-------
12
11
10
9
8
7
6
5
4
3
2
1
0
O AEROSOL VOLUME CONC. JUm /cm
A AEROSOL SURFACE CONC.
m2/cm3)
AEROSOL NUMBER CONC. (105/cm3)
RUN NO. 59
SO2 = 0.25 ppm
R.H. = 51%
0 4 20
3 min
40
m /cm tu
80 100 120
TIME (mm)
140
160
180
200
Figure 45 AEROSOL DEVELOPMENT WITH TIME FOR RUN 59
0*7
O AEROSOL VOLUME CONC. yUm3/cm3
A AEROSOL SURFACE CONC. <102 /J. m2/cm3)
D AEROSOL NUMBER CONC. (105/cm3) '.,
RUN NO. 60
SO_ = 0.26 ppm
R.H. = 38%
20 40 60 80 100 120 140 160 180 200 220
Figure 46 AEROSOL DEVELOPMENT WITH TIME FOR RUN 60
48
-------
RUN NO. 61
SO_ = 0.21 ppm
R.H. = 28%
80 120 160 200 240 280 320
Figure 47 AEROSOL DEVELOPMENT WITH TIME FOR RUN 61
(^-LIGHTS OFF AT 256 min
120 160 200 240 280 320 360
Figure 48 AEROSOL DEVELOPMENT WITH TIME FOR RUN 62
49
-------
ALIGHTS OFF AT 213 mm
r
RUN NO. 63
SO_ = 0.34 ppm
R.H, = 26%
100
200
TIME (mm)
300
400
Figure 49 AEROSOL DEVELOPMENT WITH TIME FOR RUN 63
RUN NO. 64
SO2 = 0.29 ppm
R.H. = 58%
80
120 160 200
TIME (mm)
240
280
320
Figure 50 AEROSOL DEVELOPMENT WITH TIME, RUN 64
50
-------
5.63 /Im3/cm3 hr
O AEROSOL VOLUME CONC./< m3/cm3
AEROSOL SURFACE CONC.
AEROSOL NUMBER CONC. 105/cm
LIGHT OFF AT
185 mm
40 80 120 160 200 240 280 320 360 400
TIME (mm)
Figure 51 AEROSOL DEVELOPMENT WITH TIME FOR RUN 66
from less than a minute to 10 to 15 minutes depending on the lighting and
humidity conditions in the chamber. During this initiation period, there is
no observable production of aerosol, i.e., the nuclei count remains at its
baseline level of approximately 50 particles/cm and the electrometer
current in the WAA is essentially zero. As the partial pressure of sulfuric
acid vapor builds, a critical saturation is reached and rapid homogeneous
nucleation occurs. The condensation nuclei count suddenly begins to rise
and usually exceeds 10 particles/cm within a few minutes. Almost
simultaneously the WAA begins to detect an aerosol. Doyle (1961) suggests
that the radii of the tiny embryos that form is in the neighborhood of 6A
and that the number of molecules contained in each embryo is about ZO.
Clusters of molecules which do not exceed the critical size evaporate,
whereas those which attain a critical radius become increasingly stable
and grow into larger droplets. The rate of production of new particles
decreases as the aerosol surface increases since condensation of the acid
vapor will now occur on existing nuclei. As the particle concentration
grows, coagulation becomes more important until finally the loss rate by
51
-------
coagulation is just balanced by the production rate of new particles. At
this time the maximum nuclei concentration is reached. At later times
chemical nucleation continues to produce new particles, but the rate of
production is less than the coagulation rate so that the particle concentration
continues to decrease. As the aerosol grows, new surface is produced by
chemical nucleation and by condensation on existing particles. At the
same time surface is being lost by the coagulation of particles. In most
experiments the rate of production and the rate of loss of surface achieve
a balance through a significant part of an experiment and a dynamic equili-
brium surface is achieved. It will be shown later that the equilibrium surface
achieved correlates very well with the volume rate of aerosol production.
The total aerosol surface area, S, (firm /cm ) behaves in a
somewhat different manner. It starts to rise shortly after the initial forma-
tion of nuclei but rises more slowly and approaches a steady dynamic
equilibrium value which remains essentially constant throughout the latter
part of the growth period. The volume, V, (urn /cm ) begins to grow at
the same time as the surface and grows in an essentially straight line
fashion. It will be shown below that this linear volume growth characteristic
of the SO-, photochemical system may be explained in terms of a simple,
plausible model.
The photooxidation of SO , in the atmosphere is believed to be
first order in SO-,. In the system under study here, the rate of photo-
oxidation is low, i.e., a fraction of a % per hour. Consequently, the SO-,
concentration remains essentially unchanged during an experiment (up to
12 hours in the Calspan chamber and 7-8 in the University of Minnesota
chamber) and the products of SO-, oxidation are produced at a constant
rate. The main product of SO-, oxidation in clean air is believed to be
SO,; and if moisture is present, sulfuric acid droplets will eventually be
formed.
Friend et al (1973) have suggested that the initial formation of
sulfuric acid nuclei takes place through the following reactions:
52
-------
S03 + HZ0
H2S04 + H20
(3)
A simple model for the production of an aerosol in the SO-, photochemical
system which is consistent with our experimental observations can be based
on these reactions.
The model assumes the chemical reactions listed above and that
aerosol growth is governed by the following processes:
(1) The aerosol detected consists of sulfuric acid droplets.
(2) New particles are formed by the chemical nucleation process
of reactions (l)-(3).
(3) Particles are lost in number by coagulation; that is, when
two particles collide as a result of Brownian motion, they stick together
and become one larger particle. The rate of this process is proportional
to the square of the number concentration of such particles.
(4) Particles grow in size by condensation and coagulation.
Condensation is simply the diffusion of SO, and/or H,,SO, vapor to the sur-
face of a particle and its solution in the liquid particle. The rate of this
process will be proportional to (Xv)Sh,-. where S is aerosol surface area
and h is the mass transfer coefficient for Xv (SO" and/or H?SO vapor)
diffusing to the droplet surface. It is assumed that water vapor also diffuses
to and from the particle at a rate sufficient to keep the internal water vapor
pressure of the aqueous sulfuric acid droplet equal to the vapor pressure of
the water in the overall gas phase.
(5) Wall losses and wall production effects are not important
(for small vessels of a few cubic meters and for long working times in
53
-------
larger chambers, this assumption is not valid; however, for the experi-
ments reported here this assumption appears justified during the aerosol
production phase.)
SO., production reactions will start as soon as the lights are
turned on. During the initiation period (SO,) builds up and nuclei growth
takes place by reactions (l)-(3) to sizes sufficient to be detected by the
CMC. The detection limit of most small particle counters is about 0, OOZ
jam. This chemical nucleation process continues until substantial aerosol
surface area has been developed. Once this occurs, there are two parallel
paths which allow for SO, and H?SO. vapor to participate in further aerosol
growth: continued chemical nucleation and condensation.
The expected rapid reaction rate of SO, with water vapor, as
well as the exceptionally low vapor pressure of H?SO . suggests that these
gases achieve steady state concentrations shortly after initial homogeneous
nucleation in the SO? irradiated system. The steady state conditions remain
until enough aerosol surface has been produced to allow condensation to be
an important mechanism. Consequently, (SO,) will begin to fall and the
rate of production of new particles through chemical nucleation drops. This
model is consistent with our experimental data as illustrated in the plots of
N and S against time in the SO.-, photochemical system.
Further insight into the aerosol formation process may be gained
by observing the development of the aerosol size distribution as a function
of time. Figure 52 shows time histories of number concentration for four
different particle sizes observed with the WAA for experiment no. 15 in the
Calspan chamber,, Initially, the aerosol is comprised almost entirely of
very small (*".0075 u.m) particles. As condensation and coagulation proceed,
larger particles are formed and the distribution broadens and shifts toward
larger sizes. Finally, particles will grow sufficiently large where fallout
will become important and substantial volume loss will occur. Figures 53
and 54 show typical growing aerosols for high and low volumetric conversion
rates in the University of Minnesota 600 ft chamber. Here the cumulative
54
-------
O = 0.007 /U m
A = 0.013/< m
0.023M m
0.042,// m
62 102
142 182 222 262 302 342 382 422 462 502
TIME (mini
Figure 52 TIME HISTORIES OF NUMBER CONCENTRATION FOR FOUR DIFFERENT PARTICLE
SIZES OBSERVED WITH THE AEROSOL ANALYZER. (RUN 15)
55
-------
1.0 r
0.7
0.5
0.3
J- 0.07
u 0.05
< 0.03
Q
0.01
0.007
0.005
RUN 58: HIGH VOLUMETRIC CONVERSION RATE
176V4 min
48/4 min
17'/i mm
5 10 20 40 60 80 90 95 98
PERCENTAGE
Figure 53 PERCENTAGE OF VOLUME ACCOUNTED FOR BY PARTICLES
SMALLER THAN THE DIAMETER DVG VS. DyG
1.0
0.7
0.1
0.07
0.05
0.03
UJ
0.01
0.007
0.005
0.003
RUN 60: LOW VOLUMETRIC CONVERSION RATE
2 5 10 20 40 60 80 90 95 98
PERCENTAGE
Figure 54 PERCENTAGE OF VOLUME ACCOUNTED FOR BY PARTICLES
SMALLER THAN THE DIAMETER DVG VS. DVG
56
-------
aerosol volumes at different times are plotted against particle diameter on
log-probability paper. It may be seen that as time passes the aerosol grows
and at the same time the size distribution broadens. These curves allow
particle volume mean diameter to be determined rather easily. Volume
mean diameters, the diameter at which the cumulative volume distribution
curves cross the 50% line, obtained in this manner are plotted along with
the N, S, and V values against time plots for some of the University of
Minnesota experiments as shown in Figures 33, 34, 41 through 46 and 51.
Examination of such plots shows that the volume mean diameter increases
almost linearly with time as an experiment proceeds.
Examination of the aerosol number, surface, volume and
volume mean diameter profiles for the SO-, photochemical system reveals
that the general qualitative behavior of the system is the same regardless
of SO-, concentration or humidity. In addition, comparison of Figures 41
through 44, which show the results of experiments performed in the small
bag, with the remaining figures representing large chamber data indicates
that the general features of aerosol behavior are riot influenced by bag
size within the working times of these experiments.
57
-------
D. A General Correlation
In these experiments a wide variety of conditions have been
explored. One significant point that emerges, however, is that the same
general trends in variation of aerosol nurnber, surface and volume with
time are evident in virtually all of the experiments. This is particularly
true for volume and surface development. The volume tends to grow
linearly with time and the surface attains an equilibrium value which is
maintained throughout a significant fraction of the growth period. Clark
(1972) pointed out that a strong correlation exists between the volumetric
conversion rate in the linear portion of the volume growth curve and the
equilibrium surface. He found that a log-log plot of c/v/Mt against S^
yielded a straight line. In order to further test his correlation all the
useable data obtained in the Calspan and University of Minnesota test series
have been plotted on the same coordinates. The results are shown in
Figure 55. It is seen that the general trend exists and that the data are
scattered uniformly and in good agreement with Clark's correlation line,
The scatter in the results is felt to be primarily due to fluctuations in the
WAA and is not believed to reflect on the validity of the general correlation
line.
1 x 10
7
5
£
5.
oa
O
1 x 10
7
5
A CALSPAN EXPERIMENTS 20,800 ftj
O LARGE BAG EXPERIMENTS 600 ft3
P SMALL BAG EXPERIMENTS 90 ft3
O LARGE BAG, CLARK (1972) 600 ft3
S = 314
1 x 102
0.1 0.2 0.3 0.5 0.7 1 2 3 5 7 10 2O 30 50 70 100
$;
-------
E. Observed Rates of Photooxidation of Sulfur Dioxide
In both the University of Minnesota and Calspan experiments, it
has been found that the rate of volume production approaches a constant
value, i.e., a plot of volume against time yields a straight line. The
reasons for this are as follows: Once an equilibrium surface has been
achieved, the concentration of SO, in the gas phase will approach a steady
state value. Then the rate of oxidation of SO, will be equal lo the rate of
appearance of SO-, in the condensed phase in the form of sulfuric acid drop-
lets. The rate of production of sulfuric acid aerosol, corrected for mole-
cular weight change and water concentration must be equal to the rate of
photooxidation of SO , which is a constant for these experiments. Thus, the
Tlope of the straight line volume growth curve may be related directly to
the rate of photooxidation of SO-,. The governing equation is:
_ d[S02l B dV x Q x P x^l (4)
dt dt MU2
where p is the density of the sulfuric acid droplet, P is the weight fraction
H-,SO. in the drop, MW is the molecular weight of SO,, and MW7 is the
C. ~r i £-
-------
Using equation (4) and the measured value of dvyc/"fc in each
experiment, the rate of SO-, photooxidation can be determined. These rates
are listed in Tables IV and V for the University of Minnesota and Calspan
experiments, respectively. It is rather disturbing to note that there is a
large spread between the maximum and minimum observed rate of photo-
oxidation. The range is from 0.069% to about 0. 529% per hour for the
University of Minnesota experiments and from 0.005% to 0.022% for the
Calspan tests. This variation cannot be directly tied to any obvious experi-
mental variable such as SO-, concentration, bag size, or relative humidity.
The lower values observed in the Calspan experiments are partly due to
lower light intensity; however, recent results of Friend et al (1972) have
shown that Aitken nuclei are formed via the SO-,-O atom oxidation reaction
in the presence of water vapor and that Aitken nuclei are not formed to any
extent by the interaction of SO-, excited states with O-,, H -,O, or O-, and H-,O.
If these results are correct, then any calculated SO-, photooxidation must be
due to chamber contamination. According to Friend et al, no observable
nuclei should be formed in a contamination-free SO-,-clean air system.
In the Calspan chamber, the only source of O-atom would be
from the photolysis of background NO-,. It is fairly simple exercise to
calculate what the background NO-, levels would have to be in order to
achieve the SO-, photooxidation rates reported here. Based on the light
intensity of the Calspan chamber, the SO, photooxidation rate via O-atom.
~~)
attack is equal to fNO J"j (1. 62 x 10 )%/hr. This calculation suggests that
NO-, background levels of 0. 3 to 1.5 ppm are needed to account for the
photooxidation rates observed here. Such high levels were not detected in
the experiments nor would they be expected in extensively filtered air.
A second contribution, that of SO-, photooxidation in the HO?-
SO _, reaction, is not so easily eliminated. Hydroperoxy radical concentration
is usually several orders of magnitude greater than that of O-atoms in the
atmosphere (and the chamber), and therefore may contribute significantly
to the SO-, photooxidation rate. We are presently looking into the contribution
60
-------
this reaction may
qu.es.
have on SO-, photooxidation via computer modeling techni-
Figure 56 shows an attempt to correlate the photooxidation rate
with relative humidity. Here the observed photooxidation rate in % per hour
is plotted against relative humidity for University of Minnesota experiments.
Three groups of experiments are shown: the initial work done at low
humidities by Clark, and on this program the variable humidity experiments
done in the small bag. (Since all experiments in the Calspan chamber were
conducted at 30% RH, these points were not included in the plot. The observed
SO-, photooxidation rates, however, were on the average lower by a factor of
at least ten.) All the points seem to scatter in the same general way, and
there is no trend evident with changing relative humidity. The scatter
observed probably came from some sort of contamination. The most likely
sources are atmospheric pollutants which are not removed by the charcoal
scrubbing system. Synergistic interactions between SO-, and these materials
could result in significant changes in the SO-, photooxidation rate and in the
rate of aerosol formation.
1.0
0.7
0.5
z °'3
o
K
g
x
o
6
o
a.
ff 0.1
0.07
0.05
*-- -
-
%**
o
- :
O LARGE BAG EXPERIMENTS
D SMALL BAG EXPERIMENTS "'
O LARGE BAG, CLARK (1972) *
[ <
^,
?
0-
0
D
O
<
O f
D
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!>
)
o
)
i i, i
0
D-
D
O
1
- --,
_,
- - -i
- .
1
40 60
RH (%)
80
100
Figure 56 SO2 PHOTO-OXIDATION RATE VS. RELATIVE HUMIDITY
61
-------
Another possible source of scatter in the results is the unstable
performance of WAA. The charger performance in this instrument is at
times somewhat variable and if this variation goes unnoticed, errors in the
surface and volume measurements can result. This would lead to errors in
the measured volumetric conversion rates, and thus the computed SO,
photooxidation rates.
Although the data do not show any clear dependence of the SO?
photooxidation rate on relative humidity, the quantity of aerosol formed is
strongly dependent on humidity. Volumetric aerosol production rates are
much higher at high relative humidities, as would be expected since the
developing aerosol is composed of H-,SO. droplets. The effect is illustrated
in Figure 57 which shows the dependence of '/TiO^"] dvVc/t , the volumetric
production rate adjusted for SO-, concentration, on relatively humidity.
The high volumetric production rates at high relative humidities result from
the fact that the equilibrium water to acid ratio in the aerosol droplets
increases with humidity. Thus for any given quantity of SO, produced, a
larger volume of aerosol will be formed at high relative humidity than at
low relative humidity. This is significant because in terms of environmental
impact, it is the quantity of aerosol formed rather than the rate of SO-,
photooxidation which is important.
F. The Propylene-NO System
Because of the low light intensity observed in the Calspan
chamber, only three propylene-NO experiments were performed during
X
the first year. In each of the experiments, NO-, and propylene were injected
directly into the chamber after first filtering the chamber air for 10 to 1Z
hours. In two of the three experiments, some production of nuclei occurred
prior to irradiating the samples. NO, NO-,, oxidant and aerosol growth
were monitored for each of the experiments.
62
-------
=5,
(M
O
uu
70
50
10
?
- , . O LARGE BAG EXPERIMENTS
t D SMALL BAG EXPERIMENTS
- ' : '
-
1
i
o - to
(.
©<
a
o
11
k
0
L
O
D
; 0; . ,
i ; ^ -- -i
,
i ' i ; i
0 20 40 60 80 100
RH (%)
Figure 57 NORMALIZED VOLUMETRIC CONVERSION RATE VS. RELATIVE HUMIDITY
The results are summarized in Table VI, along with the two
NO-propylene experiments that were performed in the 600 ft University of
Minnesota chamber. The table shows initial concentration of gases, volu-
metric conversion rates, peak nuclei concentration, and equilibrium surface
area, as well as chamber temperature and humidity. The University of
Minnesota tests were partially designed to study the effect of humidity on
aerosol behavior in the NO-propylene system. Examination of the table
shows that the volumetric conversion rate, <^v/a.t , is much larger for the
high humidity case, a result which is supported by more recent Calspan
tests. Also, the maximum nucleus concentration is twice as great in the
high humidity case.
63
-------
Table VI
SUMMARY OF NOX-PROPYLENE PHOTO-OXIDATION EXPERIMENTS
RUN
NUMBER
N02
ppm
[C3H6]
ppm
dv/dt
urn /cm hr
NMAX
103 cm"3
SE
2 3
(U m /cm
TEMP.
°C
RH
%
COMMENTS
CALSPAN
19
20
21
3.00
2.15
1.51
1.5
1.5
1.5
0.52
0.90
1.20
450
300
450
135
>200
240
~25
~25
~25
35
~35
~35
STIRRING
STIRRING D. Rx
STIRRING D. Rx
UNVIERSITY OF MINNESOTA
65
67
INO),
0.55
0.55
2.19
2.19
1.00
9.26
260
521
515
900
27
30
25
65
GROWTH 250 mm
DECAY 190 mm
GROWTH 260 min
DECAY 20 mm
[NO)( DENOTES CALCULATED INITIAL NO CONCENTRATION
D Rx DENOTES DARK REACTION PRODUCING NUCLEI BEFORE LIGHTS WERE TURNED ON
Examination of the NO and NCU concentration profiles for the
University of Minnesota tests (Figures 58 and 59) shows that the onset of
aerosol growth occurs when most of the initial NO has been converted into
NO^. It is possible that NO has an inhibiting effect on the production of
aerosol, which is consistent with the general belief that ozone- olefin
reactions are responsible for gas to aerosol conversion in such systems.
Time histories of aerosol number, surface and volume concen-
trations for these experiments are shown in Figures 60 and 61. It may be
seen that the general characteristics of the developing aerosol are quite
similar to those exhibited by the SO? system. In the Minnesota NO -HC-air
< 3C
experiments, the initiation period before the onset of aerosol growth is
much longer than that in the SO-, tests. Delayed aerosol formation did not
occur in the Calspan tests probably because NO? (and not NO) was introduced
into the chamber.
64
-------
Q.
a
0.7
0.6
0.5
0.4
U
8 a3
0.2
0.1
D NO
O NO.
RUN NO. 65
[NO] j = 0.55 ppm
C3H6!i =2'1
R.H. - 25%
40
80
120 160
320
200 240 280
TIME (mini
Figure 58 NO AND N02 CONCENTRATION WITH TIME FOR RUN 65
0.7
0.6
0.5
0.4
§ «>-3
0.2
0.1
a NO
O NO-
RUN NO. 67
(NO) ( = 0.55 ppm
IC3H6'i *2'19 pP
, R.H. = 65%
0 40 80 120 160 200 240 280 320
TIME (mint
Figure 59 NO AND N02 CONCENTRATION WITH TIME FOR RUN 67
65
-------
ai
0
u.
oc
LJ
§ 3
O
O AEROSOL VOLUME CONC. /*m3/cm3
A AEROSOL SURFACE CONC. (102 //m2/cm3) .
VJUMBI
yWm)
AEROSOL NUMBER CONC. (105/cm3)
O
a
~ >u
2 oo
14
12
10
E
40 80 120 160 200 240 280 320 360 400 440
TIME (mm)
Figure 60 AEROSOL DEVELOPMENT WITH TIME FOR RUN 65
O AEROSOL VOLUME CONC.//m3/cm3
A AEROSOL SURFACE CONC. (102//m2/cm3)
AEROSOL NUMBER CONC. (105/cm3)
LIGHTS OFF
266 min
O
RUN NO. 67
[NO] ( = 0.55 ppm
[C3H6li = 2.19 ppm
R.H. = 65%
40 80 120 160 200 240 280 320 360
TIME (min)
Figure 61 AEROSOL DEVELOPMENT WITH TIME FOR RUN 67
CO
5
66
-------
The above figures also show that the volumetric growth curves
in the Minnesota tests were non-linear; in the Calspan experiments one of
the three tests had a non-linear growth rate. Part of the explanation may
lie in the volume vs time curve shown in Figure 61 for experiment no. 67.
The figures show a significant loss in volume in the latter part of the lights
on period of the experiment. This decrease could be due to wall losses or
to gas phase destruction of the aerosol already formed. The processes
responsible for loss of aerosol to the wall in the NO-propylene system
should be similar to those in the SO-, system. Such losses do not occur to
any significant extent in the SO-, system, so it is unlikely that wall losses
are responsible for the observed volume decrease in the NO-propylene
system. Gas phase destruction of the aerosol is possible and this implies
that some of the species which originally condensed to form the growing
aerosol have dropped to such low concentration that they begin to evaporate
from the surface of the particles. Although this is possible, it is much
more likely that the apparent volume decrease is an experimental artifact.
Figure 62 shows two aerosol volume distribution curves near the end of the
experimental runs. Here the volume distribution function AV/A log D
is plotted against particle diameter at a fixed time. The area under the
curve between any two diameters corresponds to the volume of aerosol in
that size interval. Early in the experiment it may be seen that the volume
distribution is a more or less symmetrical curve. However, as time pro-
gresses, more and more of the righthand part of the curve appears to be
cut off. This suggests that the aerosol is growing out of the size range of
the instrument which, in turn, could lead to a deviation from linear growth
at long reaction times.
Following the analysis of the propylene-NO experiments, the
Calspan data were compared with computer simulation results using the
propylene-NO reaction model developed by Demerjian, Calvert and Kerr
(1973). This model has proven quite successful in simulating the experi-
mental chamber results of Altshuller et al (1967) and Spindt (1973) for the
67
-------
>
<
O AEROSOL VOLUME CONC. * m3/cm3
AEROSOL NUMBER CONC. (105/cm3)
RUN NO. 67
O134 mm
249.5 mm
MAXIMUM
RANGE FOR'
WAA
2
.001
Figure 62
Dp VS. Dpm
68
-------
propylene-NO system. The modeled results are shown as continuous solid
5C
lines in Figures 63 and 64 and are compared with experimental data which
are shown in symbols. The only rate constants altered from those used in
the original model were photolytic reactions depending on available light
intensities and the two heterogeneous reactions,
and
HONO2 (5)
NO2 + NO + HZO> 2 HONO (6)
The measured rate constant for NO-, photolysis determined in the Calspan
chamber was used and all other photolytic rate constants were scaled down
proportionately. The rate constants used for reactions (5) and (6) were
lower by an order of magnitude as compared to those used in the referenced
report. This is reasonable in view of the reported heterogeneous nature
of these reactions and the different surface-to-volume ratios of the chambers
used. The surface-to-volume ratio in typical smog chambers is in the range
-1 S -1
of 2 to 5 meter , while the Calspan chamber has a / /~r 0. 25 m . The
modeled results are in quite good agreement with experiment, except for
experimental ozone values which are consistently high in the Calspan
studies. Uncertainties in the initial propylene concentrations present
*
could account for this discrepancy.
'Experiments 20 and 22 were run before the hydrocarbon analytical system
was set up. Therefore, initial propylene concentrations were approximated
based on expansion of known concentrations of propylene into the chamber.
The expected concentration of propylene was 3 ppm. In light of more
recent determinations using the hydrocarbon analytical system, these
concentrations are probably closer to 1. 5 ppm.
69
-------
3.0
10
o
I
EXPERIMENTAL POINTS
a = NO
A= o3
O = NO
MODELED RESULTS
RUN NO. 20
0 10 20 30 40 50 60 70 80 90 100
50
40
30
20
10
E
i
a
Figure 63 EXPERIMENTAL AND MODELED RESULTS FOR THE PROPYLENE - NOX SYSTEM
2.0
10.0
5.0
O
0 L
J.
CM
O
z
1.0
0.5
EXPERIMENTAL POINTS
D = N02
A = 03
O = NO
MODELED RESULTS
RUN NO. 22
' A
a
A A A A
O O O O O O
0 10 20 30 40 50 60 70 80 90 100 110 120 130
TIME (min)
50
40
30
E
20 .c
a.
a
10 e"
Figure 64 EXPERIMENTAL AND MODELED RESULTS FOR THE PROPYLENE - NOX SYSTEM
70
-------
It should be noted that the above data are derived from a limited number of
preliminary experiments and are not intended for in-depth modeling. More
detailed comparisons of experimental and modeled results for the NOx-
propylene system will be made during the coming year.
G. Chamber Effects on Aerosol Behavior
So far the aerosol behavior has been discussed as though there
were no walls containing the photochemical system, although it is obvious
that in any laboratory chamber wall effects can be an important factor in
determining the behavior of the aerosol. Typical effects include: reactions
which take place directly on the walls, contaminants which remain on the
walls and catalyze a variety of photochemical reactions, and gas or parti-
culate phase materials which are lost to the walls.
In the systems used in this investigation, the influence of walls
on SO-, photooxidation rates was not an important factor. This can be partly
seen by reviewing the University of Minnesota data tabulated in Table V,
in which it is shown that the observed SO-, photooxidation rates did not reflectj
a dependence on bag size. It may be argued, however, that any observed or
calculated SO-, photooxidation is due to contamination in light of Friend et a I
(197^) suggestion that Aitken nuclei arising from SO-, photooxidation would be
formed at rates too small to be observable in a contamination-free environ-
ment. The nature of such contaminant initiated reactions are not fully
understood at this point, and so possible wall effects in this regard cannot
be evaluated. A more logical approach is to examine aerosol behavior within
the chamber as an indicator of the effects of walls on the overall system.
Three experimental techniques were used to study the importance of wall
effects: coagulation experiments, linearity checks on volume production
curves for the SO-, photochemical aerosol, and chamber reactivity tests.
These studies are briefly described below.
71
-------
1. Coagulation experiments
The rate of coagulation of an aerosol is given by
where k is the mean coagulation rate constant for Brownian coagulation. Its
value depends on the particle size distribution but for a monodisperse distri-
bution with particle diameters in the range 0.01 to 0.05 fim, (the range of
- 9 3
interest in this work), the value is slightly greater than 1.0 x 10 cm /
particle-sec. According to Fuchs (1964), as the aerosol becomes more
polydisperse, k increases.
In a smog chamber, particles may be lost by diffusion to
the walls as well as by coagulation. The governing expression then becomes
- ~jf = kN2 + a N (8)
where a is a first order wall loss coefficient and N is particle number.
Ignoring the dependence of the second term on N, integration gives
- a)
o
If wall losses are unimportant a plot of 1/N against time for a decaying aero-
sol should yield a straight line of slope k. However, if wall losses are
significant, the apparent value of k will increase as N decreases. Experi-
ments have been performed at both Calspan and the University of Minnesota
in which number concentration have been measured and the results plotted
in the form of 1/N against time. The results are summarized in Tables VII
and VIII. The plots are presented in Figures 65 to 79 for the University of
Minnesota tests and Figures 80 to 96 for the Calspan tests.
72
-------
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-------
Table VIII
COAGULATION RATE CONSTANTS FOR SEVERAL EXPS
PERFORMED IN CALSPAN CHAMBER
AEROSOL
EXP. NO.
4
12
23
24
21
13
25
3
11
20 C3H6
22 C3H6
BEHAVIOR
STIRRING
YES
YES
YES
YES
YES
NO
NO
NO
NO
-NOX YES
-NOX YES
(LIGHTS ON)
SO2 -.AIR !
K cm /sec
\*
4.1 x 10~11
8.3 x 10'11
2.1 x 10'10
3.1 x ID'10
3.2 x 10"10
9.4 x 10~11
2.6 x 10"10
2.0 x 10'10
1.2X10'10
AEROSOL
SYSTEM
EXP. NO.
4
16
14
27
12
13
14
1
16
27
C3Hg - NOX SYSTEM
6.2 x 10 10
7.8 x 10"10
BEHAVIOR
STIRRING
YES
YES
YES
YES
YES
NO
NO
NO
NO
NO
(LIGHTS OFF)
o
K cm /sec
\f
5.7 x 10'10
1.6 x 10'9
2.1 x 10~9
3.0 x 10'9
3.3 x 10'9
1.5x 109
3.7 x 10'10
4.1 x 10'10
9.3 x 10"10
3.0 x 10"9
74
-------
E
-------
15
to
'o
10
LIGHTS OFF ' ' '
0 50 100 150 200
TIME (WIN)
Figure 67 AEROSOL COAGULATION - U. OF M. RUN 51
15
10
15.9 x 10"1Q cm3/sec
40 80 120 160 200 240 280 320 360 400
TIME (mm)
Figure 68 AEROSOL COAGULATION - U. OF M. RUN 52
76
-------
(O
'o
15
10
LIGHTS OFF AT 142.5 mm
DARK SLOPE =
21.1 x 10 10 cm3/sec
50
200
100 150
TIME (mm)
Figure 69 AEROSOL COAGULATION - U. OF M. RUN 55
250
to
'o
15
19.6 x 10~10 cm /sec
150
200
100
TIME (mm)
Figure 70 AEROSOL COAGULATION - U. OF M. RUN 56
77
-------
15
10
(0
'o
~ 5
Z
RUN NO. 57
LIGHTS OFF
-T AT 70 min
LIGHT SLOPE =
0.8 x 10'10 cm3/sec ,
19.5 x 10'10 cm3/sec
50
200
100 150
TIME (mm)
Figure 71 AEROSOL COAGULATION - U. OF M. RUN 57
vo
'o
10
RUN NO. 59
LIGHT SLOPE =
0.5 x 10~10 cm3/sec
11.1 x 10"10cm3/sec
50
200
100 150
TIME (mm)
Figure 72 AEROSOL COAGULATION - U. OF M. RUN 59
78
-------
15
£ 10
CO
'o
RUN WO 60
187 x 10~10 cm3/sec
SO 100 150 200
Figure 73 AEROSOL COAGULATION - U. OF M. RUiSI 60
£
u
up
'o
0 40 80 120 160 200 240 280 320 360 400
TIME (mm )
Figure 74 AEROSOL COAGULATION - U. OF M. RUN 61
79
-------
28
24
E 20
-------
2.9 x 10 10 cm3/sec
0 40 80 120 160 200 240 280 320 360 400
TIME (mm)
Figure 77 AEROSOL COAGULATION - U. OF M. RUN 64
10
'o
10.7 x 10 10 cm3/sec
100
400
200 300
TIME (mm)
Figure 78 AEROSOL COAGULATION - U. OF M. RUN 65
500
81
-------
500
0 100 200 300 400
TIME (min)
Figure 79 AEROSOL COAGULATION - U. OF M. RUN 66
RUN NO. 1, MARCH 18, 1973
SO,, = 1.33 ppm
1.6
1.3
1.2
1.1
1.0
. of.
LIGHTS OFF
50 60 70 80 90 100
TIME (MIN)
Figure 80 AEROSOL COAGULATION SO2 - AIR SYSTEM WITHOUT STIRRING - CALSPAN
82
-------
1.6
CO 1C
E 7'5
1.2
1.1
1.0
RUN NO. 3, MARCH 19, 1973
SO2 = 0.68 ppm
LIGHTS ON
' CALSPAN
LIGHT SLOPE =
1.25 x 108 cm"3 min'1
0 42 52 62 72 82
TIME (mm)
Figure 81 AEROSOL COAGULATION SO2 - AIR SYSTEM WITHOUT STIRRING -CALSPAN
4.0
3.6
3.2
" 2.8
-------
RUN NO. 11, MARCH 2, 1973
112 122 132 142 152 162 172 182 192
Figure 83 AEROSOL COAGULATION S02 - AIR SYSTEM WITH STIRRING - CALSPAN
1.0
in
'o
0.5
RUN NO. 12, MARCH 23, 1973
t SO2 = 0.44 ppm
LIGHTS OFF AT 390 min
LIGHTS ON AT 92 min .
92 132 172 212 252 292 332 372 412 452
TIME (mm)
Figure 84 AEROSOL COAGULATION SO2 - AIR SYSTEM WITH STIRRING - CALSPAN
84
-------
8.8
- 7.6
E
10
'o
6.4
5.2
RUN NO. 13
SO2 = 0 39 ppm
LIGHTS OFF AT 250 mm
DARK SLOPE =
; LIGHT SLOPE = 5.63 x 10"9 cm3/mm
172 182 192 202 212 222 232 242 252 262 272 282 292
Figure 85 AEROSOL COAGULATION SO2 - AIR SYSTEM WITHOUT STIRRING - CALSPAN
9.0
8.0
to
'o
RUN NO. 14, MARCH 24, 1973
LIGHTS OFF : . . .
SO2 = 0.91 ppm :
K = 1.28 x 10"7 cm3/mm
0 32 42 52 62 72 82 92 102
Figure 86 EFFECT OF STIRRING ON COAGULATION IN S02 AEROSOL DECAY -CALSPAN
85
-------
RUN NO. 16, MARCH 26, 1973
SO, = 0.90 ppm
0.3
0 40 50 60 70 80 90 100 110 120 130 140 150
TIME (min!
Figure 87 AEROSOL COAGULATION SO2 SYSTEM WITH AND WITHOUT STIRRING - CALSPAN
0.7
0.6
pj~ 0.5
0.3
0.2
0.1
RUN NO. 17, MARCH 27, 1973
SO2 = 0.48 ppm
. 3.17 x 10~8 cm3/mm
0 40 60 80 100 120 140 160 180 200 220 240
TIME (mm)
Figure 88 AEROSOL COAGULATION SO2 - AIR SYSTEM - CALSPAN
86
-------
6.0
5.5
1C
'o
5.0
4.5
. RUN NO. 20, MARCH 29, 1973 i
K = 3.7 x 10 8 cm3/mm
0 112 122 132 142
TIME (mm)
Figure 89 AEROSOL COAGULATION N02 - PROPYLENE SYSTEM WITH STIRRING - CALSPAN
3.0
2.0
up
'o
1.0
RUN NO. 21, MARCH 30, 1973
SO2 = 0.58 ppm
LIGHT SLOPE =
1.7 x 10"8 cm3/mm
82 92 102 112 122 132
TIME (min)
142
Figure 90 AEROSOL COAGULATION SO2 - AIR SYSTEM WITH STIRRING -CALSPAN
87
-------
8.0
6.0
o
T-
z
4.0
2.0
RUN NO. 22, MARCH 30, 1973
1.51 ppm NO. + 1.5 ppm C,H_
£ *S O
K = 4.7 x 10~8 cm3/mm
86 96
106
116 126
TIME (mm)
136 146
156
166
Figure 91 AEROSOL COAGULATION NO2 - PROPYLENE SYSTEM WITH STIRRING - CALSPAN
RUN NO. 23, MARCH 31, 1973 ! ' ! ! :
SO2 = 0.56 ppm i :
LIGHTS ON
3.0
up
'o
2.0
1.0
LIGHT SLOPE =
1.24 x 10~8 cm3/min
162
172
182
TIME (mm)
192
202
Figure 92 AEROSOL COAGULATION S02 - AIR SYSTEM WITH STIRRING - CALSPAN
-------
I RUN NO. 24, APRIL 2, 1973;
' SO2 = 0.42 ppm
'LIGHTS ON AT 110 mm
2.0
1.9
1.8
1.7
1.6
1.5
1.4
1.3
LIGHT SLOPE =
8 3
1.87 x 10 cm /mm
136
146
156 166
TIME (mm)
Figure 93 AEROSOL COAGULATION SO2 - AIR SYSTEM - CALSPAN
RUN NO. 25, APRIL 4, 1973
, SO2 = 0.29 ppm
LIGHTS OFF .
3.0
"E 2.8
u
up
'o
C 2.6
2.4
2.2
2.0
1.8
100 110 120 130 140 150 160 170
TIME (mini
Figure 94 AEROSOL COAGULATION SO2-AIR SYSTEM WITHOUT STIRRING - CALSPAN
89
-------
3.1
2.7
I 2.3
in
'o
- 1.9
Z
1.5
1.1
0.7
0.3
j RUN NO. 27, APRIL 6, 1973
; SO, = 0.30 ppm
LIGHTS OFF
122 142 162 182 202 222 242 262 282 302 322
TIME (mm)
Figure 95 AEROSOL COAGULATION SO2-AIR SYSTEM - CALSPAN
03
'o
9.2
8.8
8.4
8.0
7.6
7.2
6.8
6.4
6.0
5.6
5.2
4.8
4.4
4.0
3.6
3.2
2.8
2.4
RUN NO. 28, April 8, 1973
SO2 = 0.80 ppm
LIGHTS OFF
K = 7.73 x 10"8 cm3/min
\ ' ' -83
K = 12.25 x 10 cm /min
60 70 80 90 100 110 120 130 140
TIME (mm)
Figure 96 AEROSOL COAGULATION, S02 SYSTEM WITH AND WITHOUT STIRRING
90
-------
The data are presented for both lights on and lights off portions
of the experiments. For the University of Minnesota tests, Table VII lists
the length of time for which aerosol decay was observed when lights were
on and off, the apparent coagulation rate constants k for both cases, and
also any departures from linearity in the 1/N against time plots. The lights
on decay data represents information that was taken after the peak number
concentration was attained in a normal aerosol production experiment,
In both the Calspan and University of Minnesota tests, it may
be seen that the slope of the 1/N against time curves suddenly increases
when the lights are switched off (see, e.g., Figures 65 and 82). This sug-
gests that generation of new particles is occurring as long as the chamber
is illuminated resulting in a smaller number concentration loss rate.
Once the lights are off, the generation of new particles ceases and only
coagulation and diffusion to the walls are operative mechanisms of aerosol
decay. The lights on rate constants as shown in Table VII exhibit wide
variation but in most cases are a factor of 3 or 4 lower than the lights off
values. The values of k in the dark exhibit significantly less scatter and
are clearly higher in the small bag than in the large bag. Averaging all
University of Minnesota experimental results gives a value of k dark,
12. 5 x 10 cm /particles-sec in the large bag and 18. 0 x 10 cm /particles-
sec in the small bag,, The difference between these values can be attributed
to larger wall losses in the small bag. It is noted in Table VII that experi-
ments 52, 65, 56, 57, and 60 all show upward curvature in the 1/N against
time plots. This upward curvature suggests that wall losses are beginning
to be important. On the average this upward curvature appears after 260 min
in the large bag and about 110 min in the small bag. Again this indicates more
surface loss effects in the small bag, as would be intuitively expected.
The coagulation rate constants shown in Table VIII (page 72)
for experiments performed in the Calspan chamber have been separated into
stirred and non-stirred cases with and without lights on. As in the Minnesota
experiments, the coagulation rates are much lower (up to a factor of ten)
when the lights are on suggesting that additional aerosol is being generated
91
-------
during this phase of the experiment. Other characteristics of the data
are:
(1) Stirring vs non-stirring. During the lights on portion of
the experiments, stirring did not appear to influence the result. When the
lights were off, stirring resulted in a coagulation rate that was on the
average twice as great as the non-stirred runs.
(2) Taking into account the particle size ranges encountered in
these experiments ( <_ 0. 024 urn) to <_ 0. 042 |j,m), the coagulation rate is
lower than the predicated theoretical value (i.e.,~10 x 10 cm /sec) for
all cases when lights are on and greater than theory by a factor of two when
lights were off and stirring was employed. The only time K was about equal
to theory was in lights off experiments in which there was no stirring.
These coagulation data for the Calspan chamber suggest that
coagulation rates differ from theory whenever the lights are on or when
stirring is employed. Experiments are still being performed with a new
aerosol sampling system to determine when wall effects begin to influence
aerosol behavior in the large chamber. Data from the University of Minne-
sota tests do not include stirred runs but show a similar trend in coagulation
rates when lights are on.
Calculations of aerosol half life were made for the large and
small University of Minnesota systems. According to theory, the coagulation
rate constant, k , measured in the dark should equal k + j-. , where (3 is a
wall loss coefficient where k is the theoretical coagulation rate constant,
-10 3
If we assume a theoretical value of k, of about 10 x 10 cm /sec, fi can be
determined according to
ke = V fe '
(11)
92
-------
The average value for N in the small bag experiments was found
53 43
to equal 1. 3 x 10 particles/cm , and in the large bag 8 x 10 particles/cm .
Substitution of these values in into the above expression gives values for
of 10.4 x 10"5 and 2. 0 x 10"5
In terms of aerosol half life,
- 5 -5 -1
of 10.4 x 10 and 2. 0 x 10 sec for the small and large bags respectively.
t1/2 = In 2/p (12)
gives values of 1.85 and 9.6 hours for the small and the large bags, respec-
tively. (A value of 14.25 hours is obtained for the Calspan chamber if the
average coagulation rate constant is used from the dark cases without stirring.
Statistically, however, there is too much scatter in the limited data to allow
this calculation to be considered valid.) These numbers represent the time
that would be required for the number concentration to be reduced to one-half
through wall losses acting alone. Additional data gathered during the coming
year will be used to refine these calculations. For aerosol volume loss, the
half lives will be longer since the large particles (which contain most of the
volume) will diffuse to the wall more slowly than small particles.
2. Linear Aerosol Volumetric Growth Curves
Clark (1972) has suggested that departures from linearity in the
volume vs time curves for the SO., photochemical system indicate the pre-
sence of wall loss effects.
The theoretical basis for this hypothesis may be understood in
terms of a simple model. It is based on two assumptions:
(1) When the lights are on, the rate of aerosol volume production
is constant and
(2) Wall losses are due to diffusion of aerosol to the walls.
Diffusion processes are first order so that the rate of loss of volume to
the walls will be proportional to the volume itself.
93
-------
A differential equation which describes the process may then be
written, giving
dV
dF
= R - aV (13)
where R is the volume rate of production of new aerosol and a is the wall
loss coefficient. Solution of this equation with the boundary condition V = 0
at t = 0 gives
V = 5- (1 - e"Qt) (14)
For small values of at this may be expanded to give
V = Rt or -^r = R (15)
dt
so that initially linear growth is expected, however, as at increases, the
rate of volume growth decreases and the volume eventually approaches a
constant value at large values of at. Departures of the volumetric growth
from linearity depend on the value of at and do not depend on R.
Figure 97 shows the general shape of the volumetric growth
curve predicted by this model as well as the no loss volume growth curve.
Here dimensionless volume, Va/R, is plotted against dimensionless time,
at. For small values of at the curves with and without losses are quite
close to one another and diverge as at increases. The curves diverge
gradually, but there is no sudden departure from linearity. This suggests
that experimentally it might be difficult to detect downward curvature of
the volume against time plot unless at is relatively large. The coagulation
-5 - 5 -1
experiments described above gave p values of 2 x 10 and 10.4 x 10 sec
for the large and small bags, respectively. The parameter p is analagous
to a except that it is for number loss rather than volume loss. For a
monodisperse aerosol, a and (3 are equal. That is not the case here and
the aerosol volume is weighted more heavily toward larger particles which
diffuse more slowly to the walls. Consequently, a will be smaller than p.
94
-------
0.9
0.8
0.7
tr
8
0.6
2
0.4
Z
O
to
0.3
0.2
0.1
NO LOSSES, Voc /R =
-------
Table IX
EXPERIMENTS TO DETERMINE THE LENGTH OF THE
LINEAR VOLUME PRODUCTION PERIOD
RUN
NUMBER
dv/dt
fj.m3/cm3 hr
TIME LINEAR
minutes
Mm
COMMENTS
LARGE BAG
18
19
20
22
24
25
27
65
66
67
0.87
1.38
2.36
6.27
22.9
16.7
0.39
1.00
5.63
9.26
280
160
115
85
> 60
> 60
370
240
> 170
180
0.060
0.046
0.045
0.045
> 0.060
> 0.053
0.065
0.055
0.10
0.080
C3H6-NO
OUT OF WAA SIZE RANGE
CsHe NO GROWS OUT OF
SIZE RANGE OF WAA
SMALL BAG
55
58
60
3.98
10.9
2.21
> 140
140
> 160
> 0.055
0.090
> 0.040
NOTES: dv/dt DENOTES THE SLOPE OF A PLOT OF AEROSOL VOLUME AGAINST TIME.
TIME LINEAR IS THE LENGTH OF THE LINEAR PORTION OF THE PLOT. DoV IS
THE PARTICLE VOLUME MEAN DIAMETER AT THE END OF THE LINEAR
GROWTH PERIOD
that suggests itself is that the non-linearity appeared when the aerosol grew
out of the size range of the instrument. This appears to be the case for
Run 67 as was explained above. Close examination of the size distributions
show that this is also the case for runs 66 and 58. Note that all of these
runs show non-linearity at rather large volume mean diameters. The
behavior of the other runs, however, cannot be explained so easily. In
fact, the only possibility that immediately presents itself is an error in
the calibration constants for the WAA. These constants are in the process
of being reevaluated and will hopefully shed further light on the problem.
96
-------
Comparing small and large bag experiments also show some
unusual results. Experiments 20 and 55 have nearly equal volumetric
production rates, but the linear growth period in the small bag is more
than 160 min while that in the large bag is 115 min, this trend is exactly
opposite from that expected from simple theory. Run 55 in the small bag
has a volumetric conversion rate which falls between runs 20 and 66 in the
large bag. The linear time of >140 min also falls between the values of 115
and > 170 found in runs 20 and 66, respectively. Again the shorter working
period expected in the small bag is not apparent in the volume production
curves.
Something other than wall losses appears to be producing the
departures from the linear volumetric production curves. This effect may
be instrumental or it may be involved in the physics and chemistry of the
SO^, -photooxidation system. In any case, departures from linearity in the
volumetric growth curves cannot be used as the sole tool for assessing
chamber wall loss at the present time. The most reliable available tech-
nique for assessing the importance of wall losses is based on the coagulation
experiments. With refinement the coagulation results should yield useful
information about volume loss as well as number loss.
3. Chamber Reactivity Tests
At the University of Minnesota several experiments were per-
formed to determine the influence of the condition of the charcoal in the
scrubber and the surface of the Teflon bag on photochemical aerosol pro-
duction. Three basic conditions were investigated; aerosol formation in
the original large bag with the original charcoal in the scrubber, both of
which had been used in the 48 experiments; aerosol formation in the old
bag with fresh charcoal; and aerosol formation in the new large bag with
the fresh charcoal in the scrubber. The results of these experiments are
summarized in Table X. Here two types of behavior, maximum nuclei
concentration and dark growth, were monitored at two different relative
humidities for each chamber condition.
97
-------
Table X
CHAMBER REACTIVITY EXPERIMENTS
RUN
NUMBER
49
50
52
53
54
61
63
64
SYSTEM
OLD BAG-OLD CHARCOAL
OLD BAG-OLD CHARCOAL
OLD BAG-NEW CHARCOAL
OLD BAG-NEW CHARCOAL
OLD BAG NEW CHARCOAL
NEW BAG-NEW CHARCOAL
NEW BAG NEW CHARCOAL
NEW BAG-NEW CHARCOAL
[so2i
ppm
0.27
0.27
0.23
0.23
0.27
0.21
0.34
0.29
RELATIVE
HUMIDITY
%
23
58
30
30
j58
28
26
58
CNCMAX
5 3
10 particles/cm
1100
1020
350
330
720
730
500
920
DARK GROWTH
YES
YES
NO
SLIGHT
YES
YES
YES
YES
The least reactive chamber condition as indicated by the least
dark growth and the lowest peak nuclei concentration was the old bag used
with fresh charcoal. This was particularly true with respect to the maxi-
mum nuclei concentration which was approximately halved. This combination
proved to be less reactive than the new chamber with new charcoal which
suggests that some sort of chamber conditioning process takes place. Some
conditioning also seems to have occurred during the series of runs performed
in the Calspan chamber during the joint workshop. In those tests, the
volumetric conversion rate steadily decreased for the first nine or ten
experiments as the chamber appeared to become conditioned to the SO-,
system. Once achieving a fairly low level only the introduction of new gases
caused any significant increase in the volumetric conversion rate. More
experiments of this type will be performed after the new lighting system is
completed in the Calspan chamber and the new teflon bag at the University
of Minnesota has had additional time to age.
98
-------
V. SUMMARY AND CONCLUSIONS
During the first program year, emphasis was placed on examining
aerosol behavior in the relatively simple SO?-clean air system using
-1
chambers of widely different surface-to-volume ratios (i.e., O.Z3 ft ),
-1 \ 3
0. 65 ft and 1. 31 ft ) At Calspan a 20, 800 ft aerosol chamber was
employed for these tests while smaller systems involving 600 ft and 90 ft
teflon bags were used at the University of Minnesota. A limited number
of experiments were also performed using the propylene-NO system.
X
Substantial effort was directed toward upgrading and testing the
Calspan chamber and its support facilities. Improvements included installa-
tion of chamber lighting and air purification systems, resurfacing of the
chamber walls with a chemically resistant fluorepoxy polymer and installa-
tion of suitable gas analysis instrumentation. Numerous tests were performed
to evaluate wall effects and establish overall chamber background reactivity.
Additional modifications to the present lighting system are being made as a
result of tests which showed a k, ~ 0.05 min or approximately 10% of noon
day sunlight. The improved system is expected to provide a five-fold
intensity increase in k, to *s 0. 24 min
Greater control of chamber conditions was available in the 600 ft
University of Minnesota chamber. Although stirring could not be employed,
fairly precise temperature and humidity control could be achieved in the
experiments. Tests were designed to study the formation mechanisms of
aerosols in the SO,-clean air system and to a lesser extent in the NO
£ x
propylene system. The effects of relative humidity on aerosol formation
as well as the influence of bag size on particle growth and decay was also
studied.
Although a number of experimental facilities were encountered during
the testing period, the overall utility of the large chamber was convincingly
demonstrated. Evidence to date points to chamber working times in excess
of 11 hours before wall effects begin to influence test results. Additional
experiments during the coming year should establish the useful working
99
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time available in the large chamber. Coagulation experiments performed
at the University of Minnesota point to effective working times of four to
six hours in the 600 ft chamber and about two hours in the 90 ft chamber.
Somewhat longer times are indicated from the linearity of the volume con-
version rates.
An important conclusion derived from the first year test results
is that aerosol formation presents an extremely sensitive indicator of
chemical reactions that occur in a smog chamber. In fact, the reaction
threshold as indicated by the onset of Aitken nuclei formation in SO-, photo-
oxidation cannot be detected by present gas analysis instrumentation. It
has been shown that the fraction of SO-, involved in the reaction is extremely
small even though particle concentrations often exceed 10 cm . In this
respect, aerosol formation has also been shown to be a sensitive indicator
of impurities existing in the chamber at concentration levels far below gas
analysis detection limits. Such highly sensitive contaminant effects occasion-
ally result in seemingly difficult to explain aerosol behavior patterns. At
this point no attempt has been made to provide a completely consistent
account of all the experimental data presented in this report.
It is concluded that the SO-, photooxidation test system studied in this
first year Calspan-University of Minnesota program has provided data on
aerosol behavior that is generally consistent with the accepted understanding
of the reaction system. A relatively simple aerosol formation model for
SO-, photooxidation is presented in this report. The observed number concen-
tration, volume and surface growth rates correlate satisfactorily with aerosol
behavior patterns predicted by the model without apparent wall effects causing
significant deviations. Particle number decay as predicted from coagulation
theory leads to the conclusion that a relatively long chamber reaction time
is available for experimentation. Precise definition of chamber working
times and evaluation of aerosol behavior in more complex HC-NO systems
are objectives of the coming year program.
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REFERENCES
Altshuller, A. P., et. al. , 1967: Environ. Sci. Tech., p. 889.
Bray, W. H., 1970: Water Vapor Pressure Control At Aqueous Solutions
of Sulfuric Acid, J. of Materials, Vol. 5, No. 1, p. Z33-248.
Clark, W. E., 1972 Ph. D. thesis, University of Minnesota, Measurement
of Aerosol Produced by the Photooxidation of SO-, in Air.
Demerjian, K. L., J. A. Kerr, and J. G. Calvert, Advan. Environ. Sci,
Tech., Vol. 3, Wiley-Interscience, N. Y.
Doyle, G. J., 1961: Self Nucleation in the Sulfuric Acid Water System,
J. Chem. Phys., Vol. 35, No. 3, p. 795.
Friend, J. P. et. al, 1973: On the Formation of Stratospheric Aerosols,
J. Atmos. Scio, Vol. 30, p. 465-479.
Fuchs, N. A., 1964: The Mechanics of Aerosols, The McMillan Co, ,
New York, N. Y.
Gorden, R. J., 1967: Pilot Study of Ultraviolet UV Radiation in Los
Angeles, J. S. Nader, El., PHS Pub. No. 999-AP-38.
Skala, G. F., 1963: A New Instrument for the Continuous Measurement of
Condensation Nuclei, Analytical Chem. , Vol. 35, p. 702-706.
Spindt, R. S., Gulf Research & Devel. Co., Pittsburg, Pa., private
communication.
Whitby, K. T., et. al. , 1972; The Minn. Aerosol Analyzing System Used
in the L. A. Smog Project, J. of Coll. & Interface Sci. , Vol. 39,
No. 1.
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1 REPORT NO,
EPA-650/3-73-002
3. RECIPIENT'S ACCESSION-NO.
4 TITLE AND SUBTITLE
DETERMINATION OF THE FORMATION MECHANISMS AND
COMPOSITION OF PHOTOCHEMICAL AEROSOLS
5 REPORT DATE
December 1973
6. PERFORMING ORGANIZATION CODE
7 AUTHOR(S)
W. C. Kocmond, D.
K. L. Demerjian
8. PERFORMING ORGANIZATION REPORT NO.
i. Kittelson, J. Y. Yang, and
NA5365-M-1
9. PERFORMING ORG \NIZATION NAME AND ADDRESS
Calspan Corporation
P.O. Box 235
44S5 Genesee Street
Buffalo, New York 14221
10. PROGRAM ELEMENT NO.
1A1008
11. CONTRACT/GRANT NO.
68-02-0557
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Protection Agency
National Environmental Research Center
Research Triangle Park, North Carolina 27711
and Coordinating Research Council, New York, NY
13. TYPE O.F REPORT AND PERIOD COVESED
Interim 6/72 - 6/73
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Calspan Corporation in collaboration with the Particle Technology Laboratory of the
University of Minnesota is engaged in a study of the formation mechanisms and growth
processes of photochemical aerosols.
Experiments are being conducted in Calspan's recently completed 20,800 ft smog
chamber and also in University of Minnesota's 600 ft3 and 90 ft^ chambers. In the
work reported here emphasis is placed on studying aerosol behavior in the S02 system
and chamber effects on aerosol behavior. Future work will emphasize aerosol formation
in representative hydrocarbon-NOx systems, with and without SO,., present. Effects of
chamber size, relative humidity and artifical nuclei on aerosol behavior will be
investigated.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
sulfur dioxide
smog chamber
hydrocarbon-NO
chamber effects
photochemical aerosols
photochemical particles
sulfuric acid
b.IDENTIFIERS/OPEN ENDED TERMS C. COS AT I Field/Group
13. DISTRIBUTION STATEMENT
Release Unlimited
19. SECURITY CLASS (ThisReport)
21. NO. OF PAGES
20. SECURITY CLASS (This page}
22. PRICE
110
EPA Form 2220-1 (9-73)
102
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