EPA-600/3-78-043
April 1978
Ecological Research Series
61"* '3
-It 3
i,
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are.
1. Environmental Health Effects Research
2 Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5 Socioeconomic Environmental Studies
6 Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8 "Special" Reports
9 Miscellaneous Reports
This report has been assigned to the ECOLOGICAL RESEARCH series. This series
describes research on the effects of pollution on humans, plant and animal spe-
cies, and materials. Problems are assessed for their long- and short-term influ-
ences. Investigations include formation, transport, and pathway studies to deter-
mine the fate of pollutants and their effects This work provides the technical basis
for setting standards to minimize undesirable changes in living organisms in the
aquatic, terrestrial, and atmospheric environments.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/3-78-043
April 1978
DYNAMICS OF AUTOMOTIVE SULFATE EMISSIONS
by
S.H. Suck, K. de Bower, and J.R. Brock
University of Texas
Austin, Texas 78712
Grant Number 803660
Project Officer
Jack L. Durham
Atmospheric Chemistry and Physics Division
Environmental Sciences Research Laboratory
Research Triangle Park, North Carolina 27711
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
-------
DISCLAIMER
This report has been reviewed by the Environmental Sciences Research
Laboratory, U.S. Environmental Protection Agency, and approved for publica-
tion. Approval does not signify that the contents necessarily reflect the
views and policies of the U.S. Environmental Protection Agency, nor does
mention of trade names or commercial products constitute endorsement or
recommendation for use.
ii
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ABSTRACT
A preliminary assessment of the potential environmental impact of
automotive sulfuric acid (or sulfate) aerosol has been made by analyzing
the aerosol dynamics. This analysis leads to the prediction of ambient
automotive sulfuric acid aerosol concentrations over and around a large,
ten-lane highway (48 m. x 20 km.), some ten or so years hence, when almost
all cars in the United States will be fitted with catalytic converters.
The attachment rate of fine automotive sulfuric acid aerosols to ambient
aerosols is examined. The dispersion and deposition of automotive sulfate
are modelled over the highway for "worst case" meteorology using K-theory.
The neutralizing effect of ambient ammonia on sulfuric acid concentrations
around the highway is examined by a direct simulation procedure for dis-
persion calculations. These calculations indicate that adverse environ-
mental effects of automotive sulfuric acid emissions may be important under
the stated conditions of this study.
This report was submitted in partial fulfillment of Grant No. R803660
by the University of Texas under the sponsorship of the Environmental
Protection Agency. This work covers the period November 1975 to November
1976. The work was completed as of November 30, 1976.
iii
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CONTENTS
Abstract ill
Figures vi
Abbreviations and Symbols vii
1. Introduction 1
2. Conclusions 3
3. Procedure 5
4. Results and Discussion 7
References 44
Appendix
A. The Rate of Reaction between Ambient Ammonia
and Sulfuric Acid Solution Aerosol 46
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FIGURES
Number Page
1 Coagulation of exhaust aerosol with ambient, Case I 9
2 Coagulation of exhaust aerosol with ambient, Case II .... 10
3 Coagulation of exhaust aerosol with ambient, Case III .... 11
4 Coagulation of exhaust aerosol with ambient, Case IV .... 12
5 Coagulation of exhaust aerosol with ambient, Case V 13
6 Coagulation of exhaust aerosol with ambient, Case VI .... 14
7 Coagulation of exhaust aerosol with ambient, Case VII .... 15
8 Dispersion of exhaust aerosol, Case VIII 20
9 Dispersion of exhaust aerosol, Case IX 21
10 Dispersion of exhaust aerosol, Case X 22
11 Dispersion of exhaust aerosol, Case XI 23
12 Dispersion of exhaust aerosol, Case XII 24
13 Dispersion of exhaust aerosol, Case XIII 25
14 Dispersion of exhaust aerosol, Case XIV 26
15 Dispersion of exhaust aerosol, Case XV 27
16 Transport and reaction of exhaust aerosol, Case XVI 30
17 Transport and reaction of exhaust aerosol, Case XVII .... 31
18 Transport and reaction of exhaust aerosol, Case XVIII .... 32
19 Transport and reaction of exhaust aerosol, Case XIX 33
20 Transport and reaction of exhaust aerosol, Case XX 34
21 Transport and reaction of exhaust aerosol, Case XXI 35
22 Transport and reaction of exhaust aerosol, Case XXII .... 36
23 Transport and reaction of exhaust aerosol, Case XXIII .... 37
24 Transport and reaction of exhaust aerosol, Case XXIV .... 38
25 Transport and reaction of exhaust aerosol, Case XXV 39
26 Transport and reaction of exhaust aerosol, Case XXVI .... 40
27 Transport and reaction of exhaust aerosol, Case XXVII .... 41
28 Transport and reaction of exhaust aerosol, Case XXVIII ... 42
vi
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LIST OF ABBREVIATIONS AND SYMBOLS
Roman Letters
!2
"2
D(y)
D
dM/dlogD
G
g.m.d.
K
K
x
K
y
K
M
-*
n(y;r,t)
v
t
u
coagulation coefficient
coagulation coefficient of the automotive sulfate with
itself
coagulation coefficient of the automotive sulfate with
ambient aerosol
number concentration of the automotive sulfate
number concentration of the ambient aerosol
particle diffusivity
particle diameter
particle mass density function
gravitational settling velocity
geometric mean diameter
eddy diffusivity tensor
x-component of eddy diffusivity
y-component of eddy diffusivity
z-component of eddy diffusivity
aerosol mass distribution
number density function of aerosol particles having
mass u in the range (v,v + dp) at r" and at time t
coordinate vector
time
time average wind velocity
deposition velocity
roughness length
vii
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Greek letters
6 removal rate
U particle mass
v. rate of production by nucleation of j-th component
4>(y;r,t) condensation coefficient
a rate of production from i-th source
a geometric standard deviation
a Gaussian dispersion parameter
a Gaussian dispersion parameter
viii
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SECTION 1
INTRODUCTION
Although the widespread introduction of catalytic converters on new cars
in the United States in 1975, reduces carbon monoxide and hydrocarbon emis-
sions, there are many uncertainties concerning possible adverse health and
welfare effects of this new technology. Among these uncertainties is the
environmental impact of the sulfuric acid emissions from catalytic converters.
Gasoline in the United States normally contains 0.03 weight percent
sulfur, which may be converted to sulfur dioxide on combustion. The catalytic
converter oxidizes part of the sulfur dioxide in the automotive exhaust to
sulfur trioxide, which in turn rapidly reacts with water vapor to form
sulfuric acid. In the atmosphere, sulfuric acid is believed to nucleate to
form a sulfuric acid aerosols (1,2).
The adverse effects of environmental sulfuric acid aerosol on man,
animals, plants, and materials have been known for some time (3). A recent
study (4) indicates the critical role of the furic acid aerosol size distri-
bution. Particles in the size range 'vO.l to 1 ym equivalent diameter are
deposited with the poorest efficiency in the lung, but are capable of pene-
trating efficiently to the critical lower lung regions.
This paper gives a preliminary assessment of the potential environmental
impact of automotive sulfuric acid (or sulfate) aerosol by analyzing the
dynamics of this aerosol and predicting the ambient automotive sulfuric
acid aerosol concentration.
It will be assumed that in approximately 10 years almost all cars in
the U.S. will be fitted with a catalytic converter. "Worst case" meteor-
ology is considered for dispersion of the automotive sulfuric acid aerosol
from a ten-lane divided highway 48 m. wide and 20 km long.
-------
First, the procedure followed in the analysis is described. Then, the
dynamics of the automotive sulfuric acid aerosol are examined. The relevant
processes included are advection, dispersion, dry deposition, coagulation,
and chemical reaction. For roadway regions, this leads to predicting the
sulfuric acid aerosol concentration under conditions of "worst case"
meteorology.
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SECTION 2
CONCLUSIONS
The results presented for the attachment of automotive sulfate aerosol
to the ambient aerosol are based on currently accepted formulations of the
relevant rate processes.
The calculations based on AROSOL of automotive sulfate concentrations
around the 48 m. x 20 km. highway are believed to represent the "state of
the art" for this particular application. For the states conditions and
assumptions, the calculated concentration levels are reliable.
The calculations based on EPOSOD of neutralization of automotive
sulfuric acid by ambient ammonia provide only a qualitative guide; however,
the results are internally consistent, and provide a basis for some qualita-
tive conclusions concerning the possible role of ambient ammonia in the
neutralization of automotive sulfuric acid.
The following forecasts are based on the stated emission rates for
automotive sulfuric acid of 1.025 x 10 gm/m-sec for the 10 lane, 48 m. x
20 km. highway under very stable meteorological conditions. Emission rates
of this order could occur in 10 years if all cars travelling the highway
are equipped with catalytic converters which produce sulfuric acid.
1. For parallel winds and in the absence of ammonia, sulfuric acid
concentrations over 90% of the length of the highway at the 2 m. level are,
3
in the range 115-240 yg/m .
2. For parallel winds and very stable meteorological conditions,
maximum sulfuric acid concentrations over the highway at the 2 m. level are
not strongly influenced by ambient ammonia concentrations at the 3 ppb
level and would be reduced by around 50% at the 30 ppb level.
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3. On and adjacent to large highways, adverse health effects may be
experienced by sensitive individuals (3). Increased rates of corrosion are
likely to occur.
These preliminary studies suggest that the adverse environmental
effects of automotive sulfuric acid emissions may be important.
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SECTION 3
PROCEDURE
Two models developed here in our group have been used: AROSOL (5) and
EPOSOD (6). The part of AROSOL dealing with atmospheric dispersion has been
adapted from the computer code of Shir and Shieh (7). AROSOL described the
transport, dispersion, and deposition of the automotive sulfuric acid aerosol.
EPOSOD accounts qualitatively for the sulfate transport and chemical reaction
with ambient ammonia.
Computer code AROSOL gives a numerical solution of the single density
function of an aerosol:
3n(y;r,t)/8t + V Un(y;r,t) = (K + K )V2 n(y;r,t)+ V K Vn(y;r,t)
x y z
y
+ 1/2 /Qd y'b(u-y',y')n(y';r",t)n(y-y';r",t)
00
- n(y;r,t)/od 'b(y,y')n(y';?,t) - |^ [*(v;r,t)n(y;r,t)]
+ G Vn (y;r,t) + Z^^r.t) + E v (v;r,t) (1)
Boundary and initial conditions are set as a particular physical system
demands, and include phenomena such as dry deposition and resuspension at
boundary surfaces.
Boundary conditions for highway calculations are:
= 0, x=0, x=x max. (2)
0, y=0, y=y max. (3)
32n
-------
nv = -(K + D(u))- - Gn, z=0 (4)
01 Z dZ
n A /CN
= 0, z=z max, (5)
o Z
n = 0, t=0 (6)
n(y;r,t) represents the number of aerosol particles having mass y in the
range y, dy at a point in space r at time t. In the atmosphere U is the time
average wind velocity. K , K , and K are the turbulent eddy dif fusivities
(a rectangular Eulerian coordinate system is used) in the x,y, and z direc-
tions, respectively. The fourth and fifth terms on the right hand side of
eq. (1) account for particle coagulation; the sixth term represents conden-
sation; and the seventh, gravitational sedimentation. The last two terms
allow for input of primary source aerosol, a., and homogeneous nucleation of
trace gaseous species v.. In eq. (4), V is the deposition velocity and G
the magnitude of the gravitational sedimentation velocity.
Eq. (1) is coupled to the corresponding conversation equations of those
trace gaseous species which act as sources for secondary aerosol (5) . The
appropriateness of eq. (1) for describing the evolution of an aerosol under
conditions of atmospheric turbulence has been verified (8,9).
EPOSOD (6) was specifically designed to study turbulent dispersion and
chemical reaction in plumes from elevated point sources. This model separates
the processes of dispersion and chemical reaction over small time increments,
and treats each process separately in a sequential, step-wise manner. The
model is formulated by defining a number of large, inf initesimally thin
grids, normal to and centered about a plume axis. Each grid is located at
a particular downward distance (x) from the source. The concentration levels
of the various chemical species at all points on a grid at xj are evaluated
as a function of the concentration levels on the grid at x = xj . A
chemically reactive species in the grid at xj is advected and diffused to the
position xj + Ax with the reaction process inhibited. The reaction is
allowed to take place for a time, Ax/U, where U is the advective wind speed.
The error introduced by this stepwise procedure is controlled by choosing
Ax (and hence At) sufficiently small.
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SECTION 4
RESULTS AND DISCUSSION
This investigation assumes that in approximately ten years, all U.S. cars
will be fitted with a catalytic converter. Hopefully, modelling of expected
roadway conditions will serve as an early indication of future problems.
The use of AROSOL for predictions of roadway sulfuric acid aerosol
concentrations eliminates the principal objections which have been raised
to previous Gaussian-type model predictions (10). Variable winds at different
heights and plume rise due to heated exhaust are explicitly accounted in
AROSOL according to best current micrometeorological (7) and roadway
information (11).
The importance of automobile traffic in regard to mixing immediately
above the highway and vertical dispersion is widely recognized (e.g. 11).
However, much more remains to be done to arrive at quantitative relationships
between mixing, dispersion, traffic speed, and vehicle characteristics.
Before presenting the results of the circulation of automotive sulfate con-
centration based on the AROSOL model, we discuss, in a highly simplified
fashion, some factors affecting the size of the automotive sulfate aerosol.
After this preliminary discussion, the results of AROSOL calculations for
roadway sulfate concentration are given. This is followed by results
obtained from EPOSOD for roadway sulfate concentrations in the presence of
significant ambient ammonia concentrations.
AUTOMOTIVE SULFATE SIZE DISTRIBUTION
Because of the important relationship between particle size and depo-
sition efficiency in the lung, some attention has been given to the rate
processes affecting the automotive sulfate size distribution.
-------
Details of the initial growth stages of the automotive sulfate are
lacking. It seems reasonable to suppose that sulfuric acid vapor is formed
in the exhaust system, in or immediately after the catalytic converter
undergoes homogeneous heteromolecular nucleation as the hot sulfuric acid
vapor (at several hundreds of degrees Celsius exits from the exhaust tail
pipe, and is rapidly mixed with the much cooler ambient air (1). The initial
aerosol growth period (including condensation of water vapor and coagulation),
to a particle geometric mean diameter of 0.01 ym is completed in a few tenths
of a second, as is evident from an order of magnitude calculation of coagu-
lation of condensation aerosol in the free molecule region (12). Of primary
interest is the subsequent growth period-when the automotive sulfate coagu-
lates with the ambient aerosol. We have frequently taken the initial geo-
metric mean diameter to be 0.02 ym; which appears to be in agreement with
data from the GM sulfate experiment (13).
Figs. 1-7 illustrate the simplest case of the interaction by Brownian
coagulation between automotive sulfuric acid aerosol and ambient aerosol.
Automotive aerosol (0.02 ym g.m.d. ag = 2.2) is mixed initially with ambient
aerosol (0.3 ym g.m.d. ag = 2.2), and coagulation is initiated. Therefore,
only the time dependent coagulation process is considered in eq. (1).
The figures giving dM/dlogD as a function of particle diameter (d) are
arranged in order of increasing ambient mode at 0.3 ym; M is the aerosol
mass distribution, of the solution droplets. The initial mode for the auto-
motive sulfuric acid solution is held fixed at a peak modal value of 155
pg/cc. The initial peak modal values for the ambient aerosol: 25, 52, 105,
200, and 520 pg/cc for Figs. 1, 2, 3, 4, and 5, respectively.
The plots of dM/dlogD as a function of log D (as in Figs. 1-7) have the
property that the area beneath a particular curve gives the total aerosol
mass concentration in that size range. In all cases for Figs. 1-5, the
automotive sulfuric acid mass < 0.1 ym decreases with time as the sulfuric
acid aerosol coagulates with the ambient aerosol that lies in the size range
> 0.1 ym. Per unit time, as would be expected, the mass of sulfuric acid
solution which attaches to the ambient mode is greater, the larger is the
mass concentration of the ambient mode at ^0.3 ym.
-------
300 -
200
0>
O-
Q
o
TJ
100
10'
lo'2 io-'
PARTICLE DIAMETER,
10
10'
Figure 1. Coagulation of exhaust aerosol with ambient, Case I. Mass
density functions (dM/dlogD) resulting from coagulation of automotive
sulfuric acid aerosol with ambient aerosol with negligible dispersion.
Initial automotive sulfuric acid: mode = 0.02 utn, ag = 2.2, dry mass
155 pg/cm^. Initial ambient aerosol: mode = 0.3 vim, ag = 2.2, mass =
25 pg/crsr. Automotive and ambient aerosol mixing time: X
O =
D =
A =
0 min.
2 min.
6 min.
10 min.
20 min.
-------
300-i
10
-3
io 10
PARTICLE DIAMETER, pm
Figure 2. Coagulation of exhaust aerosol with ambient, Case II.
as Fig. 1, except that initial ambient aerosol mass = 52 pg/cnr'.
Same
10
-------
3001
200-
m
100-
PARTICLE DIAMETER,
Figure 3. Coagulation of exhaust aerosol with ambient, Case III.
as Fig. 1, except that initial ambient aerosol mass = 105 pg/cm^,
Same
11
-------
3001
PARTICLE DIAMETER.pm
Figure 4. Coagulation of exhaust aerosol with ambient, Case IV. Same
as Fig. 1, except that initial ambient aerosol mass = 200 pg/cm^.
12
-------
600-
10 10
PARTICLE DIAMETER,
10
Figure 5. Coagulation of exhaust aerosol with ambient, Case V. Same
as Fig. 1, except that initial ambient aerosol mass = 520 pg/cm^.
13
-------
200 -i
E
\
CT
0>
o
100-
_l
^
-------
200 i
o>
0.
O
O
LU
to
s
O
100-
10' 10'
PARTICLE DIAMETER,
Figure 7. Coagulation of exhaust aerosol with ambient, Case VII. Mass
density functions (dMsuifate/dlogD) for automotive sulfuric acid as a
function of particle diameter (D). Growth of upper sulfate mode (^0.4 ym)
results from attachment of automotive sulfuric acid aerosol particles to
ambient aerosol. Initial ambient modal concentration = 200 pg/cnH.
Mixing time
X = 0 min.
o =
D =
2 min.
6 min.
10 min.
20 min.
mass concentration not
attached to ambient aerosol
3
103 pg/cm,,
94
83
75
61 pg/cm"
15
-------
Figs. 6 and 7 show the attachment process for sulfuric acid clearly.
Initially, it is assumed that there is no sulfuric acid in the ambient mode.
As coagulation proceeds, sulfuric acid begins to accumulate in the ambient
mode. The accumulation occurs at a faster rate, as the mass concentration of
the initial ambient mode at 0.3 urn increases.
From Figs. 1-6, a very simplified picture can be presented of the
dynamics of the automotive sulfate. First, neglecting advection and
dispersion, a control volume containing sources of automotive sulfate and
ambient aerosol is considered. Assume that the automotive sulfate aerosol
may be approximately represented by c, , the single number concentration,
(initial particle diameter ^0.02 urn); assume also that the ambient aerosol
is represented by c~, the number concentration (particle diameter ^0.3 urn),
the rate of change of c.. can be given by the qualitative relation:
dcą/dt = -b1;Lc* - b12c1c2 ~ 6ci + ° <7>
where b.. is the coagulation coefficient of the automotive sulfate with
itself, and b is the coefficient for coagulation of automotive sulfate
with ambient aerosol. 6 is the removal rate coefficient of automotive
sulfate from the control volume by processes such as deposition and dis-
persion, a is the source input rate of automotive sulfate. It is evident,
after an initial period that eq. (2) possesses quasistationary solutions in
which, with a steady source, a, and removal coefficient, 6, the automotive
sulfate will be maintained at a given size, and will attach to the ambient
aerosol at a particle diameter of M5.3 um.
Since the automotive sulfate aerosol undergoes advection and dispersion
over a roadway, one must consider the following partial differential equation
instead of eq. (2):
Sc-jyat + V UC;L = V K VC;L - buc* - b12clC2 (8)
where sources and removal (deposition) appear as boundary conditions. If
c~ is relatively large, it may be supposed that, for short times, it is
approximately independent of time and position, c , however, is decreasing
2
due to spatial dispersion, with the result that the term be can be
16
-------
neglected in comparison with b^c-c,.,. Therefore, dispersion decreases the
importance of coagulation of the automotive sulfate with itself. Under
these circumstances, we assume that b,.-, and cŤ are approximately independent
of position; therefore, for steady state conditions for a point source, eq.
(8) would have the solution:
C;L = F(x,y,z)exp{-b12c2X/U} (9)
where x is in the direction of the advective flow with mean speed U, and
F(x,y,z) is a function representing the diffusion processes of eq. (8).
As is to be expected, the decay of c.. owing to attachment to the ambient
aerosol is exponential.
The accurate representation of all the processes discussed here in
eqs. (7), (8), and (9) is much more complex-and is given by eq. (1).
ROADSIDE AUTOMOTIVE SULFATE ACCORDING TO AROSOL
"Worst case" meteorology is considered for dispersion of automotive
sulfate from a ten-lane divided highway with a width of 48 m. and a length
of 20 km. Dispersion is modelled under conditions analogous to F class
stability and with a mean surface wind of 0.514 m/sec (at 10 m.). For each
lane of the 10-lane highway, an emission rate of 1.025 x 10 gm/m-sec is
assumed; a figure which could be justified by extrapolating to higher traffic
densities the results of the GM sulfate experiment (2).
As previously stated, the use of AROSOL for modelling dispersion of
automotive sulfate from a highway overcomes the principal objections which
have been raised (10) to Gaussian-type models, such as HIWAY. AROSOL is a
"K-theory" model which can account explicitly for: vertical variation of
windspeed, traffic effects such as the highway "heat island" mixing over
the highway and "shelter-belt" effects of traffic (11).
Detailed knowledge of parameters entering into AROSOL for automotive
sulfate highway modelling is lacking. Consequently, we carried out model
studies in which certain parameters are varied. These input data included:
(1) Grid for highway calculation
(2) Emission data
17
-------
(3) Wind data
(4) Mixing height
(5) Surface roughness
(6) Particulate dry deposition
(7) Eddy diffusivities
These will be discussed briefly in order.
(1) Grid for highway calculation: A rectangular Eulerian coordinate
system is used with the origin at the upwind end of the straight, 20 km.
highway, at pavement level, and at the center of the highway, X increases
in the direction of the advective wind, which for the AROSOL calculations
is always considered to be parallel to the highway. Z is the distance above
the highway, and y is the distance from the center of the highway. In the
x-y plane, 26 x 13 grid points were included, with a basic grid size of
200 m. x 50 m. Two-meter intervals were considered up to the mixing height.
(2) Emissions data; Total mass emission rate used was 1.025 x 10
g/m-sec for each lane of the 10-lane highway. The sulfate aerosol diameter
was assumed to be 0.02 ym. Details of the interaction of these particles
with ambient aerosol are not considered here.
(3) Wind data: The wind speed used was 0.514 m/sec at 10 m. and
parallel to the highway. Vertical wind variation was calculated by a power
law assumption (7).
(4) Mixing height; A value of 300 m. was arbitrarily selected.
(5) Surface roughness (z ): Surface roughness was varied to reflect
possible effects of highway traffic on vertical dispersion. It started with
a value of 0.25 (15), other values used were 0.75 and 1.5.
(6) Particulate dry deposition: This process enters into the boundary
condition of eq. (1) as the deposition velocity, (V,). Several values of
V, were considered: 0, 0.5, and 2 cm/sec. The value of 2 cm/sec is the
d
upper bound for the deposition velocity for automotive sulfate from the
subsequent EPOSOD calculations. The value 0.5 cm/sec appears reasonable
under the stated stability conditions based on the correlations of Sehmel
and Hodgson (16).
18
-------
(7) Eddy diffusivities: The vertical eddy diffusivity (K ) was
z
calculated by using the techniques employed by Shir and Shieh (7) up to a
height of 1 m. Perfect vertical mixing was assumed over the highway.
Figs. 8-14 represent some of the results of our calculations employing
AROSOL and the conditions stated above.
Figs. 8-9 show the approach to a steady state of the automotive sulfate
above the highway at x = 2 km. At t = 0, the emission rate of 1.025 x 10
gm/m-sec was "switched on," and held constant. The sulfuric acid concentra-
tion at the two heights (2 m. and 10 m.), remains sensibly constant after
1-1/2 hours, and achieves around 80% of the steady state value in 30 min.
In Fig. 8, for a roughness, (z ), of 0.25 m., the acid concentration at 2 m.
above the highway is approximately 218 yg/m under conditions of no dry
deposition (V, =0). A value of V, = 0.5 cm/sec lowers the steady state
acid concentration at 2 km. to approximately 185 yg/m . The value of 140
3
yg/m for V = 2 cm/sec is believed to be a "lower bound" for the stated
consitions. In Fig. 9, the roughness (z ) has been increased to 0.75 m.;
all other conditions are the same as in Fig. 8. This results in a decrease
of the steady state acid concentration at 2 m. above the highway at 2 km. to
3 3
163 yg/m (V = 0). The "lower bound" value is now 112 yg/m (V, = 2 cm/sec).
Figs. 10 and 11 show the buildup of acid proceeding downwind along the
highway 2 hours after the start of traffic. At 2 hours, Figs. 8 and 9 indi-
cate that steady state conditions have been achieved. As is evident from
Fig. 10, acid concentration builds up very rapidly down the highway. After
1 km., around 70% of the maximum value has been achieved. Above the highway
at 2m., the maximum acid concentration for a roughness (z ) of 0.25 m. is
3 °
approximately 240 yg/m (V, = 0) with a "lower bound" value of around 140
yg/m (V = 2 cm/sec). In Fig. 11, z has been increased to 0.75 m. which
lowers the 2 m. highway concentration maximum to around 180 yg/m and the
3
"lower bound" value to 115 yg/m .
Figs. 12-15 give sulfuric acid concentration isopleths in a plane
perpendicular to the highway at 5 km. from the upwind end of the highway.
Figs. 12 and 13 are for a roughness (z ) of 0.25 m., and 13 and 14, for a
z of 0.75 m. Figs. 12 and 14 (for V, = 0) show the important effect of
19
-------
to
E
.200
d
O
o
O
<
o
o:
ID
U.
_1
ID
V)
100 -
I 2
TIME, HOURS
Figure 8. Dispersion of exhaust aerosol, Case VIII. AROSOL calculation for
parallel winds for approach to steady state automotive sulfuric acid con-
centrations over highway at 2 km downwind from beginning of highway.
Roughness (ZQ) = 0.25 m. The upper band is for a height of 2m, and the
lower band is for a height of 10m.
Dry deposition velocity
0 cm/sec
0.5 cm/sec
2 cm/sec
20
-------
CO
TIME, HOURS
Figure 9. Dispersion of exhaust aerosol, Case IX. AROSOL calculation
for parallel winds for approach to steady state automotive sulfuric
acid concentrations over highway at 2 km. Roughness (zo) = 0.75m.
The upper band is for a height of 2m, and the lower band is for a height
of 10m.
Dry deposition velocity
0 cm/sec
2 cm/sec
21
-------
*E 200
V.
o>
3.
O
o
o
0
O
100
o
2 4
DOWNWIND DISTANCE , km
Figure 10. Dispersion of exhaust aerosol, Case X. AROSOL calculation
for parallel winds of increase of automotive sulfuric acid concentration
with downwind distance from beginning of highway 2 hours after start
of traffic. Roughness (z0) = 0.25m.
Dry deposition velocity
0 cm/sec (upper curve for height
for height = 10m)
2 cm/sec (upper curve for height
for height = 10m)
2m, lower curve
2m, lower curve
22
-------
2 4
DOWNWIND DISTANCE, km
Figure 11. Dispersion of exhaust aerosol, Case XI. AROSOL calculation
for parallel winds of increase of automotive sulfuric acid concentration
with downwind distance from beginning of highway 2 hours after start
of traffic. Roughness (zo) = 0.75m.
Dry deposition velocity
0 cm/sec
2 cm/sec
23
-------
50
40
30
20
10
210
"S"
0
ROADWAY
100 200
DISTANCE (Y),m
300
Figure 12. Dispersion of exhaust aerosol, Case XII. AROSOL calculation
for parallel winds of sulfuric acid concentration (yg/m3) isopleths 5 km
downwind of beginning of highway.
Ammonia concentration = 0 ppb
Roughness (zo) = 0.25 m
Deposition velocity = 0 cm/sec
24
-------
50-
E
5
i-
C9
50-
ROADWAY 100 200
DISTANCE (Y), m
Figure 13. Dispersion of exhaust aerosol, Case XIII. AROSOL calculation
for parallel winds of sulfuric acid concentration (yg/m3) isopleths 5 km
downwind of beginning of highway.
Ammonia concentration = 0
Roughness (ZQ) = 0.25 m
Deposition velocity - 2 cm/sec
25
-------
ROADWAY 100 200
DISTANCE (Y), m
Figure 14. Dispersion of exhaust aerosol, Case XIV. AROSOL calculation
for parallel winds of sulfuric acid concentration (yg/m3) isopleths 5 km
downwind of beginning of highway.
Ammonia concentration = 0 ppb
Roughness (ZQ) = 0.75 m
Deposition velocity = 0 cm/sec
26
-------
50 -i
ROADWAY I0° 200
DISTANCE (Y), m
Figure 15. Dispersion of exhaust aerosol, Case XV. AROSOL calculation
for parallel winds of sulfuric acid concentration (yg/m3) isopleths 5 km
downwind of beginning of highway.
Ammonia concentration = 0 ppb
Roughness (z0) = 0.75 m
Deposition velocity - 2 cm/sec
27
-------
z on the vertical transfer of acid sulfate. Increasing z , decreases the
o o
acid sulfate concentration immediately around the roadway, but increases it
several hundred meters away. Comparison of Figs. 12 and 13 for z =0.25
shows the important effect of dry deposition. The deposition velocity of
V, = 2 cm/sec of Fig. 13 is perhaps unrealistically high (providing a "lower
bound"), but it shows the effect of deposition rather dramatically. Dry
deposition at V, - 2 cm/sec almost halves the acid sulfate concentrations.
A large uncertainty is the possible resuspension of acid sulfate by the
traffic. We are unable to assess this possibility, but the net effect
would always be to lower the effective deposition velocity.
The general forms of the isopleths shown in Figs. 12-15, and particu-
larly 14 and 15 for z - 0.75, resemble those found in the GM experiment with
inert tracer (10). The isopleths of Figs. 12-15, differ considerably from
isopleths based on the EPOSOD calculations discussed in the next section;
this difference is expected.
REACTION OF AUTOMOTIVE SULFURIC ACID WITH AMBIENT AMMONIA ACCORDING TO EPOSOD
The results of the EPOSOD calculation provide only a qualitative picture
of the reaction of automotive sulfuric acid with ambient ammonia. The calcu-
lations are internally consistent and indicate the variation of sulfuric
acid concentration with ambient ammonia concentrations under the conditions
of "worst case" meteorology.
Two examples are discussed: winds parallel and perpendicular to the
highway. In the former case, grids in the EPOSOD calculation are 50 m.
apart; in the latter, they are 10 m. apart. Empirically, these spacings
were found to be optimal from the standpoints of numerical error and com-
puter time. Diffusion from an upqind grid element to the next grid (50 m.
downwind) was assumed to occur with the constant Gaussian dispersion parameters:
a = 14.4m. a = 3.2 m.
y z
These values correspond roughly to the Pasquill-Gifford correlations for
F-calss stability (6). The effect of traffic on mixing was accounted for
by assuming that automotive sulfate was vertically mixed in a region 48 m.
wide and 6 m. high. Owing to the rapid reaction between sulfuric acid
28
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aerosol and ammonia, no distinction was made as to particle size in the
calculation of reaction rate. Also, dry deposition was assumed to occur at
a rate independent of particle size and to be irreversible; as in the AROSOL
calculation, the principal uncertainty is the effect of traffic on deposition
rate. Consequently, deposition rate is a variable parameter in our model
studies.
Figs. 16 to 28 are a sample of the results from our EPOSOD calculations.
Figs. 16 to 26 are the results for parallel winds and Figs. 27 to 28 are for
perpendicular winds. The neutralization of the automotive sulfate is a
diffusion limited rate process, as is apparent from the figures. The
attenuation of acid sulfate concentration has been studied for ammonia
concentrations of 0, 3, and 30 ppb. Although accurate ambient levels for
ammonia have not yet been established, 3 ppb. would appear to be a represen-
tative ambient value. 30 ppb. is believed to be unrealistically large, but
is unrealistically large, but is included for comparison. The 0 ppb. case
is studied to provide a basis of comparison with the more accurate AROSOL
dispersion calculations.
In the parallel wind case for the 20 km. highway, the origin of the
Cartesian coordinate system is located at the upwind end of the highway, at
pavement level, and at the center of the highway.
Figs. 16-17 for ambient ammonia concentration of 3 ppb. show the build-
up of sulfuric acid over the roadway at a point (y), 20 m. from the center
of the highway, and a height (z), of 2 m. This is shown for a deposition
velocity, (V ) of 0 and 0.03 cm/sec. Figs. 18-21 present sulfuric acid
isopleths for 3 ppb. ambient ammonia in a plane 10 km. downwind and per-
pendicular to the highway. These isopleths are for deposition velocities
(V,) of 0, 0.03, and 0.4 and 2 cm/sec. The value of 2 cm/sec represents
a possible upper bound based on the assumed vertical diffusion rate cor-
responding approximately to F-class stability. These values may be compared
to corresponding values for 0 ppb. ambient ammonia. The sulfuric acid
concentrations immediately above the roadway for "worst case" meteorology
are not strongly influenced by the addition of 3 ppb. ambient ammonia;
however, the roadway concentrations are significantly influenced by the
deposition rate.
29
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4 6 8 10 12
DOWNWIND DISTANCE (X), km
14
Figure 16. Transport and reaction of exhaust aerosol, Case XVI. EPOSOD
calculations of neutralization of automotive sulfuric acid by ambient
gaseous ammonia for wind parallel to highway as function of downwind
distance at 2m above surface of highway and 20m from highway center.
Ammonia concentration = 3 ppb
Deposition velocity = 0 cm/sec
30
-------
400-
320-
a.
u
o
o
240-
160-
80-
4 6 8 10
DOWNWIND DISTANCE (X), km
12
14
Figure 17. Transport and reaction of exhaust aerosol, Case XVII. EPOSOD
calculations of neutralization of automotive sulfuric acid by ambient
gaseous ammonia for wind parallel to highway as function of downwind
distance at 2m above surface of highway and 20m from highway center.
Ammonia concentration = 3 ppb
Deposition velocity = 0.03 cm/sec
31
-------
30 i
ROADWAY
50 100
DISTANCE (Y), m
Figure 18. Transport and reaction of exhaust aerosol, Case XVIII. EPOSOD
calculations of neutralization of automotive sulfuric acid by ambient
gaseous ammonia for wind parallel to highway. Sulfuric acid concentration
isopleths (yg/m3) at cross-section perpendicular to highway at 10km
downwind.
Ammonia concentration = 3 ppb
Deposition velocity = 0 cm/sec
32
-------
30-i
ROADWAY
50
DISTANCE (Y), m
Figure 19. Transport and reaction of exhaust aerosol, Case XIX. EPOSOD
calculations of neutralization of automotive sulfuric and by ambient
gaseous ammonia for wind parallel to highway. Sulfuric acid concentration
isopleths (yg/m^) at cross-section perpendicular to highway at 10 km
downwind.
Ammonia concentration = 3 ppb
Deposition velocity = 0.03 cm/sec
33
-------
301
0
ROADWAY
DISTANCE (Y), m
-Figure 20. Transport and reaction of exhaust aerosol, Case XX. EPOSOD
calculations of neutralization of automotive sulfuric acid by ambient
gaseous ammonia for wind parallel to highway. Sulfuric acid concentration
isopleths (yg/m3) at cross-section perpendicular to highway at 10 km
downwind.
Ammonia concentration = 3 ppb
Deposition velocity = 0.4 cm/sec
34
-------
30 i
0
ROADWAY
DISTANCE (Y),m
100
Figure 21. Transport and reaction of exhaust aerosol, Case XXI. EPOSOD
calculations of neutralization of automotive sulfuric acid by ambient
gaseous ammonia for wind parallel to highway. Sulfuric acid concentration
isopleths (yg/m3) at cross-section perpendicular to highway at 10 km
downwind.
Ammonia concentration = 3 ppb
Deposition velocity = 2 cm/sec
35
-------
30-i
20-
I-
X
CD
10-
o
ROADWAY
100
DISTANCE (Y), m
Figure 22. Transport and reaction of exhaust aerosol, Case XXII. EPOSOD
calculations of neutralization of automotive sulfuric acid by ambient
gaseous ammonia for wind parallel to highway. Sulfuric acid concentration
isopleths (yg/m3) at cross-section perpendicular to highway at 10 km
downwind.
Ammonia concentration = 0 ppb
Deposition velocity = 0 cm/sec
36
-------
30n
0
ROADWAY
100
DISTANCE (Y), m
Figure 23. Transport and reaction of exhaust aerosol, Case XXIII. EPOSOD
calculations of neutralization of automotive sulfuric acid by ambient
gaseous ammonia for wind parallel to highway. Sulfuric acid concentration
isopleths (pg/m3) at cross-section perpendicular to highway at 10 km
downwind.
Ammonia concentration = 0 ppb
Deposition velocity = 0.4 cm/sec
37
-------
400-,
n 320H
g 240-1
o
CJ
160-
80-
DOWNWIND DISTANCE (X), km
Figure 24. Transport and reaction of exhaust aerosol, Case XXIV. EPOSOD
calculations of neutralizatiion of automotive sulfuric acid by ambient
gaseous ammonia for wind parallel to highway as a function of downwind
distance at 2m above surface of highway and 20m from highway center.
Ammonia concentration = 30 ppb
Deposition velocity = 0 cm/sec
38
-------
0
ROADWAY
DISTANCE (Y), m
Figure 25. Transport and reaction of exhaust aerosol, Case XXV. EPOSOD
calculations of neutralization of automotive sulfuric acid by ambient
gaseous ammonia for wind parallel to highway. Sulfuric acid concentration
isopleths (yg/m3) at cross-section perpendicular to highway at 10 km
downwind.
Ammonia concentration = 30 ppb
Deposition velocity = 0 cm/sec
39
-------
20-
U
UJ
10-
36 \I2
100
DISTANCE (Y), m
Figure 26. Transport and reaction of exhaust aerosol, Case XXVI. EPOSOD
calculations of neutralization of automotive sulfuric acid by ambient
gaseous ammonia for wind parallel to highway. Sulfuric acid concentration
isopleths (yg/m3) at cross-section perpendicular to highway at 10 km
downwind.
Ammonia concentration = 30 ppb
Deposition velocity = 2 cm/sec
40
-------
40 -i
a.
o
CM
= 0
ppb
0
ROADWAY
1000
DISTANCE (X), m
2000
Figure 27. Transport and reaction of exhaust aerosol, Case XXVII. EPOSOD
calculation of neutralization of automotive sulfuric acid by ambient
gaseous ammonia with wind perpendicular to highway. Downwind distance
measured from nearest upwind border of highway. Sulfuric acid concentration
is at 2m above ground.
Deposition velocity = 0 cm/sec
41
-------
401
= 0ppb
0
ROADWAY
1000
DISTANCE (X), m
2000
Figure 28. Transport and reaction of exhaust aerosol, Case XXVIII. EPOSOD
calculation of neutralization of automotive sulfuric acid by ambient
gaseous ammonia with wind perpendicular to highway. Downwind distance
measured from nearest upwind border of highway. Sulfuric acid concentration
is at 2m above ground.
Deposition velocity = 0.03 cm/sec
42
-------
Even when, as in Figs. 24 to 26, ambient ammonia is increased to 30
ppb., sulfuric acid concentrations immediately above the roadway are only
reduced to about one-half the sulfuric acid concentrations for 0 ppb.
ambient ammonia. Again, the sulfuric acid concentration above the roadway
is most strongly influenced by the dry deposition rate.
For winds perpendicular to the highway, Figs. 27 and 28 give the sulfuric
acid concentrations at a height of 2 m. (z = 2 m.) as a function of downwind
distance. The origin of the Cartesian co-ordinate system is located at the
10 km. point of the highway, at the upwind edge and pavement level. For the
two deposition velocities, 0 and 0.3 cm/sec, it is clear that, irrespective
of the ambient ammonia concentration, the downwind sulfuric acid concentrations
are very small. The trend of these predicted concentration levels appear
to be consistent with some of the measurements from the GM Sulfate Dispersion
Experiment (14).
A comparison between AROSOL and EPOSOD calculations is given in Figs.
12 and 22.* Under equivalent conditions, EPOSOD predicts a larger roadway
3 3
concentration (^280 yg/m ) than AROSOL (^210 yg/m ). EPOSOD, when compared
with AROSOL, underestimates the concentrations some distance away from the
roadway.
* 3
For sulfuric acid, units in ppb. multiplied by 4 give yg/m .
43
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REFERENCES
1. Pierson, W.R. Sulfuric Acid Generation by Automotive Catalysts, paper
#42, Symposium on Auto Emission Catalysis, Div. of Colloid and Surface
Chem. , ACS 170th National Meeting, Chicago, August 27, 1975.
2. Macias, E.S., Fletcher, R.A. Husar, J.D., and Husar, R.B. Particulate
Sulfur Emission Rate from a Simulated Freeway, paper #31, Symposium on
Chemical and Physical Properties of Automotive Emissions from Catalyst
Equipped Cars During the GM/EPA Sulfate Dispersion Experiment, Division
of Environmental Chemistry, ACS 172nd National Meetings, San Francisco,
August 29 - September 3, 1976.
3. Air Quality Criteria for Sulfur Oxides, National Air Pollution Control
Administration Publication No. AP-50, Washington, D.C., 1970.
4. Wilson, W.E. and Durham, J.L. Atmospheric Behavior of Catalyst -
Generated Aerosols from Source to Reception, paper #33, Symposium on
Chemical and Physical Properties of Automotive Emissions from Catalyst
Equipped Cars During the GM/EDA Sulfate Dispersion Experiment, Division
of Environmental Chemistry, ACS 172nd National Meeting, San Francisco,
August 29 - September 3, 1976.
5. Suck, S.H. and Brock, J.R. To Appear.
6. de Bower, K. A Method of Modelling Chemically Reactive Plumes,
M.S. Thesis, The University of Texas, 1976.
7. Shir, C.C. and Shieh, L.J. Development of Urban Air Quality Simulation
Model with Compatible RAPS Data, Final Report, Contract 68-02-1833, IBM
Research Laboratory, San Jose, California, May 1975.
8. Brock, J.R. Processes, Sources and Size Distributions, in Fogs and
Smokes, Discussions of Faraday Society, London, 1974.
9. Lamb, R.G. Note on Application of K-Theory and Turbulent Diffusion
Problems Involving Chemical Reaction, Atmos. Environ. 7:235 (1973).
10. Cadle, S.H., Chock, D.P., Groblicki, P.J. Heus, J.M., and Monson, P.R.
General Motors Sulfate Dispersion Experiment: Results and Assessment of
EPA HIWAY Model, paper #20, Symposium on Chemical and Physical Properties
of Automotive Emissions from Catalyst Equipped Cars During the GM/EPA
Sulfate Dispersion Experiment, Division of Environmental Chemistry,
ACS 172nd National Meeting, San Francisco, August 29 - September 3, 1976.
-------
11. Dabberdt, W.F. Experimental Study of Near-Roadway Dispersion,
Presented at 69th Annual Meeting, Air Pollution Control Association,
June 27 - July 1, 1976, Portland, Oregon.
12. Hidy, G.M. and Brock, J.R. The Dynamics of Aerocolloidal Systems,
Pergamon Press, Oxford, 1970.
13. Whitby, K.T., Kittleson, D.B., Cantrell, B.K., Barsic, N.J., and
Dolan, D.F. Aerosol Size Distributions and Concentrations Measured
During the GM Proving Grounds Sulfate Study, paper #32, Symposium on
Chemical and Physical Properties of Automotive Emissions from Catalyst
Equipped Cars During the GM/EDA Sulfate Dispersion Experiment, Division
of Environmental Chemistry, ACS 172nd National Meeting, San Francisco,
August 29 - September 3, 1976.
14. Tanner, R.L. and Newman, L. Chemical Speculation of Sulfate Emissions
from Catalyst-Equipped Automobiles under Ambient Conditions, paper #22,
Symposium on Chemical and Physical Properties of Automotive Emissions
from Catalyst Equipped Cars During the GM/EDA Sulfate Dispersion
Experiment, Division of Environmental Chemistry, ACS 172nd National
Meeting, San Francisco, August 29 - September 3, 1976.
15. Ragland, K.W. and Peirce, J.J. Boundary Layer Model for Air Pollutant
Concentrations Due to Highway Traffic, J. Air Pollution Control Assoc.
25:48-51.
16. Sehmel, G.A. and Hodgson, W.H. Predicted Dry Deposition Velocities,
Battelle, BNWL-SA-5125, Pacific Northwest Laboratories, Richland,
Washington 99352, August 1974.
17. Junge, C. and Scheich, G. Determination of the Acid Content of
Aerosol Particles. Atmos. Environ. 5:165-175(1971).
45
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APPENDIX A
THE RATE OF REACTION BETWEEN AMBIENT AMMONIA AND
SULFURIC ACID SOLUTION AEROSOL
The analysis of the reaction of dispersing automotive sulfuric acid with
ambient ammonia assumed that the reaction was rapid compared to the charac-
teristic diffusion time. The diffusion time was in the range 1-10 minutes.
This section gives estimates of the reaction rate.
The automotive sulfuric acid solution aerosol is found in particle sizes
less than 1 \im diameter. For such particle sizes the rate of collision N of
ammonia molecules with a sulfuric acid solution droplet is approximately
(12):
6 x 104D2n
N z
1 + 2.3 x lO^D (A-l)
where D is the droplet diameter in centimeters and n is the number density
(cm~3) of ammonia molecules surrounding the droplet.
Under the assumption that the reaction occurs on the surface of the
droplet, a collision efficiency for reaction, a can be introduced so that oN
gives the number of ammonia molecules per second undergoing reaction with the
sulfuric acid droplet.
An upper bound for this reaction rate is given by setting a = 1. A lower
bound is not easily established. Evidence exists that in the atmosphere
sulfuric acid solution droplets can exist in the presence of ambient ammonia
(17); this phenomenon has been attributed to the presence of relatively
impermeable surface films on the sulfuric acid solution droplets.
It is not the aim of this section to explore the question of surface
accommodation and reaction of ammonia molecules with sulfuric acid solution
droplets. It will be assumed that in the absence of further information,
a = l.
Under this assumption, the characteristic reaction time is of the order
N which for ammonia concentration at the ppb level is seen to be very short
compared to the characteristic diffusion time.
In addition an estimate of the minimum time, tN, for complete neutrali-
zation of a single sulfuric acid solution droplet by ammonia according to the
overall reaction:
46
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is given by:
tN z 101?Do (1 + 2.3 x 104D)/D2n (A-2)
In this equation DQ is the diameter of the water-free sulfuric acid
droplet and D is the actual diameter of the sulfuric acid solution droplet
at a given ambient relative humidity. As the reaction proceeds D will
actually vary with time; this refinement is not included in the estimate of
eq. A-2. Also, of course, n, the number density of ammonia molecules, will
vary during the course of the reaction. However, n also varies in our
calculations because of dispersion so that integration of the rate equation
under the assumption of spatial homogeneity is meaningless. Hence, eq. A-2
provides an estimate of the minimum time for neutralization at a given ammonia
concentration.
Table A-l gives some values for neutralization times for 3 ppb ammonia
and at 50% and 70% relative humidity. The automotive sulfuric acid aerosol
has been shown to be in the size range 0.01 - 0.5 ym. Therefore, even the
time for complete neutralization is less than the characteristic diffusion
time for these particles.
Table A-l. Estimate of Time for Complete Neutralization of a Single
Sulfuric Acid Solution Droplet. Ammonia Concentration = 3ppb.
Dry Particle Diameter
T DO Relative
(sec) ym Humidity
0.6 0.01 70%
7.4 0.1 70%
254 1.0 70%
0.84 0.01 50%
10.7 0.1 50%
328 1.0 50%
47
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/3-78-043
3. RECIPIENT'S ACCESSI ON-NO.
TITLE ANDSUBTITLE
DYNAMICS OF AUTOMOTIVE SULFATE EMISSIONS
5. REPORT DATE
April 1978
6. PERFORMING ORGANIZATION CODE
AUTHOR(S)
S.H. Suck, K. de Bower and J.R. Brock
8. PERFORMING ORGANIZATION REPORT NO.
PERFORMING ORGANIZATION NAME AND ADDRESS
Department of Chemical Engineering
University of Texas
Austin, Texas 78712
10. PROGRAM ELEMENT NO.
1AA603 AE-09 (FY-77)
11. CONTRACT/GRANT NO.
Grant R803660
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory-RTP, NC
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
13. TYPE OF REPORT AND PERIOD COVERED
Interim 11/76 - 11/77
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
16. ABSTRACT
A preliminary assessment of the potential environmental impact of automotive
sulfuric acid (or sulfate) aerosol has been made by analyzing the aerosol dynamics.
This analysis leads to the prediction of ambient automotive sulfuric acid aerosol
concentrations over and around a large, ten-lane highway (48 m. x 20 km.), some ten
or so y^ars hence, when almost all cars in the United States will be fitted with
catalytic converters. The attachment rate of fine automotive sulfuric acid
aerosols to ambient aerosols is examined. The dispersion and deposition of auto-
motive sulfate are modelled over the highway for "worst case" meteorology using
K-theory. The neutralizing effect of ambient ammonia on sulfuric acid concentra-
tions around the highway is examined by a direct simulation procedure for dispersion
calculations. These calculations indicate that adverse environmental effects of
automotive sulfuric acid emissions may be important under the stated conditions
of this study.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
*Air pollution
*Aerosols
*Sulfuric acid
*Sulfates
*Automobiles
^Exhaust emissions
Catalytic converters
^Mathematical models
Highways
Atmospheric diffusion
13B
07D
07B
13F
21B
07A
04A
13. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report)
UNCLASSIFIED
21. NO. OF PAGES
56
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
48
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