United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research-Triangle Park, NC 27711
Research and Development
 EPA-600/S3-80-084  Dec, 1980.
Project  Summary
Fate  of Toxic  and   Hazardous
Materials  in  the Air
Environment
Larry T; Gupitt
  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, perchloroethyl-
ene, vinylidene chloride and benzo(a)-
pyrene (as a representative polycyclic
organic material).
  Chemical and  physical removal
processes  are discussed. Chemical
removal  processes  which  are
considered include photolytic trans-
formations  and  reactions  with
hydroxyl radicals, ozone, and  other
tropospheric  species.  Mathematical
descriptions  of  physical  removal
mechanisms  are   developed  and
applied to the volatile Type II Assess-
ment chemicals. Physical removal by
rainfall,  by dry,deposition,  and by
adsorption on aerosol  particles 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
         and of physical removal is
assessed,  and  residence  lifetimes
assigned. Probable products of tropo-
spheric oxidation processes are also
tabulated.
 Introduction
  The Office of Air Quality Planning and
 Standards (OAQPS).has recently distri-
 buted  lists  of potentially  toxic/
 hazardous chemicals being assessed in
 regard to possible regulatory action. The
 list of "43 chemicals" under Type I (pre-
 liminary) 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 chernjcalsy it-is
 important to evaluate the atmospheric
 fate  of these compounds, particularly
 those chemicals currently, under Type II
 Assessment that may possibly be found
•as gaseous emissions,  (e.g.,  acrylo-
 nitrile, ethylene .dichlpride, perchloro-
 ethylehe,   vinylidene chloride,  and
 benzo(a)pyrene  (BaP) as a representa-
 tive polycyclic organic material).
Chemical Removal Processes
   For a wide variety of molecules'/'ithe.,
most   important, chemical' removal
process in the troposphere is reaction
with hydroxyl (OH) radicals. For those

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 organic chemicals containing isolated
 double bonds, ozonolysis is also avail-
 able as a second reaction pathway. In
 addition,  reactions with minor, tropo-
" spheric species or with species known
 to exist in the stratosphere (e.g., 0(3P),
 0('D), HOI ROa, N03, 02 ('A g), etc.)
 must be considered for. chemicals that
•are-relatively long-lived.
 Reaction Wfth Hydroxyl
 Radicals
   Despite the fact that the average con-
 centration of OH radicals is much lower
 than'for many other reactive species,
 reaction with hydroxyl radicals is often
 so:rapid that it isthe predominanttrppo-
 sphBrrc removal,mechanism for a wide
 variety of organic molecules.
   Atmospheric reactions with OH are(a
-combination of abstraction and addition
 processes  leading   primarily  to
 oxygenated  o/ganic  compounds  like
 aldehydes,  ketones, and dicarbonyls.
 Halogenated  organics  tend  to  lose
 halogen atoms in the form of halo-oxy
 radicals.  Systematic   methods for
 estimating OH reaction  rates  and the
 reaction  products resulting from the
 atmospheric  oxidation o.f organic
 compounds have been published.
 Reaction With Ozone
   Organic,  chemicals  with  isolated
 double bonds may have a  significant
 atmospheric loss rate via reaction with
 ozone (Q3). While the rate constant for
 reaction of alkenes with Oa is much less
 than the rate constant for OH reaction
 (approximately  10"17 cm3   molecule"1
 sec"' compared  to approximately  10~11
 cm3  molecule"1  sec"1), ozone concen-
 trations are  so  much larger, than OH
 concenicalions that  tjie two loss pro-
 cesses  are  competitive   for  many
 alkenes. Afomatrc compounds may also
 react  with   Os,  but  their  ozonolysis
 removal mechanism is  usually  slow
 compared to.reaction with OH.
   Ozone is believed to add  across the
 double-bond  wit.hjsjo.bsequent cleavage
 to form a carbonyl compound (aldehyde
 or- k©torre)-ahd"a percarbonyl biradTcal.
 The biradical may rearrange'or react to
 form, a  variety  of  products including;
 organic acids, carbon.dioxide and a host
 of ..organic radicals. Epoxidtes have also
 •been suggested 'as minor products of
 ozonolysis reactions.
Reaction  With  Other Radicals
  Thorough understanding of labora-
tory experiments involving ozonolysis
and oxidation  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. With the exception of OH
and 03, however, their concentrations
are  low,   and  their  importance  as
reactants with T/H M's is small. None-
theless, for certain relatively unreactive
chemicals, reaction with species other
than OH   and  03  may  provide  the
predominant  (albeit,  slow)  removal
process.   Reactions  with  the  minor
species need not be considered unless
all other chemical  or physical removal
processes are ineffective.
Photolytic Transformations
  Estimates of the magnitude of photo-
chemical processes are difficult to make
because of uncertainties in light inten-
sity, quantum yields, etc. Photolysis can
be an important removal  process only
for  chemicals which absorb  strongly
within the solar radiation region, other-
wise reaction with OH or Oa is likely to
predominate the removal process. This
fact limits the compounds to be con-
sidered to those possessing a strongly
absorbing  chromophore, like  carbonyl
compounds, conjugated 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 im-
practical.
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 subsequent-
ly removed by rainfall. An estimate of
the efficacy of such a  removal process
can be obtained  by calculating the parti-
tioning   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 i  via the equation
               i = xi K,
(Eq. 1,
where:  pi = the   equilibrium  vapor
            pressure  of the  solute
            above the dilute solution
        x, = the  mole, fraction of  the
            •solute, and
        Ki = the Henryfs Law constant.

For very dilute solutions, the concentra-
tion of solute, G (in g cm"3),  is propor-
tional to xi. Assuming ideal gas behavior
at 298°  K,  the term ^.may also be
converted to a gas-phase>concentration
term,  Ci9, in units of g  cm*3 Equation
1 becomes

cig = 9.7 X 10"7 KiCi            (Eq. 2)

where the Henry's Lawconstant, Ki, has
the units of torr..
  Unfortunately,  the.  Henry's  Law
constant has not been measured for the
chemicals under assessment and must
be approximated.  For  those materials
which are only slightly soluble in water,
it is reasonable to assume that Henry's
Law holds for  all  concentrations of  i
from  infinite dilution to the
point. At saturation,
are in equilibrium with the
solution and also in equilibrium with I
saturated vapor phase,. Therefore, at
saturation, Equation 1 .becomes
            Pis =
                     i and
               K, = PJS         (Eq. 3)
where:  pis = the  saturation vapor
             pressure of  the solute,
             and
        xis =.the  saturation  mole
             fraction of the solute

The value of the Henry's" Law constant is
now expressed in  terms  of two more
commonly  measure'd  values:  the
saturation vapor pressure, pis, and the
mole fraction of i at saturation, XiS.
  The ratio at equilibrium of the concen-
tration 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  the
equations above, one- finds that
a = _£,_= 1.03 x-106 = 1 03 ;; 10s
    c,g      Ki               (Eq. 4..

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  The fi'aqtion of material i  removed
        t is.dissolved compound in rain
      product of a times the ratio of the
        of the average annual rainfall
to the volume of the well-mixed tropo-
sphere. Assuming an annual rainfall of
0.75 meter and a height of 8 km for the
homogeneous  atmosphere,  one
calculates the half-life as
 7-1,3.= 739
(Eq. 5)
  Table 1  lists the  relevant physical
parameters and estimates Tt.z for some
of the compounds under Type II Assess-
ment. The (.calculated half-lives are in
general quite long.
  The fate of the dissolved material is
subject  to question.  Hydrolysis, oxida-
tion  by  hydrogen peroxide  or  other
species, adsorption on particulates in
the rain drops  or  in contact  with the
ground  water, etc., may  all remove or
alter the toxic materials. Volatilization of
the chemicals during evaporation of the
rainfall run-off will re-introduce the T/H
M into the atmosphere. Such changes
will naturally affect the half-lives of the
T/H  M'S
      orative losses from a solution
       be  directly proportional to the
u	^entration of  the materials in the
gas-phase  above the bulk solution. The
ratio  of  the equilibrium vapor-phase
concentration of i, Cig, to  that  of water,
cwg, divided by the similar ratio for the
aqueous phase can be considered as the
"relative volatility" of the compound.


Relative volatility = Ci2/c«g= £„ jE  gj
                   Ci/Cw    Qi"
For water .at 25°C, crw — .4.34 x 10".
Values of ffw/a have also oeen listed in
Table 1 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 thatthe 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 equili-
brium considerations would indicate.
  Nonetheless, the data  in  Table  1
suggest that for those  chemicals  of
concern,  dissolution into and  removal
by raindrops is not a significant removal
mechanism.
                                         Adsorption on Aerosol
                                         Particles
                                           Toxic  chemicals in the vapor phase
                                         may be adsorbed on aerosol particulates
                                         and removed from the atmosphere with
                                         the aerosol. Since the average tropo-
                                         spheric  lifetime of aerosol particles is
                                         approximately  seven 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
                                         compound attached to aerosol particles,
                                         then the  atmospheric  lifetime of that
                                         compound is 7/

0.8 1.4 390.0 12.030.0 ITO.OUO'ti aw/a* 4.8 8.O 2.300.0 71.000.0 670,000:0 3 The theory developed above does not directly apply to soluble chemicals like acrylonitri/e; however, substitution1 of Raoult's Law for Henry's Law leads to an equivalent derivation, but with slightly different assumptions. "Equation 4 defines a as 1.03 x JO6 (xis/p,s). Since x,s and p^ are both, temperature dependent, a is aJso temperature dependent. The values quoted in the table assume a temperature of 25° C, whereas the average tropdspheric^empera- jyre.may be substantially different. This means that all half-lives calculated from this data should be considered approximations.


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This derivation is based upon assump-
tions which are valid only when p< 0.04
p0. 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.
    K can be approximated by
AT = 5.05/JJ. \
        \M/
             2/3
(Eq. 9)
                    cm
where: D  =  the  bulk  density  of the
           •absorbed  chemical  in
            g/cm3, and
       M  =  the  gram  molecular
            weight
Table 2 lists values of M, D, K, p0, 0 and
lifetime for several chemicals currently
undergoing Type II evaluation.
  Of the five compounds listed in Table
2 only B(a)P has a vapor pressure which
is sufficiently low to suggest a reason-
able value of  0.  A  particulate-to-
gaseous ratio  of 7.47 has been mea-
sured for B(a)P in an urban sample. The
resulting value of 0 = 0.88 is in reason-
able agreement with the value of 0.99
predicted by Equation 8 when K = 0.151
andp0= 5.5x 10"9torr(seeTable2)fora
reasonable urban B value of 5 x .10~6
cmVcm3.
  These   results   indicate  that  the
theoretical calculations,  while filled
with approximations and assumptions,
can provide at  least "order of  magni-
tude" 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-TO"7  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, Vd. The values
of Vd  are  highly  variable, depending
upon the type of surface, meteorology,
and composition of the material settling
out. Values of v
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3. Fates and Residence Times of Some Chemicals Under Assessment
L v 1 n1^ *, y*o18
«OH X IU /Co, A 7O
cm3 cm3 Physical
molecule'1 molecule'1 Photolysis Removal
Compound sec'1 . sec"1 Probability Probability
TYPE II ASSESSMENT
Acrylonitrile

Arsenic
Cadmium3
Eth'ylene Dichloride

Perch/oroethylene
POM (Benzo(a)pyrenef
Vinylidene chloride
TYPE 1 ASSESSMENT
Acetaldehyde"
Acrolein

Ally! chloride


Benzyl chloride


Bis(Chloromethyl) Ether
^k
^J
'Carbon Tetrachloride
Chlorobenzene

Chloroform
Chloromethyl methyl ether


Chloroprene



o-, m-, p^Cresof

Dichlorobenzene'


Dimethyl nitrosamine

Dioxane


Dioxin
Epichlorohydrin

Ethylene Dibromide
^
^F(thylene Oxide
..formaldehyde0
Hexachlorocyclopentadiene

2



0.22

0.17

4"

16.
44"

28°


3"


4*


< 0.001
0.4°

0.1
3"


46*



55.

0.3*


39°

3b



2b

0.25

2"
10.
59*

< 0.05 '

- -
— —
Possible

0.002 Possible
Possible
0.04 Possible

Probable
4* Probable

18.3 Possible


0.004" Possible


Possible


- -
< 0.00005° Possible

- -
— Possible


8° Probable



0.6

< 0.00005* Possible


Probable

- -


— Probable
Possible

— Possible

< 0.000002 Probable
8* Probable

Unlikely

Possible
Probable
Unlikely

Unlikely
Probable
Unlikely

Unlikely
Unlikely

Unlikely


Unlikely


Probable9


Unlikely
Unlikely

Unlikely
Probable9


Unlikely



Unlikely

Unlikely


—

Unlikely


—
Unlikely

Unlikely

Unlikely
Unlikely

Atmospheric
Residence
Time
Days Anticipated Products

5.6

—
~ 7
53

67
~ 8
2.9

0.03-0.7°
0.2

0.3


3.9


0.02-2.9


> 1 1000
28

120
0.004-3.9


0.2



0.2

39


<0.3

3.9


—
5.8

45

5.8
0. 1-1.2"
0.2

HzCO. HC(0)CN. HCOOH.
CN-
—
—
CIHCHO. HzCCICOCI. HzCO.
HzCCICHO
ClzCO. ClzC(OH)COCI. Cl-
B(a)P-1 ,6-quinone
HzCO, ClzCO. HCOOH

HzCO. COz
OCH-CHO. HzCO. HCOOH.
COz
HCOOH. HzCO, CICHzCHO,
chlorinated hydroxy
cardonyls. CICHzCOOH
CHO. Cl: chloromethyl-
phenols. ring cleavage
products
Decomposition products
(HCI + HzCO\ chloro-
methylformate. CIHCO
ClzCO. Cl-
Chlorophenols. ring
cleavage products
ClzCO, Cl-
Decomposition products.
chloromethyl and methyl
formate. CIHCO
HzCO. HzC = CCICHO,
OHCCHO. CICOCHO.
HzCCHCCIO. chlorohydroxy
acids, aldehydes
Hydroxynitrotoluenes,: ring
cleavage products
Chlorinated phenols, ring
cleavage products, nitro
compounds
Photolysis products.
aldehydes. NO
OHCOCHzCHzOCHO.
OHCOCHO. oxygenated
formates
—
HzCO. OHCOCHO.
CICHzO(0)OHCO
Br; BrCHzCHO. HzCO
BrHCO
OHCOCHO
CO. COz
ClzCO. diacylch/orides.
ketones. Cl-

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Table 3. (continued)
i
(onX 7012
cm3
molecule'*
Compound
TYPE 1 A SSESSMENT (cont 'd)
MaJeic Anhydride


Manganese3
Methyl chloroform
Methyl chloride
Methyl Iodide
Nickel* "' "• '
Nitrobenzene

2-Nitropropane
N-Nitrosodiethylamine

Nitrosoeth ylurea

Nitrosometh ylurea

Nitrosomorpholine

Phenols


Phosgene*
Polychlorinated Biphenyls

Propylene oxide


Toluene


Trich/oroeth ylene
o-, m-, p-Xy/ene'



sec"1

60°



0.012
0.14
0.004*
—
0.06"

55*
26*

13*

20*

28°

17*


0
< 1*

1.3


6.


2.2
~ 16.



it y rn18
t\Q /\ / U
cm3
molecule'* Photolysis
sec"1 Probability

160* Possible


- - .
— Possible
— Possible
— Possible
— —
< 0.00005* Possible

— Possible
Probable

— Possible

Possible

Possible

1*


— —
0.00005* Possible

—


0.0003


0.006 Possible
-0.001



Atmospheric
Physical Residence
Removal
Probability

Possible


Probable
Unlikely
Unlikely
Unlikely
Probable
Unlikely

Unlikely
—

—

—

—

Possible


Possible
Unlikely

Unlikely


Unlikely


Unlikely
Unlikely



Time
Days

0.1


' /
370
53
2900
~ 7
190

0.2
<0.4

<0.9

<0.6

<0.4

0.6



> 11

8.9


1.9


5.2
0.7




Anticipated Products

COz, CO; acids, aldehydes
and esters which should
photo/yze

H2CO, CI2CO, Cl-
ChCO, CO. CIHCO, Cl-
H2CO. I; IHCO. CO

Nitrophenols, ring cleavage
products
HzCO. CH3CHO .
Photolysis products.
aldehydes, nitramines
Photolysis products.
aldehydes, nitramines
Photolysis products.
aldehydes, nitramines
Photolysis products.
a/dehydic ethers
Dihydroxybenzenes, nitro-
phenols, ring cleavage
products
COz. Cl; HCI
Hydroxy PCB's, ring
cleavage products
CH^C(O)OCHO,
CHiC(0)CHO. HzCO.
HC(O)OCHO
Benzaldehyde, cresols, ring
cleavage products, nitro
compounds
ChCO. CIHCO. CO. Cl-
Substituted benzaldehydes.
hydroxy xylenes, ring
cleavage products, nitro
compounds
"Material is not expected to exist in vapor phase at normal temperatures. Residence time calculation assumes the
 chemical is substantially adsorbed on aerosol particles and that the aerosols have a residence time of approximately 7
 days.
*Rate constant calculated theoretically.
^Reaction  with O('D) is possible: k = 3.6 x 70"'° cm3 molecule'' sec"1, and (O^D)] = 0.2 molecules cm'
 tropospheric lifetime of 440 years. In addition, slow hydrolysis is expected.
"The shorter residence time includes a photolysis rate.
implies a

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rat< d no  higher than  "possible."
^Materials  with  saturation  vapor
pressures of less than 10"7 torr are
likely   to  be  adsorbed  on
atmospheric  aerosols,  and  they
were given  a  "probable" rating.
Species,   like  bis(chloromethyl)-
ether, which  have 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.
Atmospheric residence time—This
number represents the estimated
time in days,  required for a quantity
of the  individual chemical  to  be
reduced to l/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 cal-
culating the  numbers listed in the
  able, two major assumptions were
  ade:
The  room temperature  rate
constants for OH and 03 are
valid for the ambient atmos-
phere.
a.
                                     b.  Background  concentrations of
                                         OH and O3 were assumed to be
                                         constant, with values of 1  x 1 O6
                                         and 1 x  1012 molecules cm"3
                                         respectively.
                                     Anticipated -products—The  last
                                     column lists some of the products
                                     likely  to  result  from the  photo-
                                     chemical   oxidatton  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 ROO- 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.
                                     Conclusions
                                       Quantitative description of the atmos-
                                     phere degradation or photolytic trans-
formation  of  the   toxic/hazardous
material (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, esti-
mates of reasonable  residence  times
are feasible on the basis of the normally
predominant chemical  removal
mechanisms.   Based  upon .current
knowledge of  atmospheric oxidation
mechanisms,   possible   tropospheric
reaction products may be anticipated.
  For the volatile chemicals currently
undergoing  Type  II Assessment  (i.e.,
acrylonitrile, vinylidene chloride, ethyl-
ene dichloride, perchloroethylene and
benzo(a)pyrene),  chemical   removal
residence times, based  upon reaction
with hydroxyl radicals and ozone, are
estimated to range between approxi-
mately 3 to approximately 70 days.
  Estimates can also be made  of the
physical  removal rates for the volatile
Type II chemicals. Removal by dissolu-
tion 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 10~7
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.
                                        The author of this Project Summary is Larry T. Cupitt. who is also the EPA
                                          Project Officer (see below).
                                        The complete report, entitled "Fate of Toxic and Hazardous Materials in the Air
                                          Environment," (Order No. PB 80-221948; Cost: $6.50subject to change) will
                                          be available 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
                                                                                     -.- U.S. GOVERNMENT PRINTING OFFICE: 1981 -757-064/0204

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