A Symposium- The Technical Significance of Air Particulate Standards Keyed t o Visibility and Citizen Compllaints Are Interim Measure
Paper by:
Discirssion by:
Herbert C . McKee, Ph.D.
Franklin W. Church, Ph.D.
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Dr. McKee is assi,stant director of chemistry and chernicnl engineering, Southwest Resenrch Znst., Houston
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he term “particulates”-the most numerous and prevalent of all the urban atmospheric impurities-includes a variety of contaminants. However, this paper is concerned with ambient air quality standards for total particulates, without regard to effects caused by unique chemical or physical characteristics. Entirely different prohlems arise with toxic constituents such as polynuclear aromatic compounds, pesticide residues, lead, manganese, beryllium, or other heavy metals, or chemically reactive materials such as sulfuric acid mist. These separate categories, excluded from this discussion, call for appropriate standards based on their toxicity, reactivity, or other unique characteristics. Even so, the problem of setting standards for the total amount of particulates in the atmosphere still remains. Dust and other particulates can cause adverse effects independent of 542 Environmental Science & Technology
unique characteristics such as toxicity or reactivity. Such standards can be expressed in terms of the total amount present without regard to chemical composition. Most of the present standards for particulates are of this type; such standards have been adopted, or are under consideration, by several states. The popular term “inert particulates” is perhaps a misnomer, since any particle in the atmosphere may react chemically or cause physical phenomena such as nudeation, adsorption, and so forth. Therefore, in a strict sense, no particle is completely inert. In the absence of a more rigorous definition, however, the term inert particulates has been used to designate particles with no readily identifiable toxic or reactive constituents. A standard for total particulates must apply to a variety of materials with many different characteristics.
Dr. Church is n senior engineering n.s.socinte,Erso Resenrch & Enfiineering Co., Florlinrii Pork, N. . I .
Both solid particles and liquid droplets are included, and the particle or droplet size can range from slightly above the molecular level to particles, several hundred microns in diameter, which settle by gravity in a relatively short time. Complete classification based on size is of questionable value, hut some gross adverse effects may he related to particle size: * Below 0.1 micron, the maximum effects are nucleation phenomena and possible weather modification. * Between 0.4-0.8 micron, maximum light scattering and restriction of visibility occur, because these diameters approximate the wavelength of light. * From 1-5 microns, there is maximum deposition in the lung following inhalation. (Recent evidence indicates smaller particles may be more important physiologically than previously thought.) (Continued on page 5 4 6 )
(Continued from page 5 4 2 ) Particles from 10-100 microns are visible as dustfall, dirt, o r dust nuisances. These categories overlap; a particle of any size may show any of the effects listed to some degree. However, the classification identifies the most obvious reason for considering particles in each size range. Rationale for standards
Any ambient air standard should define the air quality which is acceptable to the public and which will avoid undesirable effects on health, welfare, and property. These standards can be considered as goals o r objectives for developing air conservation and air resource management programs, and for evaluating their success. To establish standards for any contaminant, the basic procedure is to identify any undesirable effects caused by a given contaminant, and then, to determine the concentration and duration of exposure required to cause each effect. An ambient air standard, then, is established on the basis of the lowest concentration and the shortest duration required to cause the adverse effects that must be limited, with due consideration for any local circumstances which would alter the values selected. To establish a general standard for particulates, in the absence of identifiable toxic o r reactive constituents, criteria for two effects are of primary importance: reduction of visibility; and soiling, o r nuisance, effects. If standards provide adequate control from the standpoint of these effects, the available information indicates that other adverse effects such as health damage, corrosion of materials, or vegetation damage, should not occur. Therefore, visibility and soiling should be discussed in detail. The attenuation of light by the atmosphere is influenced by many factors which are interrelated in a complicated manner. These include the direction of view relative to the sun, size distribution and refractive index of the particle, humidity, fog, rain, o r other natural weather phenomena. Many theoretical studies have related these factors to visibility o r visual range, but, due to a lack of complete understanding of the interrelationship of these factors, such correlations are only approximate at best. The most useful correlation for set546 Environmental Science & Technology
ting ambient air standards appears to be that developed by Charlson and his coworkers at the University of Washington. This correlation is based both on theoretical considerations and on actual atmospheric measurements in Seattle, Wash., San Jose, Calif., and New York City. In this correlation, the following assumptions were made: Humidity should be less than 70% to avoid substantial errors due to growth of particles by absorption of water from the atmosphere. The particles have been in the atmosphere for some period of time to avoid errors caused by nucleation o r other atmospheric effects. Light scattering by the particles is more important in reducing visibility than direct absorption of light by particles o r scattering o r absorption by gaseous materials. The aerosol is uniform in nature over the light path observed. This correlation relates suspended particulates in micrograms per cubic meter to the meteorological range in miles. The term meteorological range is synonymous with visual range, provided the distribution of aerosol along the light path is uniform. While containing error margins greater than that which would be desired, this correlation still is useful in establishing ambient air standards. F o r example, if 150 micrograms per cubic meter were selected as a standard, the visibility seldom would be less than three miles, and the average would be around five miles. This visibility would be reasonably satisfactory for air and ground transportation, although some objections might be raised on aesthetic grounds. Thus, in spite of the error involved in this type of correlation, such information is useful as a guide. Nuisance problems
Soiling effects-dirtiness of the atmosphere-also should be a major concern in establishing particulate standards. Certainly, soiling problems from particulate air pollution ificrease the cost of building maintenance and renovation, cleaning of clothing, drapes, and household furnishings, and many other items. Very little has been published on the relationship between measured levels of particulates and nuisanc- complaints; however, citizen complaints are a very important reflection of the attitudes of the community. Even though air quality might be adequate to prevent health effects, vegetation damage, and corrosion, an
air pollution control program would be inadequate if the average citizen still believes that the air over his city is dirty. However, a sharp line of demarcation below which no nuisance exists is impossible. Some finite limit must be set, which is tolerable and acceptable by the population. In an attempt to provide some guidance in the setting of air quality standards, a number of air pollution control agencies were contacted to see if an approximate relationship might be developed to indicate a measured level of contaminants which would correspond to a level acceptable to the public from a nuisance standpoint. I t does appear that an approximate range of concentrations can be identified below which air quality would be considered satisfactory and above which it would be considered definitely unsatisfactory. With certain important exceptions, it appears that the threshold limit, as indicated by citizen complaints, is somewhere between 200-250 micrograms per cubic meter, assuming that dust levels remain in this range for at least a day or two. This value is based both on actual air sampling data, and on the subjective evaluation of control officials who receive complaints, and who have collected longterm air quality data in areas where complaints occur. Above this range, the probability of complaints increases. Levels of 400-500 micrograms per cubic meter and above would almost certainly be considered a nuisance regardless of any circumstances which might prevail. From a less extensive body of data on dustfall, it appears that a range of 30-40 tons per square mile per month represents the equivalent threshold above which complaints may occur. A level of 50-75 tons or more per square mile per month generally would be considered definitely unsatisfactory. It is important to note that exceptions can occur because of the unusual nature of a particular emission. Carbon black is particularly noteworthy because of its high visibility. Complaints have been registered in some areas at a level of about 80 micrograms per cubic meter, a figure which would represent quite pure air if the particulates were of a general heterogeneous nature rather than the highly visible carbon black. The exceptions do not eliminate the usefulness of the 200-250 microgram threshold figure in establishing ambient
air standards. Rather, these exceptions merely illustrate the fact that evaluation of each individual problem is necessary, and that no single standard can be established for all circumstances. One major problem relating air contaminant levels to the occurrence of complaints is the statistical variation during long periods of sampling. Air sampling data must be interpreted with extreme caution in relating measured values to actual nuisances. For example, particulate levels of 100-125 micrograms per cubic meter would likely bother very few residents of a typical large urban area. But, in a city where the annual geometric mean might be 100 micrograms per cubic meter, short periods of time will occur when particulate levels are much higher. These intermittent high values might range up to several hundred micrograms per cubic meter and cause complaints; however, a long-term average measurement does not give much indication of the occurrence of conditions which lead to complaints. Thus, sampling data should be obtained for short periods of time, during the episode which causes a complaint. Existing standards
Despite the lack of knowledge and the uncertainties involved, the air quality standards already in existence represent the best judgment of many groups of people. probably the best judgment which can be applied to the knowledge presently available. Some of these standards were adopted by governmental agencies and, thus, have the force of law. Others were recommended by advisory committees or quasi-official bodies, and are intended only as guides to further planning. Most of the existing standards are based on one o r more of three different methods of measurement: Suspended particulates measured with a high volume sampler. Dustfall measured with a dustfall jar. Soiling index measured with a paper tape sampler. I n addition, California has a standard based on visibility. Dezpite differences in the method of measurement and in the statistical basis for expressing the standards, no major discrepancies appear in the various values which have been established. That is, no standard appears to be either more or less restrictive than any
Ambient air quality standards for particulates (Adopted or recommended) Suspended particulates (microgra ms/cu bic mete ra)
D u stfa II (tons/sq. mi./ month)
Soiling index
(COH/1000 lineal feet)
Colorado
120 (3-mo. average)
...
0.5 (3-mo. average)
Delaware
60 t o 125b (annual geometric mean)
...
...
Montana
95%
