Visibility: An evolving issue Visibility is not a local effect; the visual eqerience is instantaneous, and unlike health effects, visual effects
christiw s. Slosw General Motors Research Laborarories W r e n , Mich. 48090
Warren H.White Centerfor Air Pollution Impacr and nend Analysis Mwhingion University St. Louis, Mo. 63130 Clear days are an important aesthetic resource for us all. They also carry commercial value for tourism and real 760 Envimn. Sci. Technol.. Vol. 20. NO.8, 1986
estate. Thus, the appearance of layers of smoggy haze over cities and across rural vistas is one of the most widely noticed effects of air pollution. Our quest to discover the source of this haze is a twentiethentury version of an inqniry that began when humans fmt wondered why the stars disappear each day. We now know that the stars are obscured by light scattered throughout the intervening atmosphere. Even in clean air, our visibility is limited by natural airlight, which softens shadows and gives the sky a pleasing blue hue. Haze degrades visibility in two ways.
F i t , haze-mhanced aulight bnghtens all features in a scene toward a uniform level, eliminating shadow and superimp i n g a white veil. Second, light transmitted from an object is attenuated by absorption and by scattering out of the observer’s line of sight. The result is a lessening of visual range, a blurring of scenic texture and detail, and a distortion of color. The task of protecting visual air quality requires an operational understanding of what haze is, how it develops, and what the responsible pollutants are. In the past decade these issues have
M)13936x186/092U.O7~1.WlO @ 1986 American Chemical Society
been addressed through intensive modeling, field experimentation, laboratory simulation, and trend analysis. The fundamental physical and chemical characteristics of haze have been identified, with the result that many of the questions that spurred visibility research have been transformed into a second generation of inquiry. Our capacity to design effective control strategies, to mount effective monitoring efforts, and to predict with accuracy the visual effect of changing emission patterns hinges on the resolution of these new questions. This article identifies some of these critical issues in visual air quality.
Phase equilibrium One recent focus in visibility research involves the tremendous optical consequences of small shifts in the equilibrium between the gaseous and aerosol phases. Even before instrumental methods were available, it was deduced that most gaseous pollutants have little effect on visibility; they scatter light in much the same way as it is scattered by gases in clean air. The principal exception is nitrogen dioxide because it absorbs blue light. The visual effects identified with haze are predominantly caused by aerosol particles with diameters between 0.1 and 1.Opm (2). These fine particles scatter visible light much more effectively per unit mass than gases or particles of other sizes can because their size is comparable to the wavelength of visible light (3). Just as a cloud produces a dramatic visual effect when only a small fraction of the water vapor changes phase, a substantial haze results if only a fraction of the gaseous pollutant mass enters a condensed phase. In this regard, visibility is unique among air pollution effects; it depends not only on the amount of air pollution but in addition on its phase. This peculiarity greatly complicates the prediction of visibility impairment and aerosol measurement procedures because the equilibrium between the condensed and gaseous phases can be fragile. For example, when the vapor pressure of nitric acid (HN03) is too high for substantial condensation, nitrate remains predominantly in the gas phase. But if the partial pressure o€ ambecomes high enough, an monia ("3) equilibrium is established between the solid-phase ammonium nitrate (NH4N03) and gaseous HN03 and "3, so the nitrate enters the aerosol phase. When the relative humidity reaches 62% (at 25 "C), NH4N03 deliquesces (forms a solution in water) and a new equilibrium is established among gaseous "03 and "-, water vapor, and
the NH4N03 solution. Considerably more nitrate can be found in the aerosol partial presphase for the same "03 sure under these conditions. In this example, the aerosol nitrate concentration shifts with relative humidity, gaseous vapor pressures, solubilities, and chemical interactions among aerosol constituents. Experiments that are designed to explore the processes of aerosol growth and chemical evolution must strive to preserve the delicate ambient phase balance. The requirement that aerosol-sampling techniques preserve phase balance can conflict with needs for analytical sensitivity, Sampling methods must be sensitive because very small increments in concentration can have a significant effect on visibility in near-pristine areas. This follows from the observation that visual range is inversely proportional to particle concentration. To ensure adequate mass for sensitive chemical analysis, an aerosol sample is collected over several hours. But because it is not a static chemical system, the aerosol composition may shift between capture and analysis, especially if ambient conditions change during a sampling interval. Within the past five years sampling techniques have been refined to take into account the dynamic nature of the aerosol-gaseous interface. The newer techniques minimize positive and negative artifacts in measured nitrate, sulfate, and ammonium concentrations (4, 5). Precise methods for evaluating aerosol water content are undergoing refinement. Analytical methods to reduce artifacts involving volatile organics are under development; they are particularly important for sampling urban plumes in the western United States, where substantial organic mass fractions have been observed (6, 7). Aerosol-sampling techniques designed to assess visibility must meet a third criterion, which can also be operationally incompatible with the preservation of phase. This is the requirement for size resolution of aerosol composition within the range of 0.1-1.0 pm diameter. The size distribution of ambient aerosol particles can be inferred in several ways: from the geometric dimensions of their collected residues (using microscopy), from light-scattering measurements (using an optical particle counter), and from their mobility under controlled situations (using an electrical mobility analyzer, diffusion battery, or inertial impactor). Most information on the particle size distributions of individual chemical spqcies has come from samples collected in inertial impactors (8, 9). However, the sharply curved air flow streamlines required for particle sepa-
ration have sometimes been produced at the expense of changes in pressure and temperature that disturb the equilibrium between the gas and condensed phases. Refinements in the design of impactors have reduced these thermodynamic perturbations, but the degree of size resolution within the size range critical to visibility is still marginal (10, 11). As a result, it is not possible to make accurate assessments of individual species' effects on visibility. Accurate source apportionment awaits refinement of the techniques for measuring the mass dispersion of chemical constituents among aerosol sizes.
Chemical evolution Airborne particles in the size range of 0.1-1 .Opm diameter are remarkable not only for their accentuated visible light-scattering efficiency per unit mass but for two additional attributes. Each significantly complicates the process of source apportionment for visibility degradation. First, quite by coincidence, these optically effective particles are the least efficiently removed from the atmosphere by natural processes (2, 12). With airborne lifetimes of two to five days, they can be carried over extended distances by prevailing winds. This dispersion adds to the difficulty in tracking their airborne trajectories to the sources of emission. A second factor complicates the identification of pollutant emissions responsible for visibility impairment: Aerosols consist mostly of pollutants that are not emitted directly by pollution sources. Of the optically important haze constituents, only soot is a primary contaminant that is emitted directly into the atmosphere by combustion sources (13). Most haze material is formed in the atmosphere itself. A complex series of chemical reactions transforms gaseous emissions (primary pollutants) into secondary pollutants such as sulfate, nitrate, and organic condensates. These atmospheric reactions disguise the responsible emissions sources. A good deal of research has focused on measuring and predicting the volume of water condensed on aerosol particles and on the water's role in aerosol growth processes (14-19). Water is absorbed by haze particles that contain hygroscopic constituents such as sulfate and nitrate salts. These particles swell into the size range that scatters light most efficiently (1618).To the degree that water is in the condensed phase only because of these hygroscopic pollutants, it could be regarded as an anthropogenic pollutant. Water plays another role in particle growth. By providing a fluid particle Environ. Sci. Technol., Vol. 20,No. 8,1986 701
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&n&ms onto ae&~ &rticles. On the other hand, at higher relative humidities, SO, readily dissolves in aerosol droplets and is oxidized more rapidly in solution to produce aerosol sulfate, and thereby more haze (19). Were it not for the fact that pristine areas are so sensitive to small changes in aerosol mass, So, emissions would have less of an effect on visibility in arid desert h a g s . For moderate and high humidities, the umtribution of hygroscopic aemsol mnstituenta to reduced visihility extends beyond theii individual optical Dmwz On days of paor visibility the city has an urban haze, a brown cloud propertiw to include. the optical quences of the absorbed water. The for shifts between chemid species that definition of elemental and organic contribution of transient and trace aero- simply scatter light. However, the dis- aerosol fmctions (13.20). sol constituentS is also broadened. It in- tinction between elemental carbon and In summary, airborne aerosol particludes their role in accelerating aemsol organic carbon is important because el- cles are not chemically and physically emental carbon (soot) absorbs visible static entities floating in the wind. Their growthproces~. Water uptake is only one example of light e&ctively. The analytical se.para- size and composition change continuthe importance of a e m l chemistry to tion of elemental and organic carbon in ously as a result of chemical reactions visibility. Chemical composition also aerosol samples proves especially chal- and the condensation and evaporation visibility through a particle’s in- lenging. Thermal volatilization and ex- of pollutant gases and water. Some dex of refraction. The effect is small traction techniqw provide operational processes, such as the dissolution of
taction 1&a or t
monitoring strategi new stationarv so and had to submit appwpfbte-state implementationplan SIP) revrsbns to EPA. ’ Phase 2 requires state a*kn on the review of exfsting sources for visibilily impact (including consideratbn of BART, best available retrofit technology), pmcedures to Drotect integral vistas identified bv tederal land manayers. & the I d&elopment of long-term straregies for visiilii pmtection. U ir inwaded and axpe*ed melt future actions
762 Emlmn. Scl. Wchnnl., W. 20, No. 8,1988
Wed states are under r e v W Rnal &tion was Oxp&d in July lSea On-Jan. 29,lseS, E M declared 32 state slps to be deficient wlth respe* to visibility prOWUOn requirements of Phase II of the December 1980 E M rules. EPA was scheduled to ProPoSsa tedsrsl remedy by June l-; mat schedule has bben delayed.
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HNQ, reach equilibrium rapidly. 0thers, such as the oxidationof SO, to pro-
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SOIS k v e i a n i mix with one another. The mal of orOtectine visibditv has led
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rI . . . but on a clear day one can see the cify and the mountaim
one subtle but crucial issue must be addressed: whether to monitor the causes of visibility impairment or the effect itself. This issue distinguishes between measurements of the optical properties of the atmosphere and measure men@of visual effects. Some perspective on this issue can be gained by considering the analogous question for respiratory health. The PM-10 standard, which specifies a maximum aliowabie concentration of airborne particulate matter, is an example of a standard that focuses on a presumed cause of respiratory impairment. Alternativelx the standard could specified that air quality is inadequate whenever the snca of a symptom, such as coughing, exceeds a pruuetermined value. This standard could be exceeded because of natural factors such as pollen and infectious disease. For visibiiiu as for this health example, the air q U a l i and adverse symptoms are not necessarily closely related because severai agents can lead to the same symptoms. The decision to regulate health-related pollutants rather than poor health was made long ago. The corresponding decision for monitoring visibility impairment has not yet been made. It requires insight to distinguish between measurements focused on optical properties of the atmosphere and those focused on visual effects. The propagation of light through the air is governed by the optical properties of the material atmosphere. Measurements of poliutiorrrelatedcauses of visual impairment record either the optical properties of the atmosuhere or the chemical and physicai properties of airborne poilutants that determine those optical properties. The extinction mfficient (the rate at which energy is removed from a cdlimated beam of light) is an example of an atmospheric optical property. The concentration of fine airborne partides is one of the physical properties that determines the extinction coefficient. In contrast, the ambient radiation field reflects the composite effect of several factors other than the outical orow erties of the material atmosphere. Those factors are not related to air quality. They include sitespecific attributes such as reflection or shading from clouds; the color, texlure, and reflectiiily of the ground and scenic elements;
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Federalregulatory action There is no national ambient air quality standard (NAAQS) or prevention of significant deterioration (PSD)increment for visibility. The NAAQS and the PSD increment for total suspended particulates (TSP) have little bearing on visibility; much of the TSP mass is contributed by large particles, which scatter light inefficiently. These particles
varying factors shape the ambient radiation field and thereby contribute to visual effects. Several instruments are available for monitoring visibilily impairment. Those that record optical properties of the atmusphere include the nephelometer and transmissome ter. Nephelometry provides direct measurements of light ' ' scattered out of a beam by an air sample and can pm the scattering component of the extinction coefficient. measurement pertains to a finite air volume at the poi measurement. Transmissometry yields total extinctio measuring the attenuation of a direct light beam ow extended path length. Teleradiometry, photographx and human observation are used to measure features of ambient radiation C ' . Conventional teleradiometry uses telescopes and pl detectors to measure the ratio of light intensily from zon features and the adjacent sky In contrast to nepheb metry, it is a long-path measurement affected by target characteristics and weather. Photographic monitoring p m vides a record of the larger visual environment from which the relative intensities of scenic targets can be extracted. Human observations, which were recorded as far back as the 1880s in North America, yield a discrete record of the detectability or invisibility of individual targets. The fundamental advantages and disadvantages of either regulatory focus (cause or effect), and thus of the complementary monitoring instruments, depend on one's point of view. For example, nephelometry is inferior to teieradiometrv as a record of the visual environment: it is unaffected biclouds, shadows, airlight, and atmospheric layering, all of which affect our perception of a scene. For these very reasons, nephelometry is superior to teleradiometry as an index of anthropogenic impairment. In principle, the atmospheric and radiometric measure ments are precisely linked by the equations of radiative transfer. In practice, h m , the kmdary conditions (such as the clouds, the ground, and scenic elements) are so the two kinds of measure not well characterized..~~~~ . merits-those that focus on the cause of visibility impairment and those that focus on tha visual effect-are not closely linked. ~
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Envimn. Soi.Teohnd.. W. 20, No. 8.lgss ?63
are composed largely of dusts generated by mechanical processes. Their concentration tends to vary independently of the submicrometer fraction (12). A NAAQS for particles below 10 pm in diameter (PM-10) has been proposed to address differences in the inhalability of particles of different sizes. However, this size discrimination is not specific to visual effects. The Clean Air Act, as amended in 1977,establishes a national goal of preserving and, where necessary, restoring unimpaired visibility in and from nearpristine federal Class I areas. Class I areas include large national parks and wilderness areas. The fact that this mandate is formulated in terms of the human experience has broadened the scope of visibility research beyond atmospheric optics and the earlier interest of aviation in target discernment. Much of the current effort is directed toward quantifying human evaluation of vistas in terms of modern theories of visual perception (21. 22). These efforts include the identification of important vistas, the attributes (such as texture. and color in diverse targets) that make them visually pleasing, their value, and their sensitivity to air pollu-
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764 Envimn. Sci. Technol., MI. 20, No. 8, lseS
tion. The unambiguous detection and measurement of small changes in these attributes present a challenge to those who develop monitoring techniques, particularly when their goal is an instrument that can operate unattended in remote areas (23.24). Plume blight and regional haze In response to the legislative mandate of the. 1977 amendments of the Clean Air Act, EPA proposed a sequential a p proach to the protection of visibility, electing to deal first with identifiable plumes and then to address regional haze. Plumes from isolated point sources can be seen at large distances ulrough the lucid air that is characteristic of remote, pristine areas in the intermountain West. Mitigation of plume blight provided a tractable starting point for control efforts because emission sources were not in serious question. However, plumes offer measurement and modeling challenges of their own. Distinct plumes can have sharp gradients in the concentration of aemsols and gaseous species-and comequently in optid properties. The atmosphere is far from radiative equilibrium, which makes it comparatively
difficult to model accurately. The most visible plumes are. produced when emissions are trapped within an elevated, stable layer and spread horizontally by wind shear. Because such conditions are of little concern for population exposures, they r e wived little attention before interest in visibility developed. Dispersion modeling for plume visibility studies would be difficult in any case because most of the sites of interest are near complex terrain and because it is necessary to predict instantaneous, rather than timeaveraged, plume profiles (25.26). Regional haze, the second focus of visibility research, is easier to model because the sharp concentration gradients of a coherent plume are. absent. On the other hand, regional haze is much more difficult to trace to specific emission sources because of the chemical diversity of these sources and because of mixing and mction of emissions. In the eastern United States, where there is widespread urbanization and correspondinglydiffused emissions, regional hazes extending over thousands of square kilometers can be observed by satellite (27). Hence, the visual air quality of an eastern urban area is typi-
caUy a composite of local emissions and polluted air transported from other areas that lie upwind (28). Optical monitoring The demands of optical monitoring of visibility are very different for regional haze and isolated plumes. Because regional haze is associated with widespread pollution sources and wellmixed air parcels, the concentration gradients tend to be much less severe than those found in isolated plumes. Therefore, point measurements can be expected to characterize atmospheric optical properties over distances of several kilometers. Total light scattering recorded continuously by integrating nephelometers is one example of a point measuremnt (29). Light absorp tion is, as a rule, inferred from point measurementsof elemental carbon concentration (30). Ground-based point measurements are not useful for monitoring plume blight. A Of point measurements made on aircrafi tlights traversing a plume can provide useful research data for model evaluation. But aircraft sampling is not feasible for mtine monitoring. Spatially stratified effects are monitored with long-patb instnr ments that scan a scene (23.241. These long-path techniques can be confounded by interferences from clouds and other natural factors. The develop ment of long-path, color-sensitive techniques remains a priority for the operational specification of visibility impairment in extended vistas. Table 1 lists several field studies designed to explore the relationship between the physical and chemical prop erties of aerosols and their optical effects. Ideally, samples of ambient aerosol are separated by size and chemicaUy analyzed. One or more optical measurements are made simultaneously with the aerosol sampling, but the correspondence with aerosol measurements is less than perfea for two basic reasons, each tied to the uniqueness of visibility as an air pollution effect. First, visibility is not a local effect. It is a function of pollution concentrations throughout the field of view, not just in the observer’s immediate vicinity. Samples taken at a point may be completely unrepresentative of the visibility impairment observed from that point. This feature is in sharp contrast to health effects, for example. Second, the visual experience is instantaneous. Unlike health effects, visual effects are neither cumulative nor averaged over time. Most of the optical techniques for field measurements are correspondingly instantaneous, but aerosol samples repment time averages. Hence, the time resolution of
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aerosol-sarnpliig techniques in field studies continues to offer numerous research opportunities.
Source apportionment Mathematical models designed to simulate visibility effects are built from air pollution models that describe the evolution of aerosol and N@ concentrations. Visibility modeling confronts the same obstacles that are fami in the modeling of all other air pollution effects: There are inadequacies in the determination of wind fields in complex terrain, in initial pollutant concentrations, in emission source inventories, in the spatial and temporal resolution of meteorological data, and in the rates of significant chemical and physical pmcesses throughout diurnal cycles. Visibility modeling requires a description of the optical properties of pollutants. In recent years, the link between optical effects and the chemical speciation of aerosol mass has changed. Earlier, this l i was based on statistical associations within field data; now, it rests on deterministic assessments based on physical and chemical properties (16,33,49-53). These assessments have focused on the response of visibility to changes in aerosol composition and mass loading and the associated shifts in aerosol sue and number (5153). The identification of specific sources of visibility impairment has relied primarily on the tracbng of aerosol prew r s to specific sources (54). In the absence of precise deterministic models, considerable emphasis has been placed on fiding empirical relationships based on statistical associations in the variabiliry of aerosol chemical constituents and meteorological
factors (28).
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Statistical methods designed to relate mortality to pollution have been reviewed extensively (55). The effect of pollution on visibility has been explored using many of the same statistical procedures. Several of the statistical subtleties that arise in the analysis of health effects show up in visibility analysis as well. The principal differences stem from the fact that visibility is not a cumulative effect. Principal-component analysis and other forms of factor analysis have proven popular in separating groupings of factors presumed to be associated with distinguishable sources and transformation processes (28, 53, 56). The chemical signatures of several pollution sohave been measured and associated with distinguishable statistical factors. But rarely can the quantitative contribution of distinct sources to visibility impairment be resolved unambiguously using these methods. Occasionally, natural experiments present themselves. During the late 1960s and early 197Os, about 80% of the sulfur dioxide emitted in the southwestern United States was contributed by copper smelters. The proportion has since dropped as emissions from other sources have grown and as smelter op erators have begun to install air pollution controls. Visibility in Phoenix and lbcson, Ariz., improved during a ninemonth-long industrywide strike in 1967 and 1968. These observations provided a clear indication of the smelters’ contribution (57).
F‘rognosis for research Visibility research is intimately related to our understanding of the processes by which aerosols grow and chemically evolve in dynamic exchange with their gaseous environment. InEnvimn. Sci. Technol.. MI. 20, No. 8, 1986 765
cluded therein are chemical processes that may accelerate as aerosols are drawn into and through clouds during their transport over large distances prior to their removal by rainfall or by dry impaction. Therefore, the chemical and physical processes that influence visual air quality dictate the role of aerosols in acid deposition. The formulation of effective measures for protecting visibility hinges critically on the fulfillment of two important needs. The fmst is an operational understanding of the chemical and physical processes that dictate not only the visual impact of haze under existing ambient conditions but also the complex, nonlinear response of visibdity to changes in pollutant emissions. The second priority is an improvement of monitoring techniques. This includes aerosol-sampliig measures sensitive to aerosol size, phase, and composition, and it requires unambiguous optical techniques appropriate for unattended, routine operation in remote settings.
References ( I ) Sanborn,K. A nutkful Wfmum in Sourkern California: D. Appclton: New York, 1903. (2) Friedlander, S. K. S m k e , Dust and Haze: Wiley: New York, 1977. (3) McCartpey, E.J. Optics of Ihe Almsphere; Wiley: New York. 1976. (4) Appcl, B. R. et al. A r m s Environ. 1984, IR r n - l h .-,
(5) Mulawa, l? A,; Cadlc, S. H. Amos. Envi-
ran. 1985. 19. 1317-24. (6) Cadle, S. H.; Groblicki, P 1.; Mulawa, P A. Alms. Envimn. 1983,17.593-M)0. (7) Appcl, B.,R.; Toldwa, Y.;Kothny, E. L. A m s . Environ. 1983.17. 1787-97. (8) Countess, R. 1. et al.'J. Air Pollut. Control ASSOC. 1982.31, 247-52. (9) Hering, S. V.; Friedlander, S. K. Amros. Environ. 1982,16.2647-56. (IO) Hering, S. V.; Flagan, R. C.; Friedlander, S. K. Environ. Sci. Pcknol. 1978,
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12 . . , 661-7% . . -.
V. A,; Liu. 9. Y.; Kuhlmey, 0. A. J. Aerosol Sci. 1981, 12, 333-37. (12) Whitby, K. T.; Sverdrup, 0. M. In i'he (11) Marple.
Character and Origins of Smog Aerosols: Hidy, 0 . M. et al., Eds.; Wiley: New York, 1980; pp. 477-526. (13) Particulate Carbon: Armspkeric Life Cycle; Wolff, G. T.;Klimisch, R. L., Eds.; Plenum Prcss: New York, 1982. (14) Ho, W.; Hidy, G. M.; Govan, R. M. 1. Appl. Meteorol. 1974,13, 871-79. (15) Basset, M.; Seinfeld. I. H. Amos. Enviran. 1983,17, 2237-52. (16) Sloane, C. S. Amos. Environ. 1984, 18. 871-78. (17) Tang, 1. N. In Generation ofAerosols and Facilities for Exposure Mearuremnts; WilIeke, K., Ed.; Ann Arbor Science: Ann Arbor, Mich., 1980; 153-67. (18) Rood, M. J. et Environ. 1985,
EA-$.
19, 1181-90. (19) McMurray, P. H.; Wilson, 1. C. 1. Gcophys. Rev. 1983,88.5101-8. (20) Orosjean, D.; Friedlander, S. K. J. Air Pollur. Control Assac. 1975,25, 1038-44. (21) Malm, W. C . ; k i k e r , K. K.; Molenar, J. V. 1. Air Pollut. Control Assoc. 1980.30, 122-31. (22) Henry, R. In Proceedings of the Mrkshop on Visibiliiy Yalus, Department of As. ridhlre General Technical Report WO-18; Fox, D.; Loomis, R. I.; Grecn. T.G., Eds.; 760 Envimn. Sci. Technol., Vol. 20, No. 8,1986
Denarrment of Aericulture: Washinnton. - . D.&, 1979; pp. 7 4 h . (23) Malm, W.; Molenar, 1. V. J. Air Pollut. Control Assoc. 1984.34, 899-904. (24) Allard, D.; Tombach, I. A m s . Environ. 1981, I S , 1847-58. (25) . . White. W. H. et al. Amros. Environ.. in press. (26) White, W. H. et al. Alms. Environ.
1985.19, 528. (27) Wolff, 0. T.; Kelly, N. A,; Ferman, M. A. Science 1981,211,704. (28) Wolff, 0. 'E et al. A t m s Environ. 1985, 19, 1 3 4 4 9 . (29) Gordon, 1.1.; Johnson, R. W. Appl. Optics 1985,24,2721-M. (30) Clarke, A.D. Appl. Optics 1982, 21, 3011-20. (31) White, W. H.; Roberts, l? T. A m s . Environ. 1977, 11, 803-12. (32) i'he Ckaracrer and Origins of Smog; Hidy, 0. M.et al., Edds.; Wiley: New York,
(46)Richards, L. W. et al. Arms. Environ. 1985,19. 1685-704. (47) Seigneur, C. et al. A m s . Environ. 1984, 18, 2231-44. (48) Ber mom, R. W. et al. Atmos. Envimn. 2135-50. 1981, (49) Ouimctte, 1. R.; Flagan, R. C. Arms. Environ. 1982.16, 2405-19. (50) Sloane, C. S. A l m s . Environ. 1985, 19, 669-80. (51) White, W. H. Alms. Environ., in press. (52) Sloane, C. S. Paper 85-16.4, presented at the 78th Annual Meeting of the Air Pollution Control Association, Detroit, June 16-21, 1985. (53) Sloane, C. S. Atms. Environ. 1986, 20, 1025-38. (54) Macias, E. S.; Hopke, €! K. Atmospheric Aerosol SourcclAir Quality Relatiowkips: American Chemical Society: Washington, D.C., 1981. (55) Lipfert, F. W. Environ. Sci. Tecknol. 1985.19, 7M-70. (56) Rahn, K. T.; Lowenthal, D. H. Science 1985,228,275-84. (57) ltijonis, 1.; Yuan, K. Arms. Environ. 1979.13, 83343.
h,
C w e S. SIOUIM (1. j is currently a Senior staff research scientist in the environmental science department of the General Motors Research Laboratories. Her research interests include visibility impairment, aerosol optics, and processes of aerosol formation, chemical evolution, and chemical deposition. myrPn
H. White (zj is a mthem'cian
with the Center f o r Air Pollution Impact and Trend Analysis at J+fzshington University in SI. Louis. He hns been investigaring the visual efects of airpolhiion since 1972.
