ANALYTICAL CHEMISTRY
708 chloride with acetylacetone has been mentioned above. Similarly, cupric oxide, cadmium oxide, manganese dioxide, and zinc oxide react rapidly with acetylacetone after prior treatment with hydrochloric acid vapor and produced well-formed crystals. Zinc oxide and metallic zinc will react with acetylacetone without the need for preliminary acid treatment (Figure 6). The crystals shown in Figure 6 were formed from powdered zinc oxide reacting with acetylacetone. In 30 minutes or less, no zinc oxide is left unconverted. Figure 7 shows the same crystals formed from zinc present in an air-borne dust sample. As the resistance of alumina to chemical attack is well known, i t was interesting to note the reaction of acetylacetone vapor with alumina particles (Figure 8). DISCUSSION
therefore, to develop crystal reactions producing reproducible and characteristic crystal forms, without the need for too precise control of conditions. The cadmium chloride crystals formed from cadmium oxide are an example of a suitable reaction, easily and consistently produced, and characteristic in form. To be of practical use, a reaction must be reasonably rapid, unless it possesses some other advantage such as specificity. The cadmium chloride reaction meets this requirement as well. Identification based on tests carried out on the effluent gas stream may be more sensitive than visual indications given under the microscope, or may be the only indication of a substance sought. This is condition 3, illustrated by experience with arsenic. I t is not necessarily desirable that these gas phase reactions take place only a t the particle site. The huge size of some of the benzidine sulfate crystals formed may be due to the diffusion of sulfite ions to a central nucleus. Where magnification by halo formation or by diffusion to a common center occurs, submicroscopic particles may be made visible.
If allowances are made for certain limitations, the gaseous reactions described can be of help in identifying air pollutants. As a guide to the investigator, the maximum amount of information about the sample should be obtained before investigation. From a consideration of the sampling location, the probable composition of the air sample may be predicted. This should influence the method of sampling and the gassing treatment used. In such investigations, much time can be spent without result unless a t the outset it is decided the investigation of the sample will be along restricted lines. We do not look for organic material in dust from a hard rock gold mine; on the other hand, air samples from many industrial cities are largely organic and should be treated accordingly. Because organic substances are not too reactive chemically, and show no structure under the microscope, air samples containing much organic material are best taken with high-volume samplers on glass fiber filters. Such glass filters can be heated before use to ensure freedom from organic material, and the sample, after collection, can be exposed to heat and acid vapor treatment without interference from the sampling medium. Actual identification can be done by a combination of chromatography and measurements in the ultraviolet and infrared wave lengths. As it is not always easy to restrict reaction products to the original particle location, improvement is made by the use of coated sensitized slides. Sensitized films, in addition to localizing halos, crystals, etc., a t the particle site, may produce colored reaction products in favorable cases. Identification by crystal form alone is difficult, as crystal habit may vary with crystallizing conditions. It is desirable,
ACKNOWLEDGMENT
The help of T. F. Doherty, who prepared the drawings, George Ensell, who made the glassware, and Morris Katz and Ellis Kerr, who made arrangements for the photographic reproduction, is hereby acknowledged. LITERATURE CITED
(1) Cadle, R. D., A N ~ LCHEM., . 23, 196 (1951). (2) Cadle, R. D., Rubin, S., Glassbrook, C. I., and Magill, P. L.. Arch. I n d . Hug. Occupational Med., 2, 698-715 (1950). (3) Foster, W. D., and Schrenk, H. H., U. S. Dept. Interior, Bur. Mines, Rept. Invest. 3368 (1938). (4) Gitzen, W. H., ANAL.CHEM., 20, 265 (1948). (5) Lennox, D. H., and Leroux, J., Arch. I n d . Hug. Occupational Med., 8,359-70 (1953). (6) Leroux, J., “Contribution ? l’fitude i des Poussihres Atmos-
phbriques par Diffraction des Rayons-X et Microscopie filectronique,” thesis, Facult6 des Sciences, Universit6 de Paris, June 1954. Shore, V. C., and Katz, M., Defense Research Chemical Laboratories Report 154, 15th -4nnual Meeting, American Industrial Hygiene Association, Chicago, 1954. Strebinger, R., and Orth, E., Monatsh., 80, 254-9 (1949). West, P. W., and Granatelli, L., ANAL.CHEM., 24, 870 (1952). Williams, C. R., J . Ind. Hug. Toricol., 19, 44-52 (1937).
(7) (8) (9) (10)
RECEIVED for review September 3, 1954. Accepted March 15, 1955.
Instrumental Measurements of Visibility in Air Pollution Studies GEORGE D. CLAYTON‘ and PAUL M. GIEVER?
U. S. Public Health Service, Detroit, Mich. An instrument which measures transmittance (the Transmissometer) was found to be a useful tool, in air pollution studies, for measuring visibility in an urban atmosphere. It was relatively free of operating difficulties, required little attention, and was sufficiently sensitive to measure instantaneous variations in visibility. In a study of the relationship among data obtained with the instrument, the soiling power of the atmosphere, and the mass weight of air-borne particulates, it was found that there was no correlation among these three different methods of measuring aerosols. Visibility measurement therefore cannot be used as an index of the soiling power of the atmosphere or the mass weight of the air-borne particulates. Rain removed from the atmosphere some aerosols which cause discoloration, while snow had no such effect.
T
HE adverse effects of aerosols on visibility have been recorded
since the industrial age. As early as the thirteenth century, objections were raised over the burning of coal and its effect on visibility in London. During the known air pollution disasters such as Meuse Valley, Donora, and London, the reductions in visibility were reported to be the greatest during the periods when most of the deaths were occurring. In Los Angeles there is reported a definite relationship between smog and visibilitye.g., the greater the smog and eye irritation, the greater is the reduction in visibility. Reduced visibility plays an important role in other incidents of a tragic nature, such as aviation, automotive, and vessel traffic accidents. In addition to the safety and health problems, poor visibility 1 2
Present address, consulting engineer, 14125 Prevost St., Detroit 27, hlich. Present address, Detroit Edison Co.. Detroit, Mich.
V O L U M E 27, NO. 5, M A Y 1 9 5 5
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has a decided economic effect on an urban area. The cost to the aviation industry alone because of “closed” airports is considerable. The majority of air pollution ordinances in the various communities of the United States have been inaugurated as a result of the observed reduced visibility created by air pollution. From these few observations, it may be seen that reduced visibility has been associated with health, safety, economy, and the general esthetics of a community. It therefore becomes an important facet in m y comprehensive study of air pollution. It is recognized by scientists arid the public alike that weather conditions that result in fog, snow, and rain also have an important effect on visibility, and it is known that condensation nuclei which affect visibility are formed by nature as well as community activities. This presentation discusses the effect of meteorological phenomena and atmospheric particulates upon visibility. In the measurement of visibility it was first necessary to find an instrument that would accurately record atmospheric transmittance during an entire Pi-hour period. The Transmissometer developed by C. A. Douglas and associates of the National Bureau of Standards, U.S. Department of Commerce, \I;ashington, D. C., met requirements and was selected as meriting experimentation in atmospheric pollution studies. Douglas cooperated completely in the endeavor, by supplying an instrument, inspecting and giving final approval of the location of the installation, and visiting Detroit during the several months that the instrument was being operated. DESCRIPTION OF INSTRU-VENTS
The Transmissometer was designed to measure visibility by light transmittance. It consists of a 350,000candle power light source, a phototube receiver, a n amplifier, and a n indicator. The distance between the points of transmittance and reception may be up to 1.5 km. The output of the receiver is transmitted to the indicator and the recorder, which may be located several miles from the actual site of the testing. The intensity of the light is controlled by a voltage-regulating transformer and rheostats. A zero check is made each hour by an automatic cutoff and checks may be made at the recorder as indicated or desired. The expected service of a lamp is from 3 to 6 months. The receiver unit consists of a lens, a diaphragm, a photopulse unit, and an amplifier. The light, focused by
ANALYTICAL CHEMISTRY
710
ing a minimum of 1 minute of operation to ensure that the means of the lens on a pinhole in the diaphragm, strikes the photoliquid in the manometer reached equilibrium) and a reading tube receiver and there generates pulses the frequency of which is was taken in each instance. The average of the three readings directly proportional to the intensity of the light on the phot.0was then recorded. tube. These pulses are amplified and transmitted to the indiThe filter was weighed in a manner developed by Byers and cator. Keenan ( 1 ). I n the pulse generator unit, current charges a capacitor with The Wilson automatic filter paper sampler utilizes a clock sufficient voltage to cause a discharge through a neon lamp. mechanism which operates an “on and off” switch. Air is drawn The capacitor rapidly discharges through the neon lamp and a through a circle of filter paper 0.5 inch in diameter, held by an resistor until the voltage is no longer sufficient to maintain a automatic clamp. The sampling rate was 10 liters of air per current through the lamp. The voltage drop across the resistor minute in 25 minutes of each hour. In operation, one 25supplies a voltage pulse to the grid of a 6J5 tube, causing a minute sample is obtained during a 60-minute cycle, at which momentary change in the plate current of the tube. The resulttime the paper strip is automatically moved to a new position for ing momentary change in voltage drop across a transformer, the next sample. when amplified, supplies the pulse signal which is received a t the The stains obtained with this instrument were evaluated with a indicator. reflectometer using a blue light filter with a dominant length of The indicator consists of a frequency measuring unit, a two400 mp. The instrument was set at 100% reflectance for the stage amplifier, and a calibrator. The frequency meter levels paper being used and a check of that setting was made after and averages the pulses which are received through the amplifier each sixth reading, in order to minimize inaccuracies due to the and produces a meter reading which is directly proportional to varying reflecting quality of the paper. The stain was placed the pulse frequency and, therefore, to the transmittance of the over the detector element and percentage of reflectance read on atmosphere between the receiver and the light source. A recorder connected to the indicator gives a continuous record of The indicator meter reading on chart paper marked to show percentage of light transmittance by time of day. The installation in Detroit was approximately 125 feet above ground level with approximately 800 feet between the light source and the receiver. The light source was mounted on the roof of the 10story Federal Building and the receiver was located on the tenth floor of the Majestic Building, both in the center of the business district of Detroit. A high-volume air sampler was used to collect particulate matter. The essential parts of the sampler consist of a vacuum cleaner-type blower mounted in an aluminum housing, a filter paper retaining ring located on the intake side of the blower, and a U-tube manometer located on the exhaust side. The manometer was used for determining pressure differential between the inside of the VISUAL RANGE IN MILES housing and the atmosphere. The manometer was calibrated Figure 2. Visual range chart with a dry gas meter to obtain the air flow in cubic feet per minute. ,4n accordion-pleated filter paper, presenting a large surface, was used for collecting air-borne particulate matter. 60. SUNRISE 1 1 SUNSET The rate of flow through the average clean filter was approximately 65 cubic feet per w 90minute. As the sample was R -€AN .._3 a 80 I being collected, the rate of air e 70flow decreased as a result of SUNRISE SUNSET resistance caused by particuI JULY I z 60lates on the filter. In areas w 50 of heavy pollution, a t the end of a 24-hour sampling u 90 period, the flow rate often was a 80reduced to as low as 25 cubic u--69 w 70--feet of air per minute. MARCH-APRIL / In operation the filter was n 60. SUNRISE I 1 SUNSET replaced every 24 hours. Manometer readings were taken 70 M 4 a 6 before the sam ler was stopped for removal o f the filter, and 1 SUNSET SUNRISE I JANUARY immediately after a clean filI . . . . . ter was inserted. To increase 12M IAM 2 3 4 5 6 7 8 9 10 I1 1 2 N I P M 2 3 4 5 6 7 8 9 I O I I 12M accuracy in the manometer H O U R S r e a d i n g , i n s t r u c t i o n s were Figure 3. Mean half-hour light transmittance for selected months, given to stop and start the representing four seasons instrument three times (allow-
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V O L U M E 27, N O . 5, M A Y 1 9 5 5
711 inches; and Sovember, 0.8 inch. After due consideration of these factors it is believed that the lower transmittance values obtained in January and hfarch-iipril were due to aerosols rather than snow and rain. A further study of Figure 3 shows the diurnal and annual variations of light transmittance. In January there was very little variation in the mean light transmittance in 24 hours. In March-April, light transmittance increased considerably from 8:OO A.M. to 5 : O O P.M. In July the mean light transmittance was slightly higher than for the other months plotted and the values obtained for the 24 hours were rather constant. In November, the transmittance increased for a short time between noon and
N
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Figure 4.
=
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Influence of wind direction on light transmittance
the indicator. The instrument had very little lag and readings were reproducible to within 0.2%. RESULTS OF EXPERIMENT
Sutton ( 3 ) mentions other climatic factors which influence the diurnal variation of lapse rates, and in the case of Detroit, tend further to differentiate between summer and winter conditions. Sutton points out that under overcast and windy conditions the diurnal variation of the lapse rate almost completely disappearq. During December and January in Detroit, mean cloud cover is 75%, whereas in June through September it is only 50%. Mean surface wind speed in December and January is 11.8 miles per hour, whereas in June through September it is only 9.2 miles per hour. The greater frequency of cloudy, windy weather in winter will tend further to diminish the diurnal variations of lapse rate and hence of particulate pollution. In Figure 4 is shown the influence of wind direction on light transmittance. Light transmittance values on days of more than 0.15 inch of rain or other obvious meteorological conditions which caused reduction in visibility were not used in compiling the data for Figure 4. The lowest light transmittance was ohtained with winds from the north northeast through east northeast. In these sectors are a large number of small and large industries extending for several miles, as well as a large number of residents. Unfortunately, north northeast and east northeast values represent only 1 day, and only 3 days are represented by the northeast direction. Therefore, less significance should be attributed to these values than to the subsequent ones which represent the average of a larger number of readings. When the wind was from the east the visibility improved. In this direction lay the Detroit River and Lake St. Clair. Visibility improved considerably when the wind was from the south southwest. In this direction lay the Detroit River, the
Figure 1 shows typical records obtained with the Transmissometer on days of varying intensity of light transmittance. On a very clear day the transmittance record is relatively constant, while on days of lower light transmittance values, greater variations and more oscillations occurred. This figure further shows the clarity of record and sensitivity of the instrument to variation in transmitted light. Figure 2 is the visual range chart prepared by Douglas for converting the Transmissometer readings into visibility. There are two curves: one for davtime readings and one for nighttime readings. This was necessary because of the day and night definitions of visibility ( 4 ) . The instrument is most sensitive in the 0.1- to 1.0-mile range, with good sensitivity up t o 4 miles. In Figure 3 is shown the average light transmittance by the half hour for four seasonal months. The mean value given for each of the seasons was obtained by averaging the daily results for the period of observation. The monthly mean values L60J , , J 1 2 3 4 5 6 7 8 9 IO I / I 2 13 14 15 16 17 18 19 20 21 2 2 23 24 were lower in the winter and spring than HOURS during the summer and fall. This is especially interesting, because the highvolume air-sampling studies, which give the total weight of air-borne particulates, show higher mass weights in the summer and fall than during the winter and spring. To determine if the resulting lower transmittance data were due to particulates or snow and rain, the amounts of precipitation were studied. There was somewhat more snow in January and MarchApril than during November, and, of course, HOURS no snow in July. The total precipitation for the four seasons was: January, 1.9 inFigure 5 . Relation of hourly median reflectance of filter paper samples ches; March-April, 5.7 inches; July, 3.1 to hourly median transmittance readings
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,
ANALYTICAL CHEMISTRY
712
residential section of Windsor, and, farther away, a truck farming area. As the wind moved in the southwest direction, visibility decreased, which may be accounted for by the large number of industries in that area. The greatest visibility occurred when the wind was from the west through north northwest. In this direction lay mostly homes and small business establishments. The primary reason for the greater visibility from this direction, however, is that these winds are usually associated with outbreaks of fresh, clear polar air. Figure 5 shows the relationship of reflectance values to transmittance values. These data were obtained by finding the median value by the hour for the months of February and March. Tabulation of a large number of figures for each hour of the month aided in removing any bias, thus permitting a more accurate appraisal of the relationship between the two sets of data. I n addition to method of presentation in Figure 5, the data were also treated statistically and found to have a correlation coefficient of 0.365. It is apparent from Figure 5 and the correlation coefficient that there is little correlation between the two sets of data. This is expected, as the reflectance factor is primarily a measure of tars and carbon, as well as other colored material in the atmosphere, and the light transmittance is affected by all aerosols, their numbers, size, and shape, as well as the aerosol's refractive and absorption indices. The Transmissometer showed greater fluctuation than did the filter paper sampler, thus indicating its greater sensitivity. In some preliminary work, Kat2 and others (6') reported that temperature anomaly had an effect on air pollution in the Detroit area. As the temperature anomaly increased, the mass weight of the aerosols increased; conversely, as the temperature anomaly decreased, the mass weight of the aerosols decreased. To determine the influence of temperature anomaly on the Transmissometer and the tape recorder, data representing both minimum and maximum temperature anomalies were selected and are presented in Figure 6. March 12 and 13 xere selected as two days similar in respect to wind direction, wind speed, and rain. The only difference was the temperature anomaly. March 10 and 30 were similarly selected. In the following table are presented the median values for the days under study.
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Median Transmittance and ReflectanceValues for Selected Days Day March 10 March 30 March 12 March 13
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A study of Figure 6 and the table reveals that the mean temperature anomaly has little or no effect on the percentage of transmittance and reflectance. However, the data do indicate that on individual days there may be a fair correlation between
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transmittance and reflectance. At least the morning troughs of reflectance seem to follow those of transmittance with a lag of about 2 hours. There has been much discussion regarding the effect of rain on cleansing the atmosphere. Figure 7 presents the transmittance and reflectance data collected on a day of rain and a day having no rain. These days were selected after due consideration of wind direction, speed, and other meteorological factors. As would be expected, there was a decrease in light transmittance during periods of very heavy rain, as noted between the hours from 5:OO to 1O:OO A.M. on March 25, As the rain ceased, the visibility improved rapidly, reaching a maximum greater than that for the day of no rain. The reflectance follows a similar pattern for that day; during the heaviest rain, the reflectance was less, indicating the atmosphere was dirtier. This behavior would suggest that
V O L U M E 2 7 , N O . 5, M A Y 1 9 5 5
713
the rain precipitated the particles, and if the rain persisted for a sufficient length of time, there was cleansing of the atmosphere. As the reflectance factors for March 25 were constantly greater than on February 19, rain does reduce the soiling power of the aerosols. The cleansing effect of snow upon the atmosphere has also been a matter of much conjecture. In Figure 8 are shown the transmittance and reflectance data collected on a day of snow, March 29, and a day of no snow, March 31. All other meteorological factors on these two days were similar. As would be expected, snow considerably decreased visibility. However, it had little or no effect upon those aerosols which cause discoloration of the filter paper. Even during the periods of heavy snow and low Transmissometer readings, the reflectance factors remain essentially the same. Figure 9 shows the relationship of light transmittance, total weight of air-borne particulates, and light reflection from filter paper samples. For this test, a Wilson tape recorder and a highvolume air sampler nlere located next to the receiver of the Transmissometer, The filter of the high-volume air sampler was changed every 24 hours, and the weight of particulate matter was determined for the period. Data from the tape recorder and Transmissometer were tabulated and a median figure was obtained to compare with the total weight data.
There is little or no agreement between the data obtained by these three instruments. A Transmissometer measurement which records visibility cannot be used to determine either the weight of suspended aerosols or the color of the aerosols. Neither can the reflectance of a filter tape give a total weight of the aerosols nor a value for visibility. This indicates that, for a comprehensive study, all three instruments must be used. These findings may be explained as follows: The high-volume air sampler collects on a filtering pad the mass of air-borne particulates, and it is so reported. From the tape recorder are obtained the reflectance data which record the discoloration of aerosols and are greatly influenced by the tars and carbon particles therein. The Transmissometer measures the transmissivity of the air. It is known that the reason for the reduction in visibility is not formation of a barrier by the particles, but rather the phenomenon of light scattering. The number of particles per unit volume in an aerosol is very great, although the mass concentration in comparison with gas contaminants may be very small. An urban atmosphere is composed of particulates ranging in size from near the lower range of the wave length of visible light (blue haze noted in many cities) to over 40 microns. This may be the reason for the poor correlation between light transmittance values and total weight. CONCLUSIONS
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REFLECTANCE
Sensitivity of the Transmissometer is adequate for measuring small variation in light transmittance. Visibility was greater in Detroit in the summer and fall than in the spring and winter. Light rain did not appreciably affect light transmittance, while heavy snows and rain caused a marked reduction. No relationship was found to exist between reflectance values of filter paper spot tests and visibility. Temuerature anomalies had little or no effect 6n visibility or the aerosols that cause discoloration. Heavy persistent rain reduces the aerosols that cause discoloration. Snow does not reduce the aerosols that cause discoloration. No relationship was found to exist among visibility, mass weight, and soiling power of aerosols.
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LITERATURE CITED
(1) Byers, D. H., and Keenan, R. G.,
Arch. Ind. Hug. Occupational Med., 6, 226-30 (1952). (2) Katz, bI., Cave, G. A.. and Baynton, H. W., International Joint ComProgress Rept. 7 (1953).
(3) Sutton,0. G., “Micrometeorology,” p. 190, McGraw-Hill, New York,
200 150 100
,M A S S
5
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sor, Ontario, Canada, who performed the necessary laboratory work, and Howard Baynton, meteorologist, M e t e o r o l o g y Service of Canada, Windsor Laboratories, International Joint Commission, Windsor, Ontario, Canada.
mission, Ottawa, Ontario, Canada,
250
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1953. (4) Theissen, A. H., “Weather Glos-
