Chamber studies of visibility-reducing aerosols

CHAMBER

STUDIES

OF VISIBILITY-REDUCING

AEROSOLS* J. C. ELDER, H. J. Industrial

Hygiene

Group,

ETTINGER

and R. Y.

NELSON?.

Health Division. Los Alamos Scientific Laboratory, Los Alamos, New Mexico 87544, U.S.A.

University

of California,

Abstract-Interest in light scattering measurements as an index of aerosol mass concentration in the atmosphere and concomitant degradation of visibility prompted a laboratory study to define the ratio of mass concentration to particulate light scattering coefficient (M/b,,) for several individual aerosols. Simultaneous measurement of h,, with an integrating nephelometer and M with a suitable gravimetric technique provided the basic data. Several size distributions of flyash, coal dust and silica dust were generated under laboratory conditions into a 16 m3 chamber and the resulting M/h,, ratio compared on the basis of each aerosol’s size characteristics and similar data from field studies performed by others. The M/h,, values for the three individual aerosols were larger than those found in previous field studies. probably due to the relatively smaller numbers of particles in the optically active size range. A significant effect of particle size on M/h,, was observed for the coal dust test aerosol. M/b,, for an 0.70 pm count median dia. aerosol was 2.5 times the M/b,, for a 0.33 pm%erosol.

1. INTRODUCTION

Light scattering measurements have been conducted in several cities and elsewhere (Charlson, Ahlquist and Horvath, 1968; Noll, Mueller and Imada, 1968; Ettinger and Royer, 1971) to investigate the relationship between mass concentration of atmospheric aerosols and some convenient unit of visibility. In general the aim was to develop a suitable monitoring device, and a relationship between visibility and mass concentration, to aid in understanding and solving local air pollution problems. Continuous pollutant monitoring, minimization of sampling time, greater analytical sensitivity, and elimination of routine filter weighing were potential benefits. The primary objective of this laboratory study was to relate aerosol light scattering coefficient as defined by the integrating nephelometer (Charlson rt al., 1968) to the mass concentration and particle size characteristics of some individual aerosols which are of major importance in the environment. Although operational answers in the assessment of pollution were not expected or realized, useful information of a basic nature could be provided for future studies into light scattering as an index of atmospheric pollution by aerosols. The complex matter of translating laboratory measurements to the real environment was not dealt with in this study; however, the test aerosols selected-flyash, coal dust and silica dust-have physical qualities similar to aerosols commonly found in the atmosphere. Flyash is of interest because of the significant quantities released to the atmosphere. Silica * Work performed under auspices of the U.S.A.E.C., Division of Biomedical and Environmental Research. t School of Civil Engineering and Environmental Science, University of Oklahoma, Norman, Oklahoma. By acceptance of this article for publication, the publisher recognizes the Government’s (license) rights in any copyright and the Government an’d its authorized representatives have unrestricted right to reproduce in whole or in part said article under any copyright secured by the publisher. The Los Alamos Scientific Laboratory requests that the publisher identify this article as work performed under the auspices of the U.S. Atomic Energy Commission. 1035

I036

.I. C. EI.IXR.

H. .I. ETTINGI.II nncf K. 1’. NULLSO\

dust IS probably representative of airborne sand or road dust. C‘oal dust is probably not the shape or composition of common atmospheric aerosols, but probably is similar to many products of incomplete combustion (black. sooty materials) found in the atmosphere. ‘. THtOK\, Previous field studies have been based primarily on measurements of light scattered by a mixture of whatever aerosols happened to be in the atmosphere. This aerosol is difficult to describe, being composed of natural and photochemical aerosols, and in the case of urban aerosols may be bimodal or trimodal in size frequency characteristics. Assuming the existence of a self-preserving or constant size distribution (Friedlander and Wang, 1966) greatly simplifies the formula for measuring the effect’of mass concentration on light scattering. If size characteristics of the distribution (count tiedian diameter and geometric standard deviation) and the other important aerosol characteristics (density. refractive index. particle shape) do not change significantly. mass concentration should be proportional to the scattering coefficient h, (Horvath and Charlson. 1969). The Koschmieder Theory, developed in 1924, relates extinction coefficient (h) and meteorological range (L,,) by the simple formula: L,. =

9

(1)

where meteorological range is the distance at which the average human eye responds to a threshold brightness contrast of 0.02 for an ideal black object against the horizon sky in light with a wavelength of 550 nm (Middleton. 1952). The four components of the extinction coefficient (h) are: (I) h,,,, scattering due to particles; (2) h,Vy.scattering due to gases (Rayleigh scattering); (3) ho,,. absorption due to particles; and (4) h,,, absorption due to gases. In most situations the latter two components, h,,, and h_,. can be neglected since scattering is ordinarily much greater than absorption (Charlson, 1969). The integrating nephelometer measures light scattered over a wide solid angle by a small volume of flowing air and suspended particles. Since scattering due to air causes a permanent background signal (also a useful calibrating signal). output of the nephelometer is the total light scattering coefficient (h,,). which is the sum of h,,, and h,,. A relationship similar to equation (1) can be derived, differing only in the constant of proportionality, to apply to meteorological range in light of any wavelength in the visible range. The effective wavelength of the integrating nephelometer (500 nm) necessitates a correction of 550~ 500 nm. altering equation (I) to the following formula (Charlson or LI/., 1969): L,. 2

2.92 x 10 .1

4.7 M

z

17, 11, range at the new wavelength

milts,

(2)

where L,. is meteorological and h,, is the scattering coeficient (mm ‘1. To put meteorological range L,. in a slightly more familiar context. it should be explained that visual range, a more common term of visibility, is defined as the actual distance at which an ideal black object can just be seen against the horizon sky and may be the same as L,. to the average observer if h,Vis spatially uniform. Another term, prevailing visibility, is defined by the Weather Service as the greatest visibility which is attained or surpassed around at least half of the horizon circle (not necessarily in continuous sectors). Of these three terms, meteorological range is the only one readily applicable to a chamber study where the nephelometer is used.

Chamber studies of visibility reducing aerosols

1037

Although theoretical treatment of the ratio M/b, indicates that the ratio can depend on many variables, particularly in a mixture of polydisperse aerosols in polychromatic light (Horvath and Charlson, 1969) only particle density and size distribution are considered in the present study. Viewing wavelength is constant (500 nm) and only one aerosol is present at any time. The effect of humidity is assumed negligible because gravimetric determinations exclude moisture and none of the test aerosols is hygroscopic. Since the effect of particle shapes other than spherical is small (Hodkinson, 1962) this variable is not of concern. Particle density has an obvious effect on the M/b, ratio because mass concentration (M) is directly proportional to the product of particle volume and density. Particle size exerts an effect on M/b, in two ways: (1) the aforementioned effect on M relative to particle size (volume); and (2) particle size relative to the wavelength of light has a major effect on the magnitude of light scattering (Middleton, 1952). For transparent materials in light of 500 nm (0.5 pm) wavelength, maximum optical activity occurs for particle dia. between 0.1 and 0.7 pm (Horvath and Charlson, 1969). Assuming little divergence from this size range for other materials, an important generalization can be made regarding use of a fixed M/b,: large particles at one end of a size spectrum containing both large and small particles dominate M, and the small particles dominate b,. Unless the concept of self-preserving size distribution holds, measurement of b, does not provide a firm foundation for an indirect measure of M.

3. EXPERIMENTAL

PROCEDURE

A 16 m3 multipurpose chamber in a laboratory at 7300 ft above sea level was used for the study. Air density corrections to standard temperature and pressure were not made; therefore, all concentrations are based on volumetric flow rates calibrated for this altitude and 20°C. Chamber ventilation rates up to I 1 000 1 min- ’ provided the extensive dilution needed to reach the low mass concentrations required to simulate typical ambient air conditions. Room air was drawn into the chamber through a large high-efficiency particulate filter to eliminate interference of particles in the ambient air. Internal chamber circulation to promote thorough mixing of the entering aerosol was maintained by a stationary fan directed across the chamber from the midpoint of one wall. Humidity and dry bulb temperature measurements were obtained during each experimental run. A Wright dust feeder used to generate aerosols in the chamber (Wright, 1950) is a mechanical device which continuously scrapes a thin layer of material from a pressed cake of the desired material. The loosened material is carried by an air stream past an impactor plate where agglomerates are broken up and large particles (approximately 20 pm and larger) removed. The generation system is schematically detailed in Fig. 1. Particles leaving the dust feeder passed between two plates in the deionizer where fi- radiation from 4 Ci of tritium reduced the electrical charge on individual particles approximately 90 per cent, thereby minimizing agglomeration (Stafford and Ettinger, 1971). In the glass chamber acting as an intermediate volume, a portion of the feed stream was diverted into a sidestream filter to reduce the total amount of dust entering the chamber. Sedimentation tubes of several sizes (not shown) were connected to the outlet of the generation system to change the size characteristics of the aerosol by gravitational fallout of particles above a specific size.

House vacuum source House

afr

Z- cnch membrone lntegroting nephelomefer

Electrostatic

Air pump

‘Penetration plate to chamber

Chamber studies of visibility reducing aerosols

IO39

The sampling systems associated with the 16 m3 chamber are shown schematically in Fig. 2. They consist of the following: (1) Mass concentration measurements: two membrane filter (MF) streams and one Thermo-Systems, Inc., particle mass monitor, Model 3205A. (2) Particle size analysis: point-to-plane electrostatic precipitator (ESP) operated at 5 1 min-’ to provide aerosol samples directly on electron microscope (EM) grids (Ettinger and Posner, 1965). (3) Light scattering data: the integrating nephelometer, Meteorology Research, Inc. Model 1550, operated at a flow rate of 187 1 min- I. Simultaneous sampling by the particle mass monitor and nephelometer yielded continuous mass concentration (M) and scattering coefficient (h,) data. The scattering coefficient due to particulates (h,,) was then calculated by subtracting h,,. Occasional samples by membrane filter provided a means of confirming data obtained by the particle mass monitor. The particle mass monitor is a piezoelectric particle microbalance which provides continuous realtime measurements of mass concentration (Olin, Sem and Christenson, 1971). It was operated continuously during the experimental run during which mass concentration was allowed to vary slowly through the range of interest. Accuracy of the instrument is comparable to membrane filter weighing techniques if particle size is below 50 pm and if inlet losses are not significant. Experience with this instrument showed that heavy deposition of particulates on the sensing crystal affected accuracy significantly. Apparently particles deposited after several layers have adhered to the vibrating crystal are not accurately weighed due to progressively poorer adhesion between the successive layers of collected dust. For this reason it was necessary to determine for each aerosol at what point overloading occurred and assure that the crystal was cleaned routinely before reaching this point. Mass concentration measurements were routinely checked by flowing a known amount of air through a preweighed and dried membrane filter and determining the mass change by careful post-weighing. Two Gelman open face filter holders (47 mm dia.) were placed to collect samples in the chamber at 3540 1 min- ’ flow rate. Gelman Type VM-4 vinyl membrane filters were generally used to minimize interference due to moisture absorption on the filters. Calibration of the integrating nephelometer is based on the light scattering coefficient of air (h,, = 0.19 x 10e4 m-‘) and Freon 12 (h,, = 2.9 x 1O-4 m-‘). Calibration checks were performed before and after each run using filtered air as a downscale check and an internal calibration source as an upscale check. Particle size distribution of the aerosol in the chamber was determined by electron microscopy of ESP samples. Following sample collection, EM grids were shadowed by vacuum evaporation of chromium and then photographed at high magnification. Particles were classified into narrow ranges of projected area diameter (Herdan. 1960) from electron micrographs using the Zeiss TGZ3 particle size analyzer. Analysis to determine geometric median dia. and geometric S.D. were obtained by graphing on log probability paper (Whipple, 1916) the cumulative number percent of particles less than a stated diameter vs particle diameter. 4. RESULTS

AND

DISCUSSION

Aerosol characteristics

Aerosols used in the studies-flyash, coal dust and silica dust-possess different shape. color, density, refractive index and size distribution. Table 1 presents in comparative form

I040

J C’. El,l)L.K. H. J. t,-I IIN(,I II and K. L. NI IX)\

the major characteristics of each aerosol. including measured characteristics of the aerosols and specific information available in the literature. Particle size distributions for each aerosol are represented by log probability graphs in Figs. 3. 4 and 5. The solid lines in these figures were drawn by estimating the best fit of the line below 90 per cent. The upper 10 per cent of the particles were considered of minor consequence in determining effect on light scattering. These results are summarized in Table 2. Particle diameter is reported as gcomctric count tnedian projected area dia. (D,,y). Three different size distributions were obtained by using sedimentation tubes at the outlet of the aerosol generation system. The effectiveness of this method varied for each aerosol. with only coal dust providing the wide range of sizes desired. Preservation of a constant aerosol size distribution throughout an cxperimcntal test period was confirmed by taking ESP samples at the beginning and near the end of several test periods. The assumption that each si7c distribution did not change during the cxperimental period was based OH the results of these samples. ’ 1

‘OF

:

0”

0 6

’ ’

pm, cg = 2.0

l Flyosh, A Flyosh,

D,,=O.28 D,,=0,30

pm. pm.

1

: D,,

= Geometric

projected q

001

I ’ ’ ’ ’ ’ ’

D,,=O36

Note

_

’ /

. Flyosh,

02 Cumulative

=Geometrlc

4

mQ= 2.0 9 =2 3

dia.

S.D

IO 23 %

’ ’

count median

area

2

/ I I ’

smaller

40 than

Fig. ?. Flq’ash six

60

80

stated

90 size

characteristics.

98 (by

998 count)

9999

Chamber IO6- _

e_ _ 4_

/ , ,

,

,

studies of visibility ,

,

,

,

,

,

,

reducing

,

,

,

/

1041

aerosols (

,

,

,

(

nCoal dust,

~O.K)~rn, up -2.5 aCoal dust, -0.45pm, ug=28 lCoal dust, b,=0,33pm. 4 -30 Note: b,=Geometric count median projected area dia. q -Geometric

S.D.

2-

001

0,

0.51

2

Cumulative

5

IO 20

40

% smaller

60

than

80

stated

9095

98999989999939

size

(by count)

Fig. 4. Coal dust size characteristics. IO-

(, , , ,

8-

*Silica dust, D,,=O.38pm, cs -1.9 ASilica dust, DP,=0.40pm, q =I. 8 Note: D,,=Geometric count median projected area dia. erg =Geometric

e_ 4-

L

0.11 00

! I

O-I

I

/

I

05

, ,

I

i

2

5

Cumulative

, ,

I

I

IO 20

( / ,

1

40

% smaller

I

I

60

I

80

than stated

, /

1

I

/ ,

, ,-

1

I

I

size

(by count)

Fig. 5. Silica dust size characteristics Table 2. Size characteristics Count Aerosol Flyash I Flyash 3 Flyash 2 Coal dust Coal dust Coal dust Silica dust Silica dust

I 2 3 2 1

median dia.

I

9095969999899~99999

of aerosols Geometric S.D.

D,, (pm)

0g

0.36 0.30 0.28 0.70 0.45 0.33 0.40 0.38

1.6 2.3 2.0 2.5 2.8 3.0 1.8 1.9

Flyash is distinctly spherical and the two other dusts are quite irregular in shape. Shadow patterns produced by metallic (chromium) vacuum evaporation showed the particles of all three aerosols to have significant height. In the many electron micrographs observed during the studies, the number of f-lat particles was not significant. Several particles of irregular shape appearing in each flyash micrograph are suspected of being fragments ol large hollow flyash particles. Whether the smaller intact particles are hollow is not known although electron microprobe examinations at LASL of Hyash of I pm dia. and large1 dctccted no hollow particles (Mctz. 1970). The presence of many thin-walled spheres would have been of interest since one theoretical study indicates possibly less light scatter by hollow: spheres than by solid spheres (Suydam. 197 I ).

Light scattering and mass concentration data were continuously recorded as mass concentration was varied slowly upward or downward. Therefore. it was necessary to avcragc the scattering coefficient over a period (usually 3 5 min) in which mass data were also avcraged. Total area under the recorder trace divided by the time interval yielded mean light scattering coefficient (h,). An average clean air background signal (h,,,) was then subtracted from h, to yield scattering coefficient due to particles (h,,,,). The results of measurements of mass concentration (M) and particulate light scattering coefficient (h,,,) are presented as graphs of A4 vs h,,, in Figs. 6.7 and 8. A reference line describing the atmospheric aerosol (Charlson, 1969) is included and will be discussed later. Data points appear on each figure along with a line of best fit defined by linear regression analysis (Sokal and Rohlf. 1969). The data presented, although scattered. give no indication of nonlinearity. Values of M used in the linear regression analysis were assumed to be measured without error. Actual error in measurement of M was estimated to bc k IO per cent. Separation of the lines for coal dust (Fig. 6) indicates the strong dependence of &G/I,,, on distribution. The M h,,,, ratio ranged from I? x IO’ /lg m ’ for the 0.33 /lrn (D,,,, ) aerosol ‘81 1 I 1 I :-n-m7

0

I

I

40

Mass Fig. 6. Measured

/

60

/

1

1

I20

concentration

rnilss concentration

1

1

160

I

200

1

1

1

240

M. pg m-3

and light scatkrmg

coeficienr

fool-coiil dust

Chamber

studies of visibility

reducing

aerosols

1043

to 30 x lo5 /lg m2 for the 0.70 pm (D,,,) aerosol, a factor of 2.5. Particles in the range of maximum optical activity (0.1-0.7 pm) are considerably more numerous for the 0.33 and 0.45 pm coal dusts than for the 0.70 pm dust. Figure 4 shows that for 0.33 pm (Dpg) coal dust, 78 per cent of the particles in the distribution are smaller than 0.8 pm; for the 0.45 pm (Dp,) coal dust, 75 per cent are smaller than 0.8 pm; while for 0.70 pm (D,,J coal dust, only 55 per cent are smaller than 0.8 pm. Flyash results (Fig. 7) showed limited variation in h,, due to relatively small variation in particle size among the three runs. Although slopes do increase with decreasing particle size, it is doubtful that the slight difference in M/b,, is truly significant. The flyash distributions contained many small particles with 90 per cent or more below 0.8 pm. Silica dust (Fig. 8) showed (1) disappointing separation between size distributions and (2) an opposite pattern of size with respect to b,, although a significant difference between M/b,y, ratios was apparent. With size characteristics so similar, it is not clear why M/b,, should be separable, unless sizing errors interfere. This size difference need not be very large to cause significantly greater scattering by silica dust, clearly the most effective scattering agent of the three aerosols. The size distributions of silica dust, like flyash, have small median dia. and 85 per cent of the total number of particles are under 0.8 pm. Table 3 summarizes the variation in b,s, as a function of M for all the test aerosols. The form of these expressions is the equation of a regression line force-fitted through zero.

;;I] Charlson

Mass

Fig. 7. Measured

et al. (1968) k-----d

concentration

mass concentration

M.

pgm-3

and light scattering

coefficient

for flyash.

I 044

J.C.

:,:j

ELDLK.

H.J. ETTIKGLKand R. \i. NELSW

ASilica dust.

Moss Fig. 8. Measured

D,,=O 40 ,um (-----_)

concentration

mass concentratwn

M, pg m-3

and light scatter-ing coetficlent

lor sihca dust.

Values of variance of the slope in the third column are 95 per cent confidence limits. The final column (M/h,,, = inverse slope) is provided for comparison with Charlson’s values (Charlson, 1969) which were defined by the following: kiih,

= 4.5 (+4.5,

-2.2)

x 1oi ,ug Ill

2

(3)

where the two values in parentheses were stated as 90 per cent confidence limits. All the tabulated values for the test aerosols have larger ratios of M,!h,,, than those indicated by Charlson. This indicates a greater scattering coefficient for a given M of the atmospheric Table 3. Relationships

Flyash (0.36 pm) Flyash (0.30 pm) Flyash (0.27 pm) Coal dust (0.70 pm) Coal dust (0.45 pm) Coal dust (0.33 pm) Silica dust (0.40 pm) Silica dust (0.38 pm)

ol’ mash concentration IV and light scattcrmg /I,,, = K M

9.2 I 9.32 12.40 3.27 7.78 x.32

t 0.580 F_0.767 iO.180 & 0.495 + 0.904

15.4:

+

I I .XY

f 0.494

-k0.605

I .005

cocfticient

h,, uhcrc

lO.Sh IO.73 X.06 30.96 12.x5

12.1)’ 5.46

x.11

Chamber

studies of visibility

reducing

1045

aerosols

aerosol than for these test aerosols. Possible reasons for this difference can be surmised as: (1) the size distribution of the atmospheric aerosol contains more small particles in the optically active range and fewer large particles; or (2) the test aerosols may be better absorbers than the component aerosols in the atmosphere. Silica dust appears to most closely approximate the atmospheric aerosol when M/h,, is the only criterion. Examination of the data disclosed the possibility of investigating the effect of aerosol characteristics other than size by comparing the three aerosol distributions which are nearly the same size. The closest comparison in size is among flyash D,, = 0.30 pm (cs = 2.3), coal dust D,, = 0.33 pm (09 = 3.0), and silica dust D,, = 0.38 pm (a, = 1.9). Obviously, the validity of the comparison is limited by the different 0,‘s for the three aerosols. For a given M, say 100 pg rnw3, scattering coefficients based on the values in Table 3 are: b,, = 0.83 x 10m4 (m-l) for coal dust, b,, = 0.93 x 10e4 (m-l) for flyash, b,, = 1.19 x 10d4 (m-l) for silica dust. With size excluded as a factor, then the differences in these values represent a comparison of the effect on light scattering by the refractive indices of the materials. More rigorous treatment in future studies might offer better definition of effects, especially if matching of size distributions could be improved and more definite refractive indices obtained.

160

0

40

Mass Fig. 9. Meteorological

range

80

120

concentration

160

M,

200

240

260

pgrne3

as a function of mass concentration per cent confidence interval.

of silica dust, expressed

as 95

1046

.I. C. ELIXK. H. .I. ETIIW~K and R. \r. NLL.s)\

Mass concentrution and meteoroloyicul ranyc’ relationships Meteorological range L,. based on measured scattering coeficients was calculated using equation (2) and graphed as a function of mass concentration as shown in Figs. 9. IO and 11. This relationship for each size distribution is displayed as a 95 per cent confidence interval band. The width of the band was established by (1) calculating the 95 per cent confidence limits on h,, at five values of M distributed along the scale and (2) calculating L, with the value of h,, at either limit. These data bands are based on regression line slopes which were not forcefitted through zero. Presented for comparison are data provided by Charlson. Nell. and their co-workers. The limits on Charlson’s equation (equation 3) provided h,, values with which to calculate L,. using equation (2). The band attributed to No11 was calculated from the following equation (Noll, Mueller and Imada. 196X): 1’

K

(3)

Yzz

M

where Lis visual range in miles (assumed to be equal to L,.); ,L1 is mass concentration in pg m -3; and K is a constant ranging from 610 to 1190. The bands for silica dust and the smallest coal dust (0.33 Llrn) fall completely within the limits reported by Charlson. The Hyash aerosols marginally border on these field results.

0

J 40

Mass

Fig. 10. Meteorological

80

120

concentration

160

M.

200

240

280

pgm--3

range as a function of mass concentration cent confidence interval.

of tlqnsh. ~xpresscd

as 95 pc~

Chamber

studies of visibility

reducing

1047

aerosols

160-

0

40

Mass Fig. 11. Meteorological

range

60

120

concentration

160

M.

200

240

260

k4m-3

as a function of mass concentration per cent confidence interval.

of coal dust expressed

as 95

Only the 0.45 and 0.70 pm coal dust aerosols were significantly far from the band established for atmospheric aerosols. Basic differences apparently exist between the complex mixture of the atmospheric aerosol and these individual aerosols. Future studies might beneficially include individual aerosols and/or mixtures of aerosols with characteristics which provide: (1) greater separation of size distribution (possibly monodisperse); (2) better control of variation in geometric standard deviation if polydisperse aerosols are used; and (3) use of aerosols with smaller geometric median diameter and well-known refractive indices. Generation of photochemical aerosols closely approximating some urban atmospheric aerosols would possibly yield more meaningful results in the chamber. Sodium chloride aerosol might provide a predictable aerosol for similar studies of conditions near the oceans.

5.

SUMMARY

Various size distributions of coal dust, and flyash were generated individually in a 16 m3 chamber to describe the effect of particle size on the ratio of M/b,,. It was noted in each case that the M/b,, ratio maintained its constant proportionality for each aerosol size distribution, but a significant effect on the value of M/b,, was noted when different size distributions of the same aerosol were used. This ratio, due to the higher number of par-

J. (‘.

104x

ELII

K.

H. J. F nmc;r K and R. y.

NI.LSO\

titles in the optically active size range, ranged from 12 x 10 ,~cgm ’ for an 0.33 Llrn D,, coal dust aerosol to 30 x 10” pg mm ’ for an 0.70 jlrn D,,, coal dust aerosol, a factor of 2.5. Meteorological range as defined by Koschmieder’s relationship was calculated and graphically presented as a function of A4 for each aerosol. Published data of measurements of the atmospheric aerosol were used as sources ofcomparison. With the exception of silica dust (the only nonabsorbing aerosol tested). the individual aerosols revealed much larger mean M/h.,,, ratios than the mean values obtained by Charlson or Noll. Coal dust and flbash as individual aerosols scatter significantly less light than the urban atmospheric aerosol or silica dust. An effect of refractive index on light scattering coefficient was noted but not quantified. Exact matching of size distributions and more information of the extinction or scattering cocfhcient of the two absorbing aerosols. fl!ash and coal dust. would bc required to preciscly measure this effect. Since measured values of hl:h5,, were not consistent, a single value could not be assigned to apply to all aerosols and all size distributions. For this reason. w-c must be taken when applying the integrating nephelometcr to the measurement of mass concentration. Variables affecting h,. particularly particle size. can cause errors in M on the order of f 100 per cent (or greater in extreme cases of localized pollution). REFERENC’t Charlson

R. J. (I 969) Atmospheric

S

wsibilit\i related to aerosol mass conccntratlon

a trcvicfi. I:~fl,iro~. Sci. Tech-

Ml. 3, Y I.1 Y I x. Charlson R. J.. Ahlquist N. C’. and Horvath H. (196X) On lhc gcncl-alit) of ccwclatlon of atmospheric aerosol mass concentration and light scatter. 4tmo.sphrric IJnr/ro~~nw)n 2. 355 363. Charlson R. J., Ahlquist N. C.. Selvidge H. and MacC‘rcad! P. B.. Jr. I 1969) Monitoring of atmospheric aerosol paI-amcters with the integrating ncphclonic1c~-. .1. .-I/V P~~//r,r. (V~~iif~.~~/I\.v. 19. 937 4111. Ettingcr H. J. and Power S. (1965) Evaluation of part& airmg and aerosol sampling techniques. Aw. Ind. Hr‘y. ~ssoc. .I. 26, 11~ 2.

Fricdlandcr S. K. and Wang C‘. S. (1966) The selfprcset-\ mg parriclc swc distrihutwn Ibl- cuagulation by Brow&n motion. J. Colloid Iutrrfiw %i. 22, I26 132. Herdan G. i 1960) S&l Pnr~klc Stc~isric~s, p. 26. Academic Press. Nca York. Hodkinson H R. (1962) Light scattering and extinction hq irregular particles larger lhan the wavelength. In E/w ~VO~LI,~II~‘II~Sctrtwin~/, (Edited by Kerkcr M.). pp. X7 100. Pcrpanwn PI-css. Oxfot-d HoI-vath H. and Charlson R. J. (1969) The direct optical mea~urcmcnt of atmosphcl-lc air pollution. .-~vI. 1~1. Hy+ Assoc. J. 30, 5OC.509. Metr C. F. (1970) Los Alamos Scientific Laborator!. personal comlnunlca(1on. Middleton W. E. K. (1952) l’ision throuyh r/w .‘lt,lt~,sphrrc,. pp. 25: 105. Univcrsltj of Toronto Press. Canada. Moss 0. R.. Ettinger H. J. and Coulter J. R. (1972) Aerosol density measurements using a modified spiral centrifuge aerosol spectrometer. Em:iron. Gi. Tr~chd. 6, 613 61 7. Noll K. E.. Mueller P. K. and Imada M. ( 1068) Vihibilit! and acrowl concentratwn m urban air. Atmosphrru Ellr~i,orlmo~t 2, 465~~475. Olin J. G.. Scm G. .I. and C’hristenson D. L. (1071 ) PicLozlcctric-rlectl-~~s~~llic acrohol mass concentration monitor. i1w. I!,
J. 32,

209~ 220.

Suydam B. R. (1971) Light scattering tiiom aerosols. Los .4lamos Scicntllic Laboratory. LA-4641-MS. Whipple G. C. (1916) The element of chance 111 sanitation. .I. Frmkliu Just. 182. 37: 182, 205. Wright B. M. (1950) 1. Scient. Irlstrum. 27, I2 15.