Chapter 27 Induction of an Ultrafine Aerosol by Radon Radiolysis
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2
4
Kai-Dee Chu1,Philip K. Hopke, E. O. Knutson3,K. W. Tu3,and R. F. Holub 1Department of Nuclear Engineering and Institute for Environmental Studies, University of Illinois, Urbana, IL 61801 2Departments of Civil Engineering and Nuclear Engineering and Institute for Environmental Studies, University of Illinois, Urbana, IL 61801 3Environmental Measurement Laboratory, Department of Energy, New York, NY 10014 4Geophysics Division, Denver Research Center, U.S. Bureau of Mines, Department of the Interior, Denver, CO 80225 Recent advances in measurement methods have allowed for the more complete determination of the aerosol size distribution for particles incorporating Po-218. It has been found that the "unattached" fraction is an ultrafine particle aerosol with a size range of 0.5 to 3 nm. In order to initiate studies on the formation mechanism for these ultrafine particles, a series of experiments were made in the U.S. Bureau of Mines' radon chamber. By introducing SO into the chamber, particles were produced with an ultrafine size distribution. It has been found that the particle formation mechanism is supressed by the presence of radical scavengers. These experiments suggest that radiolysis following the decay of Rn-222 gives rise to the observed aerosol and the properties of the resulting aerosol are dependent on the nature and the amount of reactive gas present. 2
It has been reported for many years that condensation nuclei can be produced by ionizing radiation. Recent studies have improved the measurement of the activity size distribution of these ultrafine particles produced by radon and its daughters (Reineking, et al., 1985; Knutson, et al., 1985). It seems that the Po-218 ion is formed by the radon decay, is neutralized within a few tens of milliseconds, and then attached to an ultrafine particle formed by the radiolysis generated by the polonium ion recoil. Although there will be radiolysis along the alpha track, those reactions will be very far away (several centimeters) from the polonium nucleus when it reaches thermal velocity. The recoil path radiolysis therefore seems to be the more likely source of the ultrafine particles near enough to the polonium atom to rapidly incorporate it. These ultrafine particles have high mobility and can coagulate 0097-6156/87/0331-0365$06.00/0 © 1987 American Chemical Society
RADON AND ITS DECAY PRODUCTS
366 rapidly are
with
what
the preexisting
had been
important
to
termed
measure
distribution
i f
aerosol.
the
These
"unattached
the f u l l
range
the properties
of
of
ultrafine
fraction".
particle
these
particles It
w i l l
activity
radionuclides
be
size
are to
be
understood.
Background Chamberlain, presented induce Megaw that of
Megaw
early
the
formation
and Wiffen there
a i r
of
of
were
(1961)
as well dose
the appearance obtained
in
sources
used
themselves
produce
particles
occured
However, of
detecting
of
"particle".
counter have
to
been
there
cluster into
determine
a
of
to
i s
threshold
becomes
(Perrin,
a l ,
extended
the size
of
of
0.003
ym. They
find
to
a
larger
Martell and
average
(1977)
chemical
lead
to
the
similar more
formation
used
With
particle-free and a
features
of
ambient longer
they
become
which
the small,
thus
at
paper, can
radon-219 of
after
a i r
as they i t
molecular
"particles".
It
mobile high
nuclei
a study
use ethanol
that the
means
by Vohra,
may A et
a l .
a
pulse into i n
observable
qualitative
a simple there
up t o
theoretical
may b e
the size
the
rate i s
a
at
at
dose-dependent
clusters
observed.
rapidly In
the particles
and the word
and u l t r a f i n e
The
a r e formed
rate,
i s
increase
with
as has been
nuclei"
by a CN c o u n t e r ,
has been
they
dose
track
the radon
that
appears
clusters
order aerosol
Perrin
coagulate.
clusters
have
the
ultrafine
there
of
rapid
appears
to
potentially
their
be e x p l a i n e d
Thus,
nuclei
a
and
radiolysis. but
3.96 seconds,
a
turns
microphysical
track.
create
i n
before
observable.
the alpha
recoil
detecting
example).
ultrafine
the
the alpha
to
resulting
"condensation
of
the injection
could
of
this
the shorter
the polonium
highly
condensation
which
i n
condensation
t h e word
There in
from
a sufficiently
to
be d e t e c t e d
both
easily
occur
process
which
one that
i n
devices
a l . , 1980)
of
place
definition
for
and
particles
1983).
method
nuclei
by M a d e l a i n e et
coagulation
take
decrease
range
be reached
work
i s
a l . ,
the
These
be d e t e c t e d
ultrafine
nuclei
calculation. size
must
Recent
et
of
efficiency
the importance
a h a l f - l i f e
continuous
coagulate
of
occurring
this
coagulation
and
of
may t h e n
region
radiolysis
nuclei
that
short-lived
particles. of
diameter
processes
ionized
(1978)
rapid
(Leong,
a l . , 1983, f o r
to
could
ionization
condensation
1978; Madelaine,
has suggested
process
that
measurable
high
"particles". et
of
particles
an o p e r a t i o n a l
use a
of
b i g enough
nuclei".
et
a i r
decreasing
0.01 ym ( L e o n g ,
"condensation
coworkers
sharply
volume
Similar
the result
provide
suggested
a
the effects
sufficiently
studies
the presence
a size
atoms
where
of
in
to
a i r .
(1973)
nuclei.
generated
volume
and thereby
have
than
a
laboratory
be d e p o s i t e d
are probably
these
in
(1961)
radiation
and Scott
must
laboratory when
and Wiffen
ionizing
nuclei
neutralize
results
A l l of
less
to
of
condensation
a particular
particles
found
particles Thus,
i n
these
a n d Megaw
as Burke
that of
our
ionization density
(1957)
the ability
condensation
was a minimum
before
results
and Wiffen
reports
"particles"
this that means
particles. Subbarama and Rao
and an electron
source
to
(1966)
directly
27.
CHU ET AL.
Induction of an Ultrafine Aerosol by Radon Radiolysis
367
produce c l u s t e r s t o which radon (Rn-222) decay p r o d u c t s r a p i d l y a t t a c h . I n t h e absence o f t h e e l e c t r o n s o u r c e , they f i n d l i t t l e c l u s t e r formation. This r e s u l t i s not s u r p r i s i n g since small c h a i n a l c o h o l molecules r e p r e s e n t e x t r e m e l y good f r e e r a d i c a l scavengers t h a t c o u l d i n h i b i t t h e f o r m a t i o n o f m o l e c u l a r c l u s t e r s (Vohra, e t a l . , 1966; Gusten, e t a l . , 1981). However, f o r more normal atmospheric c o n s t i t u e n t s l i k e S 0 i o n i z i n g r a d i a t i o n has been found t o p l a y a s i g n i f i c a n t r o l e i n S 0 t o s u l f a t e c o n v e r s i o n under l a b o r a t o r y c o n d i t i o n s (Vohra, 1975). I n f a c t , Megaw and W i f f e n (1961) i n t h e i r work observed "some p r o p o r t i o n a l i t y between the s u l f u r d i o x i d e c o n t e n t o f t h e a i r and t h e n u c l e u s c o n c e n t r a t i o n produced by a g i v e n r a d i a t i o n dose". However, they a l s o found t h a t t h e r a t e o f n u c l e i f o r m a t i o n was g r e a t e r than c o u l d be e x p l a i n e d by S 0 a l o n e . I t has been suggested t h a t t h e primary mechanism o f p a r t i c l e f o r m a t i o n i s r a d i o l y s i s o f water t o form hydroxy1 r a d i c a l s (Coghlan and S c o t t , 1983) and t h a t t h e presence o f NO suppresses t h e f o r m a t i o n o f n u c l e i f i l t e r e d l a b o r a t o r y a i r presumably by s c a v e n g i n g o x i d i z i n g f r e e r a d i c a l s (Chamberlain, e t a l . , 1969). Coghlan and S c o t t (1983) have shown t h a t h y d r o x y l r a d i c a l scavengers l i k e s h o r t c h a i n a l c o h o l s suppress t h e f o r m a t i o n of p a r t i c l e s with the suppression c o r r e l a t e d with the r a t e constant f o r r e a c t i o n between h y d r o x y l r a d i c a l and t h e r a d i c a l scavenger. Thus, t h e r e i s l i t e r a t u r e showing t h e f o r m a t i o n o f c l u s t e r s and some o f t h e p o t e n t i a l i n t e r a c t i o n s t h a t o c c u r i n t h e a i r . However, t h e r e has n o t been a s y s t e m a t i c study o f t h e mechanisms o f c l u s t e r f o r m a t i o n and t h e p o s s i b l e i n t e r a c t i o n s o f v a r i o u s common atmospheric species i n that process. There i s v e r y r e c e n t evidence t h a t t h e r e i s an e x t r e m e l y f i n e mode i n t h e p a r t i c l e a c t i v i t y s i z e d i s t r i b u t i o n . Reineking, Becker and P o r s t e n d o r f e r (1985) used s e v e r a l d i f f u s i o n b a t t e r i e s t o determine a Po-218 a c t i v i t y peak i n t h e 1-3 nm diameter range. The Po-218 a c t i v i t y was a l s o a t t a c h e d t o p a r t i c l e s i n t h e a c c u m u l a t i o n mode peak i n t h e 0.1 t o 1.0 ym range. The Po-214 ( R a C ) a c t i v i t y was o n l y observed i n t h e a c c u m u l a t i o n mode and not a s s o c i a t e d w i t h t h e u l t r a f i n e p a r t i c l e s . Thus, t h e i n i t i a l motion and d e p o s i t i o n o f much o f t h e polonium-218 may be r e l a t e d t o t h e t r a n s p o r t by these u l t r a f i n e c l u s t e r s . S i m i l a r r e s u l t s were r e p o r t e d by Knutson, George, H i n c h l i f f e and S e x t r o (1985). I n t h i s s t u d y , s i n g l e s c r e e n s were used t o improve t h e s i z e r e s o l u t i o n f o r d e t e r m i n i n g t h e a c t i v i t y s i z e d i s t r i b u t i o n i n t h e 1-4 nm range. F o r low and moderate c o n d e n s a t i o n n u c l e i c o n c e n t r a t i o n s (,90,000/cnr), they f i n d t h a t t h e u l t r a f i n e mode c o a g u l a t e s t o o r a p i d l y w i t h t h e p r e e x i s t i n g a e r o s o l t o be observed. 2 >
2
2
E x p e r i m e n t a l Systems I n o r d e r t o examine t h e p r o c e s s o f u l t r a f i n e p a r t i c l e f o r m a t i o n , a j o i n t s e r i e s o f experiments were conducted a t t h e Denver Research
368
RADON AND ITS DECAY PRODUCTS
Center of the U.S. Bureau of Mines. The radon chamber i n Denver Research Center i s designed to provide adjustable humidities and a well controlled, monitored radon and CN concentration. I t has a length of 213 cm, a diameter of 152 cm, and a volume of about 3.89 m^. The walls are made of 0.5-cm r o l l e d s t e e l with welded seams. Figure 1 shows the general scheme of the test chamber excluding the transducers and data a q u i s i t i o n system. Rn-222 i s produced from a dry Ra-226 source with an a c t i v i t y of about 2 mCi housed i n a lead shield. Radon i s carried from the source by means of compressed a i r with a regulated flow rate of 200 cm^/min. The test chamber i s operated with a 75 1/min a i r pump located downstream (Figure 1)(Droullard, et a l . , 1984) that exhausts to the outside of the building. The flow rate of this a i r pump i s monitored by a mass flow transducer whose signals are converted to volumetric flow rates. A TSI Condensation Nuclei Counter model 3020 i s used to continuously monitor the aerosol concentration i n the chamber atmosphere. The chamber a i r i s drawn through a port i n the chamber wall into the counter and returned to the chamber through another port. The chamber i n t e r i o r temperature and humidity are monitored with a commercial hygrometer system. The a c t i v i t y l e v e l of Rn-222 and i t s daughters i n the test chamber i s measured. Radon a c t i v i t y levels are occasionally measured by grab sampling using s c i n t i l l a t i o n c e l l s . Radondaughter measurements f o r control or c a l i b r a t i o n purpose are made by the modified Tsivoglou method (Thomas, 1976). Samples can be collected through a sampling wand with a f i l t e r holder being put into the chamber through a sampling port. The a i r i n the chamber i s pulled through the f i l t e r and then recycled back into chamber by a pump. Working levels are continuously monitored by a continuous working-level detector developed by the Bureau of Mines (Droullard and holub, 1977; Droullard and Holub, 1980). The a c t i v i t y size d i s t r i b u t i o n of the u l t r a f i n e mode was measured using the BuMines stacked single screen method (Holub and Knutson, 1986), the EML separate single screens approach (Holub and Knutson, 1986), and the UI mobility analyzer (Kulju, et a l . , 1986) for various r e l a t i v e humidities and the presence or absence of added S0 . o
Results and Discussion A possible p a r t i c l e formation mechanism i s as follows: H0 2
S0 H 2
0
2
->H +
+
OH
0H ->S0
3
+
H
+ SOg -> H S0^ 2
The hydroxyl radicals produced by water molecule r a d i o l y s i s react promptly with oxidizable species such as S 0 i n a i r and form a condensed phase. These molecules further coagulate and become u l t r a f i n e p a r t i c l e s . The radon concentration i n the 2
27.
Induction of an Ultrafine Aerosol by Radon Radiolysis
CHU ET AL.
Compressed
air Exhaust
ν
Exhaust
Exhaust
Compressed air Room air
KEY /
Air pump
2 Moss 3
flowmeter
Radon test chamber
10
Exhaust
//
Dry radon source
radon
flowmeter
VI
Air control valve
4 Mixing chamber
V2
Chamber exhaust valve
5
Humidifier
V3
Chamber input valve
6
Aerosol generator
V4
Bypass valve
V5
Humidifier
V6
Chamber radon control valve
V7
Exhaust radon control valve
V8
Room air valve
7 Air filter θ
Chamber radon
flowmeter
9
Radon source control
Figure 1.
valve
Scheme of radon t e s t chamber.
369
RADON AND ITS DECAY PRODUCTS
370
chamber was kept at 1000 pCi/1. The experimental conditions were set to observe p a r t i c l e formation at both high and low humidities combined with high and low S 0 concentrations (Table I ) . However, i n the f i r s t set of experiments, the SO^ concentrations were only poorly controlled. We could detect only background condensation nuclei (1-80 /cm ) present when no additional S 0 was injected into the chamber at both high and low humidity. A large number of condensation nuclei were formed at about 10 ppm S0 concentration and 6% r e l a t i v e humidity. However, there was no increase i n condensation nuclei formed i n higher humidities. 2
3
2
2
Table I. Observed Condensation Nuclei Counts i n Different Atmospheric Conditions, September Experiments, 1985
S0
2
RH
Condensation Nuclei
background *\0% background 60% 10-20 ppm 6% ( r e f i l l butanol 10-20 ppm 60%
3
70 /cm 50 /cm 26000 /cm
3
3
CNC) 80
5
/cm
Figure 2 and Figure 3 show the size d i s t r i b u t i o n of these u l t r a f i n e p a r t i c l e s measured by separate single screens (EML), stacked single screen (USBM), and the mobility analyzer (UI). Since the mobility analyzer c o l l e c t s only charged p a r t i c l e s , the size d i s t r i b u t i o n derived from the mobility spectrum i s only f o r the charged p a r t i c l e s . There are some correlations among these three different methods at both high and low S 0 concentrations. At higher humidities, more neutralization occurs leading to fewer polonium ions that can be observed. Because of the uncertainty i n the S 0 concentrations i n these experiments, another set of experiments were performed i n March 1986 using only stacked single screen method. Similar combinations of S 0 concentrations and r e l a t i v e humidities were used (Table I I ) . The same lack of p a r t i c l e formation was observed when there were only a few condensation nuclei formed at 1.1 ppm S 0 concentration and 5&% r e l a t i v e humidities. In reviewing the system, we found that the design of the system was to recycle the sample gas f o r CNC back into the chamber. The TSI CNC uses butanol as i t s condensable f l u i d and the butanol i s a r a d i c a l scavenger that removes hydroxyl r a d i c a l rapidly enough to supress p a r t i c l e formation. When r a d i c a l scavengers such as butanol or n i t r i c oxide are present, a competing reaction occurs, i . e . 2
2
2
2
R0H +
OH ->R0
+ Ho 2
and consequently supresses the p a r t i c l e formation process. This reaction may explain the observation of p a r t i c l e s i n the e a r l i e r experiments when the butanol i n CNC was depleted, and after
27.
Induction of an Ultrafine Aerosol by Radon Radiolysis
CHU ET AL.
CN: 7 0 / c c S02: background RH: 10Z
CJ Œ
371
ο USBM Δ EML • UI
'>- 20
J 10 CE
ΙΕ0
IE2
IEI
2E2
PARTICLE DIAMETER (nm)
Figure 2. Size d i s t r i b u t i o n s measured with separated single screen method (EML), stacked single screen method (USBM), and mobility analyzer (UI).
CN: 2 6 0 0 0 / c c S02: 10-20 ppm RH: 6Z
ο USBM Δ EML •
UI
u Œ
or
u 10 CE
IE0
1EI
IE2
2E2
PARTICLE DIAMETER (nm)
Figure 3. Size d i s t r i b u t i o n s measured with separated single screen method (EML), stacked single screen method (USBM), and mobility analyzer (UI).
372
RADON AND ITS DECAY PRODUCTS
r e f i l l i n g butanol i n CNC, the p a r t i c l e counts dropped d r a s t i c a l l y (Table I ) .
Table I I . Observed Condensation Nuclei Counts i n Different Atmospheric Conditions
S 0
11 110 1.1 stop 11 110 1.1 11
2
R H
Condensation Nuclei
PPb 73% ppb 68% ppm 63% recycling CNC effluent 1 ppb 5% ppb
