United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
.EPA-600/S3-84-114 Jan. 1985
&ERA Project Summary
Hazardous Air Pollutants:
Dry-Deposition Phenomena
G.A. Sehmel, R.N. Lee, and T.W. Horst
Dry-deposition rates were evaluated
for two hazardous organic air pollutants
in order to determine their potential for
removal from the atmosphere by three
building material surfaces. The organic
air pollutants were nitrobenzene and
perchloroethylene; the building materials
used were cement, tar paper, and vinyl
asbestos tile. Dry-deposition experi-
ments were conducted in two stirred
chambers that were originally designed
and constructed for determining re-
moval rates to vegetation in the environ-
ment. Building materials were placed
on the bottom of the stirred chambers,
and removal rates were evaluated by in-
troducing the organic air pollutant
along with nondepositing SF6 tracer gas
into a stirred chamber. Changes in air-
borne concentrations with time were
then monitored.
Because the HAP removal rates were
small, sulfur hexafluoride tracer gas
was used to evaluate the rates of
leakage from the stirred chamber in
order to correct the deposition rates.
When pollutant wall losses were assumed
negligible, pollutant concentration
decreases (corrected using the SF6
leakage rate) ranged from near 0 to
3%/h. Dry-deposition velocities were
calculated from the rate-of-change over
time of the airborne concentrations. All
calculated deposition velocities were
approximately 10~4 cm s'1 or less.
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 docu-
mented in a separate report of the same
title (see Project Report ordering
information at back).
Introduction
The role of dry-deposition in removing
potentially hazardous air pollutants
(HAPs) from the atmosphere is not well
defined. This project was conducted to
assess the magnitude of the process,
especially deposition involving surfaces
expected to be found in urban settings.
The dry-deposition measurements
were made in stirred chambers (SCs). The
SC is an inert-surface box within which
are placed selected deposition surfaces.
The principal advantage of using the SC is
that concentrations can be controlled
within manageable limits throughout the
duration of each experiment. During the
experiments, the SCs were sealed to
minimize leakage and charged with a
preselected atmosphere of HAP and SFe
tracer gas that was mixed continuously
using an open fan. Concentrations within
the SC were monitored intermittently for
specified time periods depending on the
concentration decay rate. Deposition
rates were then calculated on the basis of
concentration-decay rates.
Seventeen experiments were conducted
to determine the maximum removal rates
for HAPs to large deposition-surface
areas. Dry-deposition velocities of per-
chloroethylene were evaluated during 12
experiments with cement, tile, and tar
paper substrates. Deposition velocities of
nitrobenzene were evaluated during five
experiments with tile and tar paper sub-
strates. The velocities were calculated
from airborne concentration losses of
HAP and SF6 tracer gas with time. The
nondepositing SFe tracer gas was used to
measure HAP leakage rates from the
stirred chambers. These measurements
of HAP dry-deposition velocities can be
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usedjn the general predictive atmospher-
ic transport, diffusion, and deposition
model MPADD (Multi-Pollutant Atmos-
pheric Deposition and Depletion), also
developed as part of this effort.
Procedure
The experimental design was based on
the assumption that HAP dry-deposition
velocities would be comparable to those
of other gases. A literature review of
reported dry-deposition velocities for
gases suggested that the deposition rate
for many of the HAPs should range from
1CT4 to 1CT2 cm s"1. The loss rates of
HAPs, as measured in SCs, were predicted
to vary from 0.5% to >10% h~1 for the
anticipated range of dry-deposition
velocities. Thus, a stirred chamber
measurement approach was selected for
the experimentation.
Deposition substrates were selected to
represent typical construction materials:
cement, tar paper, and vinyl asbestos tile.
Cement was selected to represent
construction materials in buildings,
streets, and highways. Cement patio
blocks were placed on the floor of the SCs
in two configurations with different
surface areas available for deposition,
either 6.1 or 10 m2. Similarly, vinyl
asbestos tile was selected because it is a
floor covering used in industry and
homes. Tar paper was selected because it
has a high organic content, which might
affect HAP removal rates. Both the vinyl
asbestos tile and the tar paper were
placed to cover the SC floor uniformly.
After preliminary characterization tests
with the SFe tracer were completed,
experiments were begun using both the
HAP and the SFe. Stirred chamber
experiments were conducted .with either
perchloroethylene (and SF6) or nitro-
benzene (and SFe). At time zero, the
chamber was charged with a preselected
concentration of HAP and inert tracer
gas. The concentrations within the
chamber were monitored to provide a
time series of the HAP and tracer decay.
In some experiments, both SCs were
used to conduct parallel runs to assess
the reproducibility of the experiments.
Results
Figure 1 shows the concentration
profiles of a typical experiment involving
perchloroethylene and SFe. The data are
normalized for presentation on the same
graph by dividing each measured value by
the starting concentration. Since the
concentrations in the chamber tend to
decrease exponentially with time, a plot
of the log of the normalized concentration
versus time should be linear. Loss rates of
the HAP and tracer were calculated from
the slope of the least-squares line which
best fit the data.
Both HAP and SF6 loss rates were
significant and ranged from 1.44 to
7.19% h"1. Correcting the HAP loss rates
for leakage (as measured by the tracer
losses) the resultant HAP deposition
losses ranged from near zero to 2.96%
h"1.
Deposition velocities were then calcu-
lated from the HAP loss rates. For these
calculations, HAP and tracer leak rates
were assumed to be identical, and
deposition in the SC to surfaces other
than the selected deposition substrate
(e.g., the Teflon walls) was assumed to be
negligible. The results are summarized in
Table 1. The HAP dry-deposition velocities
were consistently small, approximately
10~4 cm s~1 or less.
The observed HAP dry-deposition
velocities are near the lower limit which
was anticipated, based on values reported
for other gases. The relatively low HAP
dry-deposition velocities may have been
caused by differences in substrate (the
use of building materials in this study,
rather than the more traditional vegetative
canopies) or may result because organic
gases deposit less rapidly than the
inorganic gases for which most measure-
ments have been made. Further research
is needed to clarify these possibilities.
Neither substrate area (either 6.1 or
10.0 m2 for cement) nor increased fan
speed (experiments 4, 10, and 11)
significantly affected HAP loss rates.
Only two fan speeds were used: 520 rpm
and a motor-limited maximum rpm of
600. Nearly all experiments were conducted
at a fan speed of 520 rpm.
Although losses tended to be exponen-
tial with time, other time-dependent
effects were apparent. Concentrations
increased immediately after gas injection
and continued to increase until a maximum
concentration was observed. Subsequent-
ly, concentrations tended to be exponen-
tial with time as described by the
equations from which dry-deposition
velocities are calculated. Also, deviations
from an exponential decrease were
sometimes observed. These deviations
10°
a
10'
10'
Tracer Loss, %/h
D SFe 2.04
A C2O, 2.40
I I
I
40 80
Time, Hours
120
160
Figure 1.
Perchloroethylene andSFe tracer gas concentration decreases for 6.1 rrf of cement
block deposition surface.
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Table 1. HAP-Dry Deposition Velocities
Expt. HAP
No. SC Chemical Name
1
2
4
6
7
8
9
10
11
15
16'
17
18
19
1
2
1
2
1
2
1
1
2
1
2
1
1
2
1
2
1
perchloroethylene
perchloroethylene
perchloroethylene
perchloroethylene
perchloroeth ylene
perchloroethylene
perchloroethylene
perchloroethylene
perchloroethylene
perchloroethylene
perchloroethylene
perchloroethylene
nitrobenzene
nitrobenzene
nitrobenzene
nitrobenzene
nitrobenzene
Deposition
Surface
cement
cement
cement
cement
cement
cement
tile
tar paper
tile
tar paper
tile
tar paper
tar paper
tile
tar paper
tile
tar paper
Surface
Area
(m*j
10.0
6.1
10.0
6.1
10.0
6.1
6.0
6.0
6.0
6.0
6.0
6.0
60
6.0
6.0
6.0
6.0
Fan
Speed
(rpm)
520
520
520
520
600
600
520
520
520
520
600
600
520
520
520
520
520
HAP
Deposition]
(%/h)
0.42 + 0.31
0.36 + 0.27
0.34 ± 0.35
0.75 + 0.12
0.72 + 0.17
0.08 + 0. 14
0.41 +0.14
0.48 + 0. 14
-002 + 0.53
0.06 ± 0.42
1.16+0.61
1.57 + 0.98
2.96 +3.60
0.01 ±1.06
0.70 +1.27
1.19 +0.69
0.68 + 0.72
Deposition
Velocity]
(cm/sec)
5.2 + 3.8 x 10~5
7.3 ± 5.5 x 10~5
4.2 +4.3 x 10~5
1.5 + 0.2 x 10'*
8.9 +2.1 x 10~5
1.6 + 2.8x JO'5
8.4+ 2.9 x 10~5
9.9 + 2.9 x 10's
-0.0+ 1.1 x 70~"
1.2 + 8.6x 10~5
2.4 + 1.3 x 70~5
3.2 +2.0 x 10~4
6.1 +7.4 x 10''
0.0 +2.2 x 10'*
1.4 +2.6 x 10'"
2.5+ 1.4x W"
1.4 +1.5 x 10~4
t Tabulated uncertainties represent the 90% confidence interval.
might have been caused by temperature
effects and sorption of gases on airborne
particulates rather than on the test
substrates.
Conclusions and
Recommendations
The removal of gaseous organic air
pollutants by dry deposition has not been
studied extensively and few experimental
data are available. Hence, obtaining dry-
deposition velocities for nitrobenzene
and perchloroethylene is a significant
advance in the understanding of organic
removal rates. Measured deposition
velocities were significantly less than
those usually reported for inorganic
gases. Deposition velocities were consis-
tently very low. Many calculated values
were not significantly different from zero.
Temperature effects were not intensive-
ly investigated since all successful
experiments were conducted in warm
summer weather (ambient temperatures
from 27 to 36°C.) Because the dry-
deposition velocities measured for the
two HAPs investigated were low, we can
conclude that airborne concentrations of
HAPs are not significantly reduced by dry
deposition at temperatures characteristic
of summer months, or at temperatures
maintained within buildings. Vapor
pressure, however, is anticipated to be a
very important property influencing dry-
deposition removal rates for organic
gases.
The SC approach for evaluating HAP
dry-deposition velocities was adequate
for the HAPs and substrates investigated.
Leakage was always observed, and
leakage rates tended to increase during
the experimental series. To alleviate this
problem, the SCs were periodically
maintained by replacing critical seals or
tightening the bolts holding sections of
the chambers together to minimize
exchange with outside air.
One important advantage of the SC
technique is that a large substrate
surface area can be contained for the
deposition substrate. Even though dry-
deposition velocities might be low, they
could be a function of spatial variations in
substrate characteristics. Hence, by
using the relatively large surface areas in
an SC (6.0-m2 floor area), average dry-
deposition velocities can be evaluated
over these spatial variations.
The authors recommend that the SC
approach be used to evaluate additional
HAPs and deposition substrates. However,
other measurements techniques should
be considered. Thermogravimetric tech-
niques may be useful as a screening
method to evaluate maximum removal
rates for small surface areas on the order
of 1 cm2. This technique may be essential
for evaluating deposition velocities less
than about 10~6 cm s~1. Flow-through
chambers may be considered if the
removal rates are significantly greater
than those evaluated in the current
experiments. Instrumentation with high
sensitivity and rapid analysis techniques
would be required if flow-through cham-
bers were used.
The authors also recommend that dry-
deposition velocities be evaluated for
additional construction materials, and for
vegetative canopies and water surfaces.
Currently, no data exist for predicting
HAP dry-deposition velocities for vegeta-
tion and soils. Dry-deposition velocities of
HAPs onto construction materials appear
lower than dry deposition velocities for
other gases that were evaluated for
vegetative canopies.
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G. A. Sehmel. R. N. Lee, and T. W. Horst 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: Dry-Deposition
Phenomena."(Order No. PB 85-138 279; Cost: $11.50, 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
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