-------
r
iii
. ABSTRACT
The influence of several variables on the backscattering of laser
light by aerosols was studied. He-Ne laser light was used to illuminate
aerosol particles generated by a Collison atomizer. The backscattering
of light by the particles (effective angle: 173.5°) was measured as a
function of relative humidity. Depolarization and field studies were
also made. Potassium chloride, sodium chloride, and sodium bromide
particles all scattered more than twice as much light after becoming
droplets. The change in phase occurred at lower humidities than those
appropriate for the bulk material. These salt particles depolarized
13 to 25% of the incident polarized light intensity when dry particles
and 6 to 12% of the incident light when droplets. The effect of
changing ambient relative humidity was also studied for methylene blue
dye and uranine dye particles. Depolarization measurements were also
made for polystyrene latex and dioctyl phthalate aerosols. These
gave depolarizations of 6% and 4% respectively. The laboratory device
which measured the backscattering from the aerosols was used in a field
!_ test along with a LIDAR device. The field test, while not conclusive,
indicated that relative humidity did afreet LIDAR measurements.
i;
L
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V
TABLE OF CONTENTS
Page No.
Abstract '• iii
List of Figures--- ix
List of Tables -- xiii
General Introduction^ 1
Section 1. The Effects of Humidity on Laser Light
Backscattering from Laboratory Aerosols 3
I. Introduction 3
II. Equipment 3
A. Aerosol Generation 3
B. Aerosol Transport 8
C. Humidity Control 10
D. .Humidity Measurement 13
E. Light Scattering Measurement 14
III. General Procedure 29
IV Results and Analysis — 31
A. Size Distributions 31
B. Dependence of Backscattering on Particle
Volume 44
C. Results of Subsidiary Tests- 49
D. Backscattering versus Relative Humidity r— 57
V. Discussion 64
VI. Conclusions 72
Section 2. Depolarization of Laser Light by
Laboratory Aerosols 7t
I. Introduction • 79
II. Experimental System 79
III. Procedure 86
IV. Results - 89
V. Discussion 93
VI. Conclusions 100
Section 3. Field Experiment 101
I. Introduction 101
II. Equipment - 101
III. Sampling Plan and Instrument Description 102
IV. Results and Analysis- 105
V. Conclusions 113
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vii
- TABLE OF CONTENTS (Continued)
* Page No.
1 , Appendix 1. Correction Factor Relative Humidity
J versus Temperature 119
r '
{ Appendix 2. Pressure Correction for Humidity 121
Appendix 3. Relative Humidities at which some Salts
Change Phase 123
Appendix 4. Estimate of Weighing Errors- 125
Appendix 5. Atmospheric Lidar Measurements 127
i:
i:
i:
i.
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ir.
LIST OF FIGURES
Figure No. Page Nq.
1.1 Aerosol Generation Set up 4
1.2 Schematic of Collison Atomizer-- — 5
1.3 Aerosol Modification and .Scattering
Measurement System— 9
1.4 Scattering Chamber, Model 1 - - 11
1.5 Humidifier 12
1.6 Angular Distribution of Scattered Intensity
Polarized Vertical (1) and Parallel (2)
to Observation Plane for Spheres of
m = 1.60. - - 26
1.7 Angular Distribution of Scattered Intensity
Polarized Vertical (1) and Parallel (2)
to Observation Plane for Spheres of
* 2,2. 27
1.8 Schematic of Bendix Electrostatic Sampler 33
'1.9 Cumulative Size Distribution for Aerosols
Generated from 1% v/v NaCl, 1% v/v KC1,
81 v/v NaCl Aqueous Solutions. 38
1.10 Backscattering versus Relative Humidity for
1% v/v Methylene Blue Aerosol 73 '
1.11 Backscatterinp versus Relative Humidity for
1% v/v Uranine Aerosol 74
1.12 Backscattering versus Relative Humidity for
1% v/v NaBr Aerosol 75
1.13 Backscattering versus Relative Humidity for
1% v/v KC1 Aerosol 76
1.14 Backscattering versus Relative Humidity for
1% v/v NaCl Aerosol 77
1.15 Backscattering versus Relative Humidity for
8% v/v NaCl Aerosol 78
2.1 Polarization Modification 81
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r
xi
I LIST OF FIGURES (Continued)
r
Figure No. Page No.
3,1 Equipment Configuration for Field Test 103
3,2 Laboratory Laser Backscattering Device
(Model 2, without polarizing filter). 104
3.3 Temperature, Humidity, and Light Scattering
on May 11, 1971 107
3.4 Temperature, Humidity, and Light Scattering
on May 12, 1971 108
3.5 Product of aerosol mass concentration times
aerosol backscattering cross section
versus relative humidity for May 11, 1971 109
3.6 Backseatter and Relative Humidity versus Time
of Day, May 11, 1971 Ill
3.7 Backscatter and Relative Humidity versus Time
of Day, May 12, 1971 112
A.I Product of aerosol mass concentration times a
aerosol backscattcr cross section versus time
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xiii
LIST OF TABLES
Table No. Page No.
1.1 Model 1 Optical Pc-sprnse ai; a Function of Perpend-
icular Distance frov Optical Receiver Plane and as
t Function of Scattering Angle. 17
;
1 1.2 Size Distribution;, for 1'4 v/v KC1 Aerosol 35
1.3 Size Distributions for 1% v/v NaCl Aerosol 36
1.4 Size Distributions for 8% v/v NaCl Aerosol 37
1.5 Size Parameters of Particles Formed by Nebulizing
Salt Solutions with a Collison Atomizer 40
1.6 Net Scattering Per Unit Particle Volume for
Several Aerosols £t Low Humidities (less 40% RH) 45
w
1.7 Average Dry Scattering per Average of the First,
Second and Third Powers of the Diameters 46
»
1.8 Backscattering and Forward Scattering for Several
" Aerosols at Different Humidities 55
i»
1.9 Sedimentation Losses for NaCl Particles in
r Scattering Chamber 70
1.10 Estimated Sedimentation Losses for NaCl Particles
in Chamber 70
r
t. 1.11 NaCl Particle Stopping Distance for Velocity of
700 cm/sec., 71
2.1 Optical Response as a Function of Scatterer
Distance from Plane of Light Pipe Face 84
2.2 Net Scattering Received as a Function of the Angle
of the Receiver Polarizer T 88
2 3 Ratios of the 90' Polarization Components to the
0° Polarization Components of the Net Scattering
Received - 91
2.4 Results of Secondary Scattering Tests 94
3.1 High Volane Sampler Results 106
A.I Atmospheric and Lidar Data for May 11, 1971 136
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r
FINAL REPORT
r
A Study of the Effects of Aerosol Properties
r
on Scattering of Laser Light
I Contract #CPA 70-103
»
General Introduction:
I The objective of this work was the study of the influence of
several variables on the backward scattering of laser light by aerosol
particles. Suspended particles are an important air pollution
problem in many locations . Devices which- illuminate these particles
and measure the light scattered by them are widely used to measure
I their concentrations. Ons promising method of obtaining information
on the spatial distribution of aerosol particles uses pulses of laser
] . light as "lidar" ("light direction and ranging"), which is analogous
.- to radar (Ligca. 19f5; Clemesha, et al,, 1967; Johnson, 1969).
Hygroscopic particles are present in the air (Twomey, 1954) and
j so are droplets formed by a variety of chemical interactions, so that
one of the areas of study ^n this work was an investigation of the
| relationship between changes in humidity and resultant changes in
backscattering for some hygroscopic as well as for some nonhygroscopic
'• parti:ulates. Such changes could lead to erroneous concentration
j measurements
It has been suggested that light scattered from aerosol particles
' could be distinguished from that scattered by gas molecules by the
degree to which the particles depolarized the light they scattered
(Clemesha, et al, , 1967) Another phase of this work was a study of
-------
I
Section I, The Effects of Humidity en Laser Light Backscattering from
Laboiatory Aerosols
j • I. Introduction
-• • i
This section of the report concerns the effects of changing
the humidity of several laboratory aerosols on their backscattering
\ of laser light. This work was done using a scattering chamber
designated "Model 1" which was later altered (into "Model 2")
for the depolarization studies.
II. Equipment
The experimental set-up had to make provisions for the
following: generation of the aerosol, transport of the aerosol,
control of the aeiosol humidity, measurement of the humidity,
illumination of the aerosol, measurement of the scattering of
light by the aerosol, and measurement of the aerosol size distri-
bution ,
A, Aeicsol Generation
The overall schematic of the aerosol generation system is given
in Figure 1.1, Compressed air was piped through glass fiber filters and
a drying column (silica geli so that when it reached the pressure
regulator it was clean and dry. The pressure regulator was set at
30 Ib/in 130 p.s.i g.) or two atmospheres above atmospheric,
The air was then divided between a Collison atomizer, which is
shown in Figure 1.2, and a tube containing a critical orifice- The
orifice'"-aintairsda dilution flow into the mixing chamber of
-------
Because humidity changes could induce changes in shape and composition
for some aerosols, depolarization was measured for humidities above 7
«'
and below the "critical" humidities, the relative humidities at which
phase changes occurred,
• '
To do the experimental work, a device was designed and built
which approximated the geometry used in lidar and yet could be used
in the confines of an indoor laboratory. !f lidar were to become a
major means for monitoring pollution, a relatively inexpensive laser
backseattering device might be useful in applications where the use
of the more expensive lidar would not be justified. It would be
desirable that the less expensive device, the one used in this work,
give readings easily related to those of lidar, and it would be
desirable that such readings be simply related to the commonly used
gravimetric analysis of air samples pulled through high volume filter
samplers The third section of this report describes a limited field
comparison of the laboratory device, a lidar installation, and a high
-------
ABSOLUTE
EXHAUST
DRY.NG
INDICATOR
nujp
DILUTION
ORIFICE
DRYING
COLUMN
COMPRESSED
A/R
\
MIXING
CHAMBER
COLLISON
ATOMIZER
FILTER
TRAPS
CONDENSER
ABSOLUTE
FILTER
TO AEROSOL MODIFICATION
AND SCATTERING MEASUREMENT
SYSTEM
-------
'•V
'V,
BLANK PAGE
<-. o
.
_^ :
r :
t
-------
-1*
-5-
COMPRESSEO
AIR
-SOLUTION OR-
SUSPENSION
AEROSOL
—.16 cm DIAMETER HOLE
—.034 cm DIAMETER HOLE
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- 6 -
rate occurred in tests lun with polystyrene latex particles, as
described below, The dilution air and t.he output from the atomizer
mixed in the mixing chamber (baffled) which had a volume of 71 x
103 cm3 (2.5 ft3).
The aerosol particles used in this work were generated by
a Collison atomizer (Figur.e 1.2), which was modified by removing
the baffle. Compressed air was forced into the
Collison inlet and made to travel at a high velocity across the
top of a tube immersed in the fluid containing material which was
to become an aerosol; the air flow created i pressure drop which
brought the fluid to the top of the tube, where the fluid was
fragmented by the air into droplets. The empirical equations of
Nukiyama and Tanasawa (Green and Lane, 1964) are available to
predict the mean droplet diameter of the aerosol formed by this air-
Llast atomizer. The Jropiets emerged in a spray. Sufficient distance
was provided to allow settling out of course droplets. Only fine
droplets were discharged through the atomizer outlet.
If the droplets contained a dissolved solute, then upon
evaporation, the diameter of the solid particles was
(r/0;D3 * O/61D3 ,
which becomes
D - CD, ,
-------
I
where
D = diameter of the solid particle, cm,
C = ratio of solute volume to volume of solvent plus solute
D, = original droplet diameter, cm,
Thus an 8% v/v (i.e. 8% by volume) solution nebulized with a mean
-4
droplet diameter of one micron (10 cm) would produce a mean
dry particle size of 0.43 micron
Because particles having diameters of about one-half micron
make the major contribution to light scattering in the atmosphere
(Pueschel and Noll, 1966), and because the particles generated
by a Collison atomizer in an earlier work (Cooper and Byers, 1970)
were generally smaller than this, attempts were made to increase the
mean, particle diameter in several ways. One way was to remove the
baffle which was originally in the Collison atomizer, thus
lengthening the distance the particles had to travel before
impact ion. Some larger droplets would thus have a
t
lower probability of impacting. Early in this
work it was noticed that aerosols generated from 8% v/v solutions
formed hea\/y deposits downstream from the dilution orifice in
the tee where the two air streams met. Subsequently, for 8% v/v
aerosols, the aerosol from the atomizer was routed past
the jet issuing from the dilution orifice before releasing
the aerosol into the mixing chamber.
' The result of removing the Collison atomizer baffle was an
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- 8 -
aerosol photometer (Royco Instruments, Model No. 202)
in the aerosol generated from a 1% v/'v NaCl solution. Avoiding
impacticu by the dilution orifice jet resulted in an increase of 30%
in the backscattering from the aerosols generated by 8% v/v NaCl solution,
but only 10% increase in backscattering from 1% v/v NaCl solution. The
rerouting of the aerosol stream was done only with 8% v/v solutions.
B. Aerosol Transport
The flow path of the aerosol after it left the mixing chamber
is diagrammed in Figure 1.3. After the aerosol left the mixing
chamber, it flowed a horizontal distance of 3 meters in plastic
tubing having a diameter of 5.1 cm, from which it was sampled
by means of a vertical 1.5 meter length of 2.5 cm diameter plastic
tubing and a horizontal 0,3 meter length of 0.64 cm diameter
rubber tubing. The aerosol then passed through 10 cm of 0.15 cm
diameter glass capillary tubing into a plastic container having
a volume of 8.5 x 10 cm From this holding chamber, used to
reduce concentration fluctuations, the aerosol flowed through
15 cm of 0.64 cm diameter rubber tubing into a mixing tee where
it was mixed with filtered air of controlled humidity. The diluted
aerosol then went through 45 cm of 0.64 cm diameter tubing to the rela-
tive humidity (IU1) chamber, where its humidity was measured. The
total flow rate at the RH chamber was 13 x 10 cm per minute
(0.46 ft /min) and the RH chamber volume was 17 x 10' cm ,
so that the Average residence time of a particle was 1.3 minutes
or 78 seconds. The diluted, humidity-controlled aerosol flowed
to the tee containing a thermometer through 15 cm of 0.64 cm
-------
AEROSOL FROM AEROSOL
GENERATION SYSTEM
'HERMOMETER
\/f
SCATTERING
CHAMBER
1 ABSOLUTE
i ] FILTER
^
RH 5
THERMOMET
o/
jf RH
1 !
/ \ L
/ \
JENSOR-y
ER\ /
\/ ,1
1 i r;
1 1
L 1
1
i
CHAMBER T'
HOLDING CHAMBE
-l3ir
^
A8S(
F/(
Q^
r~
i_.
RE HUMIDIFIER
:>LUTEn=HUM
.TER 'F/E
A1
MAGNEHELIC^ ^ t
VACUUM GAGE |
ASER
»•
g i JL
fx 1\ (H
1 FLOW- \ -^- -^
— — *— ' METER ,,-,,, tr
i — np/p"
R
^4^1
• L i
VALVE
O- _jf
R ~^i t VALVE
1 /
_^JJj [pjlFLOW/Mf TFR
-, L "^
\
» \ ROOM
VALVE A/R
Figure 1.3. Aerosol Modification and Scattering
Measurement System
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- 10 - I:
aerosol went into the scattering chamber via 15 cm of 1.3 cm diameter
rubber tubing bent into a single loop of 10 on .diameter. The
aerosol entered the cylindrical scattering chamber (Figure 1.4)
through the side; turbulence resulted in mixing which appeared thorough
to the eye ir. a test u?ing a dense smoke and a clear plastic end piece
on the scattering chamber. After the aerosol left the
scattering chamber it was caught on a filter, except when its
size distribution was being determined as described below.
C. Humidity Control
The filtered air going to the Collison atomizer and dilution
jet had a humidity of less than 10% RH (relative humidity), and
the aerosol which left the mix'ng chamber (with the exception
of the polystyrene aerosols) had a humidity under 20% RH. Before
going to the RH chamber, the aerosol was joined by a flow of
filtered air of controlled humidity. The dry fraction of
the air was obtained by pulling room air through a dessicator
(containing Linde Type 5A molecular seive material) and the
moist fraction was obtained by drawing room air through the
humidifier (Figure 1.5) The combination of wet and dry air
passed through an absolute filter (Gelman glass fiber, Type E)
before being added to the aerosol. The total flow of the
humidity-controlled dilution air was monitored with a rotameter
and kept constant at 11 x 10 cm /min (0,183 x 10 cm /sec or
0-38 ft /nun). As described, the set-up gave final aerosol
{.-aridities ranging .Crom 10% RH to 85% RH . Humidities above 85% RH
were obtained by pasting the aerosol over the surface of the
water in a one-pint (0,5 x. iO' cm ) Mason jar one-quarter filled
-------
. J
I po.i^em
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- 12 -
WATER ^
O/\TM
Ofti n
«< —
_r*_^*_. ^^j-uj-tj-v^-^
^ZZZZZZZZ.
SSSSSd
..
rl
*-» ,
. —
IIL
1
. —
^~
***
•^mf
W
r ^*
— AIK
S^^iw^M^Sv
_
I-ta-._->^-«—
"^^-^^^"
L/
f
1 WATER
iPL/MP
4.
HUMIDIFICATION SET-UP
-------
A
*
' •' • - D. Humidity Measurement
The humidity of the diluted aerosol was measured in the RH
; chamber, which was made from a metal canister about 30 cm in diameter
", and 30 cm high, The aerosol entered the RH chamber from the bottom,
mixed in the chamber for an aveiage of 1.3 minutes and exited from the
chamber near the top, at measured and controlled temperature and
humidity. The humidity sensors were-glass coils impregnated with
various hygroscopic salts; the conductivities of the coils are functions
of relative humidity (the system was a Model 15-3001 Hygrometer
Indicator and Model 4-4824 Hygrosensors from Hygrodynamics, Inc.,
Silver Springs, Maryland), The sensors are rated to respond to
65% of an instantaneous change in humidity within 3 seconds and are
accurate to within ±1.5% RH according to the manufacturer.
Sensors were intercompared at the limits of their ranges and no
discrepancy with the claimed accuracy was found; the same was true
•*
(to within the psychrometer accuracy available) for comparisons with
'*
a sling psychrometer for several of the sensors; all were not tested
*> against the psychrometer. The sensors' accuracy was not dependent
F on air flow rate, according to the manufacturer, which is a major
advantage in comparison with the wet-bulb/dry-bulb thermometry
I method of determining relative humidities
The amount of water vapor air will hold when saturated is a
|. function of temperature; the relative humidity, the ratio
• of the water vapor present to the saturation amount, is also tempera-
ture-dependent. The use of two thermometers, one in the RH chamber
I and one in the inlet to the scattering chamber, allowed for a
correction for temperature change in the fashion suggested by
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- 14 -
were around a few tenths of a degree centigrade. Appendix 2
demonstrates that pressure corrections to the humdity were
negligible.
E. Light Scattering Measurement
The illumination of the aerosol and the measurement of its
scattering of the incident light was done with the scattering
chamber (Model 1) shown in Figure 1.4.
The Model 1 scattering chamber is a long cylinder lined
throughout with black velvety material to absorb light. It
is light-tight .and air-tight. The laser beam enters through
a window made from a microscope slide cover glass, and the beam
ends in a light horn made of bent glass tubing painted outside
with optical black paint, Optical stops determine how much
of the beam can be viewed by the light pipe, which transmits
light to the photomultiplier tube (RCA 7265, with 2000 volts
applied) . Ths details of the optical geometry are given in
Figure 1 4, and, in a latter part of the manuscript, a description
is included of a test which was made to determine the response
of the optical system to a target placed at various positions
along the beam.
The light was provided by a He-Ne laser (Quantum Physics
Model LS-32). The laser light was polarized with its electric
vector vertical, i e,, being perpendicular to the scattering
plane. The nominal rating of this laser is 3 milliwatts.
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- IS -
0 -10
multimode operation at the wavelength 6328 A (10 m). The constancy
of the laser output was monitored with a General Electric X-6
Photoconductive Cell, preceded by a translucent light diffuser:
i 3
/ the response of this cell, in the range used (10 x 10 ohm), is
related to the laser power output to an exponent between 1/2 and 1
(estimated from the calibration curves supplied). The measurements
reported here were made for a laser output range of 10.3 to 11.6 x 10
ohms, with the usual value near 11.0 x 10 and with changes rarely
as much as 0.1 x 10 between wet and dry scattering measurement.
Corrections were not made for these changes, which were a few
percent or less. Because the transmittances of the front and rear
mirrors of the He-Ne lasei were 99% and 99.9% respectively, the
photoccnductive cell located at the rear should have received one-tenth
the output of the laser constantly; in other words, any major changes
in output power other than those due to contamination of che mirrors
would have been noticed. The laser drew its power from a constant
voltage transformer (Sola Model CVS).
The light pipe is 0.32 cm (1/8 inch) in diameter and 30 cm (12
inches) long. The manufacturers (American Optical Co-) state that it
will accept 70% of the light incident upon it and transmit wave-
o •
lengths from 4,000 to 20,000 A, with a loss of 50% every 210 cm (7 feet).
A fairly large chamber was used to reduce the amount of light reflected
from the walls into the light pipe, which was shielded from the
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- 16 -
The light which reaches the light pipe is transmitted to the
RCA 7265 photomultiplier tube, which is in use in at least one
co-ranercial lidar. The tube is housed in a light-tight case.
The light pipe touches the center of the sensitive face of the
photomultiplier tube. The photomultiplier tube was chosen for
low dark current (<0.3 x 10" amps at our operating conditions).
The power was supplied to the tube by a voltage supply (Keithley
Model 246) which allowed adjustment to the precision of 1 volt.
The resulting current was read by an electrometer (Keithley
-14
Model 610 CR), which had a range that extended down to 10 amps.
The output of the electrometer was recorded on a strip chart
recorder.
Since light scattering from particles varies with the angle
between the directions of incidence and scattering, it is useful
to know the response of the scattering chamber to scattering
from various positions (thus various angles) along the laser beam.
This response was measured and the measurements used to compute
the effective scattering angle of the scattering chamber geometry.
To obtain the response of the receiving optics as a function of
scatterer distance from the light pipe face, and thus as a
function of scattering angle, a diffuse reflector (0.45 micron
white Millipore membrane filter) was moved along the path of
the laser beam, with its face perpendicular to the beam. The
laser light was attenuated by two filters having a total
attenuation of about 2 x 10* Table 1.. 1 gives results of these
measurements -
li
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- 17 -
Table 1.1
Model 1 Optical Response as a Function of Perpendicular Distance
From Optical Receiver Plane and as Function of Scattering Angle.
Perpendicular Distance
from Light Pipe Scattering Net
Face Plane Angle Scattering
(cm) (Degrees) (10"a A)
5.0 161.2 0.00
5.25 ' 162.0 0.255
5.5 162.8 0.288
5.6 163.1 0.592
5.7 163.4 0.97
5.8 163.7 1.25
5.9 163.9 1.40
6.0 164,2 1.50
6,25 164.8 1.46
6.5 165.3 . 1.33
6.. 75 165-8 1.25
7.0 166.3 1.19
7.5 167.2 1.04
8.0 168 0 0.932
8.5 168.7 0.847
9.,0 169.3 0.763
10.0 • 170,4 0.624
11,0 171.2 0.522
12.0 171..9 0.452
13,0 172.5 0.382
14,0 173.1 0.327
16.0 173.9 0.262
18.0 174,6 0.203
20,0 17-5,1 0.151
22.0 175.6 0,116
24 0 i75.9 0.084
26.0 176.2 0.057
28.0 176.5 0.044
340 177.1 0.017
39,0 177.5 0.008
44.0 . 177.8 0.006
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- 18 -
Tho calculation of the effective scattering angle gives the
result that if the particles scattered diffusely (Lambert's law)
near 180° , the amount of light measured in the scattering chamber
i would be equal to what would have been measured if all the particles
i '
\ ' had been placed 15.0 cm (perpendicular distance) from the light
\
pipe face. This corresponds to an effective scattering angle of
173.5°. Half the light scattered by diffusely scattering particles
would have come from positions corresponding to angles greater
than the median scattering angle of 1703. As noted below, the
1 difficulty of making exact calculations for expected scattering
| lessens the theoretical importance of having measured at an angle
/ less than 130°. It is also shown that the experimental scattering
i
angle may be greater than 173.5°, because of the angular distri-
bution of scattering from particles similar to these.
" •' The"efftctive scattering angle" is the angle corresponding to
the position along the beam at which, if all particles were placed
there, the received scattering intensity would equal the total
I
; intensity received from the sane number of diffusely scattering
I par' '.cles when spread out homogeneously along the beam path.
For this analysis, the following definitions hold:
T ffll - (flux scattered per unit solid angle in direction 9)
1 ~ (flux geometrically incident on object)
ua = projected area of particle in beam direction
. Q(z) = solid angle subtended by optical receiver as
a function of scatterer position along beam
F = flux incident on object
f = flux incident on optical receiver
-------
7
- 19 -
n(z) = particle concentration (number/volume)
N » total number of illuminated particles viewed
dz
OPTICAL STOPS
-LIGHT PIPE
tan 6 = (B/z)
6 = tan"1 (B/z)
Actually, a ray diagram for the light reach-ing the light
pipe would be more complicated, but these complications are included
in the experimental evaluation of Q(z)
Note that if
dN = differential number of illuminated particles viewed, then
dN = n(z) A(z) dz.
In what follows, n(z) and A(z) are assumed constant—that is,
homogeneous mixing and negligible beam divergence are assumed.
The total flux reaching the receiver is
f = f F Ij(z) -QCz) dN
where : and ic are the Limits of the viewed portion of the beam.
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- 20 -
Then,
f * j F IjCz) QU)
n(z) A(r) dz
j I
f = F n A Iz) Q(z) dz
zo
with the additional assumption that F is constant (no appreciable
beam attenuation).
The average scattered flux received per particle would be
f/N = [n A (zf-zo)]'1 f
zf
= F (zf'Zorl j ^(z) M(z) dz
zo
The same scattering would result from the N particles if they
were placed so that
z.
r
rV"1 J h<
FN I,Q = F N (Zf-zJ j I,(z) Q(z) dz
Zo
It is important to note that we are working with an average over
the particles, which is an average with respect to dN and thus dz,
rather than with averages weighted by d8. I Q is the ensemble average
of I.Q.
— '-1 ff '
I.Q « (Zf~Z0) I ^C2) Q(Z) dz
>
Zo
For a monodisperse aerosol we would expect to find some position z*
along the beam for which
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- 21 -
C
and this would define the effective scattering angle, 6* = tan"
(B/z*). Because the function I,(z) is a complicated funciton of
position (that is, of scattering angle) as well as particle index
j of refraction and the ratio of particle circumference to the light's
i
wavelength, the approximation is made which would hold if the sum
. of the I,(z) contributions from the polydisperse particles changes
much less rapidly than does the function Q(z), that is, we assume
1 ff
j (ZfV j
Q(z) dz
z
and get the effective position z from
zf
Q = Q(ze) = tZf-V1 j dz
zo
Th'iS defines z , which in turn gives the estimated effective
scatteiing angle 6 ,
6e -- tan"1 (B/ze)
The data in Table 1.1 come from measurements made with a Lambert's
law (I,(9j" cos o) reflector whose l.(z) is a much more nearly
constant function of z than is QU)> which varies roughly as the
inverse square of the distance from the position of dz to the
center cf the light pipe. Equal di correspond to equal dN,
thuL, the data in Table 1,1 as weighted by distance along beam
(not by angle) give 6 - 173.5;.
The flux received from the reflector is f = Fl,Q(z) which is
integrated to give
j f dz = j F IjQCz) .dz . Uf'V J (z)»
-------
- 22 -
so that
I (re) * F Ijftz,) . .
Therefore, the z corresponding to f is that corresponding to Q.
The computation for f consists of taking f(?) at each interval
times the Width of that interval and dividing the sum of these
contributions by the effective beam length, because Az is
proportional to AN.
The light scattered back from a particular volume of aerosol
will be proportional to the number of particles illuminated and
related in some way to the size of the particles and to their
composition. Particles which contribute most significantly to
natural light scattering in the atmosphere are those of about
_4
one-half micron (0.5 x 10 cm) in diameter (Pueschel and Noll, 1966).
Unfortunately, in this size region, where the particle diameter
is of the same order of magnitude as the wavelength of the
incident light, the theoretical analysis of light scattering is
quite complicated, involving the use of the Mie scattering equations,
whose solutions are infinite sums of Riccatti-Bessel functions.
Although there are tabulated solutions of the Mie equations for
many combinations of index of refraction and ratio of particle
diameter to lifht wavelc-ngth (Davies, 1966), theoretical
solutions for practical situations are difficult to calculate
for a number of reasons.
\
-------
- 23 - x
\
One complication in the use of Mie functions is that natural
aerosols are not one site but "polydisperse", i .e,.composed of
particles of many sizes. Estimates based upon approximations of the
size distribution as a sum of man/ monodisperse (single-size)
contributions are required
. Another problem in applying the exact theory is that the Mie
solutions have been tabulated almost exclusively for spherical
particles, but cannot be computed piaccically for irregular particles.
There is some disagreement about how different the results are for
particles which are only roughly spherical (Davies, 1964; Huffman
et al., 1969) ,
Finally, although much experience and data have been accumulated
about light scattering in the forward direction or at right angles
to the incident beam, ver> little work has been done on backscatter-
ing, which in this size range is roughly 1 or 2 orders of magnitude
less than forward scattering.
The behavioi o:c light -which impinges on a particle is determined
by the ratio of the particle circumference to the wavelength of
the light ( a* >-D ,'\), by the complex index of refraction of the
particle, which takes into account refractive index and absorptivity,
and by the polarization of light. When the wavelength of the light
• \
is either quite large or quite small compared to the -particle
circumference, several simplifying assumptions can be made, In
the intermediate regime, t\\:- full "Mie" theory must be used
/
-------
• 24 -
Rayleigh regime, for a'0.3: In the Rayleigh regime, the amount of
light scattered is proportional to the sixth power of the diameter and
I1''"", inversely proportional to the fourth power of the wavelength. Tins regime
includes gas molecules and Aitken nuclei (D <0 04 micron;,) and
-.-~^ is responsible for the blue of the sky and the red of sunsets.
-"•"'" The portion of light which is polarized perpendicular to the plane
of incidence and observation (I.) has no angular dependent
' • Rayleigh scattering; that which is polarized parallel to the plane
i'
/' of incidence and observation (I2) has a cosine squared dependence;
i"'.:-._•;' their sum for unpolarized incident light is (Kerker, 1969):
I2)/2
where:
1 - intensity of light received in
direction 3/unit illumination
D « particle diameter
P
m -• index of refraction of particle
\ * wavelength of light
R - distance from particle to observer
Mie regime, for 0 ? •'* 3/(m-l>; In this regime the scatter -
ing pattern loses its symmetry and becomes quite complicated.
Scattering theory computations must be used, somewhat simplified
by the use of tables which are available for a variety of indices
of refraction and circumference-to-wavelength ratios, The Mie
function does not increase or decrease monotonically with increasing
•V
lm-fr
8RV
m - 1
z
-------
>;i • .-».
•;r
•.i
particle size for a given wavelength, making monochromatic light-
scattering measurements of a variety of particle sizes very difficult
to interpret. .
Mie equations for irregular particles would seem hopelessly
difficult to solve. Although the detailed angular pattern may be
considerably different from that of spheres, the total scattering from
particles having some measure of size (e,g,; area, volume) in common
with a sphere of a given diameter of the same index of refraction
should be of the same order of magnitude as that of the sphere (Davies, 1966),
It is instructive at this point to examine some .scattering diagrams,
Figures 1.6 and 1 7 extracted f rcii the work of Pueschel and Rossano (1966).
. '
The mass mean diameter of the aerosols generated from 1% v/v solutions
was 0,2 microns; the mass median diameter was
0.6 microns; the mass-mean diameter for the 8% v/v NaCl aerosol
was 0.43 microns and the mass median diameter was 1.5 microns (see size
distribution data and analysis in the Results part of this section);
Theie diameters correspond to scattering parameters (i? of approximately
1, 3, '2, and ',5 respectively. The index of refraction of KCi is
1,49 aad for NaCl, 1-54 It is ?een from TLgurcs 1 6 ;md 1 7 that for
monodisperse spherical aerosols near these si:es, and hawng
an index of refraction near these (m = 1,6), the .scattering generally
does increase toward 180'; thus it is likely that the experimental
scattering angle is greater than 173..5', because scattering from
particle; near the back end of the chamber should be weighted more
heavily than is shown abo-e.
-------
- 26 -
a* 2.0
10
0 20 40 60 CO ICO 120 MO ICO CO
a • 4.0
»H
_1 L _1 ' ! 1 . I I
.0 CO CO CO 120 I-.O U) CO
a • 3.0
\J -0-*
0 20 40 60 10 100 120 140 130 ISO
a • 5.0
O 20 40 60 CO ICO 120 KO I6O ISO
?igure 1-6. Angular Distribution of Scattered Intensity
Polarized Vertical (1) and Parallel (2) to
Observation Plane for Spheres of m = 1..60.
-------
m • 2.0
I I I I I.I I l"->4~ !_ '
'<£ sO CO U) ICO Itf
0 20 40 pO 60 100 120 140 ISO. ISO
m • 3.0
r.i I ! I I I i ' i s/i i it ill i i '
O iO ^ «0 i>G luO J20 143 ISO 1*0
0 20 4O 60 SO 100 120 140 160 ISO
Figure 1.7. Angular Distribution of Scattered Intensity
Polarized Vertical (1) and Parallel (2) to
Observation Plane for Spheres of a = 2,2,
-------
/
/
ft - 28 -
Thus, although the scattering angle has a range of 161° to 178*,
only a small percentage of the particles in the beam are in the
first few centimeters and the estimated effective scattering
angle is 173.5°, and the median angle is 170°, for isotropic
scatterers. If I. (flux of light of vertical polarization scattered
into angle 6 per unit incident light flux) increases from 160°
to 180' (see Figures 16 and 1.7), then the larger angles are
weighted more heavily than in the above estimate, and the
effective scattering angle is even closer to 180°.
Other workers have measured the scattering of nearly
monochromatic light by laboratory aerosols, but at effective
scattering angles less than 170°. Eiden (1966) measured the
scattering of polarized light from a xenon lamp by atmospheric
aerosols for scattering angles between 50° and 160° from the
forward direction. (Matching the scattering angular distribution
with those computed from several natural aerosol models, he concluded
that the index of refraction for the dry atmospheric aerosols he
measured in Mainz, Germany, was 1,50 and for the moist aerosols
1.441) Setlzer (1969) measured the scattering from aerosols as
a function of angle in one-degree steps from 12s to 146* from the
forward direction, usiny a He-Ne laser light source and water
aerosols with median diameters of 5 to 10 micron. Huffman and
Thursby (1969) measured scattering as a functibn of angle from
10' to 150C with unpolanzed green light incident on irregular
particles. Puoschel and Rossano (1966) measured the angular
distribution from 20° to 130° of the scattering of polarized
monochromatic light from monodisperse polystyrene latex aerosols
-------
- 29 - '
.} They found discrepancies between the measurements and Mie scattering
• ; . theory which they suggest could be due to anisotropy in the latex
: I
•'. ' particles, suggested also by Maron, et al.(1963). Phillips, et al.
(1970; measured the scattering of polarized light (argon laser at
)
i 0.514 microns) from a single polystyrene latex sphere for angles
from about 10' to about lW and found that they got excellent
agreement with the Mie theory predictions for polystyrene latex
by using an index of refraction of 1.59 i 0,01 z.nd a diameter
-4
of 1.200 t 0,01 x 10 cm instead of the diameter given by the
manufacturer (1.099 i 0.006 x 10" cm).
Ill General Procedure
Aerosol generation: A volume of 550-600 cm or solution to
be nebulized was placed in the Collison atomizer jar, and the
atomizer head was screwed into place. The clean, dry air was
Jurned on. The magnetic itirrer was turned on and adjusted so
that there was turbulent mixjng in the atomizer jar, but only a
shallow vortex formed, so as not to pull the free surface below
the liquid intake of the nebulizing head. A filter was placed in the
i •
•••' filter holder following the scattering chamber; the filter was a 0.45
micron membrane filter (Miilipoie, for the microscope counts of the
polystyrene or a glass fibtr absolute filter (Gelman Type E) for the
weight determinati ns (for which determinations the filter was
' desiccated for at le?;st 12 hours and preweighed) Then the pump
i i
t
j at the end of the fiow train was turned on and sampling begun, with
»
: ' i
i j the ilows adjusted to gixi 9.5"H_0 pressure drop across the capillary
* 3 "^
• tube--this corresponded to a volumetric flew of 42 cm /sec (0 09 ft /min)
! . from the mixing chamber aercsol and a flow of 170 cm /sec (0 36 ft /min)
i '
-------
- 30 -
' •
Humidity control and measurement: By changing the ratio of
wet air to dry air in the filtered air used to dilute the aerosol
before entering the RH chamber, the humidity in the resulting mixture
could be controlled. Humidity readings were considered stabilized
after the rate of change in humidity was below about 1/2% RH per
minute. The temperature in the RH chamber was measured
with a thermometer (Thomas Model 9534) which extended into the
chamber to the level of the middle of the RH sensors.
The temperature of the mixture immediately preceding the scattering
chamber was measured with the same kind of thermometer, which had
found to read identically with the first to less than 0.1°C difference.
Corrections for the temperature dependence of relative humidity
were made, although these temperature differences were usually
0.23C or less. To improve the ability to reset the correct dilution
ratio, reference marks were made which lowered parallax errors
involved in reading the flow meter and pressure gauge.
Light scattering measurement: Before taking any readings,
the photomultiplier tube and the laser were given at least an
hour's warm-up by which time the photomultiplier dark current
had stabilized, as had the laser output. Initial readings
would be made on the "dry" conditions, that is at a relative humidity
of less than 20%. Then the humidity was raised in steps and the
light scattering measured, the net scattering being the
difference between these readings and the readings given by
filtered air (which showed a generally negligible increase in
scattering with increased humidity). Ten to fifteen minutes
-------
- 31 -
and generally two such "wet readings were taken and then a "dry"
reading was made to re-establish the baseline. If the
humidity dependence showed it was warranted, the tests were ac-
celerated by taking "dry" as being less than 40% RH rather than
20% RH. For the highest humidity tests, up to half an hour was required
to take the final humidity readings, in order to extend the upper
range of relative humidities studied. If no changes (such as
adding more fluid to the Collison) had been made in the test
set-up during the run, the "d-y" measurements before and after the "wet"
measurements were used and interpolations were made to obtain the best
estimate of the "dry" scattering; otherwise, the
"dry" test nearest in time to the "wet" test was used. The ratios of
wet to dry scattering are listed as "Scattering Ratios," meaning
the ratio of the scattering at an elevated humidity to that at a
low humidity,
IV, Results and Analysis
A, Size Distributions
The properties of aerosol particles are strongly size-dependent,
in general, so that it is useful to characterize the oizs of the
aerosols produced by the Collison atomizer. Usually such characteri-
zation is done by presenting size distribution data such as the
number distribution, which shows the percentages of the total number
of aerosol's particles having diameters which fall in a series of
-size intt-ivals. Another way of presenting size data i;: through the
mass distributions the percentages of the total mass of the aerosol
due to particles having diameters falling within a series of size
-------
- 32 -
distribution, the percentage of the total aerosol mass due to
particles of a certain diameter or smaller, plotted against
diameter, To ascertain the size distribution of the aerosol
produced, the particles were collected on aluminum fc-1 using a
Bendix electrostatic sampler. Parts of the foil were then cut
out and photographed in an electron microscope.
The Bendix sampler is shown schematically in Figure 1.8. it
was modified so that it could be used in the flow series; normally,
a small blower in the sampler pulls air samples through the instrument
at atmospheric pressure, but in this case the sampler was made air tight
and a pump and flow meter were connected to the exhaust so that air
flowed into the sampler through its inlet. The aerosol was
drawn into the sampling cylinder, which has a wire along its
axis raised to a potential of ten thousand volts, The unipolar
corona discharge from the central wire charged the particles as
they passed and the voltage gradient forced the charged particles
onto the foil which lined the inside of the cylinder, Half-hour
samples of the 1% v/v aerosols and ten-minute samples of the 8% v/v
aerosols gave a clearly visible deposit/ which ran in a band from
2 1/2 to 10 or 12 cm from the inlet edge. Samples for the electron
microscope were taken from positions 2, 4, 6, 8, 10 and 12 cr< from
the leading edge of the foil for the 1% v/v aerosol (also 14 cm
for the 8% NaCl) ,
A sample of the aerosol (at humidity less than 30% RH) was
collected on aluminum foil Electron ...icroscope pictures of the
samples on the foil were taken at a lO.OOOx magnification and
siiing was done by comparing the area of the particles to the
-------
- 33 -
SAMPLE
INLET
CHARGED AXIAL WIRE
AND
FLOW-
METER
D.C,
VOLTMETEF
/
D. C.
VOLTAGE
SUPPLY
•W^H
SAMPLE
EXHAUST
-------
- 34 -
the essential size information is given in Tables 1,2 to 1.4. The
size distributions given in these tables were obtained by counting
techniques, rather than by weighing techniques, but these have been
converted to mass distributions for the graphical presentation, since light
scattering is more closely related to particle volume (hence, mass)
than to particle diameter to the first power (Charlson et al .,1967;
Cooper and Byers, 1970; Noll et al., 1968). Because sizing small
particles by optical comparison is tedious and expensive, we decided
to use one sizing of an aerosol generated from an 8% v/v NaCl solution
and one each sizing of two different aerosols from 1% v/v solutions
(KC1, NaCl) were used to characterize the output from the nebulizer.
The cumulative mass distributions are plotted on log-normal
probability paper in Figure 1,9,
The smallest particles are least accurately sized. Those
particles listed as having diameters around 0.05microns are
actually all those that small or smaller and still visible; more
particles were not seen v»hen the magnification was doubled so that
the total number count i-.; '•><••'ieved to be nearly correct. The
number mean diameters, heavily influenced by the size inaccuracies
for the smallest particles, are likely to be less accurate than the mass
mean and mass median diameters, which are more strongly influenced
by the larger particles (\vhose si:es were determined more accurately).
Two sets of size distributions are listed in Table 1 5. The
first set is the result of this work, the second set is derived
from the work of Ccopei' and Byers (1970) Although both investi-
gations used Collison atomizers, in this work the atomizer was
-------
Particle
. Diameter
D (10~4 cm)
P
0.050
0 100
0.140
0 200
0.280
o. too
0.560
0.760
1.100
1,540
2 160
Number of Particles
ii. Si;e Interval
Centered on U
7682.
1612.
1340
962
805
722.
339.
•106.
47.
9.
2.
Fraction
by Number
in Interval
0.5613
0.1178
0.0979
0 0703
0.0o32
0.0528
0.0248
0.0077
0.0034
0.0007
0.0001
Cumulative
Number
Fraction
0.5613
0.6791
0.7770
0.8473
0.9105
0,9632
0,9880
0.9958
0.9992
0 . 9999 .
1.0000
Cumulative
Mass
Fraction
0.0032
0.0085
0.0208
0.0463
0.1094
0.2630
0.4608
0.6154
0.8232
0.9324 ,
0.9994
w
VI
Total: 13686.
-------
o.
Particle
Diameter
D (10~4 cm)
0,050
0,100
0.140
0.200
0,280
0.400 .
0.560
0.760
1.100
Number of Particles
in Size Interval
Centered on D
8611.
817.
393.
247,
216.
155,
73,
30.
13.
Fraction
by Number
in Interval
0,8158
0.0774
0.0372
0.0234
0.0205
0.0147
0 0069
0 0028
0.0012
Cumulative
Number
Fraction
0.8158
0.8932
0.9305
0 9539
0.9713
0.9S90
0.9959
0.9988
1.0000
Cumulat ive
Mass
Fraction
0.017]
0 0301
0.0472
0.0786
0.1539
0 3115
0.5152
0 7244
0..9993
Total:
10555.
Table 1.3 Size Distributions for 11 v/v NaCl Aerosol
-------
Particle
Diameter
D (10~4 cm)
0,050
0.100
0.140
0 200
0.280
0,400
0,560
0.760
1,110
1.540
2.160
3.020
4.260
Number of Particl*
in Size Interval
Centered on 1)
P
7486.
1194.
1126
527,
646.
579.
301. :
155.
81.
65.
15.
2.
3.
Fraction
by Number
in Interval
0.6146
0.0980
0.0924
0.0433
0 , 0530
0.0*75
0,0247
0.0127
0.0066
0,0053
0-0012
0.0002
0.0002
Cumulative
Number
Fraction
0.6146
0 ,7126
0.8051
. 0,8484
0.9014
0.9490
0,9736
0 , 9864
0,9930
0 , 9984
0.9996
0 . 9998
1,0000
Cumulative
Mass
Fraction
0.0010
0.0022
0.0054
0 . 0098
0.0245
0.0628
0.1176
0.1881
0.2S98
0.5437
0.7022 '
.0.7593
0.9995
11
-J
Total: 12180.
-------
- 38 -
•r"__
ex
Q
VI
Q:
UJ
2
Q
o
UJ
_j
o
h-
o:
Q.
CD
o
UJ
99
o:
r~~
O
o
^o
^O
^j
g
K
o5
99.9
99
98
95
90
80
70
60
50
40
30
20
10
05
02
01
0.2
O.I
A 1% v/v NaCl
D 1% v/v KCl
O 8% v/v ISfoCl
D
D
A °
A o
G
-
—
- A o
D
A °
D O
O
A a
_ A a
-A Q o
O
- D o
9 i i i i i i i i i i i i 1 1 _i 1
05 .07 O.I 0.2 0.3 0.5 0.7 1.0
PARTICLE DIAMETER, Dp, ( I0"4cm)
Figure 19. Cumulat.ue Size Distribution for Aerosols
Generated from 1% v/v NaCl, 1% v/v KCl,
8% v/v NaCl Aqueous Solutions,
-------
- 39 -
not routed past the dilution orifice jet, which may have led to a
preferential loss of the larger particles; finally, the flow paths
in the two experiments were different. Conparison of the size of
the aerosols in Table 1.5 lead: to the following conclusions:
1. The particle sizes inurr-as-sil in the order: 1% v/v NaCl,
1% v/v KC1, 8% v/v NaCl.
«
2. The number mean dipjncler.'j :, n the Cooper and Byers (1970)
aerosols were consistently larger.
3. The mass mean diameters in the Cooper and Byers (1970)
aerosols were larger for the 1% v/v aerosols and smaller
for the 8% v/v NaCl aerosol.
4. The geometric standard deviation of the Cooper and Byers
(1970) aerosols uere larger in two of three cases.
5. The mass median diameters of the 1% v/v NaCl and 8%
v/v NaCl aerosols'were approximately 0.5 microns and
1,5 microns lespectively in both experiments.
The aerosols sizing done in Cooper and Byers (1970) used 0.1
microns as the smallest size classification; this work used
0.05 microns thus, it is not surprising that the number means
reported htre are smaller, and '.hey are probably more accurate.
Also, more than 10,000 particles were counted for each size determina-
tion in this work in contrast to the approximately 2,000 counted
per aerosol :n the former. Since the percentage uncertainty of the
count is inversely proportional to the square root of the number
-------
Aerosols Used in This Work
Aerosols Used in Cooper and Byers (1970)
Number Mass Mass Number Mass . Mass
Mean Mean Median Geometric Mean Mear. Median Geometric
Solution Diameter Diameter Diameter Standard Diameter Diameter Diameter Standard
Nebulized (microns) (microns) (microns) Deviation (microns) (microns) (nucronsl Deviation
8% v/v NaCl 0.14
0.43
1.5
2.3
0.21
0.36.
1,54
1% v/v NaCl 0.08
0.18
0.55
2.2
0.16
0.22
0 45
2-0
1% v/v KC1 0.13
0.28*
0.62
2,1
0.20
0 32
1.26
Table 1-5. Size Parameters of Parti-cles Formed By Nebulizing
-------
T
- 41 -
determination herein should be less. All other things being equal,
the particle diameters should vary with the cube root of the dissolved
solids' volume fraction. Thus,the particles from the two 1% solutions
should have equal size distributions upon being nebulized. The
difference in size of the 1% v/v N'aCl and KC1 aerosols is unexpected.
Whitby and Peterson (1965) also produced aerosols from aqueous
2
solutions in a Collison atomizer operated at a pressure of 40 Ib/in ,
with an impactor which removed original droplets having diameters
larger than 2 microns. Their measurements foi 1% w/w (one percent
by weight) uranine dye aerosol as analyzed by electron microscopy
gave a number median diameter of 0.054 microns, a mass median of
0.103 microns and a geometric standard deviation of 1.41. Their
impactor would have eliminated droplets which would have become
dry particles 0,43 microns and larger and their concentration
corresponds to 0.7% v/v uranine for uranine particles having a
density about 1,3 to 1.5 gin/cm (Sehmel, 1967), which would lead
to a particle-size (0,7) =90% of the 1% v/v aerosol size. From
the number median of Whuby and Peterson and their geometric standard
deviation one can, assuming a log-normal distribution, compute the
number mean and mass mean diameters of their aerosol, using
equations available in the literature (Herdan, 1960; Lapple, 1968).
The concentration difference will contribute (0,7) to the
number mean and (0.7) to the mass mean differences.
Calculations from the data of Whitby and Peterson (1965), using
iormulas, for relationships between log-noimal parameters from Lapple
(1968), show an inconsistency between the number median diameter
(D in Lapple's notation), the mass median diameter (D ) and the
-------
I
I
- 42 -
D = D exp [(x-y) In o ],
mx my r l ' '
gi ill;
Dm3 " Dmo GXp U3-0)(lnl.4i)2]
Their measured mass median was 0.103 microns; that from the equation
above is 0.076 microns. .This deviation may be caused by approximating
as log-normal a distribution which is not quite log-normal. Their
number mean (based on D and o) is computed to have buen 0.057
mo
microns Their mass mean (based on 0 , and a) is 0.097 microns
(which should be raided by factor 1/(0.7) to be compared with
1% v/v case). Adjusted for concentration of 1% v/v, the size distri-
bution measured b> Whitby and Peterson corresponded to number mean dia-
meter of 0 06 microns, ma?s mean diameter of 0.14 microns, mass median
diameter of 0.15 microns and geometric standard deviation of 1.41. They
sized particles as small as 0.02 microns. An impactor which removed 0.4
Tuoron diameter particles as theirs did would have removed 74% and 69% by
mass of our 1% v/v KC1, 1% v/v NaCi respectively Thus it is not surprising
that the concentration-adjusted mass mean and mass median diameters
of their aerosol were smaller than those here. (The "mass median
diametei" for our 1% v/v aeiosols KC1, NaCl would be 0.28 microns
if only the aerosol particles smaller than 0,4 microns are
considered.) Incidentally, IVhitby and Peterson found their
tin-neutralized 1% v/v uranine aerosol to have an average of 0.75
electronic charges (i) per particle, which serves as an estimate
-------
- 43 -
In all size intervals but the smallest, the 1% v/v .NaCl has a
smaller percentage of its total number present thar. does the \\ v/v
KC1 and in all intervals the difference is statistically significant,
so that the difference in their size distributions is not due to random
i counting errors.
Green and Lane (1964) gave a formula for the "mean volume/
surface diameter" of particles generated by atomization, which they
attribute to Nukiyama and Tanas awa. The formula indicates that the
fluid density, surface tension and viscosity affect particle size; the
« . . .
dependence of droplet size on these variables is to powers of absolute
value of 1/2 or less, We do not know how different these properties
are for the KCl and NaCl solutions, though differences greater than
a few percent seem unlikely. Inertial impaction and gravitational
settling would be dependent on particle density (see Fuchs 1964 for
more detail) so that these loss mechanisms might be significantly
different for KCl (density of 1 98 gm/cm ) and NaCl (density of
2.16 gm/cm ), The density difference is 9% and the density depen-
dence is approximately linear for both mechanisms. Brownian
diffusion, another loss mechanism, is not density dependent.
Surface properties of the two salts may play a part in electro-
static loss mechanisms.
From the scatter in the data for the cumulative mass distri-
bution curves in Figure 1.9, the difference in size between the two
1% aerosols seems real, and the two 1% v/v aerosols are more like
each other in size than either is similar to the 8% v/v NaCl
aerosol. Hence, the aer;>s5ls produced from i% • »'v solutions
are estimated to hive imss n;ea>i diameter- of 0.25 trucrons, mass
median diameters of 0,58 -nicrons and geometric •=•; indard dela
-------
. 44 -
B. Dependence of Backscattering on Particle Volume
Sortie recent work has maintained that total scattering by
atmospheric aerosols is proportional to their volume concentrations
(Charlsbn et al,. 1967; Noll et al,, 1968); othei workers have found
it related to the ratio of the mean volume diameter to the mean
surface diameter, the third and second moments of the size distri-
bution (Dobbins and Jizmagian, 1966). 'Cooper and Byers (1970) found
'
that aerosols similar to those used here had backscattering which was
nearly proportional to the volume of particles per volume of air,
.for non-absorbing particles with indices of refraction near 1.5.
The data in Tables 1.6 and 1.7 .allow us to make some comparison
between size dependence relationships. The average net dry
scattering is the time average of the scattering at humidities below
the change of phase, and is in terms of photomultiplier current.
The averages were done by taking the areas under strip chart
recordings of scattering versus time, and are probably accurate to
within ±10%. The weighing errors are discussed in Appendix 4.
They are about 0,15 x. 10 gm, or less than 5%. There were some
variations in flow rate, and we estimate the error in net flow
volume to be 10 to 20%, so that the accuracy to be expected for
the dry scattering per unit aerosol volume is about ±20%. The
insults are consistent with the hypothesis that the laser light
bacVscattering tor these aerosols is proportional to the volume
fraction of the aerosol doing the backscattenng; the results do
not seem consistent with an extension of the third-to-second
moment hypothesis This ratio changed little (1,41 to 1.68 range)
-------
i V >
Table 1.6. Net Scattering Per Unit Particle Volume for Several
Aerosols at Low Humidities (less than 40% RH).
Aerosol
Total
Flow
Mass
Ratio of
Average Dry • Third to Second
Material Average Dry Scattering Per Moment of Size
Net Mass Volume Concentration Density Scattering Particle Volume Distribution
(10-3 gm) Ui3j (10-6 gm/n>3) (gm'/cm3) (10'7 A) _ (10-3 A/cm3) (d , 10"4 cm)
1% v/v KC1
6.8
IS
450
1,98 1.0
0.44
1.41
1% v/v NaCl
9.0
800
2.16 2,2
0.61
1.55
1% v/v Methylene
Blue
3.6
6.S
540
1.3a 2.8
0.67
8% v/v NaCl
47.4
13.8
3400
2.16 9.2
0.57
1.68
-------
Aerosol
Table 1.7. Average Dry Scattering per Average of the First,
Second and Third Powers of the Diameters.
Ratio
of
Ratio Ratio Ratio
of
of of
A.D.S. A.n.s. A.TV.S.. A.D.S.
D 2
a
Average Dry to
D j
P
(10'4 cm) (10~10 cm2) (10~14 cm3)
d Scattering D
0 (A.D.S.)
to
D~f
P
•y •>
j10-7 A) (lO'^A/cm) (10 A/cm')
to to
7 31 (10"7A)
1% v/v NaCl 0 077 1.37
0.59
1.55 2.2
2.8
1.6
1% v/v KC1 0.131 3.9
2.2
1.41 10
7 6
0.26 0.45 0.71
? 8% v/v NaCl 0.139 6.5
7.9
1.68 92 66.
14
1 16 5.5
' /N
b d.
u •
I P
-------
~ 47 -
assumed that the Collison atomizer output was roughly constant
in number for the various aerosols, then differences in scattering
for aerosols of the same composition (e.g. NaCl) would be due to
differences in size.
Although not within the estimated experimental error, the values of
scattering of light per unit volume of particle as shown in the next to
the last column on Table 1.6 are quite similar. This similarity is
striking especially when one considers both
the 1% v/v aerosols and the difference in chemical composition and
optical properties between the colorless, cuoic NaCl and KC1
crystals and the spherical, methylene blue particles.
What do Tables 1.6 and 1.7 indicate? If ':he number of
particles per second generated by the atomizer in these tests were
constant, then the differences in scattering would be due to particle
size, shape, and composition. If the generation rate were constant, the
size dependence of backscattering could be tested by dividing the net
scattering by the various means—mean diameter, mean surface area diai/ieter
( — r-,:/2 r _ 'i/s
(D '[ , mean volume diameter \D -*\ , and ratio of volume mean to surface
. P / » P y
mean vd )--and determining thich mean gave a ratio which was least
dependent on the aerosol stulied. This is done in Taole 1.7. The
results of these calculations make it difficult to choose which of
the lasf three of the four means mentioned is more fitting. Recall
that this table is based on the assumption of constant particle
number from the atomizer. If the particle generation
rate were constant and if particle losses were negligible, the
mass concentrations divided by particle material density (in other
-------
- 48 -
'i
original volume concentrations in the atomizer- Such a calculation
gives volume concentrations of 230, 370, 420 x 10" for the 1% v/v
KC1, NaCl, and methylene blue aerosols respectively and 1570 x 10~
for the 8% v/v NaCl. The KC1 aerosol is lower than the other two 1%
v/v aerosols, which may explain the consistently low values for
the KC1 scattering in Table 1.2. The volume concentration of the
8% v/v NaCl is not 8 times greater than that of the 1% NaCl • This
indicates that the number concentration of the 8% v/v NaCl was lower
than that of the 1% v/v NaCl by about a factor of two by the time
its scattering was measured at the scattering chamber. Assuming
that the number concentration of both the 1% KC1 and 8% NaCl
was not the same as that of the 1% v/v NaCl, as indicated by the
weight determinations in Table 1.6, then the ratios in the last three
columns for these two aerosols should be doubled . The numbers
which result have the least extreme ratios for the comparison based
on the mean volume. On the other hand, the data in Table 1.7
support the hypothesis that the scattering for these sizes was
proportional to volume fraction of aerosol without relying on
-------
- 49 - •
-1 C. Results of Subsidiary Tests
"" A variety of tests were made to check for possible problems;
other tests were made to serve as controls. Both types are described
; | here in brief.
• • i
At the experimental flow rates, the scattering chamber responded
to a step change in concentration at its inlet by reaching greater
than 90% of the ultimate scattering value in loss than two minutes.
Measurements with a Sinclair-Phoenix (forward-scattering)
Aerosol Photometer showed that less than 5% of the particles (as
measured by their dry forward scattering) at low relative humidity
and less than 15% of the particles (also as measured by their dry
scattering) at 85% RH were lost traversing the scattering chamber
for ar. 8% v/v NaCl aerosol which was generated in such a way that
x- it did intercept the jet of air from the dilution orifice; this
aerosol had size parameters between those of the 1% v/v NaCl and
those of the 8% v/v N.iCl aerosols which bypassed the dilution orifice.
The intent of this work was to measure the backscattering
from the specific aerosol particles, not scattering from molecules
in the carrier gas, nor from contaminants, nor scattering from the
walls of the scattering chamber. The control test deemed most
appropriate as the baseline was the measurement of the scattering
from the carrier gas (air) after it had been filtered by an
absolute filter (Gelman Type E glass fiber filter). Near the
• beginning of this work room air was sampled, humidified, or dried
as with an aerosol and drawn through a filter into the scattering
chamber (it had been ascertained that there were no leaks
and that the chamber was not sensitive to the small change in
-------
\ '
\
- 50 -
>
chamber) of 0,31 ±0.01 x 10~7 A, the following net
/ ' . •
current was observed:
0.23 x 10"7 A at 13% RH,
0.25 x 10"7 A at 80% RH,
- - 0,25 x 10"7 A at 85% Rll.
This same test was repeated midway through this section's work, and
with a dark current of 0.25 x 10 A the following net current
was recorded:
0.10 x 10"7 A at 8% RH,
0.10 x 10"7 A at 49% RH,
0.13 x 10"? A at 78% RH.
0.11 x 10"7 A at 14% RH.
Finally, the same test was repeated near the end of this section's
work. The ntt current (0,35 x 10" A dark current subtracted)
was:
0,06 x 10"7 A at 28% RH,
'• _7
0.09 x 10 A at 79% RH,
0.09 x 1Q~7 A. at 14% RH,
for tests done in that order. Experience indicated that the
greatest source of change in the net current for such control
tests came from minor imperfections in optical alignment and from
the use of different glass covers on the laser inlet hole, a
microscope slide early in the work and microscope cover glasses
later. The cen control tests above were specifically designed
to test for humidity dependence of th-j baseline, and they
-9
indicate that changes in the second decimal place (10 A) could
-------
- 51 -
I
reported in this work is the scattering above that which was found
-
! for filtered air; the filtered air scattering was measured several
"~~ ' •!• times for each aerosol (usually at a low relative humidity) during
7 the scattering versus humidity tests of the various aerosols. The
' -d* ' .-}
air in the laboratory gave net current of about 0.15 x 10 A
-••
. when it was sampled (undiluted, thus at its ambient humidity, about
'* t
50% RH) and the salt aerosols averaged from 1 x 10~ A to
-i 9 x 10" A net current at low relative humidities, making the
. above changes with humidity of the filtered air readings negligible
• r .
(i.e., a few percent or less).
f
The light scattering from polystyrene'latex cerosols was not
expected to change appreciably with changes in lela'.ive humidity
(Lundgren and Cooper, 1969) . The following readings were obtained from
nebulizing a suspension (0.1 cm solids in 200 cm water) of
0.71 micron diameter polystyrene latex spheres (prepared and
j sized by Dow Chemical, Midland, Michigan) . Their net current
-7
was 0.69 x 10 A and the scattering ratios (scattering at
specified relative humidity divided by scattering at less than
25% RH) were:
'/ "* • Relative Humidity Scattering Ratio
': 21% RH 1.00 r 0.02
32% RH 1.00
• 34% RH 1.02
43% RH 1.00
i 52% RH 1.03
; 75% RH 1.02
! ' 75% RH 1.03
: 80% RH 1,05
84% RH 1.06
-------
- 52 -
The slight increase at higher humidities could be due to the
huirddification of a small amount of scattering impurities, either
from the emulsifier in the polystyrene latex suvpe^sion or from
leakage or from contamination of the system from other tests.
The response of our equipment was tested with .:-.n aerosol of
0.80 micron diameter polystyrene beads. The beads were atomized
by a Collison atomizer, with one orifice, at 30 Ib/in . They
were nebulized from a solution which was lesa than 1 part per
thousand polystyrene beads in water, which gave <5% doublets
and
-------
- 53 -
For tests with the 0.71 and 0.80 micron .nonodisperse polystyrene
latex particles, the dilution flow rate in the mixing chamber was
reduced to 0.23 x 10 cm /sec(0.5 ft /min). The test for the
relative humidity dependence (0.71 micron) required dilution and
humidification of the aerosol; the other test (0.80 micron) did not.
To raise the final relative humidity of the aerosols to 85% RK
and above, two steps were required. In the first step, the aerosols
were passed through the prehumidifier (see Figure 1.3) after leaving
the holding chamber. Possible increases in aerosol losses due to the
prehumidifier were tested with both the 1% v/v NaCl aerosol and
the 8% v/v NaCl aerosol. The 1% v/v NaCl aerosol was passed through
the prehumidifier, then it.« scattering was measured at less than 30% RH.
Differences of 0%, 1%, 3%, 2% in scattering were found by alternately
passing through and by-passing the prehumidifier. It was concluded
that the prehumidifier did not contribute significantly to losses for
the 1% v/v aerosols. The same tests for 8% v/v NaCl aerosol,
however, gave scattering losses of 15% and 16%, so that the
experimental values for the scattering for 8% v/v aerosols were
increased by 16% before being plotted as the "prehumidified" data
points in Figure 1.15 (p. 78), for those measurements for
which the pre-humidifier was used.
The possibility was considered that at increased humidity
some of the scattered light xould be absorbed by water molecules.
Although there is a water vapor absorption line near the wavelength
o
of the pulsed ruby laser (6943A), water vapor absorption was not
-------
- 54 - •
found to be a problem for the continuous wave He-Ne laser, which
o
operates at a shorter wavelength (6329A). Experimental evidence
for this is that for polystyrene aerosols there was no decrease
in light back-scattering with increasing humidity nor was there
for the salt aerosols below their phase change humidities, which
would be expected if increasing the humidity resulted in increasing
tht concentration of an absorbing species. Using filtered air,
no decreases were observed in the backscattering (presumably
partly from the chamber walls) when the humidity was increased.
It is concluded that absorption of light by water vapor was
negligible.
As an experimental measure of the greater deposition (loss)
of particles at high humidities than at low humidities, the
particles were dried after exiting from the scattering chamber
and the dry, forward-direction, white-light scattering was measured
with a Sinclair-Phoenix Aerosol Photometer. Connected to the
scatteri-ag chamber outlet was 3 meters of 0.64 cm I.D. rubber
tubing, which was followed by a two-quart (about 2 x 10 cm )
metal can wrapped with heating tape, then a thermometer and finally
the Sinclair-Phoenix Aerosol Photometer. The relative humidity
in the room was about 50%, The flew through the scattering
chamber was the usual 220 cm /sec.
Table 1,8 lists the aerosols tested, their humidities and
scattering in both the scattering chamber and aerosol photometer,
the scattering ratios (wet to dry scattering), aad the ratios of
the forward scattering (aerosol photometer) measurements to the
backscattering measurements. The object of the experiment whose
results are in Table 1.8 was to get an experimental estimate
-------
Table 1.8. Backscattering and Forward Scattering for Several Aerosols
at Different Humidities.
Scattering Aerosol
Chamber Net Back- Photometer . Forward Scattering
Scattering Ratiob
Ratio of
Measurements o'f Dry
Forward Scattering to
Aerosol
1% v/v
Uranine
Dye
1% v/v
NaCl
1% v/v
Methylene
Blue
Relative
Humidity
7% RH
82% RH
8% RH
6% RH
84% RH
12% RH
5% RH
84% RH
10% RH
Scattering
(ID'7 A)
10.2
8.7
10.2
1.04
4.4
1.14
1.25
1.30
1.20
Relative
Humidity3
'10% RH
43% RH
'10% RH
"10% RH
45% RH
'10% RH
"10% RH
43% RH
*10% RH
(Arbitrary Aerosol
Photometer Units)
37.0 ± 0.3
44.0
34.5
35.0
32.2
35.0
19.0
25.5
21.0
Back- Forward Dry Backscattering
Scattering Scattering (Arbitrary Units)
3.6
0.85 1.27
3.4
34
4.0 0.92
31
15
1.06 1.28
- - 18
in
t/i
a Estimated from the temperature difference between the temperature at the inlets of the scattering
chamber and of the Sinclair-Phoenix photometer, which difference was usually 11°C.
-------
- 56 -
If the NaCl particles actually dried in the heated metal can (the
45% Rll is near the change-of-phase humidity for decreasing
huwidity, as can be seen from Figure 1.14 (p.77 }), then the value 0.92
is a measure of the fraction of the humidified aerosol which reached
the aerosol photometer compared with the originally dry aerosol which
reached the photometer. The losses (in terms of light scattering)
were on the order of 8%. From an earlier study of the dependence
of forward scattering on humidity using the aerosol photometer
(Lundgren and Cooper, 1969) no increase in scattering was expected for
the methylene blue aerosol at 45% RH when its humidity has been
raised to that value, but that work gave no information about the
behavior of. methylene blue aerosols whose humidity decreased to
that value Similarly, the same study indicated tl.at the uranine forwarl
scattering at 45% RH would be 1.5 times that at low relative
humidity for increasing humidity, in which car.e (if it is the same
for decreasing humidity) only (1.28/1.5) or 85% of the change has
been measured, and 15% lost. This loss may partially explain the
surprising decrease in backscattering with increasing relative
humidity shown in Figure l.U (p. 74 ) for the uranine aerosol.
Although the ratios of forward, white-light scattering to
backward, red-light scattering as shown in Table 1.8 do not tell
the actual ratio <-f the scattered intensities because the forward
and backward scattering measurements were in different units, the
variations of the ratio from aerosol to aerosol are illuminating.
The ratio for the methylene blue aerosol was half that for the
NaCl aerosol and the ratio for the uranine aerosol was a tenth
of that for the NaCl. The dry uranine aerosol scattered much more
-------
- 57 -
than did the NaCl anu inethylene blue aerosols. The number concen-
trations and size distributions of these aerosols should have been
similar to that of the urcnine and the forward scattering of white light
from them was not nearly so different from uranine's as was their
backscattering.
\j. Backscattering vs Relative Humidity
The results of the experimental determinations of backscattering
as a function of relative r.amidity are contained in the graphs in
this section, Figures 1,10 to 1.15 (p. 73-78). "Backscattering
rstio" is the ratio of the scattering of the aerosol at the indicated
relative humidity to the scattering at a very low humidity (-10% RH)
and is used to allow caparisons between aerosols scattering different
amounts of light.
The backscattering ratio for 0.71 micron polystyrene latex
Leads test aerosol was less than 1.10 at 34% RH.
Other test aerosols commonly used are the dyes uranine and
methylene blue; their responses to change in humidity are given in
Figures 1.10 (p. 73) and 1.11 (p. 74). Earlier work (Lundgren and
Cooper, 1969) done on the forward scattering of white light by
similar uranine dye and methylene blue dye aercsols showed a linear
increase in scattering versus RH for the uranine from 1.0 at 10% RH
to 1.8 at 80s RH, whereas under the present conditions there was a
decrease in uackscattering of monochromatic light. The earlier wovk also
showed an increase in scattering for methylene blue from 1 0 at
60% RH and below to 1,3 at SO9; RH; under the present conditions, the
backscattering from methylene blue remained constant up to
65% RH, increased to 1.26 at 76% RH, then decreased to 1.07
-------
- 58 -
forces as humidity increases (Fuchs, 1964) and thus smaller .
.•
The behavior of the uranine and methylene blue aerosols is unexplainable
although an increase in particle loss at higher humidities
could account for decreased scattering from that which would be
expected from the forward scattering observations. Microscopic
observations of uranine particles have shown that they do increase
0
<• in size with increasing relative humidity.
The behavior of the hygroscopic salt aerosols NaBr, NaCl and KC1 is
illustrated in the graphs of Figures 1.12 to 1.15 (p. 75-78 ). The theory
t of the growth of hygroscopic particles was presented quite clearly
—••-•• /in the work of Orr, et al. (1958) . Their description is paraphrased
in the following paragraph
/
' At low humidities, hygroscopic particles adsorb on theii
surfaces a thin layer of water a few molecules thick or less.
v.^
As the humidity is increased, the humidity is reached at which the
particles become dropJ.cts of a concentration ;vhich is such as to be
in equilibrium with the water vapor (relative humidity) present;
these droplets continue to grow as the relative humidity is increased.
They contract if the humidity is decreased. If the humidity
\
continues to decrease, the droplets become solids again, this change
\' ' '
of phase taking place at a lower humidity than that at which they
changed phase vhen the humidity was increasing. Additionally, the
••>
* change of phase humidity depends on particle size, smaller particles
changing phase at lower relative humidities.
In Appendix 3 the h'.'iriditics at which the change of
phase occurs for increasing humidity for salt particles so large
N
that the size dependence is negligible are lie-ted,
-------
- 59 -
t
j Although the thin film of water on a particle exists even at
low humidities, no significant changes were measured in the
scattering from the KC1 and NaCl aerosols for humidities from 10% RH
to 60% RH when they were undergoing increasing humidity. A condition
of increasing humidity occurred when the; aerosols were net "pre-
humidified" (this involved passing them through a 1/2-liter jar
one-quarter filled with water). The theory for scattering from
concentric spheres of different refractive indices has been worked
out (Aden and Kerker, 1951) and some computations have been made
(Kerker et al., 1962). No change in scattering occurred at these
', lower humidities. Since the exact scattering computations are
rather difficult, we will merely demonstrate how small a size change
is involved in the formation of this water film and use this as the
' explanation for the observation of no appreciable change in
scattering for different low humidities.
Orr et al. (1958) gave the following equation for the thickness
of an adsorbed molecular film at a given vapor pressure of the films'
constituent:
V/Vm • C'(P/Po)/(l-P/Po) (l+(C'-l)P/Po)
V = volume of gas adsorbed at P, T
I
: P = partial pressure of adsorbed gas
P = saturation vapor pressure of adsorbed gas
T = temperature of system
V = volume occupied by shell one molecule thick,
C'= exp ((ErEL)/RT)
E.= heat of adsorption of gas in first layer
E,= heat of liquification of gas
-------
- 60 -
=>~
Each water molecule occupies about 10.8 A'' (Orr et al., 1958) and
2
E. - E. can be estimated by the surface energy of NaCl, 276 ergs/cm.
, From this point, the following estimate can be made
/ C' = exp [(6 02 x 1023 molecules/mole) x (10.8 x 10"16 cm2/
molecule
(298~K)]
2 7
molecule) x (276 ergs/cm )/(8.31 x 10 ergs/mole-°K) x
C' ^ 1.38 x 103.
Thus C'> 1, and for RH = P/PQ 10%, (C'-l) P/PQ>: 1; therefore,
where RH is the relative humidity.
For relative humidity of 30%, this predicts a water shell
Q
1,4 molecules thick or about 5 x 10 cm thick, for NaCl. This
would be an increase of only one part in three hundred in dianeter
for a particle with a diameter of 0.3 microns.
From the graphs (Figures 1.12 to 1.15, p. 73-78) of backscattering
ratio versus relative humidity for the four salt aerosols tested
(1% v/'v NaBr, KC1, KaCl, and 8% v/v NaCl) the foil >wing--under
conditions in which the humidity was increasing (not pre-humidified)
or decreasing (pre-humidified) --are noted:
1. Each aerosol exhibited increased backscattering for
humidities exceeding a critical humidity.
2, This critical humidity, which is associated with a
change of phase from solid particle to liquid droplet,
was different for each salt species, was
w
lower in each case than the values given in Appendix 3
-------
-61-
~T
3. Using the arbitrary criterion for the "critical humidity" for these
j~j aerosols being that humidity at which, for increasing humidity,
their scattering ratio equals 2.0 (twice as much scattering at that
humidity than at 10% RH), we get the following critical humidities:
1% v/v KC1 75 i 3% RH
1% v/v NaCl 68 ± 3% RH
1% v/v NaBr 53 ± 3% RH
8% v/v NaCl 71 ± 3% RH
At these critical humidities, enough of the aerosol particles had
;
changed phase (and had thus grown in size) to double the backscatteriug.
\ • .. The error estimates are based on combining the uncertainty cf the
\ ;
\ " accuracy of the humidity measurements (tl.5% RH) with the uncertainty
"* from the scatter of the data (abouti 2.5% RH) by taking square root of the
sum of their squares.
4. The backscattering ratio for 1% v/v. NaCl was greater than that for 1% v/v
NaBr at all humidities above the change of phase humidity for NaCl.
5. The backscattering ratio for 8% v/v NaCl was indistinquishable from
: that of 1% v/v NaCl for all relative humidities tested above 75%, the
change of phase humidity for bulk NaCl. Thus the difference in particle
j size of these two aerosols was reflected by different critical humidities,
as expected, but it did not measurably change their scattering behavior
above the critical humidity.
6. Some fraction of the KC1 and NaCl aerosols which passed through the
-------
/• -62-
noted in Orr et al, (1958); they stayed droplets below
'"""" the critical humidity. This is an interesting result,
because the test aerosols were run through the pre-
humidifier alone at the usual flow rate (0.09 ft^/min
or 2.5 x 103 cnwmin) and humidities of 62% RH for 8% NaCl
and 59% RH for 1% v/v NaCl were measured. These humidities
are below the phase- change humidities, so that
. phase changes were not expected A test at 20% RH with
' *. 8% v/v NaCl indicated that a loss of 16% of the aerosol
\
dry scattering was incurred by adding the pre-humidifier.
Therefore the "prehumidifier" values were raised by
16% in Figure 1.15(p,78 j.
7. Neither the 1% nor 8% NaCl aerosol displayed the small
decrease (about 10%) in light scattering noticed in the
forward direction for white light in previous work
(Lundgren and Cooper, 1968).
8. In comparison with the results of Lundgren and Cooper (1968),
who also used aerosols generated from 1% \-/\i solutions
in a Collison atomizer, the laser light backscattering
ratio for the NaCl aerosols at 80% RH (a ratio of 3.6),
is considerably less than that for forward scattering of
white light at 80% RH 'a ratio of 9), suggesting that
under some circumstances lidar measurements will be less
affected by humidity interference than would measure-
mRnts by devices using the forward scattering of white
-------
-63-
/ n
'!. i
ii i
I '/
9. From the graphs presented in Orr et al.(1958), the
following change-of-phase humidities for particles
having the indicated diameters are expected:
83% RH for KC1 particles of diameter 0.1 microns
82% RH for KC1 particles of diameter 0.06 microns
73% RH for NaCl particles of diameter 0.1 microns
68.5% RH for NaCl particles of diameter 0.06 microns
At the change of phase, the volume of a hygroscopic
particle increases to approximately 1/(1-RH)times its
original volume (Neiburger and Wurtele, 1949) . These
particles would, therefore, have volume increases of three to
six times their original volume, or diameter increases
proportional to the cube roots of three to six times the
original diameters . That is, this indicates that at change of
phase to droplets they would have diameter increases of 50 to
making the 0.1 micron particies nearly as large as the
mass mean diameters of the dry 1% v/v aerosols. KC1
particles having diameters of 0.03microns would
change phase at 78% RH. It is concKided that the numerous
particles smaller than the mass mean diameter and
smaller still than the mass median diameter make a
significant contribution to the backscattering versus
relative humidity of these aerosols.
10. Those NaCl particles ^nich changed phase in the pre-
humidifier changed back to solid particles at relative
humidities below 40% Ril and those KC1 particles which
had changed to droplets changed back at relative
-------
n
-64-
I i
ii.
V. Discussion
^ The history of the aerosol particles was as follows: droplets were ;'
generated by the atomizer and dried in the mixing chamber when mixed
I '
with clean, dry air- Some particles deposited in the mixing chamber; '•
more were lost going to and passing through the holding chamber. The ]'
remainder were then humidified or kept in a dry state by the addition
of air of controlled humidity- When the particles x-r-iched the RH chamber,
they had on the average 1.3 minutes to come into equilibrium with
the water vapor present, which uas almost always at a higher relative
humidity than was the air in the mixing chamber. In coming to equili-
brium with the humidity, the particles would either adsorb a surface
\
layer of water a few molecules or less thick, or they would change
phase, becoming droplets, based on the analysis and experimental
work of Orr et al. (1958). The exceptions to this description
were those aerosols which went through the pre-humidifier. Although
the relative humidity of these aerosols was measured to be 55-60% RH
in two tests, these aerosols behaved as though at least some of
the particles had changed to droplets, even though for NaCl and KC1
the resulting humidity (55-60% RH) was below the critical humidity
foi change of phase with increasing humiuity. Two explanations are
possible: 1; Only the smallest particles change phase, in keeping
with the fact that small particles change phase at somewhat lower
humidities than the critical change of phase humidity for the bulk
material- A 0 04 micron Na.Cl cube would change phase at 68% RH,
but a 1 micron NaCl cube changes at 75% RH, the bulk critical
relative humidity. 2} There are humidity gradients in the pre-
humidifier, and some or all of the particles experience humidities
-------
K»-«cw!jfKa«**'*w»<> W.T«
If only some of the particles changed into droplets in the pre-
humidifier, it should be reflected in an abrupt change in the
scattering for the pre-humidified aerosols near the critical
humidity as the dry particles changed phase; this is not apparent
from the data.
Finally, the dry particles or the droplets flowed to the
scattering chamber> where the amount of backscattering they caused
was measured. While traversing the chamber, some would be deposited,
and the rest would loave through the exhaust where they were caught
for weight analysis or for size distribution analysis.
In terms of light scattering, the hygroscopic particles were observed
in one of three possible states: I) the very dry condition,
in which the scattering was that from crystals (colorless, cubic)
of Nad, NaBr, KC1, or dry dyestuff (roughly spherical); 2) an
intermediate condition for the salts in whic'i -hoy h«/.d adsorbed
a few molecular layers or less of watnr on t,\
-------
-66-
Although the change in scattering due to changes in size, shape
and index of refraction cannot be computed, the growth of the
J . aerosol particles on change of phase is calculated and .the
<
i
effect on their aerodynamic behavior is shown.
Once the dry particle has changed phase and become a droplet,
its volume is inversely proportional to (1-RHJ where RH is the
relati-e humidity. Any NaCl particle changing phase at 70% RH
would thus have a volume 3.3 times the original volume. A NaCl particle
changing phase at 75% RH would have a volume 4 times the original.
Let a prime denote characteristics of the droplet; unprimed
characteristics are those of the dry particle. The subscript s refers
to the salt (or dye). The subscript w refers to water.
The droplet diameter is
Dp' * Dp/d-RH)1/3.
If the scattering depended only on the particle diameter to
»he power n, :t would depend on the relative humidity as
fl/U-RH)1 3j" For changes of phase at 75% RH and 80% RH, this
would mean a change by the following factors:
1
2
3
4
5
6
75% RH
(D VD )"
P P
1.59
2.53
4.00
6.37
10. 1
16.0
80% RH
(Dp./Dp)n
1.71
2,93
5.00
8.6
14,6
-------
-67-
I
T
Work done by Pueschell et al. (1969) with artificial aerosols
generated from an aq'-.oous solution of 3% w/w NaCl by vigorous
bubbling gave a scattering increase of 3.1 at 75% RH in comparison
to the dry (40% RH) scattering as measured by their "integrating
nephelometer ." The integrating nephelomcter sums the scattering over
the angular range from 8: to 170° from the forward direction.
Lundgren and Cooper (1969) , using NaCl aerosols similar to the
1% v/v NaCl used here, found an increase in forward scattering of
about 7 at 75% RH, for white light. The backscattering ratio for
the 1% v/v NaCl aerosol used here was 3.5 at 75% RH. Scattering
is dependent on angle and wavelength • It is quite
sensitive to the humidity and small humidity errors can lead to
larger percentage errors in the ratio of wet to dry
scattering.
When the dry salt particle of density p changes phase, the
droplet density becomes:
(i-RH)-cw(RH)
A particle's aerodynamic properties are conveniently character-
ized by the particle "relaxation time" (T) in the medium. For
spherical particles moving with Stokes law resistance in a fluid,
r is given by Fuchs (1964} as
T = C(l/18) o D 2/p
-4
where C - 1 » (0.16 x 10 )/D , at STP, and is the Cunningham correction
factor in air. The terminal velocity of a particle
-------
-68-
particle having initial velocity V will go in a stationary medium
before stopping is V i. After a change cf phase from solid to droplet,
the relaxation time changes from * to '' , where -.'is given by
« HP ] C! ! D ' ^2
D
P
or
ri-RH'os* to \ (D ' * 0.16 urn)
(D\ » 0.16 urn")
VJ
For NaCl with dry D =0.4 microns, at 80% RH, D ' = 0.68 microns
and the ratio in brackets becomes 0.57. For this case, T'/T =
(0.57)(i.71}(0.84/0.56) = 1.5. The decrease in density has somewhat
offset the increase in size.
There are several reasons why the deposition of the particles
(and thus their relaxation times) is of concern. Deposition which
occurred in the $r ittering chamber or in the tubing preceding the
filter which caught the weight samples or preceding the sampler
which captured the particles for electron microscope, would result
in a sample size distribution different from the size distribution
which scattered the light. Since the tubing to the filter and the
tubing to the sampler were kept short and any bends made gradual, the
sedimentation losses in the scattering chamber are assumed to
predominate If losses of the humidified aerosols were significantly
greater than those of the dry aerosols, then the ratio of their
scattering would be biased. The losses could also make the
size distributions different from those which other experimenters
using the same atomizer but having different losses have observed.
\
-------
-69-
The latio of the final nunber concentration (n) to the initial
number concentration (n ) due to losses'by sedimentation of particles
in horizontal turbulent flow thiough a cylindrical pipe over a
distance L is (Fuchs, 1964j:
(n/n ) = exp i-ZgrL/^UR]
o
where g - gravitational acceleration, 980 cm/sec
U * average flow velocity, cm/sec
R = radius of cylinder, cm.
This can also be written
(n/no) = exp i-2g-LR/Q]
where
. Q = volume rate of flow, cm /sec.
For the flow in the scattering chamber, Q •= 220 cm /sec, L = 65 cm
and R = 7.1 :m. Using this last formula and - and T' for
NaCl at low relative humidity and at 80% Rll, respectively, the
fractional losses il-n/n ) due to .the sedimentation in the
scattering chamber have been calculated and are given in Table 1.9.
The difference in the losses af. h_gh \80% Rl!) and low humidities
from settling out in the scattering chamber is less than 10% for
NaCl paititles with original diameteis smaller than 20 microns
A similar loss estimate can be made fo: the losses in the RH
chamber foi which n/n = rxp[-L'/h] in which L' is the distance
0
the particle would fall in the 78 seconds it is in the RH chamber and
-------
-70-
Table 1.9. . Sedimentation Losses for NaCl Particles in Scattering
Chamber.
Diameter
of Particle
When Dry
(microns)
0.1
0.2
0.4
l.C
2.0
4.0
Dry Relaxation
Time , T
(10~6 sec)
i
0.19
0.49
1.49
7.7
28.
109.
Relaxation
Timeb, r'
at 80% RH
(10~6 sec)
0.24 •
0.67
2.2
12.
45.
177.
Fractional
Loss
(1-n/nJ
Dry °
0.00
0.00
0.01
0.03
0.11
. 0.36
Fractional
Loss
d-n/n )
at 80%°RH
0.00
0.00
0.01
O.OS
0.17
0.51
, Based on Fuchs (1964) table on page xiv, including Cunningham
.correction facto.rs .
b\ r« - t(0.57)(1.71)(D ' * 0.16 x 10"4 cm)/(D + 0.16 x 10"4 cm)
Table 1.10. Estimated Sedimentation Losses for NaCl Particles
in RH Chamber,
ticle Diameter
IVhen Dry
(microns)
0.1
0.2
0.4
1.0
20
4.0
Fractional
Loss
(l-n/n0)
Dry
0.00
0.00
0.00
0.02
0.07
0.24
Fractional
Loss
(l-n/r.Q)
at 80% RH
0.00
0.00
0.00
0.03
0.11
0.36
The difference in the losses due to settling in the RH chamber is less
than 4% for NaCl particles with original (dry) diameters less than
-------
i
,
I-
-71-
Another possible source of losses of particles is impaction.
A rough estimate of the importance of .this effect can be obtained
by comparing the particle stopping distance (the product, here,
of the flow velocity and the particle characteristic time) with
the scale of the dimensions of changes in the flow field, for
example the 0.64 cm I.D. of the tubing. The flow velocity through
this tubing was the largest velocity in the system (excluding the
2
capillary tube in the holding chamber inlet) and was 6,9 x 10 cm/sec.
Impaction losses are on the order of 50% when the particle
stopping distance is the same order of magnitude as the length
characteristic of major changes in the flow field. Such
losses to a first approximation seem insignificant for .NaCl particles
with initial dry diameters of 2.0 microns or smaller(Table 1.11). In all
calculations of T, the non-sphericity of the particles was
ignored because the difference in r is less than 10% even for a
cube compared to a sphere..
Table l.ll. NaCl Particle Stopping Distance for Velocity
of 700 cm/sec
Particle Stopping Distance Stopping Distance (S.D.)1
U 4. dill W VW L
When Dry
(microns)
0.1
0.2
0.4
1.0
2,0
4.0
(S.D.) Drya
(10~3 cm)
0,13
0 34
1.04
5.4
19.5
76,0
at 80% RHb
(10~3 cm)
0.17
0.47
1.53
8,4
Jl,0
124,0
a) Based on (S.D.) = VQI - (700 cm/sec) .T
-------
X
•fl
VI. Conclusions ;
U
The backscattering of light from a He-Ne laser by hygroscopic 31
aerosols was considerably altered by changes in humidity. The *r
results presented here should be useful in estimating the influence
of humidity on aerosol particles present in the atmosphere (Narl) j;
as well as aerosols often used in laboratory work. If humidity
is ignored as an interference, serious errors of measurement can 1:
-------
i!
il
-73-
I2
is
i i r
O NOT PRE-HUMID/RED
I % v/v METHYLENE BLUE AEROSOL
00 OCPOOOOO
I I
J 1
J I
0 10 20 30 40 50 60 70 80 90 100
RELATIVE HUMIDITY (%)
Figure 1,10. Backscattering vs, Relative Humidity for
-------
-74-
O 3
P
2
5 2
Ul
H
I-
<
o
<0
*
o
-------
-75-
r
• i>
o
cc
£ 3
S
o NOT PRE-HUM/D/F;EO
1% v/v NoBr AEROSOL
1 1
0 JO 20 30 40 50 60 70 80 90 100
RELATIVE HUMIDITY (%)
Figure 1,12, Backscactering vs. Relative Humidity for
-------
-76-
if
4
O
K
< 3
tc.
0
Z
£
Ul
0
CO
o
CD
'
\ \ \ \ 1 1 1 1 1
V PRE- HUMIDIFIED
O NOT PRE- HUMIDIFIED
JT ~
o
/% V/V KCI AEROSOL tn
_,-_. ...
O
o
Vc^
^v ~
- O\7 O O o (&O 0^ O ~"
il i i ii i il
i
**'
I
I
I
—
«•.
O IO 2O 3O 4O SO 6O 7O BO 9O IOO
RELATIVE HUMIDITY (%)
Figure 1.13. Backscattering vs. Relative Humidity for
-------
-77-
6
V PRE-HUMIDIFIED
O NOT PRE-HUMIDIFIED
O
V
V
oc?
1% v/v /VoC/ AEROSOL
O
Z
o:
UJ
8
<
to
00
V
V
57
V v O
- O V V O
J I
j I
IO 2O 3O 4O SO 6O 7O 8O 9O IOO
RELATIVE: HUMIDITY (%)
Figure 1 14 Backscattering vs. Relative Humidity for
-------
-78-
Ill
5 -
o
I3
IU
o
en
x
O
<
CO
V PRE- HUMIDIFIED
O NOT PRE-HUMIDIFIED
8%\f/vNaCI AEROSOL
- COD v
o o
O
_i____J 1_
IO 2O 3O
L
j
I
I
I I
40 SO 6O 7O
RELATIVE HUMIDITY (%)
8O 9O /OO
Figure 1.15. Backscattering vs. Relative Humidity for
-------
-79-
; I
— Section 2. Depolarization of Laser Light By Laboratory Aerosols
.''•- t
\
. I. Introduction
\ . . *" The primary focus of the work in this section was to investigate
\ f the effect several aerosols have on the polarization of the light
which they scatter. The scattering chamber was modified so that a
I rotatable polarizer could be mounted in front of the receiving light pipe.
The puipoie of this procedure was to study the received scattered
/ F •
/ • light as a function of angle between the receiver polarization direction
(x" f and the direction of polarization of the light incident on the particles.
The rc-.civcd scattered light was measured as a function of four
| different angles between incident polarization direction and
receiver polarization direction (0J, 30", 60°, 90°), and for aerosols of
I five different chemical compositions (NaCl, NaBr, KC1, pci/styrene latex,
•'•'" t dioctyl phthalate) . For two of these aerosols (NaCl, NaBr) the scattering
i
was measured not only as a function of polarization angle, but also
I a; a function of humidity The control test baseline in all instances
was the measuiement of the scattering received when the chamber was
I filled with filtered air.
II, Experimental System
In order to bring the effective scattering angle closer to ISO3
than it was with the Mode}, i scattering chamber without decreasing th
-------
-CO- i.
r
2. The light pipe was moved as close to the laser inlet hole as
construction permitted. ig
i
i 3. A blacK felt baffle was placed 13 cm from the back (light horn)
i
end of the chamber. This prevented some or all of the
light pipe from receiving light scattered from the intersection
of the laser beam with the light horn. The beam almost touched
the baffle edge.
\
y t
To study the depolarizing characteristics of the aerosols used, another
modification was made:
4. A polarizi.:g filter was placed on a mount so that it could
serve as an analyzer, allowing the light pipe to receive only
the component of light intensity polarized along a given
direction. The polarizer rotates; the light pipe is
stationary.
Because tome air leaked in around the mounting of the polarizer,
another change had to be made:
5. A glass co^er slide was put ever the rectangular window
through which scattered light must pass to reach the
light pipe,
A sketch of the polarization modification constitutes Figure 2.. 1.
; figure 2 2 is a cross-sectional side view of the chamber,
\
After the first two modifications listed above were made (the
replacement of the light pipe and the charge in its position), the
/
beam profile was measured with the method described above . This
involved mo/ing a diffuse reflector along the beam axis and
-------
{ » J
iENTATION INDICATOR
POLARIZER MOUNT
LIGHT PIPE
oo
*p*
I
LASER BEAM
-------
t -~."t 1 • » ' •
r
Q76 cm
A "T7
" L
LIGHT PIPE
0.63 cm
POLARIZER
J
I.Otm
- 4.85 cm
'— 0.16cm
0.28 cm
vLASER BEAM
k—51
cm
OUTLET
"X
41
r
0.24 cm
072 cm
'v N^ WINDOW
\ 14.3 cm
-COVER SLIDE
^-OPTICAL STOP PLATE
IT
20cm
I3em -
r
-BAFFLE
15.6 cm
-INLET
70cm
-2.5cm
LIGHT HORN
Figure 2-2, Scattering Chamber, Model 2,with Polarizer.
\- >
-------
1
-83-
Results are shown in Table 2.1. The beam was measured at positions
starting from the optical stop plate and going as far from the
optical stop plate as the geometry of the test equipment allowed .
The farthest distance perpendicular from the light pipe face plane
was 64 cm, The light horn was 20 cm from the back plate of the scattering
chamber, or 90 cm from the front plate. Hence, the beam profile
measurements did not include the last 25 cm (approximately) of the
beam length. The position (median) along the be-un which divides
I
J
the total scattering received into halves (half the light
! came from more distant scatterers and half from those nearer) is 14 cm
from the plane of the light pipe face. The separation between the laser
•• beam axis and the light pipe axis is 0>76 cm (19/64 inch) The
median effective scattering angle corresponds to 6 = tan" (14/0.76)
•*
or 8 - 177°, Adding to the beam profile an overestimate for the
• »
i contribution of the last 25 cm by assuming a net scattering of
0.1 x 10~ amp throughout, shifts the position of the median to 16 cm;
) this corresponds to 177.3°,. Thus the median effective
scattering angle is estimated to be 177'.
It was necessary to determine the fraction of the incident light
transmitted when the polarization direction of the analyzer was
perpendicular to the laser polarization, (The polarizer was a
~! Klinger Cat. No, 063410 Polarizing Filter,) It was also necessary
to determine this fraction when the analyzer polarization was
parallel to the laser polarization. To determine these fractions,
the polarizer was mounted on the front of the laser and the light
from this combination was made to fall onto a black felt target placed
inside the scattering chamber. The polarizer was rotated to get
-------
-84- !:
Table 2.1. Optical Response as a Function of Scatterer
Distance from Plane of Light Pipe Face.
\
Perpendicular
Distance from
Light Pipe Face
Plane (cm)
4.0
5.0
6.0
6.4
6.8
7.2
7.6
8.0
8.4
8.8
9.0
10.0
11.0
13.0
15.0
17.0
21.0
25.0
26.0
34.0
42.0
50.0
. 60.0
64.0
Scattering
Angle
(Degrees)
169.8
173.8
174.6
175.6
176.6
177.1
177.4
178-2
179.1
179.3
Net Scattering
(10~6 Amp)
0.005
0.020
0.44
1.26
1.86
2.26
2.41
2.46
2.46
2.41
2.36
2.11
1.86
1.56
1.37
0.87
0.63
0.45
0.43
0.2&5
0,175
0.145
0.105
-------
T
7]
i i
-85-
maximum and minimum readings and the photomultiplier dark current was
measured. The averages of three such measurements were:
Polarizer polarization parallel to laser polarization:
64 x 10"' A.
Polarizer polarization perpendicular to laser polarization:
0.23 x 10"7 A.
Photo-multiplier dark current: 0.14 x 10" A,
The net scattering without the polarizer on the laser was 105 x 10"
amp.
These readings indicate that the amount of light which is transmitted
when the polarization directions are perpendicular compared to that
which gets throught when they are parallel is (0.23 - 0.14) / (64 - 0.14)
or 0.0014, For the present work this 0.14% transmission Vleakage" is
negligible. Once these tests were finished, the polarizer was
mounted as in Figures 2,1 and 2-2
The system for generating the aerosols is shown in Figure 1,1 and
the rest of the flow path for the aerosols is shown in Figure 1,3.
After being generated from a solution or suspension by a Collison
atomizer, the aerosols were mixed with clean, dry air in a mixing
chamber, ducted to a point where they were sampled in a holding
chamber, mixed with clean air of controlled humidity, had their
humidities measured in the RH chamber, were illuminated with a He-Ne
laser and had their scattering measured. The only exception to
this general description was the aerosol generated from a polystyrene
latex (PSL) suspension . It did not undergo a second dilution by
being mixed with air of controlled humidity.
-------
-86-
III. Procedure
When the polarisation tests were to be made, the electronic
equipment (laser, photomultiplier power supply, electrometer) would
be allowed to warm up. The photomultiplier dark current would
usually behave erratically at first and then settle down to a
value less than 0.2 .x 10 A in about an hour, after which time tests
would begin. An aerosol was then generated and passed through the
entire apparatus flow path; when its concentration had stabilized, as
shown by a stabilization in the scattering readings, the polarization
measurements would begin. Almost all the aerosols used in the polarization
study here were generated from a three-holed Collison atomizer at a
pressure of 30 Ib/in (2 atm) above atmospheric and diluted with
a 62 x 10 cm /min flow of clean, dry air. The exceptions were:
the ;>olyst>iene latex aerosol was generated from a six-hole Collison
atomizer anu diluted with 18 x 10 cm /min clean, dry air and the
dioctyl phthalate (DO?) aerosol was generated from a one-holed Collison
atomizer at a pressure of 20 Ib/in , and diluted with about 50 x 10 cm /min
of clean, dry air. All the aerosols except the polystyrene latex
beads (PSLj aerosol passed through the same flow path used before
(through a mixing chamber, holding charaber, mixing T where humidified
or dried filtered air would be mixed with them, RH chambsr, z.i
-------
j ' -87-
it
Dilution air was reduced to 18 x 10 cm /rain in the mixing chamber.
None of the aerosols were pre-humidified by using the prehumidifier.
: j Instead the humidities above 80% RH were obtained by heating slightly
the filter holder which is upstream from the RH chamber. This evaporated
moisture condensed on the filter causing an increase in the moisture
content of the dilution air.
The chamber was checked for light leaks and made light-tight.
The dark current was measured (the laser was off) and found to be,
as it should, independent of the polarizer orientation.
After the aerosol had entered the scattering chamber, the
•* * •
polarization measurements were taken by turning the polarizer to
^ four different angles: parallel to the laser polarization (0*),
P . perpendicular (90°), and oblique (30°, 60°), The amount of
scattering received (measured as current from the electrometer
connected to the photomultiplier) was recorded for each of the four
different angles, and this was repeated four times, giving a total
of sixteen measurements for the aerosol The dark current was
measured. The humidity was recorded. The control experiment was
to take the same readings after putting an absolute filter
(Gelman Type E glass fiber filter) in-line between the RH chamber and
the scattering chamber, so that the readings were being made on the
carrier gas from which the aerosol was filtered. The control
readings were subtracted from the total scattering readings to give
the net scattering received; these data are presented in Table 2.2-
The readings from the filtered air were a bit higher at high humidity
than they were at low humidity (see Table.2 2), possibly because of
-------
Table 2.2. Net Scattering Received as a Function of the Angle of the Receiver Polarization.
Aerosol
1% v/v NaCl
1^ v/v NaCl
8% v/v NaCl
8% v/v NaCl
8% v/v NaCl '
8% v/v NaCl
8% v/v NaCl
8% v/v KC1
8% v/v KC1
Polystyrene
Latex Beadsc
Relative
Humidity
(percent)
12
76
12
77
92
12
S4
12
12
46
,46
Dioctyl Phthalateui2
Fiiteied Aird
Filtered Aird
8% v/v NaBr
8% tf/v NaBra
8% v/v NaBr
8% v/v NaBr
8°6 v/v NaBr
8% v/v NaBr
8% v/y NaBr
8% v/v NaBr
76
12
12
12
12
12
76
?o
82
82
Net Scattering^ Received (10~7 A)
0"
Range Average
0-5
0.4
1.0
10
3 0
0.6
2.0
0.4
0.02
0.2
0.3
4.
0.03
0.01
0.4
02
1.1
0.6
20
4.0
1,7
0,9
1.99
4.20
16.4
42.9
45.0
10 9
43-0
14.25
9.05
1.95
2,11
83
1.02
0.59
7.3
7.3
23.0
31.2
97,0
67.1
22.1
23.0
30°^
Average
1-84
3.34
14.7
34.6
35.2
10.0
36 2
12,88
8-32
I ,64
I .81
70.6
0.82
0 48
6.4
62
20.4
27.8
75 8
53.2
17.6
18.5
60"
Average
1.04
1 44
8.4
13 6
IS 2
5 62
14 5
.6.89
4.54
0 /9
0.83
31 b
0 36
0 23
32
3.1
10 4
15 4
32 1
23.2
7.2
7.6
90"
Average Range
0.45
0 37
4.07
4.49
5.61
2 U
2 57
2.03
1.91
0.10
0,14
3-0
0 *3
0 10
1 1.01
0 94
i 39
5-43
11 3
6.52"
1-6)
1-71
0 06
0 01
0 15
0.30
0 20
0 i5
0 10
0- 1
0.0'
O.OJ
0.01
0.1
0.01
0 01
0 03
0.03
C.35
0 20
0.30
0 , 30
0,25
0.10
Polarization Ratios
30'"/0'
0.925
0 79S
0 896
0 807
0 782
0.917
0.842
0 904
0-9)9
0 840
0 856
0 850
0 804
0 814
0.876
0 850
0 . 88 :
0.891
0 '81
0. /92
0. ?96
0.805
60'-/IV"
0 523
0 343
0 512
0-31 •'
0 338
0 516
0 33 /
0 484
0.50^
0-405
0.393
0 381
0 353
0 390
0-438
0 425
0 452
0-493
0 330
67T46
0 326
0.330
90 /O'"
0.226
0 088
0.?48
G.1"".b~-
0 , 09 '
"0~OTT
0.074
I
OP
OQ
I
a Baffle in scattering chamber removed,
2
b One-holed Collison atomizer at 20 Ib/in
i4
f. Diameter = 0.50 x 10'. cm.
d This includes scattering from chamber walls, etc., and is the "backgiound" which is subtracted to get
-------
n
-89-
the aerosols . Such contamination presumably would b-i from the
salts used and would thus be hygroscopic. Except for the use of
an altered flow path (as noted above) the procedure for the PSL aerosols
was the same ,
The laser output power changed during the tests, over a period
of several months, so that valid comparisons from the data may only
be drawn from the polarization ratios, not from the absolute
scattering intensity data.
IV. Results
The results of the polarization measurements are contained in
Table 2.2. The first coiumn indicates what aerosol was used. The
percentages associated with the NaCl, KC1, and N'aBr aerosols in
this column are the volume percentages of the salts in the aqueous
solutions from which they were nebulized.
The polystyrene latex beads were from Dow Chemical Company (Midland,
Mich )and were sized by Dow as being 0-500 microns m diameter with
a standard deviation of 0 0027 mi:r?ns The readings for filtered
air are the readings from the photomultipner tube (RCA 7265 at
2000 voits) when filtered air was illuminated by the laser beam
minus the readings of dark current for the photomultiplier tube
(determined by blocking the l^ser light, from reaching the scattering
chamber) and the readings reflect the amount of light scattered by the
chamber walls, etc The net scattering received values are the
readings obtained with the specified aerosol minus these readings
obtained when the aerosol was passed through an absolute filter
upstream from the scattering chamber The relative humidity was
-------
-90- ji
""
temperature was less than 0.2°C different from the temperature
at the outlet end of the scattering chamber. The anglas listed are T'
• i
the angles between the direction of polarization of the analyzer
polarizer in front of the light pipe and the incident Ij. er light !•
• t
polarization The ranges are the differences between the largest
and smallest of the four readings at each angle indicated; the
ranges are intended to give a measure of the reproducibility of the
readings. The averages are taken cf the four readings at
the given angle. The polarization ratios are the ratios of the net
scattering received with the analyzing polarizer at the given
angle to the net scattering received with the analyzer polarization
parallel to the direction of incident polarization.
Comparison of the depolarization ratios (the 90"'/0s ratios)
is facilitated by Table 2.3 which lists these ratios and contains
che measurement ranges as percentages. This table also gives the
square roots of '-he sum; of the squares of the range percentages
and the products cf these square zocts with the corresponding
depolarization ratios as an indication of the amount of the depolari-
ist.ion ratio uncertainty rtiated to problems or' reproducibiiity..
Frcm the information in Tables 22 and 2.3 it _s noted:
1. The salt c.erosoJui, when dry; depolarized about one-fifth
of the incident polarised light intensity,
2. When humidified, :he salt aerosols depolarized less light
man they did when they were dry (The values for
humidity "ther than 12% RH for che NaCl and NaBr aerosols
art above the critical humidifies at which the;e salti
char.ge phase.- as determined earlier in this work and by
-------
\/
Table 2.3. Ratios of the 90° Polarization Components to the 0° Polarization
Components of the Net Scattering Received.
Aerosol
1% v/.v NaCl
1% v/v NaCl
8% v/v NaCl
8% v/v NaCl
8% v/v NaCl
8% v/v NaCl
8% v/v NaCl
8% v/v;'KCl
8% v/v, KC1
Polystyrene
Latex Beadsc
Dioctyl Phthalate
Filteied Air*1
Filtered Aird
8% v/v NaBr
8% v/v NaBr
8% v/v NaBr
8V v/v NaBr '
8% v/v NaBr
8% v/v NaBr
8°6 v/v NaBr
8°6 v/v NaBr
Relative
Humidity
Lpoi cent)
12
7b
12
12
77
84
92
12
12
46
46
12
12
76
12
12
12
12
76
76
82
82
90°/0°
Polari-
zation
Ratio
0.23
0.09
0.25
0.25
0.11
0.06
0.12
0.14
0.21
0.05
0.07
0.04
' 0.17
0.13
0.14
0.13
0.15
0.17
0.12
0.10
; 0.07
0.07
% Range
of Oe
Readings
25
10
6
6
2
5
7
3
0
10
14
5
2
3
5
3
4
2
2
6
7
4
% Range
of 90°
Readings
13
3
4
6
7
4
4
5
4
20
7
3
<10
<10
3
3
10
4
3
5
15
6
Square Root
of sum of
squares of
O5 and 9CC
% Ranges
28
10
7
8
7
6
8
6
4
22
16
6
-10
<10
6
4
11
4
, 4
! 8
! 17
7
Column 3
times
Column 6
0.06
0.01
0.02
0.02
0.008
0.004
0.01
0.008
0,008
0.01
0,01
0.002
<0.02
*0.01
0.008
0.005
0.02
O.OC7
0.005
! 0,008
' . 0.012
0.005
No baffle in scattering chamber.
One-holed Collison atomizer at 20 Ib/in
r -4
Diameter = 0.50 x 10 cm.
This includes cattcring from chamber walls, etc., and is the "background" which is subtracted to
-------
rl
-92- . li I
3. There were no measurable differences between the depolarization
made by the 1% v/v NaCl aerosol and the depolarization made by
the 8% v/v NaCl aerosol, either at 12% relative humidity or at 76%
\ relative humidity.
4. Removing the baffle (see Figure 2.2) did not significantly
alter the scattering recorded from the 8% v/v NaBr aerosol at
12% RHand made an average difference of 5% for the
scattering from the same aerosol at 82% Ri',
5. The dry NaCl aerosols depolarized light more effectively
than did the dry 8% v/v NaBr aerosol, but when -humidified
into droplets these aerc>ols depolarized the light to about
the same degree.
6. The random ertor in the measurement of the depolarization by
the 8% v/v KC1 seems insufficient to explain the discrepancy
between the two (12% RH) figures listed. The depolarization
by 8% v/v NaCl measured a". 84% RH seems anomalous.
7. In all cases the polarization ratios were greater than those
pied ic ted by the law of Malus (cosine squared dependence of
the intensity on the angle between the polarization directions)
for no depolarization by the targets; if the Malus law had
held, the ratios in the li'.st three columns (30°/0°, 60'/0°,
90 VO5) of Table 2.2 would be 0.75, 0.25, 0.00.
8. The nearly spherical aerosols fPSL, OOP, salts at humidities
above their change of phase humidities) gai/e depolarization
measurements which were non-zero within the estimated
-------
-93-
Since it was possible that some of the light scattered by the
particles struck the inside of the chamber and scattered again in
appreciable amounts into the light pipe, a test was run tc estimate;the
magnitude of this secondaiy scattering from the scattering chamber
walls. A long, thin strip (64 cm long, 0.64 cm wide, 0.16 cm thick)
of balsa wood was painted flat black and positioned so that it ran from
the space between the light pipe and the laser inlet hole along a line
essentially parallel to the laser beam to the back plate of the
scattering chamber, At the back it was placed so that it touched the
circumference of the light horn holder at the light pipe side of the
laser beam. The obstacle thus blocked the light pipe from the rays
of light scattered directly back f.'om the particles, but it did
not keep the light pipe from being exposed to scattering from almost
as much of the chamber walls as it would have been exposed to had
the obstacle not been there- Readings were taken on filtered air
before and after they were taken at 8% v/v NaBr. The range and
average of four such readings at each condition ate presented in
Table 2.4, along with measurements taken the same day of 8% v/v NaBr
scattering without the obstacle, from which data secondary scattering
may be estimated.
The secondary scattering from the chamber for 8% v/v NaBr was
(0.10/7,3) * 1.4% and (0 02/0 92) * 2% for scattering analyzed with the
analyzer polarizer parallel and peipendicular to the laser beam polari-
zation at 12% RH The corresponding values at 82% RH were 1.2% and 3%;
all arc negligible for thii work
V, Discussion
The greater ability of aerosol particles to depolarize light
-------
-94-
Table 2,4. Results of Secondary Scattering Tests
I Ml
Aerosol
Filtered Air
8% v/v NaBr
Filtered Air
Filtered Air
8% v/v NaBr
Filtered Air
8% v/v NaBr
Filtered Air
Filtered Air
£% v/v NaBr
Relative
Humidity
(% RH)
12
12
12
Average Ne
12
\i
Average Ne
82
S2
82
Average Ne
82
82
Obstacle
yss
yes
yes
: Scattering
. no
no
: Scattering
yes
yes
yes
Scattering (10* )
0° M 90°
Range
0 08
0,05
0 03
0,01
0 02
0.08
0.08
0.06
t Scattering:
I
no
no
0.07
1.7
Average Net Scattering:
Average
3.03
3.05
2.86
0.10
4.62
11.9
7,3
4.75
4.95
4-62
0.27
7.24
29 4
22,1
Average
0.42
0.43
0 40
Range
0.025
0.02
0.01
0.02
0.90
1.82
0.01
0.03
0.92
0.68
0.72
0.66
0.01
0.01
0.01
0.05
1.11.
2.72
0.03
0,15
1.61
T
I
molecules and from aerosoj particies (Ciemesha, et al. 1967; Cohen,
et al 1969), Homogeneous spheres made of molecules which do not
depolarize light would not deiioiari'e light, but irregular particles
and radially inhomcgeneous spheres would depolarize light.
This work measured the dc-po..arii45uon by ccloriess, cubic
/
«.
-------
-95-
crystals (NaCl, NaBr, KC1) with a variety of nor.-absorptive
refractive indices (1.54, 1,64, 1.49). The two sizes of NaCl
particles used had mass mean diameters of 0,4 microns and 0.2 microns
and gave depolarizations of 0.25 i 0.02 and 0.23 ± 0.06. The
NaBr and KC1 are assumed to have had mass mean diameters of 0.4 microns
and gave depolarizations ( 90°/0°) of 0.15 ± 0.02 and 0.18 ±0.04 respectively.
Significantly, this means a considerable fraction of the light was
depolarized by the aerosol particles. In contrast, clean air has given
measured depolarization of 0,015 (Cohen et al., 1969). Polluted air
has given depolarizations up to 0.7 (Cohen et al.,1969).
The full Mie scattering equations have not been solved for
generally irregular particles and have been solved for only a few
non-spherical cases. Rayleigh scattering (a= n D A < 0.03) has
been studied more fully,however, and we include a Rayleigh-scattering
depolarization calculation' which may indicate the order of magnitude
of depolarization to be expected for particles that are approximately
spherical.
Kerker (1969) gives the following exact analysis of the
problem of depolarization by scattering by a collection of randomly
oriented ellipsoids in the Rayleigh scattering approximation. This
approximation requires that the particle dimensions be much smaller
.than the wavelength of the light, a condition met by only a fraction
of the particles used in this work.. The depolarization ratio p
is defined as the ratio of the horizontally polarized component (90°)
of scattered light intensity to the vertically polarized lighi. intensity
component (0~) scattered for incident light having vertical polarization-
-------
-96-
pv * 12/11 = (fj[ cos2 a «• f2 sin7a;/f*
Here, f is the average of the square of the scattered electrical field
horizontal component at right angles to the direction (z) of the
incident radiation. At 180 ' backscattering this expression becomes
(sin 9*0) after mathematical manipulation
j;
pv * (L' - L")2/[3(L'j2 * 4(L'L") * 8(L")2j ,
v
for a non-absorbing particle. L1 and L" are related to the polariza-
biiities •*' and a" by
:>' * (3V/4Tt)L' -- V(m2 - l)/[4it » (m2-!) P^J
a" = (3V/4-)L" = V(m2 - Ij/t4^ » (m2-!) Pjj]
where V is the particle volume and mis the refractive index. For the case
of oblate (flattened) spheroids the depolarization factor P^
is given on the figure axis (length B) by
'-5
and by
on the two equal axes (diameter^), A - OB,
The eccentricity is e.,
e. . (A2 - B2,1/2/A.
As a sample calculation fci o it was supposed that a salt particle
-------
-97-
spheroid with A = C = 5, B = 4; that is, approximated by an oblate
spheroid whose figure axis was 80% of its diameter. Then
e - - (A2 - B2)1/2/A =0.6
s
P' B 5.0
e
P" =3.8
e
Thus,
L' = (4it/3) (m2-l)/[4Tt + (u2-!)
0.278,
i 4 x 10"4 ,
and
L" = 0.302
Therefore,
p
or the I- component (90°) would be 0.04% of the I. component (0°).
The depolarizations measured for the dry salt particles were
two orders of magnitude greater than this estimate. One possible
reason for the discrepancy is that a significant fraction of the
particles was larger than the Rayleigh scattering regime, although
the similarity in the dry depolarizations of the 8% v/v NaCl
aerosol and the 1% v/v NaCl aerosol, which was smaller, weakens
the case for an explanation based on aeroso! particle size. Another
possible reason is that the depolarization from angular (roughly
cubic) crystals is poorly approximated by that from somewhat oblate
spheroids, which are curved rather than angular. The .limiting cases
for backscattering depolarization from randomly oriented oblate
spheroids (circular disks) is 0.125 and from randomly oriented prolate
spheroids (circular cylinders) is 0.333, according to Kerker (1969).
Even the spherical particles (salt solutions, polystyrene,
diocty. phthalate) depolarized the laser light. Why was non-zero
-------
-98- «»
Several possibilities present themselves:
1. The molecules which comprise the spheres individually
depolarize the light.
2. There are small-scale (greater than a molecule, much
smaller than the particle) inhomogensities such as
layers in the polystyrene or concentration fluctuations
in the salt solution. • •
3. The particles are not quite spherical.
4, The depolarization figures are due to a systematic error.
Because the control test should have eliminated most systematic
errors and the test of secondary scattering from the chamber walls
eliminated secondary scattering as a significant problem and no other
interference is apparent, the non-zero depolarizations for spherical
particles are evidently due to experimental error.
The information in this research should be of use to those
doing woik in Light Detection and Ranging (LIDAR) measurements of
atmospheric aerosois: using the backscattering of pulsed ruby
laser light to measure concentration profiles of suspended particulates.
Such woik had been described in papers by Barret and Ben-Dov (1967);
Clemesha et al. (1967); Johnson (1969j ; Reagan (1968), who pointed
out humidity as a possible interference, and many others in a growing
number of papers on the topic,
Since particle behavior depends strongly on size, there is
always the problem cf the degree to which a laboratory aerosol
approximates the natural aerosols. One car.not exactly reproduce
the ambient ae^isoij i;i thi iaborax ;r> . They miy only be
-------
jj similar in essential characteristics. The particle size distributions
of the aerosols used in this work are given above.
7i
.'1 Another characteristic one would want to match between atmospheric
. •: and test aerosols is that of refractive index. Hanel's (1968) measure-
t ments ascribed to the atmospheric aerosol a real part of the refractive
' i
1 index of 1.57 at 40% RH, with values of real refractive index ranging
from 1.7 to 1.33 as the relative humidity went from 0% RH to 100% RH.
'. . As noted above, NaCl has & refractive index of 1.54, Water
' /
/ • • has a refractive index of 1.33. As relative humidity increases, the
" ' refractive index of the salt droplets will approach that of water.
Thus, NaCl would seem well suited with regard to index of refraction
* •
to model some atmospheric aerosols. There remains the question of the
,. degree to which the change-of-phase characteristics of NaCl
successfully model the relative humidity dependence of the atmospheric
scatterers for aerosols of other than maritime origin.
The applicability of the NaCl data must not be overstated
4 •
in the case of continental aerosols. The work of Meszaros (1968)
shows that the water soluble fraction of these aerosols have
a much greater mass fraction of sulfate and ammonium particles
than chloride particles, and that the mass fraction of water
soluble materials in the particles over Budapest in the summer was
20% for particles with radii smaller than 0.14 microns and
8% for particles with larger radii. Thus water soluble particles
are a fraction of the atmospheric aerosol, and chlorides a
-------
-100-
/ VI. Conclusions
Laboratory aerosols were illuminated by vertically polarized
, light from a He-Ne laser; the backscattered light contained a
horizontally polarized component of intensity as well as a vertically
polarized component of intensity. The ratio of the horizontally
polarized component to the vertically polarized component is the
depolarization ratio, and this ratio was 0.04 for dioctyl phthalate
oil droplets, about 0.1 for droplets and aqueous salt solutions,
• . and as high as 0.25 for dry NaCl particles. These depolarizations
are greater than the 0.015 characteristic of air and could be used
to distinguish particulate scattering from scattering by the air.
Humidity can influence the degree to which hygroscopic particles
depolarize polarized light most obviously by controlling the phase,
liquid or solid, of the aerosol particle. The data from this work
do not demonstrate whether or not changes in relative humidity
during which the particle remains a droplet change the depolarization
i
ratio., Tho data indicate that the depolarized light from droplets
- ,. may be a significant fraction (5-10%) of the scattered light received
by LIDAR probes using polavized laser light and that scattering from
irregular particles should have even greater depolarization.
Depolarization measurements of particles large enough to be in the
Mie scattering range showed a difference between spherical and
non-spherical particles. Such measurements could be useful in
-------
-101-
Section 3. Field Experiment
I. Introduction
/
A one week study with two days of sampling was undertaken the
week of May 10th. The purpose of the field test was to study quali-
tatively the relationship between lidar backscatter measurements and
relative humidity. The site selected for the field test was the
General Electric Space Center at King of Prussia, Pennsylvania.
King of Prussia is located about ten miles West North West of
Philadelphia. This site was selected to provide for safe operation
of the lidar probe relative to the general public
II. Equipment
A Podge van equipped with a 5000 watt generator supplied the
sample station with power. The G.E, lidar was located near a
building from which power-for its operation was acquired. The
overall range from the lidar to a grassy embankment used as a stop
was approximately 130 meters. Four standard General Metals high
volume samplers were operated with glass fiber filters (Gelman, Type C).
A Sinclair-Phoenix Aerosol Photometer (Model Ko. JM-3000-AL) and the
laser backscatter device used in the earlier experiments were set up
inside the van.
Three methods were used to record temperature and relative
humidity. The Hygrodynamics Electric Hygrometer-Indicator (Model
15-3001) used in the earlier experiments was operated in conjunction
with the laser device, a continuous recording hygrothermograph
(Beifort Instrument Company) was placed outside the van, and
-------
I'
-102- ' -•
III. Sampling Plan and Instrument Description
A sampling probe for the instruments inside the van extended
approximately one meter out in front of the unit and two meters from
{ the ground. Figure 3.1 illustrates the layout of the range and the
i
location of the field sampling equipment. The four high volume
("hi-vol") samplers were started consecutively at two hour intervals
beginning at approximately 0700 in order that an estimate could be
made of changing concentrations. The G. E. LIDAR probe was activated
• at random internals at the rate of three to four shots per hour,
weather permitting. Figure 3.2 shows the arrangement of the laser
backscatter device inside the van.
The weather on Tuesday, the lith of May, was sunny and warm
with light and variable winds in the morning and a light westerly
wind in the afternoon. Wednesday was cloudy with intermittent
light rain over the entire day. Winds were moderate out of the west
with occasional gusts.
A calibration curve for the flowrate of each high volume sampler
was made in the laboratory prior to the field experiment. The glass
fiber filters were put in the dessicator snd dryed for 24 hours before
obtaining the initial weight and the same procedure was followed prior
to the final weighing. The Sinclair-Phoenix aerosol photometer measures
the total forward scattering of the particulate matter and has its own
internal calibtation. Filtered air was used as the zero point for the
Model 2 laser device and the scattering above this point was that
attributable to the particles in the air.
The G E, LIDAR probe was operated by Dr, G. W. Bethke and
Dr. C, S. Cook of the Space Sciences Laboratory of the General Electric
-------
-103-
G.E LIDAR PROBE
HIGH VOLUME SAMPLERS
LIDAR FOCAL POINT
f
4m— >
1 m-J
E
CVJ
VAN }
,^.-_ ,_,__,,- . ,...-_-,...,.,.—.,.',.-. ,,-H^I
Q
I
I3m
HIGH VOLUME SAMPLERS
Figure 3.1, Equipment Configuration for Field Test
£
-------
LASER
o
**t
i
FLOW METER
SCATTERING CHAMBER jj
PUMP
OUTLET
INLET
HYGROMETER
Figure 3.2. laboratory Laser Backscattenr.g Device (Model 2, without
-------
-105-
and analysis of zesults is presented in its entirety as Appendix 5
in the form of a final report. Figures 3.6 and 3.7 contain information
based on Figure A.I of Appendix 5.
i
' IV. Results and Analysis
Table 3.1 summarizes the high volume sampler results. On Tuesday
sampler number 4 operated four hours and the loading was too light to
give significant results. A wind gust upscl samplers two and four on
the afternoon of Ma/ 12th thus these results were not considered to
be valid.
The hi-vol data from samplers two and three for May llth indicate
that from 0900 to 1700 the particulate loading was about 58 micrograms
per cubic meter (u-gm/m^). For sampler number one to record 65 tgm/m
for ths period 0700 to 1700 the loading would have to be of the
order of 95 ugm/m from 07 DO to 0900.
Figures 3.3 and 3.4 are plots of the relative humidity and the
light scattering data from the van instruments as a function of time
of day.
Of major interest is the relation of the backscatter from the
G.E. L1DAR probe to relative humidity and particulate loading. Figure 3.5
is a graph of iclative humidity versus .the backscattering signal received
plotted as a function of the total mass backscattering section for
particulates, n-o..(180:r) [See Appendix 5] for May llth. The similarity
of the relationship shown in Figure 3,5 tc that obtained for laboratory
aerosols can be noted by comparing this figure to Figures 1 12 through
1.15,. which shew increasing backscatter with increasing humidity.
A similar giaph to Figure 3-5 is not given for May 12th since there
was little change in humidity for that day. See Figure 3.7 for a
-------
-106-
_1> \.
.
' TABLE 3.1
High Volume Sampler Results
. s. u/n
Sampler No . !
1 ?{ 3' 4
TIME
0700 j
ObOO
0900
iOOO
*100 S5 _£m,m
•1200
1300
i-iOO
1500
:.600
1 700 "*
58. 6. .
1
gm/m
58.2
-gm/m
' No Da
•
b/Ui/i.
1234
i
;
, 3
98 6 ugm/m
ta
f ' >
i
No Data
' N
101-
i \
i >
3
1 ugm/m
No Data
-------
T
i
-107-
1001—
90
i
80
70
60
50
40
30
- - iiELATIVE HUMIDITY (% RH)
— TEMPERATURE (°F)
_ X \
lOi-
o
CO
li-
lt.
o
I _L
BACKSCATTERING
(laboratory device, Model 2)
FORWARD SCATTERING
(Sinclare-Phoenix photometer)
D
D
O
D
I
1
O
O
00
O
1
1
O
o
0
1
1
o
o
CO
s^
\
1
o
o
~^~
l\.
o
o
u>
1
o
o
CO
TIME OF DAY ( hours)
Figure 3,3. Temperature, Humidity, and Light Scattering on
-------
-108-
T
100
90
80
70
60
50
40
30
*
>
0
'lO
LU
_l
<
o
V)
^5
u.
LL
O
JjS
°C
c
t
c
I .
/\
/ \ X ^
>
-,,— *' ^^^ / ^-N
_ X^. ^'X ^
^
RELATIVE HUMIDITY (%
TEMPERATURE (°F)
i*
^
1 1 1 1 I 1 i 1 1 1 1
RH)
1
FORWARD SCATTERING
— (Sinclare-Phoenix photometer)
V^A -/Y-V^V
^ BACKSCATTERING
( laboratory device, Model 2)
/
\ \ \ \ I I II i 1 I
D O O O O O
3 O O O O O
D CO O CM ^j- t£>
3 0 - - — -
|
o
o
00
TIME OF DAY ( hours)
Figure 3.4., Temperature, Humidity and Light Scattering on
May'12, 1971,
-------
-109-
3.5
3.0
2.5
V)
TE
o
r 2.0
o
CO
1.5
1.0
0
\_
0
10
o
I
20 30 40
%RH
50
60
Figure ?,5. Product of aerosol mass concentration times
aerosol backscatter cross section versus
-------
-110-
Figure 3.6 shows both backscattering and relative humidity as
a function of time of day for May llth. The two spikes noted in the
i
1 backscatter, one at 0900 and one at 1500, are believed-to be caused
by periods of high particulate loading resulting from increased {
traffic flci* on the adjacent Pennsylvania Turnpike intsrchange «.
and the Sc.huylkil.1 River Expressway during these hours. The higher
particulate loading for the 0900 hour can be inferred from the hi-vol data I
of Table 3.1 (see discussion on page 105), Increased particulate
T
loading does not show in the hi-voi data for the 1500 hour since JL
hi-vol sampler No 4 produced no measurable result due to the short
sampling period :•£ four hours.
The backscatter versur relative humidity relationship for May 12th
is shown in Figure 3.7. Considering the gusty wind conditions the
backscatter remained essentially uniform throughout the day. For the
fame period the humidity remained relatively constant. The dependence
between backscatter and humidity is not as obvious in Figure 3.7
but the results do not negate the relationship shown in Figure 3.6.
Seme laboratory aerosols show changes in light scattering at a
relative humidity of about 50% (see Figure 1.12-1.15). Figure 3,6
shows a similar change, ai-.hough less pronounced.
X-..
-------
3.0
2.5
jr"
ui
TE
o
+*m*
~ 2.0
0
O
CO
*-*
^
* | £L
E '-D
1.0
C
c
11
c
I
-•--•• BACKSCATTERING
\ (G.E. LIDAR PROBE)
~ X ' -A- ^- RELATIVE HUMIDITY
O
t
1 -
1
I t
— 1
1
', 1 S(? o
III /'. —
, 1 1 / «
' , . 1 \
. 1 • 'I
0 \ /\ / \
A 1 A '
fcA.. i ' ™NX / I
VP ®" ' £) '
/ \ ft
V. / » A'' \ ~
*^^*^^H
-------
• - •« • * . - . *
I —1^» i - ! - t I -1 • t I . - • . .
-------
-113-
Light scattering data were collected using the forward
scattering Sinclair-Phoenix and Model 2 of the laboratory laser
backscattering device. Figures 3.3 and 3.4 indicate the same general
relationship between forward scattering and humidity was found as
j existed for the atmospheric lidar probe. Unfortunately, tlu signal
i '
from the Model 2 laboratory laser (see Figures 3,3 an-J 3.4) vas so
weaic that meaningful results could not be obtained. The weak signal
is believed to be the result of the much lower number concentration
of particles experienced in the atnosphere as compared to laboratory
aerosols.
V. Conclusions
The limited number of measurements ,nade in this field study
were meant only to study qualitatively the atnvspheric relative
humidity and lidar backscatter relationship. !t has been found
that this relationship follows the trends shewn by quantitative
laboratory measurements Lidar measurements of atmospheric part^culates
seem to be affected by relative humidity as well as by particulate
concentration but additional d'.ta are needed to r.onfi.rm this
supposition A clear definition of this humidity backscatter
relationship needs to be established before iidar techniques will
adequately be able to assess size or concentration properties of
-------
a
BLANK PAGE
• !
:H
i i
iU!
ft
'i!
'I
•4.
-------
-115-
REFERENCES
Aden, A. L. and M. Kerker (1951), J. Appl. Phys., 22, 1242.
Barret, E. C. and 0. Ben-Dov (1967), J. Appl. Met., 6_, 500.
Born, M. and E. Wolf (1965), Principles of Optics, Perjjamon Press,
Oxford.
/ Charlson, R. J. and, H. Horvath, and R. F. Pueschel (1967), Atm.
Environment 1^, 469.
Clemesha, B. R. and G. S. Kent, and R. W. H. Wright (1967),
J. Appl. Met., 6_, 386.
Cohen, A., J. Neuntan, W. Low (1969), J. Appl. Met., £, 952.
Cooper, Douglas W, and R. Lee Byers (1970), J. Air Poll. Control
Assn., 20_, 43.
Dave, J. V. (1969), Appl. Optics, £. 155.
Davies, C. N. (1964), Recent Advances in Aerosol Research, The
MacMillan Company, London.
Davies, C. N. (1966), editor, Aerosol Science, Academic Press,
Inc., New York.
Dobbins, R. A., and G. S. Jizmagian (1966), J. Opt. Soc. Am., 56,
/ 1345.
Dobbins, R. A and J. G. Stephen (1966), J. Opt. Soc. Am., 56,
1345.
Eiden, R. (1966), Appl. Optics, 5, 569. PRECEDING PAK
Fuchs, N. A. (1964), The Mechanics of Aerosols, The MacMillan
Company, New York.
. Green, H. L. an-^ W. R. Lane (1964), Particulate Clouds: Dusts,
Smokes, and Mists, D. Van Nostrand Company, Inc., Princeton,
-------
-116-
Hanel, G, (1968), Tellus, 2\_. 377-379.
llerdan, C. (1960), Small Particle Statistics, Academic Press, London.
ilolliday, U. and R. Rcsnick (1966), Physics, John Wiley and Sons,
. . —
New York.
Huffman, Paul J., and William R. Thursby, Jr. (1963), J. Atm.
Sci., 26_, 1073.
Jackson, J. D. (1962), Classical Electrodynamics, John Wile; and
Sons, New York.
Johnson, W. B. (1969), J. Air Poll. Control Assn., 1.9_, 176.
Junge, C. (1966), Air Chemistry and Radioactivity, Academic Press,
London.
Kerker, M. (1968), Ind. and Eng. Chem. . 60_, No. 10, 37.
Kerker, M., J. P. Kratohvil, and f Matijevic (1962), J. Opt.
Soc. Am., 5£, 551.
Lange, N. A. (1952), editor, Handbook of Chemistry, Handbook
Publishers, Inc., Sandusky, Ohio.
Lapple, C. E. (1968), Chem. Ens., 149.
Ligda, M. G. H. (1965), Discovery. 26_, 30.
Lundgren, D. A. and D W. Cooper (1969), J. Air Poll. Control
Assn., 1_9, 243.
MacKinnon, David J. (1969), J. Atm. Sci., 26, 500.
Maron, S. il., M. E. Elder and P. E. Pierce (1963), J. Coll. Sci.
1£, 733,
Meszaros, E. (1968), Tellus, 20_, 443.
Mie, G. (1908), Ann. der Physik, 25_, 377.
Neiburger, M. and M. G. Wurtele (1949), Chem. Rev., -U_, 321.
Noll, K. E., P. K. Mueller, and Miles Imada (1968), Atm. Env.,
-------
I
f
f
f
-117-
Orr, C., F. K. Hurd, and W. T. Corbett (1958), J. Coll. Sci.,
13, 472.
Phillips, D. T., P. J. Kyatt, and R. M. Berkraan (1970), J. Coll.
and Tnterf. Sci., 34_, 159.
Pueschel, R. F. and K. E. Noll (1966), "Comparison of Observed
and Calculated Visibilities Based on Measured Size
Distributions", Paper presented at the 1966 annual meeting of
the Pacific Northwest International Section of A.P.C.A.,
Seattle, Washington.
Pueschel, R. F., R. J. Charlson, and N. C. Ahlquist (1969),
» J. Appl. Met-, B, 995.
| Pueschel, R. F. aid A. T. Rossano (1966), "Light Extinction by
Mixed Aerosol Systems", Paper presented at 1966 Air Pollution
I Control Association Annual Meeting, San Francisco, California.
Reagan, J. A. (1968), Laser Focus, 4_, 26.
I 9ehmel, G. A. (1967), Amer. Ind. Hyg. Assoc. J., £8, 491.
Seltzer, D. E. (1969), Appl. Optics, &_, 905.
Twomey, S. (1954), J. Meteo,, JJ^, Z34.
Whitby, K. T. and C. M. Peterson (1965), I. $ E. C. Fund., 4, 66.
-------
I
I
1
-119-
APPENDIX 1
Correction Factor Relative Humidity vs. Temperature
1
The relative humidity of the air at a certain temperature is the
I ratio of the amount of water jp.pnr in the air to the amount the air
i -
' would have when saturated at that temperature. This is the amount of
1
a. water vapor the air would ha"e when in equilbrium with an infinite ;
| plane surface of pure water. Air having a certain amount of water vapor
would have different relative humidities at different temperatures. ;
1 The saturated vapor pressure over water in millibars is the following: :
i
| Temperature (°C) Saturation Vapor Pressure (millibars) |
20 23.373 I
1 21 24.861 i
22 26.430 |
23 28.086 }
24 29.831 ]
- -25 31.671 1
1, 26 33.608 !
27 35.649 :.
| 28 .37.796 PRECEDING PAGE BLANK. \
29 40.055 i
I 30 42,430 i
Most of the present tests were made at about 23 SC, where an increase of 1°C >
1-1
gives a change of (28.086 - 29.831) / (28.086) or - 6.2%. A decrease of I
- 13C gives (28.086 - 26.4^0) / (28.086) or + 5.9%. Thus 80% R,H at 23°C <
" would be [80 - (0.62; (80)] = 75% at 24°C . \
i
I The temperature of the mixture of aerosol and dilution air was taken ;
both in the R H; measuring chamber and at the inlet to the scattering 1
chamber The temperature difference was usually less than 0.5°C, and the
correction was applied. [The inlet temperature was usually the sane
^ temperature as the room within a few tenths of a centigrade degree.]
-------
I
I
I
I
I
1
I
-121-
APPENDIX 'i
Pressure Correction for Humidity
The humidity sensors were calibrated for atmospheric pressure rather
than for 10" H20 less than atmospheric, which was the R.H. chamber pressure. To
estimate the effect of thii; pressure difference, the actual
humidity was calculated for atmospheric pressure for three examples
(10% R.H., 50% R.H., 90% R.H.) at AP = 10" H221) ' (OA * ° 90 ' '022)
* (Rx - 2.21) / (.98)
Thus
RX = 50%
30
30
S.O'i
PRECEDING PAGE BUNK,
The differences are small, although they may be important for some
purposes. Corrections have not been made for them.
I
-------
I
I
I
I
1
L
1
1
J
I
I
I
-123-
APPENDIX 3
Relative Humidities at Which Some Salts Change Phase
Different salts change phase from solid to liquid at different rela-
tive humidities as listed below. This table is taken from Twomey's (1954)
experimental work and does not include the effect of particle size on
transition humidity. \
i
Salt Transition Point (20°C)
Nad 75-77
KC1 86
NaBr 57
NH.C1
4
91
80.3
80
45
-------
F
I
-125-
APPENDIX 4
Estimate of Weighing Errors
The mass concentrations of the aerosols were determined by the
weight gain of glass fiber filters. To estimate weighing errors, one
•»
weighing was made per day for ten days of both a 2" diameter glass fiber
filter (Gelman Type E) and of a 47 mm membrane filter (Millipore 0.45p)
both of which were kept in a dessicator between weighings.
Glass Fiber Membrane
1
1
L
I
*-
.
0,1552 gm
0,1552
0.1554
0.1554
Ocl553
0.1555
0.1554
0.1555
0.1554
0.1555
0.0907
0.0907
0.0906
0.0904
0.0907
0..0907
0.0904
0.0905
0.0903
0.0905
Mean: 0.15538 Mean: 0.9055
Stcir.darJ deviation: ,00012 Standard deviation: 0.00015
Thus, the weighing error is about ± 0,15 mgm.
PfifCEDfNC PAGE BUM
L
-------
-127-
I
I
i
APPENDIX 5
ATMOSPHERIC LIDAP. MEASUREMENTS
L
"LIDAR MEASUREMENTS"
FINAL REPORT
May 24, 1971
1
L
i
1
1
Prepared by: G. W, BETHKE and C. S. COOK
General Electric Company
Spac; Sciences Laboratory
P. 0. Box 8555
Philadelphia, Pa. 19101
Prepared for: Center for Air Environment Studies
The Pennsylvania State University
University Park, Pennsylvania 16802
-------
SYMBOLS:
-128-
T
A Clear acceptance area of lidar receiving -jDjective (cm }
c Velocity of light in air (2.9979 x 10 cm/sec)
C Total capacitance of photomultiplier tube output lead to
oscilloscope plus oscilloscope input capacitance
E Total energy per laser pulse (joules)
E Total optical efficiency of lidar system (unitless)
m Particulate (aerosol) mass concentration in air, (ygm/cc of air)
n Number concentration of air molecules or of scatterers (cm" )
nQ n for air at 0° C and 1 atm pressure (2.6871 x 1019/cc)
P Atmospheric pressure near lidar (inches of mercury)
I' One atmosphere (29.921 inches Hg.)
r Range from lidar receiving objective (cm)
R Lidar detector (photomultiplier tube) load resistance (ohms) {•
I j
R Clear acceptance radius of lidar receiving objective
a
S Lidar photomultiplier tube sensitivity at anode (?.mps/we.tt) ; ;
t Average time since the laser beam was, produced (sac)
T Air temperature near lidar (degrees Rankine) '-':'
T Freezing point of water (491.7°R) r
i-<
V Voltage drop across lidar detector load resistor (volts)
< j
V Integrated photomultiplier tube output voltage obtained j i
from calibration run L:
p Total reflectance of diffuse white target 1'
L.
o.(180s) Mass normalized backscattering cross section for particulates
in air (cm2 ugm"1 sr'1) jj
I '
cR(18CT) Rayleigh backscattering cross section^per molecule for air
(1,9925 x l
-------
j
L
-129-
•\
INTRODUCTION
As part of a program to investigate the effect of relative humidity
I
on aerosol scattering, a series of ground level lidar (Light Detection
I , And Ranging) backsoattering measurements were made of ambient atmospheric
J i •
i aerosols which were also being sampled by other techniques. These [
i\ ,
3. measurements were made on May 11, 1971 and May 12, 1971 at General .
Electric Company's Valley Forge Space Center which is located in King :
of Prussia, Pennsylvania. King of Prussia is an an outer suburban j-
1
*
I light industrial-residential area WNW of Philadelphia. The first day *
was sunny and warm with the relative humidity dropping from 60% to 1
i 1
J 29% during the lidar measurements. The second day was mostly cloudy, ;
(developed gusty SW winds and frequent light showers, and had humidity
i.
ranging from 64% to 79% during the lidar measurements. '
I ' . )
EXPERIMENTAL SET-UP . !
The lidar system was focused, aligned, and aimed at a point 366.S
. feet away and about seven feet above the ground, this point being inside
I '
*- an open cluster of four high volume samplers. About 100 feet beyond j.
*
j this measurement point, the iaser beam was terminated by a grassy ]
*
bank. The high volume samplers as well as a Sinclair-Phoenix Photometer -j
j and point-sampling laser light scattering device were operated by ]
personnel from the Pennsylvania State University Center for Air j
Environment Studies. '<
! The truck-mounted lidar system used for these measurements consists j
of a ruby laser beamed through a transmitter telescope which is mounted »
I ••
I alongside a scattered light receiving system. The Q-switched ruby las^ fr
emits a short pulse ( i-30 nanoseconds wide) of ^0.5 joules at 6943A * \
I ' ^
I wavelength. A laser-mounted photodiode monitors the exact energy of ::
-------
.
/r
each laser pulse. After passing
-130-
through the Galilean collimating
telescope, the laser light pulses have a divergence less than 1
milliradian. The adjacent parallel-mounted receiving system consists
of a 6 inch diameter refractive telescope with 4 nilliradian diameter
field stop, a small collimating lens, a thermally controlled narrow
band pass interference filter, and a photomultiplier tube detector
with S-20 spectral response. A double beam oscilloscope plus camera
is used to simultaneously display and record both the photomultiplier
tube output and the photodiode laser monitor output.
ANALYSIS AND RESULTS i
lidar Range Equation ;
As already indicated the lidar backscatttr data is displayed as
an oscilloscope voltage (V) which decreases with increasing time (t)
as the laser light beam propagates away from the lidar system. The
"range equation" which permits the extraction of particulace macs
concentration in air (m) from the lidar data was developed elsewhere
(Bethke, et al. 1970), with a somewhat restricted result being,
EA
V = c R (SEo)
[n(r) • oR(180°) * m(r) • cM(180°)].
waere
n =
(TQ/T) (p/po)
(1)
(2)
Also, since the light received from range r must first travel out,
be scattered, and then return to the lid»r system, we have,
r = c t/2 (3)
The symbols for the above and all other equations are defined at the
front o'f this appended report,
3
I
ii
11
a
II
\\'
LJ
I ;
Li
i '
LJ;
ii
ii.
i
-------
A.
1 -131-
I
This form of the range equation includes the effects of both parti-
I . culate (Mie) backscattering and molecular (Rayleigh) backscatterirg.
„ Due to the short range used for these measurements (366.5 feet), neither
I
• atmospheric extinction effects nor change-of-altitude effects are
1 included in equations (1) and (2). Reference 1 gives the complete
form of these equations which includes all of the mentioned effects.
I All parameters of equations (1) through (3) ars well known constants
or are readily obtained from the lidar systron, except for S, E , and the
I
JL particulate backscatter cross section c.,(180°'). The terms S and E are
M O
| both lidar system efficiency parameters which can be considered as one
parameter, (SE ), that varies with photomultiplier tube power supply
I voltage. The parameter (SE ) is further discussed and its experimental
evaluation described later. The term aM(180°) is the mass normalized
1 particulate backscatter cross section.
j If equation (1) is solved for the product m(r) «oM(180°) at
a specific range r, we have,
2
m ' °M(1800> * C'R! (SEO) A - n ' V180') (4)
We thus see that the lidar data leads us tc the product of aerosol
Imass concentration and aerosol backscatter cross section, m • ou(180°).
M
By combining this result with mass densities obtained by other means
J.
J such as high volume samplers, we can extract o.,(180°) from the
I
lidar results.
Lidar System Calibration
At the single photomultiplier tube voltage used for all lidar
shots, the lidar system sensitivity, (SE ) in equations (l)and (4),
-------
-132-
was determined by the following method: During the first day of
measurements (5/11/71), the lidar system was fired at a diffuse white
i target placed 366.5 feet away, this target consisting of a 2 ft. x 2 ft.
i
/ Masonite board undercoated with flat white spray paint and then over-
coated with 3M "Nextel" Velvet White Coating (a spray paint). The back-
scattering diffuse reflectance (p) from this surface when placed
perpendicular to the lidar axis has been determined to be about 0.90
at the laser wavelength.
Since the return signal from such an effective scatterer would
grossly overilluminate the photomultiplier tube (PMT) and perhaps even
damage it, a Schctt absorbing glass neutral density filter was placed
near the PMT between the narrow band interference filter and the
receiving telescope small collimating lens. The transmir.tance of this
filter has been measured by Infrared Industries as a function of
wavelength. At 6943A, the filter used for these calibration measure-
ments has a transmittance (T) of 0 0116 (optical .density = 1.934)
AlsOj the 6 inch diameter objective len:, of the receiving system was
masked down to 3/16 inch clear aperture further reducing the light
intensity at the PMT. These signal attenuating procedure kept use
of the PMT well into its linear response region for pulsed signals.
Since the backscattered return from such a flat target lasts
about 30 ranoseconds (the laser pulse period) the combined PMT plus
recording system response of also about 30 nanoseconds was not
sufficient to oermit sufficiently accurate presentation of the
i'
backscatter signal. Thus, the PMT output signal (V ) was integrated
•
through use of the PMT output coaxial cable capacitance plus other L
stray capacitances (C), A 9.3 K ohm load was used as the bleed resistor. j:
-------
i
I -133-
When this PMT integrated output is divided by the laser output energy,
I the resulting normalized signal yields (SE ) via equation (5).
2 (5)
C Vc ir.rVCE p T A)
I (SEo) = C Vc/[E pTSin2 (Ra/r)]
I
_ Here, A and R are the clear acceptance area and radius of the receiving
'• objective, respectively.
! Using the values C = 334 pf (measured with a capacitance bridge),
p = 0.90, T = 0.0116, r = 4398 inches, and R = 3/32 inch, the lidar
3>
j system SE was determined as described while using several laser
energies (E). through the range 0.25 to 0.75 joules. Although we would
not normally expect SE to be a function of E, the reduced results show
SE to linearly increase from 71 amps/watt at E = 0.25 joule to 93 amps/watt
at E = 0.75 joule. Such a variation of SE would normally be caused by
a nonlinear PMT or an incorrectly calibrated laser monitor. However,
the PMT linearity had previously b?en checked in detail, and the FMT
was used well within its linear range for all of these measurements.
Also, the iasei monitor had previously been calibrated against a
calorimeter type laser energy monitor. Although the reason is not
jcnown for this apparent variation in SE as a function of E, the lidar
/ runs were reduced using the SE value appropriate for the laser energy
L of each shot. This procedure is equivalent to the (SE ) E method
discussed in the next paragraph,
L
which is also to be found in equation (4), we see that the lidar
L
Since equation (5) can be slightly rearranged to solve for (SE ) E 1
calibration does not actually depend upon an absolute calibration of '\
I
-------
r
134-
the laser energy monitor. Thus, in the absence of a laser energy
monitor calibration, the lidar system is still correctly calibrated
by determining (SE ) E at several laser outputs, and plotting
(SE ) E versus monitor output voltage (as a reference).
RESULTS
The results from these measurements are tabulated-in Tables A.I
and A.2 for May 11, 1971 and May 12, 1971, respectively. Tables A.I jj
and A.2 list for each lidar shot the time of day, relative humidity,
I!
p/p , T/r, and m • o.,(1800). The relative humidity and T /T values •]!
are interpolated and calculated from data supplied by Mr. J. Davis i-
of Pennsylvania State University, The p/p values are calculated '-
for the correct altitude and time from hourly data supplied by the j
i
Philadelphia Inquirer Newspaper Weather Bureau. The m • o.,(180°) values
2 I
are calculated from equation (4) using R = 93.1 ohms, A = 181.5 cm , ('
r = 366. 5 feet = i.. 119 x 10 cm, and SE as described in the previous . .
section. In Figure A.I m • o..(i80'} is plotted versus time of day 'J
for both 5/11/71 and 5/12/71.. - ]i I
SJ ;
All of the data and reduced results are considered equally valid )
except for m • c..ti80i) from the 8:27 AM shot on 5/12/71. An accuracy ij j
analysis of the lidar measurements shows 20 possible sources of errors •
which could combine such that the m • oM(180:) values of Tables A., 1 and I'-
A, 2 generally have maximum possible relative (precision) errors of +25% j i
and maximum possible absolute errors of +_70%, Of course, individual
errors will tend to offset each other such that actual total errors
will be considerably less than these maximum possible values.
-------
I
I
I
.1
1
1
I
I
I
-135-
The 8:27 AM result from 5/12/71 is more than ten times smaller than
all other results from that day. In this case, the maximum passible'
-8 811
relative and absolute errors are ^0.16 x 10 and _+ 0.46 x 10 cm sr" ,
respectively. We do not know why this one m • a..(180°) result is so
M
low compared with the others, but the difference is sufficiently large
that the data point should be discarded. When making this lidar shot,
we possibly used the wrong oscilloscope sensitivity or PMT voltage
without realizing it.
Although there were frequent scattered light rain showers or
sprays during the afternoon of May 12, 1971, there was no noticeable
1
*• rain or spray in the air during the actual times of lidar shots. Even
1
1
1
*- 5/11/71 and 5/12/71, particulate backscatter cross sections can be
1
Tables A.I and A.2. These particulete backscatter cross sections :
] (2-5 x 10* cm sr' Lgnf } are in approximate agreement with the \
1 "'
equivalent cross sections calculated by Mie theory using the Junge !|
L size distribution function parameter, v, = 3.0 (d~ size distribution), 1
L index of refraction 1.5 and average mass density of 2.0 gm/cm . These
',
values are generally used in describing continental aerosols. j
a slight rain or spray in the air would strongly scatter light and ",
i
invalidate the results. I
t
Given the high volume sampler mass concentration results (from ;
J.. Davis, Pennsylvania State University) of about 60 - 100 jjgm/m for j
calculated from tne m • oM(180i>) values determined here and listed in
REFERENCES
1. Bethke, G,, C. Cook, and f. Mezger, "Laser Air Pollution Probe,"
-------
-136-
Table A.I, Atmospheric and Lidar Data for May 11, 1971
These measurements were mad,; at General Electric Company's
Valley Forge Space Center, King of Prussia, Pa.
H
-U
il
Time
of day
9:05
9:24
9:40
9:56
10:18
10:37
10:40
11:03
11:20
11:42
12:02
12:25
12:42
13:05
13:19
Rel,
hum. *
60%
54%
51%
49%
46%
44%
n
41%
39%
3B
37%
35%
34% '
33%
32%
13;42 1 "
ii:01 ! 30%
n
n
CP/PO)
1.0003
n
i,
n
"
n
it
.
1. H'W
1.0001
1.0000
0.9395
0.9991
0,9937
"
(T0/T)-
0.943
0.938
0.935
0.932
0.930
0,929
11
0.927
0,926
0.924
0,921
m-oM(180n)
(cm^sr'1)
3.32 x 10"8
2.92 "
1.98 "
2.39 "
2,30 "
1,78
1.82 "
2.07 "
1.89 "
L80 "
1.82 "
" 1.38 "
0.920 ' "1.38 "
0,918
0.916
" i 0.912
ii
0 9 .". i
.
i
"
1.18 "
1. 16 "
1.17 »
1.35 "
1.26 "
Lida:: calibration runs during ln:0l - 15:00 hours.
15:04
15:22 '
15:45
15:59
16:20
16:40
16:55
29%
ti
;'o%
n
31%
32%
.
0 9980
0,99 '8
0 99 M
0 99 :3
0,99 72
0, 997:1.
0,99'C
0 9C9
0. 9-0
it
0, 9?.i
0,91i
2,26 x 10
1.77 "
1.56 "
1.S4 "
1,56 "
0,913 | 1,71 "
it
1.43 "
Comment?
y
V.
Fair
Weather
\
k
• The=& results are based on diti supplied by -J, Davis, Finn State Univ.
-------
3.0
2.5
TE
o
~ 2.0
o
O
00
V?
1.0
0
^
I
o
o
<£>
O
O
o
CO
o
o
o
o
-138-
BACKSCATTERING
-0-0- MAY 11,1971
-0--0-MAY 12,1971
RELATIVE HUMIDITY
-A-A- MAY 11,1971
-A--A-MAYI2.I97I
I I I
o
o
CVJ
O
o
o
o
CD
90
80
70
60
50
40
30
20
o
o
CO
ii
II
i! i
TIME OF DAY (hours)
Figure A,1. Product of aercsol mass concentration times aerosol
backscatter cross section versus time of day for
May 11, 1971 and May 12, 1971
U
-------
-137-
Table A.2. Atmospheric and Lida; Data for May 12, 1971
These measurements were made at General Electric Company's
Valley Forge Space Center, King of Prussia, Pa.
Time
of day
7:34
8:00
8:27
Rel.
hum.. *
78%
74%
70%
0.9936
ft
tt
cvn-
0.943
0.942
0.941
ui-UMliov )
2.75 x 10"8
2.31 "
0.160 " **
Rain fr:m 8:30 - 9:30
10:15
10:30
10:46
ii
11:00
11:18
11:40
12:03
12:21
72%
69%
66%
ii
64%
it
0.9932
ii
0.9931
it
0.9930
0.9928
it
0.9926
0.9925
0.938
0.936
0.935
It
0.933
0.932
ff
0.931
0.930
2.50 x liT8
2.40 "
2.17 "
2.06
2.27 "
2.06
2.27
1.99 "
1.93
Scattered light rain showers
12:55
13:18
• 64%
II
0.9923
0.9922
0.928
0.927
2.12 X 10"8
2.07 "'
Scattered light rain showers
13:50
67%
0.9920
0.930
2.28 x 10"8
Scattered light rain showers
14:30
76%
0.9915
0.933
2.07 x 10"8
Scattered light rain showers
15:22
15:45
74%
72%
0.9908
0.9905
0.931
0,930
2.16 x 10"8
2.14 "
Comments
!• no wind
light
gusty
wind
Increasii
strong g\
Wind
no rain
ii it
no rain
no rain N
no rain
tigly
asty
/
* These results »re based en data supplied by J. Davis, Penn State Univ.
-------
•>v-.'iST 3wynwf B
-------