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4 Patterns and Trends in Data for Atmospheric Sulfates and Visibility John Trijonis INTRODUCTION In this chapter, we examine the geographical patterns and historical trends in atmospheric sulfate concentra- tions and visibility. The relationship between particu- late sulfate and acid deposition is a close one. Sulfur dioxide emissions, which contribute to acid deposition, also produce particulate sulfate. In fact, the sulfate particles are a fundamental component of acid deposition. In the eastern United States, sulfate particles are also the dominant contributors to visibility reduction (light extinction or light attenuation). Thus, visibility in the East is indirectly related to acid deposition. Particulate sulfate is an important component of visibil- ity reduction for two reasons. First, sulfate particles tend to form in the 0.1- to 1.0-micron size range, by far the most efficient size range for scattering visible light. Second, since sulfates are hydroscopic, they attract a substantial volume of water into the particulate phase, which further increases the mass of light- scattering aerosol. It is now well established that, on average, sulfates and associated water account for about 50 percent of visibility reduction in the East, slightly less in urban areas but somewhat greater in nonurban areas (Weiss et al. 1977; Trijonis and Yuan 1978; Leaderer and Stolwijk 1979; Pierson et al. 1980; Ferman et al. 1981; Wolff et al. 1982; Trijonis 1983, 1984). In fact, the most definitive program conducted at a nonurban site during the summertime concluded that sulfates and asso- ciated water accounted for three-fourths of the total light extinction during the study period (Ferman et al. 1981). 109

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110 Some have even suggested that visibility observations, i.e., light-extinction levels, can serve as a quantitative surrogate for sulfates in the eastern United States on a day-to-day basis (Leaderer et al. 1979). More recent studies, however, indicate that visibility is not neces- sarily an accurate daily predictor of sulfates (TrijoniS 1984). Nevertheless, the relationship still appears to hold for longer-term averages; geographical patterns and historical trends in average light-extinction levels in the East often reflect corresponding changes in average sulfate concentrations. DESCRIPTION OF DATA BASES Three types of nationwide data are examined in this chapter: visibility observations at a large number of airports, turbidity measurements at a few monitoring stations, and sulfate aerosol measurements at a few nonurban Environmental Protection Agency (EPA) sites. The visibility data consist of visual range estimates made by trained observers at airport weather stations. To help to ensure the quality of these data, we have followed several steps discussed and recommended in the literature, such as using daytime measurements only, plotting cumulative percentiles according to appropriate methods, prescreening airports for adequate visibility markers, and checking patterns for consistency among various stations in a region (Trijonis and Yuan 1978; Husar et al. 1979; Sloane 1982). Some questions remain, however, regarding possible inconsistencies in the data because of variations in reporting practices, observer detection thresholds, and visibility markers. Because spatial patterns in visibility usually involve much greater differences than historical trends, airport visibility data are considered to be of better quality for detecting spatial patterns than for detecting his- torical changes. Nevertheless, airport visibility data are generally considered to be fairly good for historical trend analysis, especially if care is taken to check for a general consistency for trends at similiar locations. In this chapter, we often convert the visual range data (V) into estimates of light-extinction coefficient (B), using the Koschmeider formula, B = (-in Co)/V. This relationship holds for a uniform atmosphere with an observer able to detect a minimal contrast of Co. We have assumed a value of C0 = 0.02, the so-called

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111 standard observer, although several studies have shown that visibility data from airport observers more closely correspond to Co = 0.05 (Aerovironment, Inc. 1983; Malm 1979; Middleton 1952; Douglas and Young 1945). The extinction levels reported here can be corrected to the latter value by dividing by 1.3. The turbidity data were obtained from the EPA/NOAA network of Volz sunphotometers. Sunphotometers measure variations in the amount of direct sunlight lost before reaching the Earth's surface. Because the measurement is made only with an unobstructed line of site to the Sun, data are missing whenever clouds obscure the Sun, so the the number of observations varies with location and season. The turbidity data are similar to the visibility data, except that the sight path is vertical in the former rather than horizontal. Thus, turbidity depends not only on atmospheric extinction but also on the vertical depth of the extinction layer. The sulfate data come from the EPA's National Air Sur- veillance Network (NASN), which not only collects par- ticulate (Hi-Vol) filter samples at nonurban stations but also performs chemical analysis of those samples for sulfate ion and other constituents. A potential problem with the sulfate data concerns an artifact caused by sulfate formed from S°k absorbed on glass-fiber filters The error introduced by this factor should not be of great concern in this study, however, for two reasons. First, the data are from nonurban sites, where gaseous SO2 concentrations should be relatively low. Second, in terms of acid deposition, it is total sulfur loadings that are of interest and not just sulfate aerosol per se. It is not necessarily a liability to include SC2 in the measurements unless the SO2 collection introduces sig- nificant biases in either the regional distributions of sulfate or the trend of sulfate over time. Because the results of these analyses are compared in Chapter 1 with other measures of overall acid deposition patterns in eastern North America, the data bases cited here pertain, whenever possible, to rural locations (as defined in World Aeronautical Charts, 1976). Since metropolitan areas can produce significant changes in sulfate concentrations and sometimes in visibility, it seems best to avoid large urban centers when trying to characterize overall regional patterns and trends. . Many of the data presented in this chapter represent gross annual or seasonal statistics with no presorting for the influence of meteorological factors. There are

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112 questions concerning the effect of climatic variations on geographical patterns and, especially, on historical trends. For example, the work of Sloane (1983, 1984) suggests that shifts in relative humidity and to some extent in temperature over the past 30 years may have affected visibility trends in the eastern United States substantially. Husar and co-workers, in providing the visibility data examined in this chapter, sought to minimize the influence of humidity by eliminating observations with fog or precipitation and normalizing for relative humidity. Nevertheless, as is noted in Chapter 3, questions remain regarding the potential effect on visibility trends of changes in stagnation episodes and temperature. Unfortunately, we cannot definitively resolve the issue of climatological trends and visibility with our present state of knowledge. For the most part, this chapter relies on published data and analyses. The main exception is that Husar and coworkers have updated and revised some of their studies for this report. Their new work extends the analysis to 1983 and restricts it to nonurban sites. GEOGRAPHICAL D I STRIBUTION Figures 4.1-4.3 summarize the geographical distribu- tions for visibility, atmospheric turbidity, and sulfate concentrations in the continental United States. Figure 4.1 is based on median values of visibility at 100 rural airports. Figure 4.2 presents annual averages from 26 stations of the EPA/NOAA atmospheric turbidity network. Figure 4.3 is based on annual averages for sulfate con- centrations at 22 nonurban EPA monitoring sites. The main spatial features of all three figures are similar. The best air quality occurs in the mountain/ desert West, with fairly sharp gradients to worse air quality toward the east and the west. The highest sulfate concentrations and atmospheric extinction (lowest visibility) occur east of the Mississippi River and south of the Great Lakes. The location of worst air quality varies somewhat from figure to figure, but the Ohio Valley generally exhibits nearly the worst, if not the worst, air quality.

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116 {~} ' ~-~ At: \~ FIGURE 4.4 (a) Locations of 35 sites at rural airports for determination of visibility (1949-1983). (b) Locations of 15 rural sites of the NASN for determination of concentrations of airborne sulfate (1965-1978). Designated areas A-E represent the 5 regions defined in Chapter 1 of this report. Figure 4.5 indicates that light-extinction levels decreased from the early 1940s to the early 1950s, then after a short level period increased from the late 1950s to about 1970, and finally fluctuated without any apparent trend between 1970 and the early 1980s. The overall increase in extinction from 1950 to the early 1980s is highly significant (t = 6.1). However, the apparent changes in slope after 1950, e.g., the seeming change around 1970, do not pass the 95 percent confidence level. Figure 4.5 also shows that rural sulfate concentra- tions increased from the 1960s to the 1970s. The percentage increase in sulfates, however, was greater

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117 (b) ~ \~ FIGURE 4.4 (continued). than the percentage increase in light extinction. Recog- nizing that sulfates account for only about half of the total extinction in the East, this suggests that the sulfates may have been the dominant particulate component producing the light-extinction increase. Figure 4.6 presents lengthy historical records that Husar et al. (1981) have compiled for two sites. One of these records--visibility observations at Blue Hill, Massachusetts--covers the years 1889 to 1958 with a recent addition in 1981-1983. The other record-- turbidity at Madison, Wisconsin--covers the period 1916 to 1977. Comparing the two records reveals three notable features. First, the trends at the two locations are dissimilar between 1916 and the early 1940s. Second, there is fairly good agreement from the early 1940s to the late 1950s. Third, although the two trends seem divergent from the late 1950s to the present, it is hard to interpret a trend from the single Blue Hill data point for 1981-1983.

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120 Part of the disagreement in the trends might be explained by noting that the two sites are widely separated geographically and that a single site may not be representative of its own region. The trend at Madison appears to reflect the trend for the North Central region (Region E) from the 1950s to the 1970s (see Figure 4.7). However, the apparent decrease at Blue Hill from the late 1950s to the present does not seem consistent with the nearly constant overall trend for the Northeast region (Region B) during that period (see Figure 4.7). Visibility data from the 35 airports were subdivided into the four regions of the eastern United States defined in Chapter 1 (Regions B. C, D, and E). The data were then plotted and analyzed to determine regional trends. As shown in Figure 4.7, the Northeast region (Region B) did not participate in the overall extinction increase from the 1950s to the 1970s shown in Figure 4.5 but apparently underwent a slight decrease in extinction over the three decades. The trend for the Northeast region is different from those of the other three regions at a 95 percent confidence level. The Midwest region (Region D) underwent a moderate increase in extinction levels. While the North Central region (Region E) also seemed to undergo a moderate increase, this conclusion is tenuous because the data base includes only two sites. The Southeast (Region C) demonstrated an especially pronounced rise in extinction levels during the 1960s. The increase in extinction for the Midwest, the North Central region, and the Southeast are each highly significant statis- tically (t > 6). Figure 4.8 presents historical trend data for light extinction and sulfate concentrations during the summer quarter (July-September). It is apparent that an especially sharp rise in summertime light extinction and sulfate concentrations occurred from the 1950s to the early 1970s. In fact, this particular seasonal trend is one of the most salient features of air quality history and has been pointed out by most workers who have studied trends in visibility (Miller et al. 1972, Munn 1973, Trijonis and Yuan 1978, National Research Council 1979, Husar et al. 1979, Sloane 1982, Trijonis 1982), turbidity (Flowers et al. 1969, Robinson and Valente 1982), or sulfates (Trijonis 1975, Environmental Protection Agency 1975, Altshuller 1976, Frank and Possiel 1976, National Research Council 1979) in eastern North America. The summer was a season of relatively good visibility during the 1950s but became the season of distinctly lowest

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123 30 - E - ~ 20 u, in a: Lid - cot UJ 6 cr > Is 10 /\ Extinction Total Fine Particle Mass Fine Sulfate Mass (as NH4HSO4) 1 1 1 1 /\ A \ \ \ 2 1 1 st Quarter 2nd Qua rter Jan.-Mar. Apr.-Jun. 3rd Qua rter 4th Quarter Ju l.-Sep. Oct.-Dec. FIGURE 4.9 The seasonal patterns of fine-sulfate mass per unit volume, total fine-particle mass per unit volume, and light extinction for rural areas of the eastern United States. Fine-particle mass and fine- sulfate mass data are averages over five rural EPA dichotomous sampler sites (Will County, Illinois; Jersey County, Illinois; Monroe County, Illinois; Erie County, New York; and Durham County, North Carolina). The light-extinction data are averages over eight rural airports. SOURCES: Trijonis 1982, Trijonis and Yuan 1978. to - z o z - x IL

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124 visibility and highest sulfate by the early 1970s (see Figure 4.9). We are quite certain of the observed seasonal trend because artifacts from measuring incon- sistencies are canceled out when trends for one season are compared with those for other seasons. The summer- time deterioration of air quality has been attributed to the rapid growth of summertime sulfur oxide emissions from coal-fired power plants caused by air conditioning demands (Holland et al. 1977, Husar et al. 1979, Sloane, 1981). CONCLUSIONS Data for atmospheric sulfate concentrations provide one means of investigating large-scale patterns and trends in sulfur oxide emissions and deposition. Another useful measure is the rich historical data base for atmospheric visibility, since sulfate particles are a major contributing factor to reduced visibility in the eastern United States. Although the atmospheric data bases for sulfates and visibility involve significant uncertainties, they can provide productive comparisons with data bases for emissions, precipitation, water quality, and other effects. The national spatial patterns for light extinction, turbidity, and sulfates are in general agreement. The best air quality occurs in the mountain/desert Southwest, with the worst air quality east of the Mississippi River and south of the Great Lakes, particularly in the Ohio Valley. Light extinction (a measure of inverse visibility) in nonurban areas of the East decreased substantially from the 1940s to the 1950s, increased from the 1950s to about 1970, and has been about constant since then. The south- east quadrant of the United States (Region C) experienced the greatest deterioration of air quality from the 1950s to the 1970s. The trend of increasing extinction during the 1960s is consistent with trends in ambient sulfate. An especially marked increase in extinction during the summer season occurred from the 1950s to the 1970s.

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125 REFERENCES Aerovironment, Inc. 1983. Comparison of visibility measurement techniques: eastern United States. EPRI-EA 3292 Research Project 862-15. Electrical Power Research Institute, Palo Alto, Calif. Altshuller, A. P. 1979. Regional transport and transformation of sulfur dioxide to sulfates in the United States. J. Air Pollut. Assoc. 26:318-324. Douglas, C. A., and L. L. Young. 1945. Development of transmissometer for determining visual range. U.S. Civil Aeronautics Administration. Tech. Development Report No. 47. Environmental Protection Agency. 1975. Position on regulation of atmospheric sulfates. EPA 450/2-75-007. Environmental Protection Agency, Washington, D.C. Ferman, M. A., G. T. Wolff, and N. A. Kelly. 1981. The nature and source of haze in the Shenandoah Valley/Blue Ridge Mountains area. J. Air Pollut. Control Assoc. 31:1074-1082. Flowers, E., R. McCormick, and R. Kurfis. 1969. Atmospheric turbidity over the United States 1961-68. Appl. Meteorol. 8:955-962. Frank, N., and N. Possiel. 1976. Seasonality and regional trends in atmospheric sulfates. Paper presented at the meeting of the American Chemical Society, San Francisco, Calif., August 30 to September 3. Holland, T. C., R. L. Short, and A. H. Wehe. 1977. SO2 emissions trom power plants over the 1969-1975 period. Environmental Protection Agency, Washington, D.C. Husar, R. B., D. E. Patterson, and J. M. Holloway. 1979. Trends of eastern United States haziness since 1948. Paper presented at the Fourth Symposium on Atmospheric Turbulence, Diffusion, and Air Pollution, Reno, Nevada, January 15-18. Husar, R. B., J. M. Holloway, and D. E. Patterson. 1981. Spatial and temporal patterns of eastern U.S. haziness: a summary. Atmos. Environ. 15:1919-1928. Leaderer, B. P., and J. A. Stolwijk. 1979. optical properties of urban aerosol and their relation to chemical composition. Ann. N.Y. Acad. Sci. 338:70-85. Leaderer, B. P., T. R. Holford, and J. A. Stolwijk. 1979. Relationship between sulfate aerosols and visibility. J. Air Pollut. Control ASSOC. 29:154-157. Malm, W. C. 1979. Visibility: a physical perspective. Paper presented at the Workshop on Visibility Values. Fort Collins, Colorado, January 28-February 1.

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126 Middleton, W. E. K. 1952. Vision Through the Atmosphere Toronto: Toronto University Press. Miller, M. E., N. L. Canfield, T. A. Ritter, and C. R. Weaver. 1972. Visibility changes in Ohio, Kentucky, and Tennessee from 1962 to 1969. Mon. Weather Rev. 100:67-71. Munn, R. E. 1973. Secular increases in summer haziness in the Atlantic provinces. Atmosphere 11:156-161. National Research Council. 1979. Controlling Airborne Particles. Washington, D.C.: National Academy Press. Pierson, W. R., W. W. Brachaczek, T. J. Truex, J. W. Butler, and T. J. Korinski. 1980. Ambient sulfate measurements on Allegheny Mountains and the question of atmospheric sulfate in the northeastern united States. Ann. N.Y. Acad. Sci. 338:114-173. Robinson, E., and R. J. Valente. 1982. Atmospheric turbidity over the United States from 1967 to 1976. EPA 600/S3-82-076. Environmental Protection Agency, Washington, D.C. Sloane, C. S. 1982. visibility trends--II: mideastern United States, 1948-1978. Atmos. Environ. 16:2309-2321. Sloane, C. S. 1983. Summertime visibility declines: meteorological influences. Atmos. Environ. 17:763-774. Sloane, C. S. 1984. Meteorologically adjusted air quality trends: visibility. Atmos. Environ. 18:1217-1230. Trijonis, J. 1975. The relationship of sulfur oxide emissions to sulfur dioxide and sulfate air quality. Pp. 233-275 in Air Quality and Stationary Source Emission Control. Prepared for the Committee on Public Works, United States Senate. Serial no. 94-4. Washington, D.C.: U.S. Government Printing Office. Trijonis, J. 1982. Existing and natural background levels of visibility and fine particles in the rural East. Atmos. Environ. 16:2431-2445. Trijonis, J. 1984. Analysis of particulate concentrations and visibility in the eastern United States. Prepared under contract 68-02-3578 for Environmental Protection Agency, Washington, D.C. Trijonis, J., and K. Yuan. 1978. Visibility in the Northeast: long-term visibility trends and visibility/pollutant relationships. EPA 600/3-78-075. Environmental Protection Agency, Washington, D.C. Weiss, R. E., A. P. Waggonner, R. J. Charlson, and N. C. Ahlquist. 1977. Sulfate aerosol: its geographical extent in the midwestern and southern United States. Science 197:977-981.

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127 Wolff, G. T., N. A. Kelly, and M. A. Ferman. 1982. Source regions of summertime ozone and haze episodes in the eastern United States. Water Air Soil Pollut. 18:65-82. World Aeronautical Charts. 1976. U.S. Department of Defense, Federal Aviation Administration and U.S. Department of Commerce, Washington, D.C.