oxy radicals. Hendry and Kenley describe a systematic
method for estimating reaction rates and reaction products resulting from
the atmospheric oxidation of organic compounds.
REACTION WITH OZONE
Organic chemicals with isolated double bonds may have a significant
atmospheric loss rate via reaction with ozone (0_). While the rate
constant for reaction of alkenes with 0 is much less than the rate constant
for OH reaction (approximately 10 cm molecule sec compared to
approximately 10 cm molecule sec ), ozone concentrations are so much
larger than OH concentrations that the two loss processes are competitive
for many alkenes. While aromatic compounds may react with 0_, their ozonolysis
removal mechanism is usually slow compared to reaction with OH.
Ozone is believed to add across the double bond, with subsequent
cleavage to form a carbonyl compound (aldehyde or ketone) and a percarbonyl
biradical. The biradical may rearrange or react to form a variety of
products including organic acids, carbon dioxide and a host of organic
radicals. Epoxides have also been suggested as minor products of ozonolysis
reactions.
Reaction with Other Radicals
Thorough understanding of laboratory experiments involving ozonolysis
processes is clouded by the formation, during reaction, of reactive radicals
that may interact with the reagents, intermediates and products. A wide
variety of such reactive species, including those produced during ozonolysis
-------
and hydroxylation reactions, exists in the ambient atmosphere (Table 1).
With the exception of OH and 0~, however, their concentrations are low, and
their importance as reactants with T/H M"s is small. For certain relatively
unreactive chemicals, reaction with species other than OH and 0- may provide
4 J
the predominant (albeit, slow) removal process. Reactions with the minor
species listed in Table 1 need not be considered unless all other chemical
or physical removal processes are ineffective.
PHOTOLYTIC TRANSFORMATIONS
Solar radiation in the troposphere covers the region from 2900 to 8000
angstroms. Photochemical fragnentation or rearrangement processes are
possible for those chemicals that absorb radiation of the appropriate
wavelength. Estimates of the magnitude of photochemical processes are
difficult to make because of uncertainties in light intensity, quantum
yields, etc. Photolysis can be an important removal process only for
chemicals which absorb strongly within the solar radiation region, otherwise
reaction with OH or 0~ is likely to predominate the removal process. This
reaction removal pathway limits the compounds to be considered to those
possessing a strongly absorbing chromophore, like carbonyl compounds, con-
jugated alkenes, nitro and other nitrogen-containing compounds and halides.
Compounds which may form from photolysis of these absorbing groups can be
suggested although predictions of efficiencies and specific yields are
impractical.
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SECTION 4
PHYSICAL REMOVAL PROCESSES
DISSOLUTION
Toxic hazardous materials in the gas-phase may be removed from the
air environment by dissolving them into cloud or rain droplets to be
subsequently removed by rainfall. An estimate of the efficacy of such a
removal process can be obtained by calculating the partitioning of
material between the aqueous and gas phases.
Henry's Law relates the equilibrium vapor phase and liquid phase
concentrations of a dilute solution of material 1 via the equation
(Eq.
where: p. = the equilibrium vapor pressure of the solute above the
dilute solution
x. « the mole fraction of the solute, and
K = the Henry's Law constant.
For very dilute solutions (x « x ), the concentration of solute, c.
A j. WCIL.GL X
(in g cm ) is proportional to x . Assuming ideal gas behavior at 298° K,
the term p. may also be converted to a gas-phase concentration term, c. ,
1 _3 *8
in units of g cm . Equation 1 becomes
, - - if (18) (273)
"ig ~ ci Ki (22414)(298)(760)
-------
c, = 9.7 X 10~7 K. c. (Eq. 2)
ig 11
where the Henry's Law constant, K., has the units of torr.
The ratio at equilibrium of the concentration in the aqueous phase to
the concentration in the gas phase is a dimensionless number, a, describing
the partitioning of the material between the two phases. Rearranging
Equation 2, one finds that
a = c. = 1.03 x 106 (Eq. 3)
"ig
For materials which are only slightly soluble in water, it is reasonable
to assume that Henry's Law (Equation 1) holds for all concentrations of i
from infinite dilution (x. -* 0) to the saturation point (x. = x. ). It is
X ly IS
also reasonable to assume that, at saturation, bulk quantities of i are in
equilibrium with the saturated solution and also in equilibrium with a
saturated vapor phase. At saturation, therefore, Equation 1 becomes
p. = x. K. and
is is i
Ki = is (Eq'
where: p. = the saturation vapor pressure of the solute, and
x. = the saturation mole fraction of the solute
is
The value of the Henry's Law constant is now expressed in terms of two
experimentally measurable values: the saturation vapor pressure, p. ,
IS
and the mole fraction of i at saturation, x. . Expressing p. in torr,
IS IS
Equation 4 may be substituted into Equation 3 to yield
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1.03 x 106 x (Eq. 5)
The fraction of material i . removed each year as dissolved compound
in rain is the product ofa times the ratio of the volume of the average
annual rainfall to the volume of the well-mixed troposphere. Assuming an
annual rainfall of 0.75 meter and a height of 8 km for the homogeneous
atmosphere , one calculates
dc = - q 0.75 A (yr"1) c.
--^ 8000 A
0.5 CIQ t - T 1/2
f dCi - - 9.375 x 10"5
CiO ci ?r
/
t - 0
T1/2 - 7.39 x 103
where: A the surface area of the earth,
TI ,_= the half-life of the species i,
t time, and
c._ = the concentration of i in the troposphere at time zero.
Table 2 lists the relevant physical parameters and estimates T. , for
some of the compounds under Type II Assessment. The calculated half -lives
are in general quite long.
The fate of the dissolved material is subject to question. Hydrolysis,
oxidation by hydrogen peroxide or other species, adsorption on particulates
in the rain drops or in contact with the ground water, etc., may all remove
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or alter the toxic materials. The assumed equilibrium is perturbed if
chemical interaction does occur within the suspended droplets. The more T/H
M absorbed by the droplet, the lower the values of T.. , are reduced.
Volatilization of the chemicals during evaporation of the rainfall run-off
will re-introduce the T/H M into the atmosphere. Any subsequent return of
the T/H M's to the air will lengthen the half-lives listed in Table 2.
Evaporative losses from a solution should be directly proportional
to the concentration of the materials in the gas-phase above the bulk
solution. The ratio of the equilibrium vapor-phase concentration of i,
c. , to that of water, c , divided by the similar ratio for the aqueous
18 wg
phase can be considered as the "relative volatility" of the compound.
Relative volatility = Cig °wg aw (Eq. 7)
c. /c a.
i w i
o 4
For water at 25 C, a = 4.34 x 10 . Values of a /a. have also been listed
w w i
in Table 2 and indicate that evaporation of even small amounts of rain
water are likely to reintroduce the T/H M into the air.
Estimation of actual evaporation rates from physico-chemical parameters
is very complex ' and depends in large part upon the model system selected.
Even in simple systems, laboratory experiments have shown that the actual
evaporation rate is diffusion limited in the liquid phase for species
with high relative volatilities. This result implies that, while the
evaporative losses are still fast, they are not so fast as equilibrium
considerations would indicate.
Nonetheless, the data in Table 2 suggest that for those chemicals of
concern, dissolution into and removal by raindrops is not a significant
removal mechanism.
10
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11
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ADSORPTION ON AEROSOL PARTICLES
Toxic chemicals in the vapor phase may be adsorbed on aerosol par-
ticulates and removed from the atmosphere with the aerosol. Since the
average tropospheric lifetime of aerosol particles is approximately seven
I Q
days, adsorption on natural aerosols may establish a limit for the gas-
phase lifetime of various toxic compounds. If 0 is the fraction of a com-
pound attached to aerosol particles, then the atmospheric lifetime of that
compound is 7/0 days.
Unfortunately, 0 has not been measured for many compounds. Those
measurements which have been made are subject to questions about sampling
artifacts and often lack adequate characterization of the ambient aerosols.
Using adsorption theory 0 can be estimated, but such theoretical cal-
culations are severely limited because of approximations and assumptions
involved in deriving useable equations. They should be considered "order
of magnitude" estimates only.
19
Brunauer, Emmett, and Teller (BET) modified Langmuir adsorption theory
to overcome the problems of multilayer adsorption and heterogeneous surfaces.
Their results take the form
V = Cp_ (Eq. 8)
m
^E
CPO - P) (i + (c - i) P/PO)
where: V = the volume of gas adsorbed,
V = the volume of gas adsorbed when one complete monolayer has
been formed,
= the pressure of the gas,
= the saturation pressure of the gas, and
C = a constant.
The volume terms in Equation 8 represent a specific mass for a defined
compound, pressure and temperature. Lettingn represent a "surface density"
P
12
-------
2
term (in units of g/cm ), the equation becomes
2- = Cp_ (Eq. 9)
nm (pQ - p) (1 + (C - 1) p/po)
where: n = the number of grams of gas adsorbed per square centimeter of
surface area of adsorbing material, and,
n « the similar surface area density term when a complete monolayer
m
exists on the adsorbing surface.
In the environment, aerosol particles are characterized by their number
count, surface area, and volume (or mass) characteristics. If 0 is the
surface area density of the aerosol particles in the atmosphere, then the
ambient concentration of the T/H M adsorbed on aerosol particles is given by:
(Eq. 10)
2 3
where: 9 = the number of cm of aerosol surface per cm of ambient air, and
p » the number of grams of T/H M adsorbed on aerosol particles per
cm of air.
From ideal gas laws, the gas-phase density is:
p = Mp_ (Eq. 11)
g RT
where: p = gas phase density, and
D
M = the gram molecular weight
Combining Equations 9, 10, and 11 yields:
RT «= K 6 (Eq.12)
M (P0 - p) (l+(C-l)p/po) (po-p)
where: K = a combination of constants.
13
-------
Equation 12 can be greatly simplified via two assumptions (p » p and p >
Cp) to become
Po
(Eq. 13)
Since C usually has a value of 5 to 25, both assumptions above are valid
only when p < 0.04 p . For most pollutants, the requirement that the ambient
pressure be less than 4% of the saturation pressure is easily met, especially
on the global scale.
Phi is related to the expression in Equation 13 via
p +p = p +1 = p + 1 = K 9 + P (Eq. 14)
g a g o _ o
P p K9 K 6
a a
0 = K e (Eq. 15)
Evaluation of K will permit calculation of 0 as a function of 9 and p ,
From Equation 12
* = nm C ^ (Eq. 16)
n is in units of g/cm and is the surface density when a complete monolayer
exists on the particle surface. It is estimated by determining the number
of molecules adsorbed per unit area and multiplying by the mass of each
20
absorbed molecule. Emmett and Brunauer calculate the surface area
covered by each molecule, S, as
S = (4) (0.866) (MM-v/7 N D)2/3 (Eq. 17)
where: N = Avogadro's number, and
D = the bulk density of the adsorbed chemical in g/cm .
14
-------
n is therefore given by
in
n = 1 /M ^ = _(4_
m I
_
S N (4) (0.966)
/DNoN2/3/ M
\ H J \ N
m
0.917 / D2M \ 1'3 (Eq. 18)
I ' « 1
\ N J
^ n '
O
The value of C is generally significantly greater than 1. Values of C
19 o
reported by BET range from 3 to 26 at 298 K, with the larger value given
for butane and the smaller values given for inorganic gases like nitrogen
and argon. Assuming a value of 25 and a temperature of 298 K, K is evaluated
as
2 1 /"}
0.917 /DM\ ' (25) (22414) (760) (298)
M V N J (273)
>.05 / D N2/3
V M )
torr-cmj (Eq. 19)
cm
Table 3 lists values of M, D, K, p , 0 and lifetime for several chemicals
currently undergoing Type II evaluation.
An average value of K = 0.25 was used, together with Equation 15 to
calculate 0 as a function of particle surface area, 9, for a variety of p
values. The results are shown in Table 4. On the global scale, 9 should
7 6 2 S 18
range between 10 and 10 cm /cm . For a reasonable value of 6 = 3 x
-723
10 cm /cm , 0 is small for any chemical with a saturation vapor pressure
greater than 10 torr.
Of the five compounds listed in Table 3, only B(a)P has a vapor pressure
which is sufficiently low to suggest a reasonable value of 0. Cautreels and
21
Van Cauwenberghe measured a particulate-to-gaseous ratio of 7.47 for B(a)P
in a urban sample. The resulting value of 0 = 0.88 is in reasonable agreement
15
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-------
with the value of 0.99 predicted by Equation 15 when K » 0.151 and p = 5.5
x 10~ torr (see Table 3) for a reasonable urban 9 value of 5 x 10 cm /cm .
TABLE A. FRACTION (0) OF MATERIAL ADSORBED ON AEROSOLS FOR VARIOUS
VALUES OF AEROSOL SURFACE AREA (0) AND CHEMICAL SATURATION
VAPOR PRESSURE (p ) K = 0.25
po
(torr)
io-8
io-7
io-6
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0.962
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0.200
These results of Table 4 indicate that the theoretical calculations,
while filled with approximations and assumptions, can provide at least "order
of magnitude" estimates of the adsorption of toxic chemicals on atmospheric
aerosols. The results further suggest that adsorption will be a reasonable
vapor-phase removal mechanism only for materials with saturation vapor
pressures of 10 torr or less.
DRY DEPOSITION
Gaseous toxic/hazardous materials will come in contact with soil and
water at the earth's surface and may be removed by adsorption or absorption.
The ratio of the deposition flux divided by the airborne concentration is
defined as the deposition velocity, v . The values of v are highly variable,
depending upon the type of surface, meteorology, and composition of the
material settling out. Values of v have been reported for a few gases, and
22
they range from 0.01 to 3 cm/sec, implying residence times of 3 to 900 days.
17
-------
The deposition velocity of only one of the 43 chemicals on the
Type I Assessment list, methyl iodide, has been measured. Its v, was
/ 9
reported as 10 to 10 cm/sec. From reports of photodecomposition of
23
carbon tetrachloride absorbed on silica surfaces , one may estimate v,<
M A
10 cm/sec.
Many of toxic and hazardous materials of concern (e.g., perchloro-
ethylene, dichloroethylene, vinylidene chloride) can Ee expected to
have deposition velocities comparable to those for CH,I and CC1,.
-3
Assuming a mid-range value of 10 cm/sec, an estimated half-life of 25
years is predicted.
Considering the variability of the data and the likely ineffectiveness
(T, ,~ ~ 25 years) of dry deposition for removing or transforming many of
the toxic or hazardous chemicals, this removal mechanism should be con-
sidered significant only under unusual circumstances.
18
-------
SECTION 5
FATE OF CHEMICALS UNDER ASSESSMENT
The atmospheric chemical and physical removal processes are evaluated
in Table 5 for 46 individual chemicals currently under assessment regarding
regulatory action.
For each chemical listed, the following items are evaluated:
(1) Hydroxyl radical reaction rate constant - Experimentally measured or
theoretically-estimated gas-phase rate constants for the initial
'reaction between OH radicals and the volatile chemical are given
in units of 10 cm molecule sec
(2) Ozone reaction rate constant - The reaction rate constant is given in
units of 10 cm molecule sec for those species whose rate has been
measured and for all other species likely to react with 0,. The
theoretically-estimated rate constants in Table 5 (denoted by a
superscript b) are subject to substantial uncertainty and should be
used cautiously. In some cases, the estimates are for chemicals in
structural classes far removed from those used to develop the estimation
techniques in Reference 1. Caution is especially warranted in the case
of ozonolysis of alkenes with strong electron withdrawing substituents
and for OH reaction with substituted aromatics.
(3) Photolysis probability - Any chemical possessing a chromophore likely
to absorb light within the solar spectral region was given at least
a "possible" rating. If photolysis of the specific compound, or of
several structurally similar compounds, had been reported, then the
chemical was given a "probable" rating. Quantitative values for
photolytic decomposition are extremely difficult to predict, and
accurate numbers require experimental determination. Because experimental
19
-------
data are not available for many of the subject compounds, photolysis
was largely ignored in the calculation of the species' lifetimes.
(4) Physical removal probability - The ratio of the aqueous solubility
density of the material to its saturation vapor density, a, (defined
4
in Section 4), was determined. If the value exceeded 2 x 10 , implying
a rainout lifetime of about 0.5 years, the chemical was rated as
"possible." Because revolatilization of the materials is likely,
removal by dissolution could be rated no higher than "possible."
Materials with saturation vapor pressures of less than 10 torr are
likely to be adsorbed on atmospheric aerosols, and they were given a
"probable" rating. Species, like bis(chloromethyl)ether, which have
24
been reported to decompose rather quickly in the environment, were
also rated as "probable." A priori prediction of decomposition is
difficult, and it was considered as a physical removal mechanism only
in cases where decomposition had been reported.
(5) Atmospheric residence time - This number represents the estimated time,
in days, required for a quantity of the individual chemical to be reduced
to 1/e of its original value. As it is not a concentration lifetime
it does not include a dilution term. For species with likely photolysis
or physical removal mechanisms, the residence time was expressed as a
range, and a comment about the smaller lifetime value was included in
the footnotes to the table. In calculating the numbers listed in the
table, two major assumptions were made:
(a) The room temperature rate constants for OH and 0. are valid for
the ambient atmosphere. Since rate constants are temperature dependent,
the actual removal rates will depend on time of day, season of the year,
altitude, and any other parameter affecting the temperature. In
addition, some OH reactions show positive temperature dependencies
while others show negative effects. Because the temperature dependencies
of so many of the species are unknown, room temperature values were used
throughout.
(b) Background concentrations of OH and 0_ were assumed to be constant,
with values of 1 x 10 and 1 x 10 molecules cm , respectively.
20
-------
Obviously, the OH and 0. concentrations depend on a complex set of
chemical and physical conditions (light intensity, pollutant con-
centrations, temperature, altitude) and are not actually constant.
The choices of concentrations used are reasonable averages for the
1 2 25
lower troposphere. ' ' (Some data obtained in a private communication
from H.B. Singh 1980).
(6) Anticipated Products - The last column lists some of the products likely
to result from the photochemical oxidation of dilute quantities of the
specific compound in the atmosphere. The product list is not intended
to be all-inclusive but is suggestive of the kinds of materials likely
to be produced. Many of the products can be expected to react further,
producing still other chemical species. Often reactive species like
radicals are listed as products in order to suggest that whatever
reaction scheme is available in their immediate locale will dictate.the
specific products. In addition, all the reaction schemes proceed
through RO* and ROD' radicals which may add nitrogen oxides to form a
wide variety of nitrogen-containing species. These species are not
tabulated, but they should be considered as possible products for
every reactive material.
21
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REFERENCES
1. Hendry, D.G. and R.A. Kenley, Atmospheric Reaction Products of Organic
Compounds, Environmental Protection Agency Report EPA-560/12-79-001,
1979. 81 pp.
2. Sprung, J.L., Tropospheric Oxidation of H.S, In: Advances in Environ-
mental Science and Technology, VII, (J.N. Pitts, Jr. and R. L. Metcal,
eds.) John Wiley and Sons, New York, New York, 1977. pp. 263-278.
3. Graedel, I.E., Chemical Compounds in the Atmosphere, Academic Press,
New York, New York, 1978.
4. Atkinson, R., G.M. Breuer, J.N. Pitts, Jr. and H.L. Sandoval, Tropo-
spheric and Stratospheric Sinks for Halocarbons: Photo-oxidation, 0( D)
Atom, and OH Radical Reactions, J. Geophys. Res., 81: 5765-5770, 1976.
5. Leighton, P.A. Photochemistry of Air Pollution, Academic Press, New
York, New York, 1961. 300 pp.
6. Fairbridge, R.W. (ed.) The Encyclopedia of Atmospheric Sciences and
Astrogeology, Reinhold Publishing Corp., New York, New York, 1967.
1200 pp.
7. Tverskoi, P.N. Physics of the Atmosphere, NASA/TT-F-288, Published for
the National Aeronautics and Space Administration and the National
Science Foundation, Washington, DC, 1965. 561 pp.
8. Mackay, D. and A.W. Wolkoff, Rate of Evaporation of Low-Solubility
Contaminants from Water Bodies to Atmosphere, Environ. Sci. Technol.,
7(7): 611-614, 1973.
9. Liss, P.S. and P.G. Slater, Flux of Gases Across the Air-Sea Inter-
face, Nature, 247(5348): 181-184, 1974.
10. Mackay, D. and P.J. Leinonen, Rate of Evaporation of Low-Solubility
Contaminants from Water Bodies to Atmosphere, Environ. Sci. Technol.,
9(13): 1178-1180, 1975.
11. Billing, W.L., Interphase Transfer Processes, II. Evaporation Rates
of Chloro Methanes, Ethanes, Ethylenes, Propanes, and Propylenes from
Dilute Aqueous Solutions. Comparisons with Theoretical Predictions,
Environ. Sci. Technol., 11 (4): 405-409, 1977.
26
-------
12. Fuller, B.J., J. Hushon, M. Kornreich, R. Ouellette, L. Thomas and P.
Walker, Scoring of Organic Air Pollutants, MITRE Technical Report
EPA/MTR-7248, Rev. 1, 1976.
13. Pupp, C., R.C. Lao, J.J. Murray and R.F. Pottie, Equlibrium Vapour Con-
centrations of Some Polycyclic Aromatic Hydrocarbons, As.O, and SeO?
and the Collection Efficiences of these Air Pollutants, Atmos. Environ.,
8 (9): 915-925, 1974.
14. May, W.E., J.M. Brown, S.N. Chesler, F. Guenther, L.R. Hilpert, H.S.
Hertz and S.A. Wise, Development of an Aqueous Polynuclear Aromatic
Hydrocarbon Standard Reference Material, Polynuclear Aromatic
Hydrocarbons (P.W. Jones and P. Leber, eds.) Ann Arbor, Mich: Ann
Arbor Science Publishers, Inc., 1979.
15. Verschueren, K. Handbook of Environmental Data on Organic Chemicals,
Van Nostrand Reinhold Company, New York, New York, 1977. 695 pp.
16. Windholz, M., (ed.) The Merck Index, Merck and Co. Inc., Rahway, New
Jersey, 1976.
17. Hushon, J. and M. Kornreich, Air Pollution Assessment of Vinylidene
Chloride, MITRE Technical Report MTR-7230, 1978. 71 pp.
18. Junge, C.E. Basic Considerations About Trace Constituents in the
Atmosphere as Related to the Fate of Global Pollutants, Fate of
Pollutants in the Air and Water Environments, Part I, 8, (I.H. '
Suffet, ed.) John Wiley and Sons, New York, New York, 1975. pp. 7-25.
19. Brunauer, S., P.H. Emmett and E. Teller, Adsorption of Gases in
Multimolecular Layers, J. Am. Chem. Soc, 60; 309-319, 1938.
20. Emmett, P.H. and S. Brunauer, The Use of Low Temperature Van der
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