effects of plumes

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INTRODUCTION

Evaluation of Black Smoke by Means of Reflectance Standards(Ringelmann Charts)

It is nearly 70 years since the French engineer MaximilianRingelmann 1 devised this famous procedure for assessing blacksmoke. The luminance of the plume is compared to the luminance offour white charts (numbered 1, 2, 3, and 4) on which are black gridsobscuring respectively 20, 40, 60, and 80 percent of the chart surfaces.The comparison is between (1) the amount of light transmitted tothe observer through the black smoke from the portion of sky on its farside and (2) the amount of light from a different and wider area of skyand from the sun, in whatever position it happens to be, reflected tothe observer from the white areas of the chart. Even if the smoke does

not scatter an appreciable amount of sun- and sky-light toward theobserver, the limitations of such a comparison between totally differentquantities has long been recognized. 2 Nevertheless, the Ringelmanncharts remain the basis o f smoke legislation and control in all indus-trial nations.

Evaluation of Black Smoke by Means of Transmittance StandardsA black smoke is better evaluated by comparing its luminancewith the luminance of an adjacent and like portion of sky viewed througha series of neutral filters of known transmittances. When the lumi-nances are equal, so are the transmittances. The U.S. Public HealthService Smoke Guide 3,4 and several commercial instruments oper-ate on this principle.lt must be emphasized that the true optical transmittance ex-presses the full effect of the smoke in attenuating the light that would

have come directly to the observers eye in its absence, both byscattering of this light from the particles and by absorption within them.The measurement of transmittance by comparison with neutral f ilters iserroneous if the smoke is not black and if it scatters appreciable lightfrom the sun or other parts of the sky to the observer.


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Figure 1. Laboratory smoke stock.

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Figure 2. Smoke stack transm~ssom eter.

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The black smoke was produced with a domestic oil furnace bychoking off the air supply to cause incomplete combusion of the oil; thewhite smoke was produced from fuel oil by an insecticide fogger. 9The transmittance of both smoke sources was adjustable from 100% tobelow 20%.

An experimental smoke stack was also constructed at a gas- andoil-burning steam electric plant of the Pacific Gas and Electric Com-pany at Mnrro Bay, C:tlifornia. Part of the effluent to the main stackwas diverted at the base of the stack to a convenient place on the roofof the plant, where it was accessible for study both in the duct andafter discharge to the air. The experimental duct was designed andconstructed to simulate real stack conditions of retention time, temper-ature gradient, turbulence, and discharge velocity so that the concen-tration and size distribution of the particulates within the plume wouldmatch those in the real plume as closely as possible. The duct was 31cm in diameter at the exit. Figure 3 is a view of the plant showing thetake-off on the induced-draft-fan suction durt, the discharge above theroof, and the location of the observation towers on the roof.

To measure the luminance of distant objects, a narrow-angletelephotometer was developed. The complete unit (Figure 4) consistedof a photo-multiplier-photometer lo and a pair of telescope lens sys-tems (Figure 5) mounted parallel to each other, one for viewing andaiming and the other for focusing 311 image of the target on the entrancepupil of the photomultiplier tube. The effect ive focal length of thetelephotometer was 1. 324 meters and its angular field of view was0.280.During the latter part of the study, a commercial telephotometer(Figure 6) was purchased lf for the field work. This unit was batteryoperated and its angular field of view was 0. 50.

Spectral Responses ol the Telephotumetrr and TransmissomrterThe spectral responses of the 0. 28O telephotometer and in-stacktransmissometer with visual correction fil ters were measured withthe aid of a calibrated, tungsten-filament, quartz-envelope, iodine-filled lamp 12 and a grating monochromator to disperse its spectrum.The responses (Pi) of the instruments to light of wavelength \i werecalculated using the relation Pi = Oi,:Li, where Oi was the observedresponse of the sensors to the standard lamp at ii and Li was the lampirradiance at \.i . The relative response curves are shown in Figure 7

along with the desired relative luminosity curve for the human eye.

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Figure 3. Expersmental smoke stack ot o power plant.

Telephotometer CalibrationFor calibration of the telephotometer, a self-luminous laboratorytarget was constructed and its luminance was determined with the aidof a Weston Model 614 illumination meter. This laboratory target wasused to determine the effect ive luminance of a field light source, whichwas placed on the front of the telephotometer for periodic checks of

calibration in the field.The laboratory target was composed of a 61-cm-long, 31-cm-diameter cylindrical container, two sheets of ground glass, and a 350-

6 OPTICAL PROPERTIES AND VISUAL EFFECTS


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watt flood lamp. The double layer o f ground glass diffusing screenswas placed over the open end of the cylinder, and the flood lamp wasplaced at the opposite closed end of the cylinder to illuminate them.

Flgure 4. Telephotometer assembly.

After it was determined that the ground glass was evenly illumi-nated by the flood lamp, it was masked down until a 64-cm2 area wasexposed at the center. The intensity of this area was determined bymeasuring the illumination received at various distances from thesource with the illumination meter. A curve of the illumination versusmeter distance from the light source is shown in Figure 8. Where theslope of this curve was linear and equal to -2, the illumination wasvarying with distance as though the 64-cm2 light source was a pointsource. The intensity of the source can be determined from any pointon the linear part of the curve by multiplying the illumination at thispoint by the square of the corresponding distance. The intensity of thetarget was 320 candles. The luminance of the target, which is definedOF SMOKE-STACK PLUMES 7


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12.7 mm D,AMLlHT STOP

44.8 cm

Figure 5. Telescope lens system for telephotometer.


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as the intensity of the target divided by the target area, was 50 x 103candlesimeter2. Although the luminance of the target was higher thanthe luminances that will be encountered in the field, by a factor of about10, the extra brightness was needed so that the illumination measure-ments could be taken at a distance great enough for the intensity of thesource to be determined. A neutral density filter with a transmittanceof 10% was placed in front of the telephotometer objective to give anapparent luminance of 5000 candles/meter2 for calibrating.

The field light-calibrating source was bui lt into a cylindricalcontainer 7.6 cm in diameter and about 25 cm long. A ground-glassdiffusing screen located about 7. 6 cm from the open end of the tube

Figure 6. Telephotometer.

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was illuminated by a 40-watt incandescent lamp with voltage regulationlocated at the closed end of the tube. The effect ive luminance of thisfield calibrating source was found to be 5200 candles/meter2 whencompared with the laboratory target.

WAVE LENGTH, m,l l ,m,~r~n~

Figure 7. Spectral response of telephotometer and smoke stack transmissome ter.

VISUAL EFFECTS OF SMOKE PLUMESVision involves physiological factors and cannot be defined byphysical measurements alone: however, in the absence of color con-trasts, visibili ty of objects depends among other things on the per-ception of luminance contrasts between the objects and their surround-ings. For scenes of normal brightness in daylight, the eye can usually

distinguish an object from its background when their relative contrast(defined below) exceeds *O. 02 to 0.05; in general, the greater the con-trast of the object with its background, the greater its visibil ity. 13 Inthis study we take photometric measurements of contrast betweenplumes and their background (usually the sky) as an index of the visi-bility of the plumes themselves; and we take the reduction in contrastbetween objects viewed through plumes as an index of obscuration bythe plumes. The relationships can be expected to be simplest when theplumes are viewed with a restricted field of view as in a telescope. Fornormal unrestricted vision, perception of contrasts between parts ofthe field can be influenced significant ly by the brightness of the field asa whole.

The other common criterion of vision obscuration is the reductionin meteorological visual range. This range is usually defined as the10 OPTICAL PROPERTIES AND VISUAL EFFECTS


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\ \\

\ INVERSE SOUARE LAW RELATIONc

\

1

DISTANCE. metersFigure 8. llluminonce as o function of distance from the telephotom eter calibration targe t.

distance at which the contrast of a black object, relative to the horizonsky, is 0.02 and its value is 3. 9/K, where K is the atmospheric tur-bidity coefficient defined below. This criterion is more appropriate asa measure of the visual nuisance of the smoke plume after it has beendispersed in the atmosphere. Since the relationships involved in visi-bili ty through the atmosphere are. more complex than in the contrastreduction between objects seen at a shorter distance, where naturalatmospheric attenuation and scattering may be neglected in comparisonwith the smoke, evaluation by contrast-reduction is preferable for ourpresent purpose.



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Contrast Between Targets Viewed Through Smoke PlumesThe luminance contrast between two equal-size targets withluminances of Bl and B2 (Bl > B2) may be defined asc : Bl - Bz

Bl (1)I f the targets are not self-luminous, their luminances depend on theirreflectances (Rl and R2), and on the illuminance (E) such that

BI = klRlEand B2 = k2R2E (2)where kl and k2 are factors depending on the diffusing characteristicsof the target surfaces and viewing direction. For perfectly diffusesurfaces (that obey Lamberts law) k= l/n and their luminances areindependent of viewing direction, a good approximation for the behaviorof many surfaces. If the targets have similar, even if not perfectlydiffusing surfaces and are illuminated in the same way kl = k2 and Cbecomes

(3)an intrinsic property of the targets independent of ambient lightingconditions.

When the targets are viewed through a light-scattering plume,their luminance will appear to change because of the attenuation of lightbeing transmitted through the plume and to the addition of air-lightresulting from the scatterin, 0 of ambient light by the-plume in the direc-tion of the viewer. If , in addition to the foregone assumptions, theplume transmittance and the scattered light are the same along bothlines of sight, then the apparent luminances of the targets (Bi and B2)may be writtenB=B +BT1 a 1

and B2 = Ba + B2T (4)where B1 and B2 are the luminances of the targets viewed clear of theplume, T is the plume transmittance, and B, is the plume air-light.Plume air-light is defined here as the limiting apparent luminance of ablack target viewed through the plume as its size goes to zero. Theapparent contrast (Ca) between the targets is

Bi - B2ca =- Bi (5)12 OPTICAL PROPERTIES AND VISUAL EFFECTS


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which becomes(B1 - B2) T

a = Ba + BIT(6)by substitution of (4). For targets with similar diffusing properties,substituting relations (2) with kl = k2 = k reduces (6) to

ca=ci&& EG (7)where B, is the plume air-light, E is the illumination on the targets,T is the plume transmittance, and A is a constant equal to kR1. Thusthe luminance contrast between targets will always be reduced whenthey are viewed through a plume that scatters light. If the plumescatters a negligib le amount of light, B, - 0 and the target contrastremains unchanged. Equation (7) also shows that the apparent contrastbetween the targets. besides being a function of plume air-light (B,), isnow a function of the target illumination (E) and the plume transmittance(T). An increase in air-light results in a decrease in contrast, where-as an increase in target illumination or plume transmittance results inan increase in the apparent contrast between the targets.

Contrast Between Smoke Plumes and Their BackgroundThe contrast Cp between a plume of luminance BP viewed againstan extended background of luminance Bb is

Bp - BbcP =- Bbbut by relations (4), the luminance BP of a light-scattering plumeviewed against a background of luminance Bb is

(8)

where B, is the plume air-light and T the plume transmittance. Sub-stitution of (9) into (8) gives

for the contrast between a plume and its background. Thus, the plumeair-light also plays an important role in determining plume-to-back-ground contrast. For a plume that scatters a negligible amount oflight, B, 2 0 and Cp = T-l.OF SMOKE-STACK PLUMES 13


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Contrast Reduction by the Experimental Rlack aud White PlumesTo illustrate how the contrast between objects can vary when theobjects are viewed through plumes from different directions relative

to the sun on clear days, the apparent contrast between targets wasmeasured through the experimental black and white plumes on clearand overcast days for periods of 6 to 7 hours from fixed plume-viewingpositions. The viewing directions chosen were east and west, sinceextremes in the angle between plume, viewer, and sun occur whenplumes are viewed throughout the day in these directions, Runs wereconducted from about 9:00 a. m. to 4:00 p. m. with plumes having in-stack transmittances of 60 and 40 percent. The plume directions weresouth or north, according to the wind. The targets were located about3 meters behind the plumes, and the telephotometer was located about12 meters east or west of the plumes. Measurements of the inherentcontrast between the targets and of intensity of solar radiation on ahorizontal surface, as indicated by an Eppley globe pyrheliometer, l4were taken concurrently during each run. Tests were also conducted toillustrate how the apparent contrast between targets can vary with plumetransmittance when the targets are viewed through plumes under illu-minating conditions that result in high and low plume-air-light luminances

Two types of target-pairs were used. The first pair of targetsconsisted of 31- by 62-cm black and white panels, The surfaces ofthese targets were of matte finish and exhibited similar diffusing char-acteristics. Consequently, the contrast between them showed littlevariation when they were illuminated from different directions as theday progressed, even though their luminances varied considerably.The targets in the second pair were self-luminous; each con-sisted of a circular, 13-cm-diameter frosted-glass sheet located in theback of a cylindrical 93-cm-long, 31-cm-diameter black box target.The luminance of the frosted glass was controlled by illuminating itfrom behind with four loo-watt lamps and one 25-watt lamp. Its lumi-nance was 10700 candles/meter2 with all lamps on and 800 candles,/meter2 with the 25-watt lamp on; it served as a black box target with

the lamps off. The lamp housing was cooled with a small blower. Theluminances of these targets, unlike those of the panel targets, wereindependent of ambient illumination and remained constant as the dayprogressed.The apparent contrasts between the panel targets were measuredthrough white and black plumes with 60 and 40 percent transmittanceduring clear days from both east and west (Figures 9a and 9b), andduring overcast days from the east only (Figure 10). The apparentcontrasts between the self-luminous targets were measured throughwhite and black plumes with 60 and 40 percent transmittance during

clear days from the east (Figures lla and lib). Measurements ofplume air-light were taken concurrently during the latter runs by usingthe target as a black box to eliminate transmitted light from the back-ground.



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The different contrasts between both sets of targets were meas-ured through black and white plumes of various in-stack transmittanceslate in the afternoon from the east (high plume air-light) and from thewest (low plume air-light). The measurements were taken within aperiod of about 45 minutes, between 3:30 and 4:15 p.m. Figure 12ashows the results for white plumes; Figure 12b, for black plumes.

100 rRAST/EST-

i 0900 1000 1100 1200 1300 1400 1500 1600h4 LOCAL TIM E OF DAY, hr(01 WHITE PLUME!zE loor

40

30 ii APPARENTCONTRASTVIEWED FROM kEST20 40% TRANSMITTANCE60% TRANSMITTANCE10-

II APPARENTCONTRASTVIEWED FROM EAST/ 40% TRANSMITTANCE60% TRANSMITTANCE01 I I I 1 I I 1

0900 1000 1100 1200 1300 1400 1500 1600LOCAL TIME OF DAY, hr(b) BLACK PLUME

Figure 9. Variation throughout the day of apparent contrast between panel targets viewed fromeast and wes t through experimental plumes on clear days.



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100 I I I I I 1

90 - .INHERENT CONTRAST

a0 -

70- APPARENT CONT RAST60% TRANSMITTANCE60-

50 - 000 0

40 -

30 - 40% TRANSMITTANCE20 -

5 j , , , , , I 1a . 900 1000 1100 1200 1300 1400 1500 1600

tG-=l LOCAL TIME OF DAY, hrE (0) WHITE PLUME$ loo- , I i I I Iu .90 -4 AINHERENT CONTRAST

80 - APPARENT CONTRAST60% TRANSMITTANCE

60 -50 - APPARENT CONTRAST40% TRANSMITTANCE40 -30 -

20 -

lo-0 I I I I 1 I900 1000 1100 1200 1300 1400 1 so0 1600

LOCAL TIME OF DAY, hr(b) BLACK PLUME

Figure 10. Variation throughout the day of apparent contrast between panel targe ts viewed fromthe east through experimental plumes on overcast days.



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-60% TRANSMITTANCE40% TRANSMITTANCE - -u 2,000 -5

90 - INHERENT CO NTRAST / - 10,000PLUME AIR-LIGHT

80 - 60% TRANSMITTANCE405 TRANSMITTANCE .

60 - 7,000

APPARENT CONTRAST

LOCAL TIME OF DAY, hr((11 WH ITE PLUMEz 100 2,400 ;

52.200 -0 -

80 -

70 -

60 -

I 50 -

40 -

30 -

20 -

IO -

APPARENT CONTRAST60% TRANSMITTANCE40% TRANSMITTANCE

PLUME AIR-LIGHT60: TRANSMITTANCE40% TRANSMITTANCE

u.- 2,000- 1,800

- 1,600

- 1,400

- 1,200

- 1,000

- 800

- 600

0 I I I I I I J 400900 1000 1100 1200 1300 1400 1500 1600LOCAL TIME OF DAY, hr(b) BLAC K PLUME

Figure 11 . Variation throughout the day of apparent contra st between self-luminous targets viewedfrom the cost through experimental plumes on clear days.

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ILaYI o . - SELF-LUMINOUS TAR GET Snn A A - GRAY-SCALE PANEL TAR GET S ,

Figure 12.

lY 0 IO 20 30 40 50 60 70 80 90 1005 IN STACK TRANS MITTAN CE, percents (a) WHITE PLUMEI- loo-z2g 90-a% 80 -

70 -

60 -50 -

40 -

30 -

VIE\

VIEWED FROM

VED FROME WEST

41 , $ : 1 ;~k:!~ ::~~E ; , 1

0 10 20 30 40 50 60 70 80 90 100IN STACK TRANS MITTAN CE, percent(b) BLACK PLUME

Apparent contrast between panel and self-luminous targets viewed from easthrough experimental plumes on clear days (3:30 - 4:15 p.m.), os a functiontransmittance.

t and WBLof DI ume



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Contrast Between the Experimental Black and White Plumesand Their Sky Backgrounds

To illustrate how the contrast between black and white plumesand their sky background can vary when viewed from different direc-tions relative to the sun on a clear day, tests similar to the contrast-reduction experiments were conducted. Here the contrast betweenplumes with transmittances of 60 percent and the sky behind them weremeasured over 6- to Y-hour periods on clear days from the east andwest. The results are shown in Figure 13a for white plumes andFigure 13b for black plumes. The low plume-to-sky contrast measure-ments from the west early in the morning in Figure 13a resulted fromatmospheric haze, which gives a high plume background luminanceparticularly when the observer is viewing in the direction of the sun.The hazy condition cleared by about 11:00 a. m. The contrast betweenblack and white plumes with a range of in-stack transmittances andtheir sky backgrounds were measured late in the afternoon from theeast and from the west between 3:30 and 4:15 p.m. (Figures 14a and14b), and from the west on overcast days (Figures 15a and 15b). Thefigures also show values of plume air-light, measured concurrently byusing a black box target to eliminate directly transmitted light.

The results of the contrast obscuration and plume-to-sky con-trast experiments showed that, as expected, the contrast betweenobjects viewed through a white plume and the contrast between a whiteplume and its background are both highly variable with respect to theplume-viewing direction on a clear day. As the angle between theviewer, plume, and sun increased, the light scattered by the plume inthe direction of the viewer increased. The increase in scattered lightcaused a decrease in contrast between objects viewed through the plumeand an increase in contrast between the plume and its sky background.If the objects were not self-luminous, and most objects are not, thecontrast between them was further affected because the illumination ofthe object decreased as the viewer-plume-sun angle increased.

The contrast obscuration experiments with black plumes showedthat this obscuration of contrast was also variable with respect toplume-viewing direction on clear days. The variation was similar tothat found with white plumes, but less pronounced. The plume-to-skycontrast experiments with black plumes, unlike those with white plumes,showed that the contrast between black plumes and their sky backgroundvaried little with respect to plume-viewing direction on a clear day.

This is to be expected with an ideal black plume, whose luminanceis a constant percentage (the transmittance) of the sky behind it; how-ever, a perfectly non-scattering plume cannot exist. An analysis ofthese data by Stoeber 15 suggests that with this black plume, the light itscatters forward toward the observer from the area of sky seen adja-cent to the plume, predominates over sunlight, which must be scatteredthrough a wider angle to reach the observer when the sun is not seennear the plume. Thus luminance of the plume light will change in pro-portion as luminance of its sky background changes. This result isOF SMOKE-STACK PLUMES 19


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AIR-LIGHT, VIEwEDFROM EAST

1000 1100 1200 1300 1400 1500 1600LOCAL TIME OF DAY, hr(0) WHITE PLUME

J 11,OOC

/

10,0009,000

1

8,0007,000

16 000

120r 12,600

AIK-LIGHT. VIEWEDFROM EAST

P AIR-LIGHT, VIEWEDinn,, WCCT

CONTRAST, VIEWED FROM WEST AAACONTRAST, VIEWED FROM EAST l l l

J- 2,400 5

- 1,800- 1,600- 1,400- 1,200- 1,000- 800- 600- 400

0 I I I I I II 1 20000 1000 1100 1200 1300 1400 1500 1600LOCAL TIME OF DAY, hr(b) BLACK PLUME

Figure 13. Air-light and plume-to-sky contrast of experimental plumes with 60 percent transmittanceviewed from east andwest on clear d ays, as a function of time.



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-- FROM EAST - OY=,

10 I I I I 1 t I tlo 20 30 40 50 60 70 80PLUME TRANS MITTAN CE, percent(a) WHITE PLUME

-90

- 80 AIR-LIGHT, VIEWEDFROM EAST

12,00020,00018,00016,00014,000

12,00010,000

8,000

6,000>\lO(

4,0002,000

0)

5,000

4,000

3,000

2,000

1,000

70 - CONT RAST, VIEWED

60 -

CONTRAST, VIEWE3o - FROM EAS T

L 1 I I I I I 10 10 20 30 40 50 60 70 80 90 100

PLUME TRANS MITTANC E, percent(b) BLACK PLUMEFigure 14. Air-light and plume-to-sky contrast of experimental plumes viewed from cost and wes t

on clear days, (3:30 4:15 p.m .), 05 0 function of plume transmittance.



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-60 -

- 50

- 40I

- 20

- 10

010 1

0 10 20 30 40 50 60 70 80 90 100PLUME TRANS MITTANC E, percent(a) WHITE PLUME

- 900 23-800 a.

- 700

- 600

- 500- 400

- 300

- 200

-0 10 20 30 40 50 60 70 80 90 100PLUME TRANS MITTANC E, percent(b) BLACK PLUME

Figure 15. Air-light and plume-to-sky contrast of experimental plumes viewed on overcast days.



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physical ly reasonable because the scattering by absorbing particlesdif fers from that by transparent particles chiefly in the reduction inscattering at angles outside the main forward lobe in the scatteringpattern. This ef fect would not be expec,ted if the plume backgroundwere a dark object.

On overcast days when illumination of plumes was by diffusedlight and not strongly directional, the contrast between objects viewedthrough black and white plumes and the contrast between the plumesand their background did not show the extreme variation with viewingdirection that was apparent on clear days; rather, the contrasts fluctu-ated randomly as overcast areas of varying density passed in front ofthe sun. The plume-to-background contrast of a white plume and itsobscuration of contrast on an overcast day can be extremely complexand vary in no predictable manner as overcast conditions vary fromday to day and minute to minute.Evaluation of Plumes by Trained Observers

Since tests have shown that the visual appearance of a plume asmeasured instrumentally by telephotometer varies with direction ofview relative to the sun, we must ask whether the subjective evaluationof plumes by trained observers also varies with direction of view rela-tive to sun. Or can the trained observer compensate for the observedvariation in the luminance contrast of plumes with viewing direction?Evaluation tests were made at the smoke school of an air pollu-tion control district. A group of five observers was positioned about18 meters east of the training stacks and a group of four observers was

positioned about 18 meters west of the stacks. The observers wererequested to allot Ringelmann numbers to the plumes in accordance withtheir training and to allow for any variation in plume appearance due toviewing direction. A telephotometer was stationed with both groups.The observers had been trained to recognize plumes of a given Ringel-mann number as indicated by appropriate readings of the in-stacktransmittance under various lighting conditions. Both black and whiteplumes with in-stack transmittances of 15, 35, 50, 65, and 85 percentwere generated in random sequences. On signal, the observersassessed the plumes and the telephotometer operators measured theplume and background luminances.

Eight runs were made with the groups located east and west ofthe stacks, four runs with dark plumes and four with white plumes.Six of the runs were made with alternating dark and white plumesbetween 9: 30 and 11:OO a. m. The other runs were made between 12:30and 1:15 p.m. A run consisted of 10 assessments, with each of thetransmittance levels occurring twice in each run, Each run lasted about15 minutes, The average assessments by the observers for the fivetransmittances of each run are shown in Figures 16 and 17, for whiteand dark plumes, respectively. The assessments were made in termsof Ringelmann numbers for the dark plumes and equivalent Ringelmann0 F SMOKE -STACK PLUMES 23


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numbers for the white plumes. The calibration scheme (Ringelmannnumber: in-stack transmittance) by which the observers were trainedis described by Yocum and Coons 6, 7 and is shown on the ordinates ofFigures 16 and 17. The concurrent plume-to-sky contrast measure-ments from east and west of the plume are shown for each run inFigure 18.I I I I I I I

- 900:

-70z-f -2 -60E 2-z --z -50z?

izi -40

5 3-.?CT -- 30

TIME GROUP A GROUP BEAST WESTr:30 a.m . o o o . . .lo:oo a.m. AAA AAA t310:30 a.m. 0 0 0 n n l12:30 p.m. vv v VW- A

EAS T: 9:30, lO:OO, lo:30 a.m .

FAST AND WEST

WEST: 9:30, l@nn JO:30r-0 o.m

0 10 20 30 40 50 60 70 80 90

!IN-STACK TRANSMITTANCE, perceniFigure 16. Variation in evaluations of white plumes by groups of trained observers viewing from

east and west on o clear day.

Plans to continue assessing the plumes with the group positionedeast and west o f the stacks were abandoned shortly after 1:00 p.m.because clouds began to form on the eastern horizon and a change inthe wind direction shifted the plume direction from south to east.During the afternoon, runs were made with each plume with both panelscombined and positioned north and south of the stacks. The results areshown in Figures 19a and 19b for white and dark plumes, respectively.Additional tests were conducted with trained observers to deter-mine how accurately they could evaluate smoke plumes when allowedto choose their own viewing conditions. For these tests a panel of six



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observers were shown black and white plumes with in-stack transmit-tances of 15, 30, 45, 60, 75, and 90 percent. As before, the plumeswere generated in random sequence and on signal the observers assessedthe plumes. Five runs were made, each consisting of 12 assessmentswith each of the transmittance levels occurring twice in each run. Norestriction was placed on viewing direction. The tests were conductednear noon. Results are given in Tables 1 and 2, which show the meanof the IO estimates by each inspector for the six transmittances assessedwith both plumes, the standard deviation of the estimates from the mean,and the error of the mean.

EAST: 9:45, 10:15, lo:45 a.m.EAS T AND W EST : 1:OO p.m.

WEST: 9:45, 10:15, lo:45 a.m.

GROUP A GROUP B9:45 a.m. 0 o 0 l l l

10: 15 a.m. AAA AAA10:45 a.m. 0 Cl 0 l DD

1:oo p.m. vvv www5 I I I I I I I I

0 10 20 30 40 50 60 70 80 90 100IN-STACK TRANSMITTAN CE, percent

Figure 17. Variation in evaluations of block plume s by groups of trained observers viewing fromeast and west on a clear day.

These tests of trained observers show that the observers thatviewed white plumes on a clear day facing the sun assessed the plumesat a higher Ringelmann number (lower transmittance) than did observ-ers that viewed the plumes with the sun to their backs. For darkerplumes, the ef fect was less pronounced. Group assessments showedgood agreement for similar sun-plume-viewer geometries.OF SMOKE-STACK PLUMES 25

. ..-. -.-~--_ ..--


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100 1I I I I 1I CONTRAST OF VrHlTE PLUMEFROhl

T IME EAST W E S T9:30o.m 0 0 0 . . .

10.00 a.m. AfTA **.70 IO:30 a.m . 000 n l l

12:30 p.m . oov . . .60

WE ST, 9:30, 10.00, 10.30 a.m.

2 - 10 -dt- -2o-s. i2 - 30 -

40 -

-50 -

60 -

70 -

80 -

90 -

EA ST . 9:45, 10 15, lo:45 a.m .&J7rM 1

WE ST: 9:45, 10.15,

.1:00 p.m. RESULTSAFFECTED BYCLOUDS INBACKGROUND

CONTRAST OF BLACK PLUME. FROMTIME EAST W E S T

9:45 a.m. 0 0 0 . . l10.15 a.m. noA . . .10:45 0.m . 0 0 0 l m m

l:oo p.m . vov . . .

- ml I I I I I I0 10 20 30 40 50 60 70 80 90 100IN-STACK TRANS MITTANC E, percent

Figure 18. Variation in plume-to-sky contrast of plumes from east and west on (1 clear day duringevaluation by trained o bserv ers.



32/92

7 ESTIMATES BY GROUP B PPP -0 _. CONTRAST OF PLUME on0 8Drn80 i

l- L-70 ;;

$- E

4 ,20 2- w--10

S-

GROUPS A AND B. SOUTH

CONTRAST, FROM NORTH

CONTRAST, FROM SOUTH

- 80

- 60

- 50

-40

-30

- 20

0 10 20 30 40 50 60 70 80 90IN STACk TRAN SMITTAN CE, percent

(a) WHITE PLUME

I I I I I 5 I I IO- NORTH SOUTH2:oo p.m. 3: 15 p.m.

--90 ESTIMATES BY GROUP A 000 . . .ESTIMATES BY GROUP B hhP &AACONTRASTOF PLUME q nn ,..l--80 ;%

--70 pj GROUPS A AND B ,7 K NORTH AND SOUTH

4 10 x CONTRAST, FROM NORTHr .

E.I002e-90 u;5-80 2-70-60

-50

-40

-30

-20

-10

00 '0 20 30 40 50 60 70 80 90 100IN-STACK TRANS MITTAN CE, percen+

(b) BLACK PLUMEFigure 19. Variation in evaluations of plumes by groups viewing from north and south on (1 clear

day.OF SMOKE-STACK PLUMES 27


33/92

Tablel. EVALUATION OF BLACKTRAININGPLUMESBY TRAINEDSMOKEINSPECTORS

In-stack %rons, 15 30 45 60 75 90Equiv. Ring. No. 3.50 3.00 2.40 1.90 1.30 0.50

Imp. No. 1 3.68 3.08 2.65 2.14 1.65 1.20Std . Dev. 0.34 0.34 0.37 0.24 0.33 0.38Error t 0.18 t 0.08 + 0.25 + 0.24 t 0.35 + 0.70Imp. No. 2 3.58 2.95 2.53 2.03 1.55 1.10Std . Dev. 0.32 0.35 0.31 0.34 0.24 0.37Error + 0.08 - 0.05 t 0.13 t 0.13 + 0.25 t 0.60hp. No. 3 3.58 3.00 2.67 2.30 1.68 1.23Std . Dev. 0.24 0.35 0.33 0.29 0.34 0.39Error t 0.08 0 t 0.27 t 0.40 t 0.38 + 0.73Insp. No. 4 3.38 2.70 2.10 1.75 1.45 0.84Std . Dev. 0.42 0.33 0.30 0.19 0.31 0.24Error - 0.12 - 0.30 - 0.30 - 0.15 to.15 t 0.34hp. No. 5 3.70 2.94 2.43 1.78 1.35 1.03Std. Dev. 0.50 0.28 0.32 0.26 0.20 0.21Error + 0.20 - 0.06 + 0.03 - 0.12 t 0.05 t 0.53Imp. No. 6 3.50 3.00 2.35 1.98 1.43 1.10Std . Dev. 0.34 0.29 0.33 0.26 0.24 0.12Error 0 0 - 0.05 + 0.08 + 0.13 t 0.60Table2. EVALUATION OF WHITE TRAINING PLUMES B Y TRAINED SMOKE INSPECTORS

In-stack trans, % 15 30 45 60 75 90Equiv. Ring. No. 4,20 3.30 2.60 1.80 1.00 0

Imp. No. 1 3.90 2.94 2.42 1.70 1.03 0.03Std . Dev. 0.28 0.10 0.40 0.33 0.34 0.09Error - 0.30 - 0.36 - 0.18 - 0.10 t 0..03 + 0.03Insp. No. 2 3.92 2.83 2.59 1.78 1.13 0.16Std . Dev. 0.31 0.30 0.41 0.20 0.41 0.21Error - 0.28 - 0.47 - 0.01 - 0.02 t 0.13 t 0.16Imp. No. 3 3.73 2.97 2.47 1.70 0.97 0.17Std. DE-V. 0.42 0.38 0.46 0.42 0.56 0.17Error - 0.47 - 0.33 - 0.13 - 0.10 - 0.03 t 0.17Imp. NO. 4 4.28 3.55 3.15 2.55 1.98 1.18Std . Dev. 0.31 0.44 0.28 0.37 0.21 0.23Error t 0.08 t 0.25 t 0.55 + 0.75 + 0.98 + 1.18Insp. No. 5 4.18 3.58 2.78 2.38 1.80 1.06Std. Dev. 0.23 0.35 0.24 0.32 0.29 0.11Error - 0.02 + 0.28 + 0.18 + 0.52 t 0.80 t 1.06hp. No. 6 4.08 3.58 3.08 2.53 1.90 1.10Std. Dev. 0.37 0.46 0.30 0.51 0.30 0.17Error - 0.12 t 0.28 t 0.48 t 0.73 t 0.90 + 1.10



34/92

A similar series of tests conducted with white plumes on anovercast day did not show any variation with plume-viewing direction.This was to be expected, since on an overcast day the plume illumina-tion is not directional.

OPTICAL PROPERTIES OF SMOKE PLUMESA Brief Outline of Scattering by Fine Particles

The optical properties of smokes could be studied to some extentwithout reference to their connection with the size, composition, andconcentration of the constituent smoke particles; however, some knowl-edge of this connection is of great practical interest, and can aid ininterpreting and understanding the observations.If in the presence of an interposed aerosol the observing eye orinstrument receives a flux F direct from a source of light (which canbe a portion of sky or an object such as a lamp) and in the absence ofthe aerosol a flux F,, then the aerosol transmittance T is given byBouguers law (often called the Lambert-Beer law), which may bewrittenT E i = exp (-naQt)

0

where n = the number of particles per unit volume of air in the lightpath of length t through the aerosol; a = the projected area of one ofthese particles; Q = the particle extinction coefficient or extinction-efficiency factor defined asQE total flux scattered and absorbed by a particleflux geometrically incident on the particle

If particles of different sizes and extinction coefficients arepresent then a summation over all values of a and Q must be taken, oralternatively, appropriately taken average values aand & must be taken,so the law may be written

T-exp(-txniaiQi)-exp(-naQt).The product na& = K is sometimes called the turbidity coefficientor the extinction coefficient of the aerosol as a whole, and has thedimensions (length) -1.The particle extinction coefficient or extinction eff iciency factor

Q depends on the particle refractive index relative to the surroundingmedium, its shape, and its size relative to the wavelength usuallyexpressed as r* z 7$, where d is the particle diameter and 1 is thewavelength of light in the medium surrounding it.



35/92

Particles of transparent materials, i. e. , materials with negli-gible electrical conductivity, have real values for the refractive index,e.g., 1. 33 for water, 1. 55 for quartz. Absorbing materials, i.e.,those having appreciable conductivity, have a complex refractive index,e. g., (2 -i) for carbon (moderately absorbing) in much of the visiblespectrum 16 or (0.89 - 2.23i) for copper at 5, 500 A. 17

In visible light, in air, Q 7 d4 when d 6 0.05 microns. This isthe Rayleigh or dipole scattering regime for very small particles inwhich Q seldom exceeds lo- 2. When d = 2 microns, Q exceeds by lessthan 50 percent, sometimes by less than 10 percent, the limiting value2 to which it tends at larger diameters. At larger diameters the totalextinction by the particle, regardless of how it is divided betweenscattering and absorption and regardless of particle composition andshape, is simply proportional to its projected area. I f the particle isa transparent sphere, then as d increases above 0.05 micron, Q rises,attains a maximum value about 3 or 4 somewhere between 2/3 and 1micron diameter, and settles after some oscillation to the limitingvalue 2 (Figure 20, curves A and B). With an irregular transparentparticle averaged over all orientations (rotational Brownian motionphysically accomplishes this), the Q curve settles to 2 after passingthrough a simple maximum whose position and size depend on the par-tic le shape. For an absorbing particle of any shape, Q settles to 2without oscillation and with only a weak maximum or none, completingthe rise when 2 1 l/2 to 1 (Figure 20, curve C).

The Q curves for transparent, non-spherical particles averagedover all orientations resemble those for absorbing particles, exceptthat the rise is slower than with transparent spheres. The maximumis less marked the more irregular the particle shape. I8Thus, if the aerosol transmittance is found not to vary with wave-length, its projected-area-concentration na or n5 can be deduced fromBouguers law taking Q = 2 without serious error, and without needingto know its particle size or composition. I f the transmittance does varywith wavelength, then an estimate of size and area-concentration can be

derived by fitt ing measurement at two wavelengths to the theoreticalcurves of Q; the better the particle composition is known, the moreaccurate the estimate.It is interesting to express the Bouguer law also in terms of themass concentration of the particles, Cn,=nsPd3/6, where P and d arethe density and diameter of the particles in consistent units, giving formonodisperse particlesT = exp ( - 3CmQt/2Pd).

Even when Q = 2 and the mass-concentration is constant, the transmit-tance will decrease with decreasing particle size. A family of curvesshowing the relation between plume transmittance and mass concentra-tion in a plume 3 meters in diameter with black and white particles ofdifferent sizes is shown in Figure 21.30 OPTICAL PROPERTIES AND VISUAL EFFECTS


36/92

OF

I I I-, -4 %.I ND lN313143303 NOIL3NllX3

SMOKE-STACK PLUMES 31


37/92

MASS CONCEN TRATION, graitdft 3

\\\

' ' ' ' '1

h' L\ -I----

PLUME THICKNESS + = 3 METERS ( IO ft)

iiPARTICLE SPECIFIC GRAVITY P = UNITY

II

Q \1 ,II

I

\ \ \ \ \ \ \\ \ \ .I I \ \ \

i iii0 0.1 0.2 0.3 0.4 0.5 0.6 u./ U.8MASS CONCENTRATION, grams /m 3

Figure 21. Relationship of transm ittance and mas s concentration for plumes containing particlesof various diameters and irregular shapes.

In preparing this figure we assumed that the particles were irreg-ular in shape, and of unit specific gravity; that those in the white smokewere transparent with refractive index 1.5 (many common transparentmaterials); and those in the black smoke were like carbon with complexrefractive index about 2(1-O. 5i). Values for the extinction coefficientQ, assuming a mean wavelength 0.5 micron, were as follows, takenfrom values established by one of the authors. 1932 OPTICAL PROPERTIES AND VISUAL EFFECTS


38/92

d,cl : 0.1 0.2 0.4 0.8 1.6 3. 2 6.4z = rrd/i : 0.63 1. 26 2. 52 5.04 10. 8 21. 6 43.2Q (white): 0.03 0.14 0.88 1. 87 2 2 2Q (black): 0.88 1.87 2 2 2 2 2Black smokes and white smokes yield different curves for particlessmaller than 0.8 micron diameter because the light extinction by blackparticles below this size is greater than by white.

The white smoke shows a maximum extinction per unit mass con-centration at about 0. 6 micron particle size, because below this size theparticle extinction coefficient decreases faster than its surface per unitmass increases. The black smoke shows this minimum at about 0.15micron. Transparent spheres smaller than about 2 microns extinguishmore light than irregular particles of the same projected area. So ifthe particles in the plume are polydisperse spheres, as with mists, theread mass concentration will be 2 or 3 times too high for white plumesbetween 0.2 and 1.6 microns in mean particle-size, but not appreciablydifferent for black plumes.The angular distribution of light scattered by particles smallerthan 0.05 micron diameter does not change with size; only the totalamount changes. The scattering for both polarizations combined isequal in the forward and backward directions and is only 50 percentless at 900. With increasing size above 0. 05 micron the pattern

becomes more forward-directed, and in visible light the scatteringpatterns of particles larger than 1 micron show a strong forward lobeof angular half-width about 35/d degrees, the particle diameter d beingexpressed in microns. At the same time subsidiary lobes appear in thescattering pattern, by number approximately 2%. This evolution of thescattering pattern with increasing particle size, for transparent spher-ical particles of refractive index 1. 5 in monochromatic light, is illus-trated in Figure 22. The particle diameters for light of wavelengthabout 0. 5 micron have been marked as well as the a- values.The strong forward scattering that all particles develop as their

size increases above 0. 5 micron is much the same for all materials:but transparent particles scatter much more strongly than absorbingones at all angles outside the forward lobe just mentioned. There is,in fact, no such thing as a black smoke-particle in the sense of a par-ticle that scatters no light whatever. In fact , particles of absorbingmaterial (e.g., carbon) that are smaller than about 0. 1 micron, as wellas absorbing light, can at the same time scatter in all directions asmuch and sometimes more light than transparent particles of the samesize. A smoke of absorbing particles can only appear black, i. e., notscattering significantly, either when it is not seen close to the sun, ornot against a dark background (as from an aircraft above it), or whenit is so dense that most of the light entering it is eventually absorbedfrom repeated encounters with particles, being part scattered and partabsorbed at each encounter.



39/92

In white light the oscillations in the angular scattering pattern ofa spherical particle (Figure 22) would be diminished because of thesmoothing over a range of 12 values, and a range in particle size wouldhave the same effect. The angular scattering patterns for an assemblyof irregular particles in random orientation are similar to those of anassembly of spheres of similar size-range in white light. Even withsuch smoothing, there remain large differences between the patternsof particles with different refractive indices. Figures 23 and 24 showhow the intensities of scattering through 4S and 90, respectively, bysingle spheres vary with particle size and refractive index. 2O Thecurve for the complex refractive index (2 - i) would be appropriate forcarbon; a curve for water, 1. 33, may be interpolated between the curvesfor 1.2 and 1.4; and many transparent minerals, such as quartz, havea refractive index about 1.6.

The angular scattering pattern of an assembly of particles, suchas portion of a smoke plume, is given by the sum of the patterns of theindividual particles in it only so long as its transmittance exceeds atleast 80 percent. The lower the transmittance, the greater is theproportion of light that is scattered more than once before emergingfrom the plume. This secondary scattering modifies the overall scat-tering pattern, reducing the angular variation and especially the forwardlobe. The scattering process at the individual particles in the plume isnot affected by increasing particle concentration until the mean inter-particle distance is reduced to a few particle diameters; these condi-tions could not persist in an aerosol because of the rapid coagulationthat would ensue even if it arose.

20 60 ,dO 180 20 60 100 140 80SCATTE RING ANGLE, degrees

Figure 22. Scattering by transparent spheres, calculated from Mie theory.



40/92

For a fuller account of light scattering, one may refer to thereview by Hodkinson 13 and the theoretical treatise of Van de Hulst. 17Because the angular scattering phenomena are much more complexthan the extinction phenomena, it is considered impracticable to evalu-ate plumes by measurements of scattered light, whether from naturalor art ificial sources.

I

0.1

0.01

0.001

:* - , . . . . . .-

2-iULATED FROM REFRACTION

1.6 -----mm1 8 -. - . -2.0 -..-.*2.4 ----

:: PARTICLE DIAMETER FOR MEAN A- 0.5 micronO.S# 11 511 JOEII I I I

1 10 100PARTICLE SIZE PARAMETER ( LI =+)

Figure 23. Scattering at 45 degrees by transparent spheres in white light, calc ulated from Mietheory.



41/92

. . . . . . . . . . . . . . . . . . . . - SCATTERING CALCULATED FROM REFRACTIONAND REFLECTIONI, 2 . . . . . w . . . . .

1.4 . ..v...- ...-.-.-.

PARTICLE DIAMETER FOR MEAN X = 0.5 micron

PARTICLE SIZE PARAMETER (a =+)Figure 24. Scattering a+ 90 degrees by transparent spheres in white light, calculated from Mle

theory.

Transmittance-Wavelength (Characteristics of theExperimental Plumes

The out-of-stack transmittances of the plumes were determinedby viewing a 500-watt incandescent reflecting flood lamp through the



42/92

plumes with the telephotometer. This procedure permitted direct out-of-stack transmittance measurements in daylight because the intensityof the lamp was high enough that the light scattered by the plume wasnegligible when the sun was not directly behind the plumes. Withoutinterference from scattered light, the transmittance is given by theratio of the telephotometer reading of the lamp sighted through theplume to the telephotometer reading of the lamp sighted clear of theplume, The transmittance measurements were made for white lightby using the telephotometer with the visual correction filter and forblue (No. 47), green (No. 58), and red (No. 29).

The effective overall spectral distribution of the telephotometerwith the color filters and lamp was measured with the aid of the mono-chromator by using the telephotometer lP22 phototube with each of thethree filters. The relative distributions are shown in Figure 25. Themean responses (r) of the distributions were calculated from the rela-tion

c RiXi;~ =-;i Rii

where Ri is the spectral response at the corresponding wavelength 7 i.The mean responses of the telephotometer to the flood lamp with blue,green, and red filters were at the wavelengths 0.438, 0. 531, and0. 651 micron, respectively.

300 400 500 600 700WAV E LENGTH, mllamicron.

Figure 25. Effective spectral response of telephotometer-filter-lamp combinations used for plumetransmittance measurements.



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For the measurements on the black and white experimentalplumes, the lamp (12-cm diameter) was placed about 3 meters behindthe plumes and the telephotometer was placed about 12 meters in frontof the plumes. The out-of-stack transmittances of red, blue, green,and white light through the experimental white and black plumes arecompared to concurrent measurements of in-stack white light trans-mittance in Figures 26 and 27, respectively.

100 ) I I I I I I I I I

60

40

20

0

WHITE LIGHT

BLUE LIGHTGREEN LIGHT

// I I I I I I I I I0 20 40 60 80 100

IN - STACK TRANSMITTANCE, percentFigure 26. White plume: transmittance measurements by tronsmissometer inside stock andtelephotometer with color filters slghted on lamp outside stock.

The variation in transmittance with wavelength observed withthese plumes shows that the particles are so small that the particleextinction coefficient (Q) varies with wavelength, i. e., they are onaverage smaller than about 1 micron.Linear plots, as in Figures 26 and 27, give curved lines passingthrough the points (lOO%, 100%) and OS, 0%). A straight line (slope = 1)in Figures 26 and 27 results only when T2 = Tl, which is when 11 = 12,or when Q is independent of wavelength, i. e., for polydisperse smokesof particle-size exceeding about 1 micron.



44/92

100

90

8.0

70

60

50

40

30

20

10

0

Figure

-

GREEN AND WHITE LIGHTS

0 10 20 30 40 50 60 70 80 90 100IN-STACK TRANSMITTANCE, percent

7. Black plume: transmittance measurements by transmissometer inside stack endtelephotometer with color filters sighted on lamp outside sto ck.

The correlation between in- and out-of-stack transmittances,both measured in white light, for the experimental plumes showed thatthe in-stack transmittance of the white plume was higher by as much as6 percent and the relation was not linear. This deviation from the ideal1:l relation for the white light curve and the failure of all the curves topass through the (0, 0) point is probably due to a difference in the con-ditions of air flow between the in- and out-of-stack measurement posi-tions.For the transmittance measurements on the experimental powerstation plume, an experimental arrangement was devised to permitmeasurement through a greater thickness of the effluent because the

transmittance of the 30-cm-diameter plume was around 98 percent.The effluent from the stack exit was diverted into a horizontal duce 30cm in diameter and 4.9 meters long. The effluent entered the ductnear one end through a side arm; because of the angle and high velocityat which the effluent entered the duct, it continued to flow through theOF SMOKE -STACK PLUMES 39


45/92

duct and out at the far end. By viewing the lamp through the duct, theobservers measured transmission through a section of the effluent 5meters long or more. The average transmittance measurements forfour runs with blue, green, and red filters were 81. 5, 86.0, and 93,Opercent, respectively.Angular Scattering (Characteristics ofthr Experimental Plumes

The angular scattering patterns of the plumes were measured atnight by viewing the plumes with the telephotometer while rotating alamp around them. A lamp-holding mechanism was attached to thestack exits to permit the lamp to be rotated at a fixed distance fromthe center of the plumes in a plane normal to the direction of the plumes.The plumes were viewed along the lamp rotation plane and as close tothe plumes as practicable to eliminate interference from light scatteredby the intervening air between the telephotometer and plumes.

Before the angular scattering of the black and white plumes wasmeasured, it was necessary to attach a 23-cm-diameter, 46-cm-lengthof duct to the stack exit to change its rectangular cross section to acircular one. For these measurements, a 500-watt flood lamp wasrotated at a distance of 1. 2 meters from the center of the plumes andthe telephotometer was located about 50 cm from the plumes. Theangular scattering patterns of black and white experimental plumeswith transmittances of 60 and 90 percent were measured in white light.Because of the low plume density of the experimental power plant plume,it was necessary to rotate two 250-watt spot lamps around the 30-cm-diameter plume at a distance of 1. 1 meters from its center. Thescattering patterns are shown in Figure 28.

Optical Properties and Particle Sizes of Experimental PlumesThe mean particle sizes of the experimental plumes were esti-mated by fitting the ratios of the mean particle extinction coefficientsof the plumes for red, green, and blue light to theoretical extinctioncurves plotted from Mie-theory computations for the correct refractiveindex. I f the plume transmittances are Tl and T2 when measured attwo different wavelengths 11 and X 2, it follows from Bouguers law that

Q2log, T2 = q loge T1

where Ql and Q2 are the mean particle extinction coefficients at Xl andx2. Consequently, as the particle concentration is varied a log-logplot of T2 versus Tl would give a straight line of slope Q2/Ql throughthe point 1, 1 (1000/o, 100%).

If Tr, Tg, and Tb are the measured transmittances for red,green, and blue light, then from Bouguers law40 OPTICAL PROPERTIES AND VISUAL EFFECTS


46/92

Il- 0

.I\ I I I I I-\T

-

-

-

a A 60%0 . 90%0 98%

TRANSMITTANCETRANSMITTANCETRANSMITTANCE

POWER STATION PLUME

NHITE (OILi PLUME

I J30 60 90 120 150

SCAT TERING ANGLE FROM FORWARD DIRECTION, degreesFigure 28. Angular scattering measurements of experimental plumes.

180



47/92

Qr : Qg : Qb = logeT, : IogeTg : 1ogeTbwhere Qr, Qg, and Qb are the corresponding particle extinction coef-ficients. The corresponding ratios of the particle size parametersdr, 59, and o.b are

cl : c :a =A -1r g b r : A -1 : Lb-lgwhere A,, X and kb are the mean spectral responses of the tele-photometer a%d filter combinations used for the measurement.

To obtain the estimate, the theoretical curve of particle extinc-tion coefficient Q against particle size parameter 2 = -d/X is plottedon a log-log scale; on an identical graph sheet are plotted the points(log Tr/log Tr = 1, T/Xr), (log Tg./log Tr, /Xg), and (log Tb/log Tr,rr,/Xb); the second sheet is slid over the first, with the axes kept paral-lel until the three points are fitted to the portion of the theoretical curvewith the appropriate curvature; then the correspondence of the loga-rithmic abscissae of rd/Aand 7/h. gives the value of particle diameterd. With this value of d the actual value of Q for any of the wavelengthsmay be read from the theoretical curve for the appropriate ~1 and hencethe projected-area concentration also derived, from Bouguers equation.A second estimate of the particle sizes of the plumes was obtainedby comparing the angular scattering patterns of the plumes with theo-retical patterns for different particle sizes plotted from Mie-theorycomputations (Appendix A),

Transmittance-Wavelength Chararteristics and Particle-Sizeof the White (oil) Plume

The published Mie-theory extinction computations for sphericalparticles that came nearest to the refractive index of the oil, approx-imately 1.45, were those for 1.44 by Penndorf 21 and are plotted asthe dashed curve in Figure 29. Since the droplets were not monodis-perse but ranged in size, a sliding average of this extinction curve wastaken over a 2-to-1 range in diameter for an equal number frequencyof particles through this range. I f the range is considered to run from2D/3 to 4D/3 with a frequency of 1 particle per unit size-range, thenthe area-mean particle diameter is given by

d; = J2$,'" D2dD +J ,$( dD = ; D2 3da = 1.02D zD.

The area-mean diameter is the appropriate measure of mean particlesize because the particle extinction coefficient Q is defined with respect



48/92

MIE THEORY FOR REFRACTIVE INDEX1.44 AFTER PENNDORF

/SINGLE SPHERES, DIAM d 7 /

AVERAGEDO VER 2:l DlAhlRANGE OF SPHERES,AREA-MEAN DIAM d

/I:

///

:PLOT OF TRANSMITTANCEDATA FOR SLIDING OVERQ: =CLlRVEIN -STACKi TRANSMITTANCE

/:

:

/

3 sI I I IIIl 80

. 70. 60

/2 .50 -

B l 40

/ :0 l 30l 200 ;t

4 5 67 8 910WA,/4 -

I . I I I I I

I-

0.1 - I I I I I I I I I

IO I I I I I I I I I

l-l1 1.5 PARIlL.5SIZEPAR&R(a%; 7 8 910Figure 29. White plume: transmittance measurements fitted to Mie-theory extinction curve.



49/92

to the particle area. The solid curve in Figure 29 represents thissliding average, the 1 values corresponding to -id/X. It differs verylittle from the monodisperse curve and so, although the size distribu-tion on which it is based represents the oil drops only approximately,a sliding average using the actual distribution would result in a curvenot very different.

The transmittance measurements (Figure 26) were used to con-pute the extinction ratios of the particles in the experimental white(oil) plume for light of mean wavelengths 0. 651, 0. 531, and 0.438micron. The ratios between extinction coefficients (Table 3) areplotted in the insert of Figure 29, providing three points on the slidinggraph since three wavelengths were used. Each trio of points wasfitted in two ways, using the red and blue points only and using allthree: but the resulting estimates of area-mean particle diameter(Table 3) do not differ significantly. The increase in particle size withincreasing smoke concentration is consistent with reports on the per-formance of such smoke generators; it may be due to increased coagu-lation of droplets in the generator or to reduced evaporation of dropletscaused by increased oil vapor pressure in the smoke, or to both.

Table 3. EXTINCTION COEFFICIENT RATIOS AND PARTICLE SIZE ESTIMA TES OF THEEXPERIMENTAL WHITE (OIL) PLUME

In-stacktransmittance, % Qr : Q, : Qb

Area-mean port~cle diameter, microns3-point Fit 2.point Fit

80 1 : 1.69 2.60 0.32 0.3270 1 : 1.67 2.28 0.37 0.3960 1 : 1.58 2.10 0.44 0.4350 1 : 1.50 1.93 0.47 0.4740 1 .: 1.47 1.80 0.48 0.4930 1 1.39 1.56 0.54 0.5420 1 : 1.34 1.56 0.58 0.57

Transmittancr-~avclen:rth Cherarteristics and Partirle-Sizeotthe Black ((:arhon) Plume

Complex refractive indexes of 1.90 (1-O. 36i) for wavelength0. 436 micron and 2.00 (1-O. 33i) for 0.623 micron are cited for carbonparticles by McDonald, 22 At these two wavelengths, McDonald hasalso published Mie extinction effic iency factors for carbon spheres withparticle size parameters from 0. 2 < z 6 8. The extinction curve(Figure 30) was constructed from an average of the extinctions at thetwo wavelengths. For particle size parameters greater than about 3,the extinction factor was not different at the two refractive indexes and



50/92

was only slightly different below 3. As with the white plume the extinc-tion curve was averaged over a 2:l range in particle diameters.The extinction coeff icients of the particles in the black plume -

computed from the transmittance measurements of Figure 27 - did notvary with plume density as did the coeff icients of the white plume. Con-sequently, an average of the extinction ratios at in-stack transmittanceintervals of 10 percent gave 1: 1.23: 1. 37 as the extinction ratios of theparticles in the black plume for light of wavelengths 0. 651, 0. 531, and0.438 micron. The extinction size parameter ratios are plotted in theinsert of Figure 30. The points were fitted to the averaged Mie-theorycurve in the manner described above and gave an area-mean particlediameter estimate of 0. 23 micron for the black experimental plume.

10 I I II I IIIl ll I I II I IIl l-AT POSITION OF BEST F IT d = 0 23~

MIE THEORY CURVE AVERAGE OF INDEXES1.90 (l-O.36 11 AND 2.00 (1.0.33 i)

o., I III I111111 I III IIIIIJ0.1 .I5 .2 .25 .3 .4 .5 .6 7 .a .9 l.C 1.5 2 2.5 3 4 5 6 789PARTICLE SIZE PARAMETER (e =F)

Figure 30. Block pl ume: transmittance measurements fitted to Mle-theory extinction curve.



51/92

Comparison of Particle-Size Estimates with DirectMass-Conrentration Measurements

The mass concentration of an aerosol with monodisperse spher-ical particles of diameter d, specific gravity P, and number concen-tration n is Cm = nn d3 p/6, and the projected area of the particles isa = vd2,/4 = 3Cm/2nd . Consequently, the mass concentration of aplume of spherical particles may be written from BouLqers law as

C 2d PIn = 3$t loge; [gg3]

where t is the thickness of the plume in meters, d is the mean diam-eter of the particles in microns, and Q and T are the mean particleextinction coefficient and transmittance of the plume, both functions ofwavelength.For calculation of the mass concentration of the white and blackexperimental plumes, their transmittance data (Figures 26 and 27)were fitted by the procedure described above, to the Mie-theory curvesof Figures 29 and 30 to obtain d and Q. The thickness of the plumeswas 0. 2 meter, and the specific gravity of the fuel oil used in the whitesmoke generator was 0. 87. A specific gravity of 1.95 was used forparticles in the black plume since a specific gravity 1. 8 to 2. 1 isreported for amorphous carbon.To obtain direct measurements of the mass concentration of theplumes, isokinetic samples of the eff luents were collected on membranefilters and weighed with an analytical balance. Samples were collectedat plume transmittance intervals of about 10 percent. The comparisonsare shown in Figures 31 and 32.The measured mass concentration of the white plume was higherthan the calculated concentration by 44 percent at 70 percent trans-mittance, and 25 percent at 20 percent transmittance. This agreementis as close as can be expected, since much mass may be contributed by

a few large particles (mass = d3). In terms of the particle size esti-mate from light extinction, this agreement implies an error of only 8to 15 percent over the range of transmittances.

Almost perfect agreement was obtained between the measuredand calculated weight concentration of the black plume. Such an agree-ment is undoubtedly coincidental and is probably due to a fortuitouscompensation of the effects of using both a high extinction coefficientand a high area-mean diameter in the computation due to matching thetransmittance data to a Mie extinction curve for spheres. Irregularparticles less than about 1 micron extinguish less light than sphereswith the same projected area, and the volume to area diameter forirregular particles will be less than for spheres. Nevertheless, theagreement again illustrates how the transmittance of a plume can berelated to the amount of nongaseous material in the plume.



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90

80

70

60

50

40

30

20

I I I I I I I I I I I

MEASURED BY DIRECT SAMPLING

CALCULATED FROM PLUME TRANS MITTANC E

0 .I .2 .3 .4 .5 .6 .7 .8 .9 1.0 1.1 1.2MASS CONCENTRATION, grams/m3

Figure 31. Mass concentration of white plume CIS calculated from transm ittance and measured bydirect sampling.



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100

90

80

70

60

1C

n An - CALCULATED FROM LIGHT TRANSMISSION DATA0 0 0 - MEASUR ED BY DIRECT SAMPLING

MASS CONCENTRATION, groms/m3Figure 32. Mass concentration of black plume as calculated from transm ittance and measured by

direct sampling.



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Transmittance-Wavelength (Iharacteristirs and Particle-Sizeof the Oil-Burning Power Station Plume

The solid particles in the smoke consist primarily of metalsulfates with an indeterminate degree of hydration. An estimate of0.14 micron diameter, based on electron microscopy, has beenreported. 23 Inspection of the refract ive indices of various sulfatessuggests values of 1. 45 to 1. 5, with hydration making little difference. 24A refractive index of 1. 5 was therefore used in this analysis, sinceespecially good Mie-theory tables for this refractive index have beenpublished by Giese and others. 25 In the analysis of the transmittancemeasurements of the white experimental plume it was remarked thatsmoothing the Mie curve over a 2:l range in particle size did notgreatly change the shape of the initial rise in the curve, on which thepresent observations also fal l. The experimental extinction ratios forthe power station plume were therefore fitted by the same procedure tothe unsmoothed Mie extinction curve for refract ive index 1. 5 (Figure33), and yielded an estimate of 0. 3 micron for the area-mean particlediameter.

Because of the very wide range in particle size suggested by theanalysis of the scattering patterns (Appendix A) and the overwhelmingpreponderance by number of the finest particles, which because of theirvery small extinction coefficients contribute less than half of the totalextinction by the model aerosol, fitt ing the transmittance measurementson the power plant smoke plume to the Mie-theory extinction curve givesan estimate of area-mean particle size considerably larger than theactual size. Nevertheless, such an estimate is of value; it is the meansize of the particles that chief ly contribute to the attenuation and thescattering and therefore is useful as a description of these properties.When we bear in mind the ignoring of the smaller particles, this esti-mate of 0. 3 micron is reasonably consistent with the model size-distri-bution inferred from the scattering measurements (Table Al) asdescribed in Appendix A. I f any of the larger particles present werein fact condensed droplets, then their sizes might have been affected bythe different air flow conditions at the exit of the smoke stack in thetransmittance and scattering measurements.

INSTRUMENTAL TECHNIQUESFOR EVALUATING SMOKE PLUMES

The experimental data accumulated on plume visual effects con-firm theoretical expectations of great variabil ity. Vision obscurationby smoke plumes and the visual appearance of smoke plumes are fartoo dependent on environmental conditions of plume illumination to bereliable measures for characterizing the plume as an aerosol. Aplume that is assessed by a visual ef fect could be condemned whenviewed on one day and accepted on another, or condemned when viewedfrom one direction and accepted from another, even when its contentshad not changed.OF SMOKE-STACK PLUMES 49


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1.0

1

I-I?

PLOT OF TRANSMITTANCEDATA FOR SLIDING OVER0: c( CURVE

4 5 6 7 8 9 IO

I .1 I I I I I I I III,1 1.5 2 2.5 3 4 5 6 7 8 9 10PARTICLE SIZE PARAMETER (a=+)Figure 33. Experimental power-station plume: transm ittance measurem ents fitted to Mie-theoryextinction curve.OPTICAL PROPERTIES AND VISUAL EFFECTS


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The solid and liquid particles in a plume may be characterized bytwo intrinsic optical properties, their angular scattering pattern andtheir extinction coefficients. However, the light transmittance of aplume is more simply related to concentration, particle size, composi-tion, and plume dimensions, and is more readily measurable than aplume scattering pattern. Plume evaluation according to the angularscattering pattern would require not only measurements of the relativeangular distribution of scattered light which, as evidenced by suchmeasurements described here, are cumbersome, but also a quantitativecomparison of the illuminating and scattered light.

We have indicated that, under conditions in which a measure ofaerosol concentration can be derived from a determination of the plumetransmittance, this measure is the projected-area of the solid andliquid particulate material per unit volume of air, termed the projected-area concentration. Conversely, the light-transmitting and light-scattering properties of the plume are in general related more closelyto this measure of concentration than to any other, such as the numberconcentration (e.g., particles per cm3) or the mass concentration(e.g., grams per m 3, or loading as grain per cu ft). The mass con-centration of solid particles is the only one that has been used routinelyas a criterion of the maximum permissable level of particulate materialin a smoke plume. Its relation to the area concentration depends onthe particle mean-size and size distribution. Thus a given mass con-centration of a given substance can correspond to a wide range of plumetransmittances depending on the particle size. It is partly for thisreason that the mass concentration is an incomplete criterion forassessing the particle content of a plume, although it is a useful sub-sidiary criterion; in the past it has often been the sole criterionbecause it was the simplest quantitative measure of particulate con-centration that could be obtained.Transmittance Measurement by Means of (Contrasting Targets

The transmittance of a plume can be obtained, even when it isscattering much light from other sources, by measuring the luminancedifference between a pair of contrasting targets through the plume(Bi - Bi) and clear of the plume (Bl - Bz). The transmittance is cal-culated from the relationB; - B;

T =Bl - B2With this procedure, interference from light scattered by the plumecancels out. Luminance difference ratios between contrasting targetsviewed through and clear of a plume may be obtained by direct tele-photometry of targets or by photographing the targets and obtaining themeasurements from the negative in the laboratory with a densitometer.

For the photographic method, a series of neutral density filterswould be positioned along one side of the camera film plane to produceOF SMOKE-STACK PLUMES 51


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a calibration scale on the negative. For the photograph, the camerais orientated to position the filters in the brightest part of the scene,usually the sky. A calibration curve is obtained from the negative byplotting the optical density of the images as a function of the filtertransmittance (relative negative exposure). Then by measuring thedensities of the contrasting targets, their relative luminances areobtained from the calibration curve.

Contrasting targets that may be viewed through plumes from theground mav be a portion of blue sky and white cloud, or, where avail-able, highground or a building and a portion of sky.Contrasting targets on the ground may also be used to measureplume transmittance by use of a helicopter. The contrasting targetsmay be distant land and horizon, plowed field and wooded areas, water

and sandy beaches, etc. I f the density of the smoke or ambient lightingconditions are unstable, the photographic technique is preferred becauseit gives a permanent and instantaneous record of the measurements.The camera must be equipped with lenses that will produce images ofsizes suitable for densitometer measurements. Direct telephotometryof the targets has the advantages of greater simplicity and faster datareduction.The technique of obtaining plume transmittance by direct tele-photometry of contrasting targets has been illustrated in all the con-trast reduction experiments on panel targets viewed through the blackand white experimental plumes. Computations of transmittance fromthese measurements are compared with in-stack transmittance meas-urements in Appendix B.The transmittance of plumes was also measured with a telepho-tometer by viewing distant hills and horizon skies through the plumes.Table 4 shows transmittances of the white experimental plume measuredby sighting on a distant hill and adjacent sky. The measurements weremade on a clear day and the viewing direction was northwest. Theagreement between the two sets of transmittance measurements is

acceptable.Table 5 shows transmittance of a coal-burning power plant plumemeasured by sighting on a distant hill and adjacent sky. The measure-ments were made at various times over a 5-day period. Illuminatingconditions were highly variable throughout the entire period because ofthe varying overcast conditions that persisted. A coal-cleaning opera-tion located in the area also caused variable luminance measurementsand was probably responsible for the particularly high luminance of thewooded hillside when viewed clear o f the plume. The viewing directionwas southwest.Figure 34 illustrates the photographic technique of measuring thetransmittance of a smoke plume. The coal-burning power plant plumewas used for the test and the contrasting objects were the distant hilland adjacent sky. The sky was overcast. In Figure 34 the calibration



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Table 4. TRANS MITTANC E OF WHITE PLUME MEASURED BY SIGH TING ON HILL AND SKYTHROUGH THE PLUMEWITHATELEPHOTOMETER

(luminances ore in candles meter2)

2900 600 3100 2600 22 222900 600 2800 2000 45 362650 600 3050 2400 32 282600 600 2800 1800 50 482600 600 2700 1600 55 502600 600 2650 1250 70 682550 600 2600 950 85 822650 600 3600 3250 17 142650 550 3100 2400 34 292700 600 3000 2600 19 24

0942323355

E, = 3.7

T = calculated plume transmittance6, - Bhz-Bs -%

T = in-stock plume tronsmlttonce(measured with transmissom eter)

E, = error in the calculated transmittance

curve of density versus relative exposure is shown in upper right andthe calibration scale is shown in the upper left. The calibration curvewas obtained from densitometer readings of the negative image of thecalibration scale. Densitometer readings of them% images of thetargets indicated a plume transmittance of 28 percent. Direct tele-photometry of the targets indicated the transmittance of the plume wasvarying between 23 and 30 percent.

Transmittance Mrasuremrnt by IIleans of Single TargetaIf the target is so bright that light scattered by the plume is neg-ligible when compared to the transmitted light, the transmittance canbe obtained directly as the ratio of the target intensity when viewedthrough the plume to the target intensity when viewed clear of the plume.At night almost any light source behind the plume that is brightenough to be measured with a telephotometer is suitable, since theplume is not scattering light from its surroundings. The targets may

be the moon, light scattered from an intense searchlight beam behindthe plume, light reflected from illuminated objects behind the plume,or similar sources. In the daytime, the sun offers a suitable target.A simple sun photometer sighted on the sun, first clear of the plumeand then through the plume, gives the transmittance directly, since anyOF SMOKE-STACK PLUMES 53


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Toble5. TRANSMITTANCEOFACO AL-BURN ING POWER PLANT PLUMEMEASURED BYSIGHTINGO N HILLAND SKY THROUGH THE PLUMEWITHATELEP HOTOMETER

(luminonces in candles,'me+er')Time (EDT) Bs Bh 8s'

1115 10000 3100 100002 1120 11000 3200 8600> 1136 10000 3400 8200x 1200 12000 2800 7900

1235 10000 2200 62001240 9300 2600 6400

0935 5500 1800 5500? 0940 5700 1800 55002 0942 6000 1800 5800a 0947 6900 2100 6900

0951 6500 2100 67000950 7400 1500 51001030 8100 2400 58001059 8600 2100 6500

3 1130 5000 1000 34002 1300 8200 1500 5300s 1314 7500 1500 5500

1342 7400 1400 55001432 8200 1400 62001514 11500 1400 69001530 14500 2100 100000923 5800 1000 2700

q 0930943 3200200 100010 3400000w 1000 4600 1000 24001015 6900 1000 43001045 5800 860 2900

Bh'___-T '

7200 237200 186700 236400 164600 215100 194600 244600 234500 315500 295500 273600 254500 235500 152700 1B3900 214300 204100 234500 254100 188200 15210027001500170034002200

13171919I514-

scattered light is faint compared to the brightness of the sun. Figure 35shows two modified Volz sun photometers 26 that have been combined topermit simultaneous measurement of the transmittance of blue and redlight through plumes. The spectral responses of the photometers areshown in Figure 36.Table 6 shows the transmittances of red and blue light throughthe experimental white plume measured with the sun photometers. Themeasurements have been corrected for the angle of view through therectangular plume, about 45 degrees. The results are compared withthe in-stack transmissometer readings and the transmittance measure-ments o f red and blue light as measured with the telephotometer andlamp (Figure 26).



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Figure 34. Transmittance of power-plant plume, measured by photography of contrasting targe ts.

Table 6. TRANS MITTANC ES OF RED AND BLUE LIGHT THROUGH THE WHITE EXPERIMEN-TAL PLUME MEASURED WITH A SUN PHOT OME TER

Transmlttonce with Transmittance within-stack sun photometer, 5 Extinction telephotometer and lamp, % Extinction

tronsmlttonce, 5 Red Blue ratio, Qb,Q , Red Blue ratio, QbQ,

40 52 31 1.78 48 26 1.8544 57 33 1.97 53 28 1.9749 61 36 2.07 58 34 1.9763 69 50 1.90 70 46 2.1665 74 54 2.03 72 48 2.2290 a7 a4 1.25 90 75 2.90



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5.6 cm, f3LENS

,/

SIGHTINGPIN HOLE

\ 5.6 cm, f3LENS

Figure 35. Modified sun photometer for measuring light-transmission characteristics of smokeplumes.



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too 500 600WAVELENGTH, m,ll~m,crans

Figure 36. Spectral responses of dual sun photom eter.

Transmittance Neasurement by Means of ComparatorsComparators may be used by an observer to estimate the trans-mittance of smoke plumes. For a comparator to be effect ive, it mustcontain particles with scattering properties similar to those of theplume being assessed; then when the comparator and plume are viewedunder the same lighting conditions, their luminances should also bethe same.Black smoke comparators of transparent neutral filters forassessing the transmittance of black smoke only have alreadv been

developed in the form o f the Public Health Service Smoke Guide. Whitesmoke comparators might consist of small cells containing liquid sus-pensions of fine transparent particles of different concentrations andsizes.

Transmittance Measurement by Means of LasersA laser technique for measuring the transmittance of smokeplumes appears feasible. The method is based on a relative measureof backscatter signals of a pulsed laser beam by aerosols in the airbeyond a plume. A plumes transmittance is obtained by measuring thebackscatter signal from the air behind the plume when the beam isdirected through the plume relative to the backscatter signal obtainedfrom the air when the beam is directed beside the plume.



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The laser technique of measuring smoke plume transmittance wastested at the oil-burning power plant at Morro Bay. The tests wereconducted by the Stanford Research Institute with their Mark I lid&. 27The measurements were made on the number one stack, which carriesthe eff luent from two of the four generating units of the plant. The gen-erating units can burn oil or gas. When the effluent contained 50 percentoil combustion products from one unit and 50 percent gas combustionproducts from the other unit, laser measurements indicated that thetransmittance of the plume was around 82 percent. A slightly highertransmittance of 85 percent was indicated when the gas-burning unitwas shut down and the plume contained only oil combustion products.Sun photometer measurements on the plume when it was composedof equal parts of oil and gas combustion products indicated a transmit-tance of 85 percent for red and blue light. Sun photometer measure-

ments on the plume when the plume contained only oil combustionproducts indicated a transmittance of 92 percent for red light and 89percent for blue light.Although field testing of instrumental techniques for measuringplume transmittance has been limited, the tests have served to illus-trate the techniques and demonstrate their feasibility . The methodsthat require targets behind the plume, although completely objective,are not always applicable. They may be classified as two-ended tech-niques of measuring plume transmittance. The sun photometer methodis most applicable in areas where overcast conditions are few and the

sun may be viewed through the plume. The telephotometer method ismost applicable in mountainous or metropolitan areas where a hill orbuilding may be viewed through the plume or in areas with a large num-ber of days where the sky offers areas of suff iciently contrastingbrightness. The telephotometer, sun-photometer, and neutral fil termethod for black smoke could be combined in a single, inexpensivefield instrument. A schematic for such an instrument is shown inFigure 37. The accuracy with which a nonblack plume may be assessedby use of a comparator has not been demonstrated, though the methodis promising in principle. The laser method represents an objective,single-ended. general technique for measuring the transmittance ofplumes, and may offer a standard for evaluating smoke plumes withwhich other less expensive methods can be compared.

CONCLUSIONSThe visual appearance of smoke plumes and vision obscurationby smoke plumes are closely related to the amount of light the plumesscatter in the direction of the viewer from their surroundings and thesun. Consequently, evaluations of plumes by a visual effect are more

stringent for nonblack plumes than for black plumes and depend onplume illuminating and viewing conditions. Because of this, nonblackplumes could be evaluated differently when viewed on different days orviewed from different directions, though their aerosol content had notchanged.58 OPTICAL PROPERTIES AND VISUAL EFFECTS


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SW I

EYEPIECE

NEUTRAL DENSITY FILM ANNULUS

MICROMETER

60% FIELD OF VIEW

GLASS PLATE

TCHPHOTOCELL MIRRO

Flgure 37. Schematic for combination smoke meter .

I f a plume is regarded as contributing to an air pollution problemmainly by virtue of the nature and amount of its aerosol content, itwould be desirable to evaluate the plume by an intrinsic property thatcan be related to that content, independent of environmental lightingconditions. Then the plume could be evaiuated according to emissionstandards based on the composition, diameter, and velocity of theplume, the prevailing meteorological conditions, and other pollutionsources in the area.I f the main objection to the plume is its appearance and thedecrease in visibil ity that it will cause, it would still be desirable to

evaluate the plume by an intrinsic property. In such a case emissionlimits in terms of some intrinsic property of the plume (e.g., trans-mittance) will have to be formulated from a consideration of the visualeffects of plumes under the most unfavorable conditions. Then plumesmeeting this emission limit would also be acceptable under all illumi-nating and viewing conditions.Also, if plumes are evaluated by a property that can be relatedto the composition, size, and concentration of particles in the plumeand a measurement technique is available, the operator of a stack candetermine more readily for himself when his emission is excessive and

he can better define specifications for control equipment to make theemission acceptable.The optical property of a plume that is easiest to measure andmost simply related to concentration, particle size, composition, and



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dimensions of the plume is its light transmittance. Although no general,inexpensive instrumental technique is available for objectively measur-ing the transmittance of plumes, there are several special techniqueswhich collectively under most circumstances will provide an objectivemeans of measuring the transmittance of a plume.

It appears that an observer may be trained to estimate the trans-mittance of a plume from its visual appearance. Results presented inthis report indicate, however, that plume illuminating, background,and viewing conditions must be considered in training of the inspectorand by the inspector when he is assessing plumes in the field. Theevaluations of a nonblack plume by observers even with training, mayvary significantly if they assess plumes without regard to plume illumi-nating, background, and viewing conditions. A valuable aid to theinspector of a nonblack plume would be comparators with light-scatter-ing characteristics similar to those of the plume being assessed. Suchcomparators would help the observer compensate for variations in theappearance of a plume due to illumination and viewing conditions.

A completely general technique that uses a pulsed laser beam formeasuring the transmittance of plumes appears feasible. A laser mayoffer a standard technique for measuring the transmittance of plumeswith which less expensive methods can be compared.

ACKiVOVk7LEDGMENTSWe wish to express our thanks to the Steering Committee of thestudy for their support and interest. especially to Mr. V. F. Estcourt

of the Pacific Gas and Electric Co. and Dr. J. H. Ludwig and Mr. J. S.Nader of the Public Health Service. We extend thanks for valuablescientific discussions to Mr. E. S. Johnson of the Pacific Gas andElectric Co. and Dr. Werner Stoeber (now at the Max Planck Institute,Goettingen, Germany), who was a consultant to the Edison ElectricInstitute for this study. One of us (Dr. J. R. Hodkinson) was a con-sultant to the Public Health Service for the study. It

effects of plumes

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