United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-84-113 Jan. 1985
Project Summary
Hazardous Air Pollutants: Wet
Removal Rates and Mechanisms
M. Terry Dana, R. N. Lee, and J. M. Hales
Fourteen hazardous organic air pol-
lutants were evaluated regarding their
potentials for wet deposition by precipi-
tation scavenging. This effort included
a survey of solubilities (Henry's Law
constants) in the literature, measure-
ment of solubilities of three selected
species, development of a general
deposition model (MPADD) that in-
cludes dry deposition and plume deple-
tion, and performance of scavenging
field experiments to provide a data base
for testing the model. Solubility param-
eters (dimensionless ratio of aqueous
concentration to air concentration) for
ethylene oxide, nitrobenzene, and
methyl chloroform were measured in
rainwater at two temperatures each;
the values obtained agreed generally
with previous work except those for
methyl chloroform, which were some-
what lower than previous experimental
and calculated values. Four field experi-
ments were conducted: three used
nitrobenzene and one involved methyl
chloroform. Agreement of measured
concentrations with model-calculated
values was good for nitrobenzene,
despite larger-than-desired experiment-
al uncertainties during two of the
releases. Analytical difficulties resulted
in only a few measurements of methyl
chloroform rainwater concentrations;
these, however, were in general agree-
ment with model calculations and
expectations on the basis of methyl
chloroform's much lower solubility
than that of nitrobenzene.
This Project Summary was developed
by EPA's Atmospheric Sciences Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The U.S. Environmental Protection
Agency is evaluating the environmental
fate of a number of possibly hazardous
organic chemicals that are being consid-
ered for regulation. The removal from the
atmosphere by precipitation scavenging
of a selected list of these chemicals,
shown in Table 1, is the focus of the
present project. The major objective is to
provide a reliable and convenient meth-
odology for evaluating the wet removal of
the listed pollutants, which is based on a
sound understanding of the pertinent
atmospheric interactions.
The research directed toward fulfill-
ment of this objective was divided into
three components: deposition modeling,
solubility determinations, and field
experimentation. The modeling effort
resulted m the development of the Multi-
Pollutant Atmospheric Deposition and
Depletion (MPADD) Model, a versatile
computer code for predicting the wet and
dry deposition behavior of reactive or non
reactive species. The solubility component
included a survey of known Henry's Law
behavior of the listed hazardous air
pollutants (HAPs), and measurements
were made on three of them. To provide a
data base for evaluating the MPADD
model, field experiments employing
controlled releases of two HAPs were
conducted.
The following sections summarize the
results from these components. Conclu-
sions and recommendations are based
upon the project as a whole
Modeling
The MPADD model is a complete
revision of the former Scavenging Model
Incorporating Chemical Kmetics(SMICK),
developed for the EPA by Battelle, Pacific
Northwest Laboratories m the early
1970's The major change is a completely
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Table 1.
Hazardous Air Pollutants of
Present Interest
Acetaldehyde
Acrylonitrile
Carbon Tetrachloride
Chloroform
Cresols
Epichlorohydrin
Ethylene Oxide
Methyl Chloroform
Methylene Chloride
Nitrobenzene
Perchloroethylene
Phenols
Phosgene
Polychlormated Biphenyls
new numerical integration subroutine,
which is more efficient and offers a
considerable reduction in computer time.
Other advances include the inclusion of
dry deposition as well as wet deposition,
and an accounting for plume depletion.
MPADD is modularized, allowing for a
variety of source configurations, plume
descriptions, chemical reaction mecha-
nisms, and pollutant physical properties
(including gaseous or aerosol).
A schematic of the macroscopic features
of MPADD is shown in Figure 1. The
plume is subject to diffusion, possible
chemical reactions, and dry and wet
deposition. The mass transfer between
the aqueous phase (raindrop) and gas
phase is integrated numerically along the
raindrop's trajectory (determined by fall
velocity and wind speed), and resulting
ground-level concentrations are evaluated
at the receptor This process is repeated
for a selectable number of raindrop sizes,
and the bulk rain concentration at the
receptor is computed by weighting over
the calculated or measured raindrop size
spectrum. The above procedure is done at
a selectable number of cross-plume
positions, and cross-plume integrated
fluxes (wet and dry) are evaluated. At
each downwind distance, the process is
repeated, and depletion of the plume by
wet and dry deposition is accounted for.
The result is a deposition pattern and
values for removal rates in the downwind
area
Elevation
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Solubility
The Henry's Law constant is probably
the most important physical property of a
gaseous pollutant affecting precipitation
scavenging. A literature survey was
conducted to assess the knowledge of
solubility of HAPs, and to assist in
selecting species for measurements
during the current project. The results of
this survey are shown in Table 2.
Figure 1. Schematic elevation and plan views of model layout.
Additional measurements were performed Law constant, for the three species are
for ethylene oxide, nitrobenzene and also listed in Table 2. Each experimental
methyl chloroform. Measured results for value entered in Table 2 is the average of
a, a dimensionless form of the Henry's three separate runs.
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Table 2. Solubility Parameters a For Hazardous Air Pollutants
Species
Acetaldehyde
Acrylonitrile
Carbon tetrach/oride
Chloroform
Cresols
p-cresol
4-6. din/tro-o-cresol
Epic hi or oh ydrin
Ethylene oxide
Methyl chloroform
Methylene chloride
Nitrobenzene
T
°K
298
298
293
298
293
298
298
298
298
278
283
288
293
303
280
288
293
298
298
280
288
a. aimensi
Calculated
^•1
9x1 03
1 04
0.86
8.35
7.57
760
5.3
79
11.2
6.28
5.99
1.5,8.04,9.1
oniess
Measured
0.81..87
7.21
2.8x103
1.7x10*
3.8
6.2
4.7
24
4.97
1.4,767.10
S.SxIO3
1.7X103
Perchloroethylene
Phenol
Phosgene
Species
293
298
293
298
293
298
278
T
°K
1.88X103
1.016x10*
1.18
0.85,2 4
3.4x10*
4.06
0.85.2.0
1.88x10*
a, dimension/ess"
Calculated
Measured
Po/ychlorinated biphenyls
Arochlor 1242
Arochlor 1248
Arochlor 1254
Stovh/ot 1260
298
298
298
298
42.9
6.99
873
34.4
"Ratio of aqueous phase concentration to gas-phase concentration
^Present project
Field Experiments
A test of the efficacy of MPADD to
predict HAP rainwater concentrations
under actual field conditions was provided
through controlled-release field experi-
ments. The release system provides a fine
mist of the liquid pollutant that volatilizes
quickly before encountering raindrops
over the sampling positions. Samplers
were placed in arcs at distances of 200
and 400 m downwind of the release
tower. To simplify the plume description
employed in MPADD and to minimize dry
deposition, the source point was elevated
26 m. Two species with (presumed)
widely different solubility parameters
were chosen as the materials to be
released. Nitrobenzene possesses the
desirable characteristics of high aqueous
solubility and low volatility. In contrast,
methyl chloroform is representative of a
large number of industrial halo carbons
characterized by low solubility and high
volatility.
Rainwater samples were collected in
Teflon and amber glass funnel/bottle
collectors. Before experiments, the bottles
were charged with volumes of pentane
sufficient to cover the collected rainwater
and thus minimize gas-liquid phase
transfer prior to collection. This was
particularly important for the experiment
employing the hrghly volatile methyl
chloroform.
Four release plume experiments were
conducted during the early months of
1983, three involving nitrobenzene, and
one with methyl chloroform. Figure 2
shows the comparison of the calculated
and experimental results for the first of
the four
Conclusions and
Recommendations
The Multi-Pollutant Atmospheric Dep-
osition and Depletion (MPADD) code is a
significant advance in the area of model-
ing precipitation scavenging and dry de-
position of hazardous and other types of
pollutants. It contains a new timesaving
integration scheme and is versatile in
that it produces both dry and wet fluxes,
describes depletion of the pollutant
plume, and can accept a variety of plume
model and chemical reaction mechan-
isms. Applications during the present
project were to gaseous species with
physical properties and chemical reaction
behaviors that could be described rela-
tively simply. As knowledge of the proper-
ties of hazardous air pollutants increases,
MPADD should be tested and the relevant
subroutines refined accordingly.
The literature of the solubility behavior
of hazardous air pollutants is not extensive
and measurements of Henry's Law
constants do not involve so wide a range of
temperatures as one would like for
atmospheric assessments Experimental
values for acetaldehyde and acrylonitrile
apparently do not exist. Measurements of
solubility during the present project were
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0.4
0.2
Q
c. 1.0
8
§ 0.8
0.6
0.2
0
ArcB
Arc A
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figure 2. Observed and calculated rain-
water concentrations, run HI -
A. The shaded area represents
the uncertainty in source term
input to MPADD.
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made for three species: ethylene oxide,
nitrobenzene, and methyl chloroform.
These measurements were performed in
rainwater, and they filled temperature
gaps in the Henry's Law constant record
for these species. The results generally
agreed with previous work except for
methyl chloroform, for which the magni-
tudes were somewhat lower than previous
results.
Solubility is a very important property
influencing precipitation scavenging
behavior. It is therefore important that
more measurements be made on species
of particular interest. In particular, data at
more temperatures are needed, and
dependence on pH and other chemical
variables should be investigated for
species for which for this could be
chemically significant.
Four controlled-release experiments
employing nitrobenzene and methyl
chloroform were conducted during the
early months of 1983. The results were
valuable in providing data under realistic
atmospheric conditions for testing the
MPADD model and solubility information.
Although a difficulty with the release
solution led to above-normal uncertainty
in released mass during two of the three
nitrobenzene experiments, agreement
with MPADD predictions was good
(generally within a factor of two) for this
species. However, the calculated rain-
water concentrations were generally
higher than those observed. This result
could be due to the mass-transfer
description in the model or to uncertainty
in nitrobenzene solubility, which is
apparently a strong function of tempera-
ture. Analytical problems led to a loss of
many of the concentration measurements
from the methyl chloroform release. The
surviving values, though not sufficient to
provide a measure of cross-plume
integrated flux, do show that the scav-
enging follows the predicted behavior in
being much less efficient than nitroben-
zene, due to much lower solubility.
The results of the field study provided
strong support for the validity of the
MPADD model, but additional experiments
should be performed using particular
species of environmental concern and/or
species with chemical properties different
from those previously tested.
M. Terry Dana, R. N. Lee, andJ. M. Hales are with Pacific Northwest Laboratory,
Rich/and, WA 99352.
Larry T. Cupitt is the EPA Project Officer (see below).
The complete report, entitled "Hazardous Air Pollutants: Wet Removal Rates and
Mechanisms," (Order No. PB85-138 626; 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:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
* U.S. GOVERNMENT PRINTING OFFICE. 1985 - 559-016/7879
United States
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Center for Environmental Research
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