current research
Atmospheric Visibility Related to Aerosol Mass Concentration A Review R. J. Charlson Department of Civil Engineering, University of Washington, Seattle, Wash. 98105
presence of particles in the atmosphere reduces visibility. This phenomenon is dramatically evident in polluted air, where the visual range often falls below a few kilometers. This amount of degradation is frequently objectionable and hazardous. This paper summarizes the present and recently acquired knowledge of the relationship between the mass concentration of aerosol and the visual range. Five main topics are covered: the significance of the self-preservingor constantshape size distribution; the integrating nephelometer for measuring atmospheric visibility degradation; light-scatter and extinction related to aerosol characteristics; and the relationship between mass concentration and visual range; and the application of this result. IThe
R
went improvements in theory and instrumentation have resulted in a better understanding of the relationship between some of the physical-chemical properties of atmospheric aerosols and visibility degradation. In particular, an approximate proportionality between the mass concentration of atmospheric aerosol and the extinction coefficient due to light scattering has been demonstrated in a wide variety of cases by Charlson, Horvath, et al. (1967) and Charlson, Ahlquist, et af. (1968), and a reciprocal relationship between mass concentration and visual range was shown by Fett (1967) and Noll, Mueller, et al. (1968). That these simple relationships should exist is somewhat surprising, for several reasons. First, the pertinent aerosol properties (density, refractive index, size distribution, etc.) are frequently thought of as being extremely variable and unknown. Second, both the physical process of light scattering and the theory used to describe it, the Mie theory, are complicated and difficult to visualize. Finally, there exists a backlog of experiments in which a low correlation coefficient between mass concentration and visual range was found or suggested (Burt, 1961; Clayton and Giever, 1955; Robinson, 1968). The substance of this paper is a review of the current knowledge of the relationship between the visual properties of air and the aerosol characteristics, particularly mass concentration. The goal of the discussion is to demonstrate that useful methods and data have evolved which relate, at least semiquantitatively, cause to effect-that is, the amount of particulate matter to the degree of visibility reduction. t Circle NO. 90 on Readers’ Service Card
Visibility Theory Before discussing specific relationships between optical properties (such as visual range) and a wide variety of physical and chemical aerosol properties, it is necessary to review classical visibility theory and define terms. Theoretical aspects of the relationship of optical properties of air to the aerosol properties will be considered later. Koschmieder (1924) derived a relationship between extinction coefficient, b, and a distance now called the meteorological range, L,. For an average human eye with a threshold of brightness contrast of 0.02 (Middleton, 1952), at a wavelength of 550 nm and for an ideal black object against the horizon sky in daytime: -In 0.02 3.912 L , = ___ = (1) b b
where the extinction coefficient is defined by the usual BeerLambert (Bouguer) law, dI/I = -bdx for intensity, I, and distance, x . Unfortunately, Koschmieder found it necessary to make several assumptions, some of which we now know to be unrealistic (Charlson, Horvath, et al., 1967). Of particular importance is his assumption that the extinction coefficient is constant in a horizontal plane. Experimental results of horizontal profile measurements by Ahlquist and Charlson (1968) show that the horizontal scale of variations of the extinction coefficient is of the same magnitude as the visual range and not much larger, as would be required for Koschmieder’s assumption. This spatial variability combined with the variability of human eyes makes necessary a differentiation between meteorological range as defined by Equation 1 and visual range as observed over a long atmospheric path. These two distances may be the same to an average observer if the extinction coefficient is spatially uniform. Whereas the visual range (sometimes called visibility) is the actual distance at which an ideal black object can just be seen against the horizon sky, the meteorological range is the distance yielding a contrast threshold of 0.02 with the extinction coefficient, b, independent of the variable, x . An excellent discussion of the entire optical aspect of visibility is given by Middleton (1952). Accepted definitions of this terminology are given by Huschke (1959). Recent Work Two major inroads toward understanding the behavior of well-aged, low humidity atmospheric aerosols (relative humidity less than ca.7 0 x ) make possible gross simplifications in understanding and using the visibility-mass concenVolume 3, Number 10, October 1969 913
tration relationship. First, numerous efforts in the study of size distribution have resulted in a strong confirmation of the concept of a self-preserving size distribution. Tentatively proposed on the basis of observation by Junge (1952), studied theoretically by Friedlander (1960a,b, 1961, 1969, Hidy (1965), Hidy and Lilly (1965), Friedlander and Wang (1966), and Liu and Whitby (1968) and experimentally by Cartwright, Nagelschmidt, et al. (1956), Friedlander and Pasceri (1965), Clark and Whitby (1967), Peterson and Paulus (1967), Pueschel and No11 (1967), and others, this concept removes many of the uncertainties in the knowledge of particle size distribution in the atmosphere. This realization is particularly important to atmospheric visibility, since particle size dominates the other aerosol system properties-e.g., refractive index, etc.-in determining the extinction coefficient due to scatter. The basic concept simply implies that the aerosol size distribution always has the same form or shape. This is not to say that it is invariant, but rather that for many purposes it is useful and realistic to consider it to be so. This constant shape-size distribution can be described mathematically in several ways; perhaps the most straightforward is that of Junge (1952) in the form of a power law:
where N is the number of particles per volume, r is radius, /3 is approximately constant and about 3.5 += 0.5, and c is a quantity dependent on the total concentration of aerosol. If this form is used, upper and lower size cutoffs are necessary and are approximately 0.02 and 5 microns, respectively. It is thus unrealistic to consider the atmospheric aerosol t q be monodisperse for the problem of visibility. Rather, it is necessary to consider consistently a group of size classes covering perhaps two or more decades of radius. The significance of the constant shape-size distribution to the relationship between atmospheric visibility and aerosol mass concentration was discussed in detail by Horvath and Charlson (1969). Briefly, if the size distribution always has the same shape and if all the other pertinent aerosol characteristics are fixed, the mass concentration, M , will be proportional to scattering coefficient, banst.This can be illustrated as follows: If N , , M,, and S , are the number (cm.-a), mass (g), and scattering cross section (sq. cm.) of individual particles in a size class, i, and f,is the fraction in this class of the N total particles in the system, then:
i = __-
Em i
which is clearly independent of the total number, N . Now, iffi, M,, and S, remain constant, M/b,catis constant. The constraint on M , and s,amounts to a statement that these properties of individual size classes do not change from one system to another, while a fixed fi implies a constant shape-size distribution. Interestingly, this is independent of the actual shape of the size distribution. In the: case of the power law distribution, the mass is distributed over a wide set of size classes, while scattering is attributable to a narrow range of sizes around 0 . 3 - ~ mradius (Pueschel and Noll, 1967). Nonetheless, these two quantities can be proportional if there is a fixed relationship between the number of particles in the various size classes-Le., f,is constant. 914
Environmental Science & Technology
The second major development is in general due to the widespread use of light scattering for studying aerosols and specifically the development of an integrating nephelometer of sufficient sensitivity and stability for long-term studies of the atmosphere. The basic principle of this instrument is due t o Beuttel and Brewer (1949); other such instruments were developed by Ruppersberg (1959), Crosby and Koerber (1963), and Ahlquist and Charlson (1967, 1968). Commercial versions of these devices are available. Such instruments measure scattering over the entire sphere rather than over a narrow solid angle; as a result, the quantity measured is proportional to the extinction coefficient due to scattering. Thus, it is possible to measure at a point a component of the extinction coefficient. As a result of this instrumental development, it has been possible to approach several atmospheric aerosol problems experimentally. The primary experiment to date has centered on correlating the mass concentration of aerosol to the extinction coefficient due to scatter (scattering coefficient) (Charlson, Ahlquist, et al., 1968). A correlation coefficient of 0.82 was found for 238 low humidity cases in a wide variety of locations. The regression equation corrected to a wavelength of 550 nm. via the method of Ahlquist and Charlson (1969) with 90% confidence limits is:
1(4? where M represents the mass concentration, represents the measured scattering coefficient, and the subscript and superscript represent the 90 % confidence limits. This result alone has obvious applications, discussed below. Before doing so, it is necessary to establish the magnitude of the extinction coefficient due to scatter in relation to other extinction components. Besides the correlation to mass concentration, the integrating nephelometer has proved to be useful for mapping the scattering coefficient spatially for studies of the motion and diffusion of atmospheric aerosols on a mesometeorological scale (Ahlquist and Charlson, 1968). Despite the apparent dominance of such spatial variation and other quantities, Horvath and No11 (1969) demonstrated that the measured light scattering coefficient and visually observed visual range were nearly in perfect agreement with Equation 1. Still other applications include a study of the automotive contribution to urban haze (Buchan and Charlson, 1968), measurement of the relative magnitudes of the Rayleigh scattering of air, Con, and Freon 12 (Charlson, Horvath, et al., 1967; Horvath, 1967), and several basic aerosol experiments (Horvath and Charlson, 1969). The latter effort concluded that the observed amount of variation in the mass-concentrationlight-scattering proportionality can be explained by the combination of variations in the exponent of the size distribution, p, refractive index, the upper size limit of the size distribution, and the density of the suspended particles. Light Scattering and Extinction as Related to Aerosol Characteristics Generally, the extinction coefficient can be depicted as the sum of several components :
b
= becat
+
blsylaiph
f
babagaa
f
babs-serasol
(5)
where bscatrepresents the component due to light scattering by aerosol, b R s y l e i p h the scattering due to gaseous air (the blue-sky scatter), and b.ba-laaand bsbs-ssroaol represent the
absorption due to gases (like NOS and particles (such as carbon black), respectively. Middleton (1952) and others have suggested that b,,, is dominant especially in situations where the visual range is somewhat degraded due to haze. Fenn (1966) suggested that b,, could be of the same magnitude as bRaylsiph in many tropospheric cases. Data acquired at Mt. Olympus, Wash., with an integrating nephelometer showed that, for 39 days during February and March 1968, about two thirds of the light scatter was due to the gaseous components and one third due to aerosol (Charlson, 1968; Radke and Hobbs, 1969). This may represent a tropospheric background value due to the remote location and prevailing westerly air flow. Much higher values of bsoathave been reported for even clean continental air and, of course, for polluted air. Thus, for urban cases it is safe to assume b,c.t >> bRSylaigh. In any case, it is now possible to measure the total light scatter with an integrating nephelometer and subsequently to subtract the constant b R a y l s i g h to get baCst.It is possible to measure b R s y l s i g h with a signal-to-noise ratio of about 50, as reported by Charlson, Horvath, el af. (1967), by simply filtering out the particulate matter. The absorption of light by gases is, of course, well understood. Although almost no published data exist for such absorption in the atmosphere at visually detectable wavelengths, instruments such as the Barringer correlation spectrometer have been used recently for detecting NOZ, for example, in a polluted environment. Still, the magnitude of may be relatively small. Hodkinson (1966b) reports an absorption coefficient of 0.7 p.p.m.-l km.-' for NOn at 500-nm. wavelength. Since the atmospheric concentration of NOz rarely exceeds 1 p.p.m., an absorption coefficient of 0.7 m.-l seems adequate as an upper limit of km.-l or 7 X b.bs-o.l in Los Angeles, for example. Now, in a situation where NO2 is present in such quantities, the meteorological conditions are always such that aerosol is also concentrated. Buchan and Charlson (1968) report the ratio of beoatto NO, due to automobiles alone at about the same 500-nm. wavelength as approximately 5 X lo-' m.-l p.p.m.-' If other sources of aerosol are present and if automobiles are the dominant source of nitrogen oxides, it seems probable that b,, would dominate baba-gas in the case of NOZ. No other light-absorbing gases appear to be as important as NOz. If simultaneous measurements of bBCatand either bsba-gas or the light-absorbing gas-e.g., NOz-were made, it would be possible to evaluate the effect of each extinction component. Clearly, extensive studies of this sort would help to settle the question raised by Hodkinson (1966b) whether the brown color of polluted air is due to NOz, wavelength-dependent light scattering, or possibly both. A preliminary experimental comparison by Charlson and Ahlquist (1969) of these two extinction components suggests a typical ratio of bScat/ b a b b ~ ~=, 7 at 500 nm. and 10 at 550 nm. Finally, the light absorption by aerosols-the fourth extinction component-must be considered. Unfortunately, this quantity has thus far eluded accurate measurement, in contrast to that of Whereas the integrating nephelometer measures light that is scattered, the absorbed light has not yet been measured directly. The alternative involves measuring total extinction by a transmission method and subtracting the other three extinction components: baba
= b
- heat - bRaylsigh
- bsba-gar
(5a)
Several factors in atmospheric measurements introduce inaccuracies in excess of those required to make this difference meaningful. First, the extinction coefficient is small (typically to IO-' m.-l), so that it is necessary to depend on either a
very long path-e.g., kilometers-or transmissions close to 100%. If a long unfolded path is used, spatial variation of the extinction coefficient and multiple scatter may be important, whereas in the case of a short path and a high percentage transmission, it is necessary to take the ratio of two nearly equal numbers. Folded paths require mirrors exposed to the elements, a source of experimental difficulty. Even if it were possible so to achieve a value for the total extinction coefficient, 6, the subsequent subtraction could well involve another case of subtracting numbers of the same magnitude, since baba-aerorol may be a small quantity. As a result of these difficulties, no really adequate evaluation of bsbcaeroaol is currently possible. Further, it is difficult to estimate the amount of light absorbed by calculating absorption cross sections from chemical composition and size distribution data. Some information obtained from recent studies of visual range (Horvath and Noll, 1969) and measured bsoatsuggest that in their location all the observed extinction can be attributed to scattering and that absorption might be neglected. However, the human eye is a notoriously poor instrument, leaving the assumption of negligible light absorption by atmospheric aerosols as a continuing necessity. If it is possible to assume that light scattering by aerosols is dominant in determining the visual range, then it is possible to state those aerosol characteristics which are relevant : 1. Wavelength of light 2. Size distribution 3. Refractive index (including its complex part, if any) and its size variation 4. Particle shape and physical structure and orientation with respect to the sight path, and their size variation 5. The variation of 2 through 4 along the sight path and in time. A similar list can be made for the variables controlling mass concentration : 1. Size distribution 2. Density 3. Shape and physical structure 4. The dependence of 2 through 3 on particle size 5 . The variation of 1 through 4 in time and space Of these two sets of variables, only a few can be treated mathematically and even then only in a cursory way. The effect of nonspherical shape and heterogeneous physical composition might be understood in the case of mass concentration but certainly not in the case of light scatter. Interestingly, the calculations by Horvath and Charlson (1969), while concerned only with size distribution, density, refractive index, and wavelength, seemed to account for the observed amount of variation in the proportionality of mass concentration and light scattering. In the measurements reported by Charlson, Horvath, et af. (1967) and Charlson, Ahlquist, et nl. (1968), care was taken to eliminate the effect of temporal or spatial variations in both quantities. The light scattering and mass concentration were measured at the same location and either time or volume averaging of the light-scattering record was used to eliminate the dependence on time. The wavelength remained a fixed band of about 50-nm. width at about 500 nm. This implies that the other variables are somewhat less effective in controlling the relationship of mass concentration and light scatter-that is, that particle shape and physical structure and their size dependence, as well as the dependence of density and refractive index on size, are perhaps unimportant to this particular relationship. Although it would be difficult to approach theoretically, the situation may be analogous to that demonstrated by Hodkinson (1966a) for the case of polychromatic us. monochromatic light scatter. It may be, for Volume 3, Number 10, October 1969 915
instance, that the averaging effect over particle orientation removes to some extent a dependence on particle shape. This same line of thought could be extended to spatial variations as well. Noll,Mueller, et al. (1968) found for their location that visual range was correlated in a linear fashion to mass concentration measured at a point. Only if the extinction coefficient were constant in space or if the scale of spatial variation were very short and of random character might this result be expected. However, the spatial variation data of Ahlquist and Charlson were hardly of this character. Further, the extinction measurement of Clayton and Giever (1955) over about 250 meters showed an apparently poor correlation. Thus, the problem of the way in which the extinction coefficient varies in space (and time) remains to be studied.
IDEAL BLACK OBJECT AGANST HORIZON SKY
bFigure 1. Mass of particulate matter in a box oriented along the sight path
Applications
Noll, Mueller, and Imada (1968) in Berkeley, Calif. The modal value of -1.8 grams per square meter obtained by these workers using visual methods agrees with corrected data (Figure 1) utilizing the light-scattering approach. The consistency of these different experimental approaches was demonstrated by Horvath and No11 (1969). It is interesting to compare the 1.8 grams per sq. meter quantity with that calculated by Robinson (1968) for monodisperse particles. He estimated that 0.34 and 0.67 gram per sq. meter of 0.6-pm. diameter oil and water aerosol, respectively, would be required to determine the visibility. This demonstrates that the atmosphere contains a substantial amount of material that is less effective to light scattering per mass than these 0.6-pm. diameter particles. The result illustrated in Figure 1 has several potential applications. Since the extinction coefficient due to scatter is measurable with an integrating nephelometer, it is possible to obtain records for interpretation in terms of both meteorological range and (within the given error band) mass concentration. Further, historical records of visibility at suitably low humidity may be used for studying trends in air pollution. In fact, even without a substantial knowledge of this relationship, Holzworth (1961) concluded that visibility records reflected changes in pollution levels that had occurred over a severalyear period in Columbus, Ohio, and elsewhere. The obvious dependence of visibility on humidity at once poses limitations on utilizing this mass concentrationmeteorological range relationship and provides a means for
Given the foregoing as a starting point, it k possible to state what applications of the published information seem justified. Presuming that light scattering dominates atmospheric extinction, it is possible to relate meteorological range, Lo,and aerosol mass concentration, M , via the previously mentioned correlation of M and b,,$ and the Koschmieder expression. If both sides of this expression are multiplied by the mass concentration, a product with physical significance is formed:
M = !Z 3.9 x 0.45-,:,, $0 45 L, x M E 3.9 g./sq. m. b
$1 8 1.8-69 g./sq. m.
(6)
This product represents the mass of aerosol in a I-sq.-meter column oriented along the sight path of length L,, as shown in Figure 1. Where 0.45 gram per square meter resulted in decreasing the intensity to lle of its original value, 3.9 times as much material is required just to obliterate the view of an ideal black object against the horizon sky. Clearly, this result is independent of where the particulate matter k in the box, so that this result is not constrained to a spatially homogeneous case. This quantity does not seem to depend on an urban location and, therefore, might represent a kind of tropospheric property. The high, negative correlation between mass concentration and visual range and the quantitative result of Figure 1 have been corroborated by Fett (1967) in Berlin, Germany, and by
9 JANUARY, 1967 SEAlTLE, WASHINGTON y4
4M
m xx)
IM
2403
-
1203
0
0
TIME
Figure 2. Light scattering as a function of time Heavy line: Time dependence of light scattering in Seattle, Wash.
Ordinates: Meteorological range (km.),light scattering coefficient at 500 nm. (m. -l) and approximate mass concentration &g./cu. m.). Error band (light lines) represents 90% confidence limits for the mass scale. Vght scattering was measured, other ordinates and error band calculated from Erluetions 4 and 6, wth b.wt corrected to 550 nm. per Ahlquist and Charlson (1969). 916 Environmental Science & Technology
inferring the presence of deliquescent materials. Aerosols composed of hygroscopic salts--e.g., NaC1, (NH&S04, etc.undergo a phase transition accompanied by a radical increase in light scatter at relative humidities below 100% (Pilat and Charlson, 1966); however, Charlson, Horvath, et al. (1967) suggested that air can be heated prior to optical evaluation. Further, since the relative humidity at which aerosol particles deliquesce is a function of their chemical composition, studies of the light scattering as a function of humidity can provide useful composition information (Lundgren and Cooper, 1969). Fortunately, many (if not most) common atmospheric aerosols exhibit deliquescence only above 70% relative humidity (Acheson, 1964). Thus, a humidity criterion exists for utilizing the above mass concentration-meteorological range product (Lo X M). The State of California has already adopted 70x relative humidity as the upper limit for application of its visibility standard (State of California, 1960). Records of light-scattering coefficient as a function of time reveal a variability which can be interpreted as changes in the mass concentration. Figure 2 shows a previously unpublished record taken in Seattle, Wash., by the author to illustrate the character of such data. The error band for about 90% confidence is given, as is a mass concentration ordinate scale. Recognizing that the traditional filtration method of determining mass concentration (the high volume sampler) utilizes a typical sampling period of 24 hours, considerable short-period detail is gained by the use of a light-scattering record. Spatial data such as those published by Ahlquist and Charlson (1968) can be interpreted in a similar manner to yield mass concentration profiles. Perhaps the most important application of this information arises because of the need for quantitative relationships between the amount and effect of a pollutant. Decreased visibility is one of the obvious consequences of the presence of aerosols in air; however, no instrumental means has been available until recently for objectively and continuously measuring this effect of pollution. The quantities and concepts given in Figure 1 can be used to formulate a criterion or standard for visibility, as was done in the “Air Quality Criterion for Particulate Matter” ( U S Department of Health Education and Welfare, 1969). The 1.8 gram per square meter quantity represents a refinement on the earlier data in this document. Conclusions Atmospheric aerosols of relative humidity less than about 70 result in physically understandable visibility reduction. Of particular practical importance is the relationship between the mass concentration of atmospheric aerosol and the extinction coefficient due to scatter. Although the relationship between these two quantities is dependent on many variables and is complicated, an approximate proportionality between them has been demonstrated both experimentally and theoretically. Of 238 cases measured with an integrating nephelometer, 90 had a mass concentration-visual range product (Lo X M ) of 1.82;:; grams per sq. meter. Even though this represents only a factor of 2 in terms of accuracy, the application to such problems as air quality standards seems possible. In view of the scarcity of such relationships or of better accuracy, this application seems valid.
Nomenclature
meteorological range, km. extinction coefficient, m.- 1 scattering component of extinction, m.-l Rayleigh scattering component of extinction coefficient component of extinction due to light absorption by gases component of extinction due to light absorption by particulate matter intensity distance number of particles per cc. particle radius, microns proportionality coefficient in size distribution Junge exponent mass concentration, fig./cu. m. number of particles in ith particle class, cc.-l mass of individual particles in ith particle class, grams scattering cross section of individual particles in ith particle class, sq. cm. particle class index fraction of particles in ith particle class Literature Cited Acheson, D. T., in “Humidity and Moisture,” A. Wexler, ed., Reinhold, New York, 1964. Ahlquist, N. C., Charlson, R. J., Atmospheric Environ., in press, 1969. Ahlquist, N. C., Charlson, R. J., ENVIRON. SCI. TECHNOL. 2,
363-6 (1968). Ah1quist;N. C., Charlson, R. J., J . Air Pollution Control Assoc. 17. 467-9 (1967). Beutiel, R. G., Biewer, A. W., J . Sci. Znstr. 26, 357-9 (1949). Buchan, W. E.,Charlson, R. J., Science 159,192-4 (1968) Burt, E. W., J. Am. Znd. Hyg. Assoc. 22, 102-8 (1961). Cartwight, J., Nagelschmidt, G., Skidrnore, J. W., Quart. J . Roy. Meteorol. SOC.82, 82-6 (1956). Charlson, R. J., J . Air Pollution Control Assoc. 18, 652-4 (1968). Charlson. R. J.. Ahlauist. N. C.. Atmosoheric Enuiron.. in press, 1969. Charlson, R. J., Ahlquist, N. C., Horvath, H., Atmospheric Environ. 2, 455-64 (1968). Charlson, R: J., Horvath, H., Pueschel, R. F., Atmospheric Environ. 1, 469-78 (1967). Clark, W. E., Whitby, K. T., J . Atmospheric Sci. 24, 677-87 (1967). Clayton, G.D., Giever, P. M., Anal. Chem., 27,708-13 (1955). Crosby, P., Koerber, B. W., J. Opt. SOC.Am. 53,358-61 (1963). Fenn, R. W.,Appl. Optics 5,293-5 (1966). Fett, W., Beitr. Physik Atmosphiire 40,262-78 (1967). Friedlander, S. K.,in “Aerosols,” K. Spumy, ed., pp. 11530,Publishing House of Czechoslovak Academy of Sciences, 1965. Friedlander, S. K.,J. Meteorol. 17, 373-4 (1960a). Friedlander, S. K., J . Meteorol. 17, 478-83 (1960b). Friedlander, S. K., J . Meteorol. 18, 753-9 (1961). Friedlander, S.K., Pasceri, R. E., J . Atmospheric Sci. 22,571-6 (1965). Freidlander, S. K., Wang, C. S., J . Colloid Sci. 22, 126-32 (1966). Hidy, G. M., J . ColloidSci. 20, 123-44 (1965). Hidy, G.M., L a y , D. K., J . Colloid Sci., 20, 862-74 (1965). Hodkinson, J. R., in “Aerosol Science,” Chap. X,pp. 287357. C. N. Davies. ed.. Academic Press. London. 1966a. Hodkinson, J. R., Air Water Pollution inter. J., 10, 137-44 (1966b). Holzworth, G.C., in “Air Over Cities,” SEC Tech. Rept. A 62-5 (1961). Horvath, H., Appl. Optics 6, 1140 (1967). -
.-~~
I
Acknowledgment The author is deeply indebted to Norman C. Ahlquist, not only for designing the nephelometer, but also for being a continuous source of ideas.
I
-
~
Volume 3. Number 10, October 1969 917
Horvath, H., Charlson, R. J., J. Am. Znd. Hygiene ASSOC.,in press, 1969. Horvath, H., Noll, K. E., Atmospheric Environ., in press, 1969. Huschke, R. E., “Glossary of Meteorology,” American Meteorological Society, 1959. Junge, C. E., Ber. Deut. Wetterd. 35, 261-77 (1952). Koschmieder, H., Beitr. Phys. Freien Atm. 12, 33-53, 171-81 (1924). Liu, B.Y. H., Whitby, K. T., J . ColloidSci. 26, 161-5 (1968). Lundgren, D. A., Cooper, D. W., J. Air Pollution Control ASSOC. 19, 243-7 (1969). Middleton, W. E. K., “Vision through the Atmosphere,’’ pp. 60-82, University of Toronto Press, Toronto, 1952. Noll, K. E., Mueller, P. K., Imada, M., Atmospheric Environ. 2, 465-75 (1968). Peterson, C. M., Paulus, H. J., Paper 67-133,Air Pollution Control Assoc. Meeting, Cleveland, Ohio, June 1967. Pilat, M. J., Charlson, R. J., J. Rech. Atmosphkrique 2, 165-70 (1966). Pueschel, R. F., Noll, K. E., J. Appl. Meteorol. 6, 1045-62 (1 967).
Radke, L. F., Hobbs, P. V., J. Atmospheric Sci. 26, 281-8 (1 969). Robinson, E., in “Air Pollution,” A. C. Stem, ed., Vol. 1, 2nd ed., Chap. 11, pp. 349-99,Academic Press, New York, 1968. Ruppersberg, G. H., Jahrbuch der WGL, 230-36, FFM Bericht Nr. 29 (1959). State of California, “California Standards for Ambient Air Quality and Motor Vehicle Exhaust,” Dept. of Public Health, 74-5 (1960). U.S. Dept. of Health, Education and Welfare, “Air Quality Criterion for Particulate Matter,” Chap. 3, 1969.
Received for review September 16,1968.Accepted June 30,1969. Symposium on Colloid and Surface Chemistry in Air and Water Pollution, Division of Colloid and Surface Chemistry, 156th meeting, ACS, Atlantic City, N.J., September 1968. Research sponsored in part by the National Air Pollution Control Administration under Grants No. AP 00336-05 and 06.
Sorption of Phenol and Nitrophenol by Active Carbon Vernon L. Snoeyink,’ Walter J. Weber, Jr., and Harry B. Mark, Jr. Departments of Civil Engineering and Chemistry, University of Michigan, Ann Arbor, Mich. 48104
rn Equilibrium measurements of the sorption of phenol and p-nitrophenol from aqueous solution by active carbon suggest a heterogeneity of active surface sites with respect to energy of adsorption. Desorption studies show the presence of significant hysteresis effects when long equilibration periods are involved, although these effects are much smaller when adsorption-desorption equilibria are attained more rapidly. Differences in surface properties for different carbons is suggested by more extensive sorption of phenol at lower surface coverages on a coconut carbon than on a coal carbon of similar surface area. Further, again for low surface coverages and the same coconut carbon, p-nitrophenol is sorbed more extensively than phenol. At higher surface coverages the sorption is apparently less specific, and the sorption isotherms tend to converge. Studies at various pH levels indicate that the capacity of active carbon for adsorption of the anionic forms of both phenol and p-nitrophenol is less than for the corresponding neutral species. There is no marked effect of pH on the sorption of the neutral form ofp-nitrophenol in the pH range from 2.0 to 6.5.The capacity for the neutral phenol molecule decreases significantly with decreasing pH in this same range, however, suggesting that the hydrated proton competes effectively with phenol for active surface sites.
A
t least one fact has emerged quite clearly from recent efforts to characterize the processes of adsorption on active carbon in aqueous systems; namely, the structural and surface complexities of this material result in a diversity of sorptive reaction mechanisms for organic compounds of interest in water and wastewater treatment applications. Phenol and various substituted phenols comprise a group of comPresent address, Department of Civil Engineering, University of Illinois, Urbana, Ill. 61801 918 Environmental Science & Technology
pounds of particular concern for both water and wastewater treatment. The present report describes an investigation of the sorptive reactions of phenol and p-nitrophenol on active carbon. As noted, the structural characteristics and surface properties of an active carbon are essential determinants of the nature of the sorptive behavior of the carbon. Active carbon is generally considered to be comprised of rigid clusters Qf microcrystallites, each microcrystallite consisting of a stack of graphitic planes. The diameter of the planes, as well as the height of stacking, is normally less than 100 A. Each carbon atom within a particular plane is joined to three adjacent carbon atoms by u bonds, with the fourth electron of the atom participating in a x bond. It is likely that part of the carbon within a microcrystallite is highly disordered, thus deviating from the ideal graphite structure. Carbon atoms at the edges of the graphitic planes have highly reactive “free” valences (free radical sites). It is probably these free radical carbon atoms, in conjunction with van der Waals forces, which serve to bind the microcrystallite into a rigid unit. The porosity of active carbon results from the “burn-out” of intermicrocrystallite material and planes of the microcrystallite by oxidizing gases during the activation process. The extensive intraparticle surface which results is very heterogeneous in nature, consisting of basal planes and edges of the microcrystallites. A diversity of functional groups undoubtedly forms on the edges of the microcrystallites in commercial carbons because of the high reactivity of the free valences and the variety of substances used in the preparation of such carbons. Not all of these groups have been characterized, but some of the oxygen-containing functional groups which have been identified are proving very important in various applications of active carbon. Extensive discussions of the structural and surface characteristics of carbons have been given by Boehm (1966),Garten and Weiss (1957a,1957b), Snoeyink and Weber (1967),and Mattson, Mark, et a(. (1969). The presence of oxygen-containing functional groups on the surface of carbon markedly affects the adsorption of certain
