Health effects of acid aerosols on North American children: air pollution exposures.

Air pollution measurements were conducted over a 1-year period in 24 North American communities participating in a respiratory health study. Ozone, particle strong acidity, sulfate, and mass (PM10 and PM2.1) were measured in all communities. In 20 of the communities, sulfur dioxide, ammonia, nitric acid, nitrous acid, and particulate nitrate were measured. The sampler was located centrally in the community whenever possible and samples were collected every other day. Concentrations of particle strong acidity, mass, sulfate, and ozone were highly correlated both in the region of the country defined as a high-sulfur source area and in the downwind transport regions. These regions of the eastern United States and southern Canada experienced the greatest particle strong acidity, sulfate, and particle mass concentrations during the spring and summer months (May-September). The particle strong acidity concentrations were highest in regions close to the high sulfur emission areas of the United States; that is, in the area immediately to the west of the Appalachian Plateau and west of the Allegheny Mountains (western Pennsylvania, eastern Ohio, and West Virginia) up through southern Ontario. The frequency of particle strong acidity events decreased with transport distance from the region of highest sulfur emissions. Low particle strong acidity and sulfates were found at the western and midwestern sites of both the United States and Canada. Substantial concentrations of nitric acid were found in two of the California sites as well as many sites in the northeastern portion of the United States. Sites selected for the epidemiologic study provide a range of annual mean particle strong acidity exposures from below the limit of detection to more than 50 nmol/m3. Imagesp492-aFigure 1.Figure 2.Figure 3.Figure 4.Figure 4.Figure 4.

Although recent epidemiologic studies have suggested that increased morbidity (1) and mortality (2,3) are associated with exposures to ambient particulates, the chemical or physical property of particulate matter which is responsible for these associations is not clear. Many of the potentially toxic components of the particles, including particle strong acidity (H+) and many trace metals (e.g., vanadium, manganese, lead, and arsenic), are associated with fine particulates (4). Changes in size and chemical composition of particulates could lead to differences in particle toxicity that can vary between sites and over time, for equivalent mass concentrations. In the northeastern United States and southeastern Canada, the highest particle exposures occur during the summer (5,6) with sulfate representing a large fraction (40%) of the particle mass. Atmospheric conditions favoring acid aerosol formation also enhance ozone formation. Nitrogen oxide emissions are converted to gaseous acids (such as nitric and nitrous acid) and particle nitrates under conditions conducive to the formation of ozone. Thus the epidemiologic findings of adverse health effects associated with PM10 may represent an underlying casual association with some specific characteristic of the particles or effects associated with some other pollutant that is correlated with the mass concentration.
Ambient concentrations of PM1O, PM2.1, particle strong acidity, sulfate, nitrate, and ammonium, nitric and nitrous acid, sulfur dioxide, and ozone were measured for approximately 1 year in each of 24 communities in the United States and Canada as part of a study of health effects of particle strong acidity (7). Respiratory illness and pulmonary function of approximately 15,000 children were measured in these 24 cities. This paper presents the results of the air pollution monitoring in these communities. The results of the respiratory health effects assessments, along with the observed associations with air pollution exposures, are presented by Raizenne et al. (8) and Dockery et al. (9) in this issue.

Methods
We selected study communities to provide a range of exposures to acid aerosol and ozone to limit confounding from other air pollutants, and to limit possible confounding b) population characteristics. Specifically, communities were targeted with the followin1 characteristics based on 1980 census data populations between 20,000 and 30,000 foi adequate numbers of schoolchildren, racia homogeneity, low poverty levels, high resi dential stability, and balanced use of gas for cooking. From the demographically acceptable communities, specific sites were selected based on records of ozone, sulfate, acid deposition, and wherever possible particle strong acidity concentrations.
Communities were selected to contrast ambient exposures to ozone and particle strong acidity. We identified three of the four combinations of high and low expected ozone and particle strong acidity conditions. Communities with high particle strong acidity and low ozone concentrations could not be identified. The original study design also called for a third selection dimension based on high and low gaseous acids (nitric and nitrous acid). It was not possible to identify communities with gaseous acid exposures independent of ozone, and this constraint in the design was relaxed.
Eight communities were monitored in the first year of the study (1988)(1989), nine in the second year (1989)(1990), and seven in the third year (1990)(1991). Six communities were in Canada, and 18 were in the United States (Fig. 1). The communities were grouped into four broad geos graphic clusters to clarify the presentation: high particle strong acidity regions in the f east near sources of sulfur emissions (Sulfate I Belt) and downwind (Transport Region); high ozone but low particle strong acidity regions in the west (West Coast); and low .r d Figure 1. Locations of the study sites by year of study: 24 cities, United States and Canada, 1988-1991 codes are provided in Table 1. 17.3 aMinimum value in all communities was less than the limit of detection. bMaximum sample size for any of the four parameters. cSouth Brunswick, NJ sampling was for 21 June 1989-25 August 1989. dMean particle strong acidity level below limit of detection; set to zero. pollution regions in central United States and Canada (Background).
Ambient air pollution concentrations were measured for approximately 12 months in each community. Ambient ozone concentrations were measured continuously in each community or at a location representative of ozone concentrations in the general area. Samples of other pollutants were collected every other day using the Harvard/EPA Annular Denuder System (HEADS) sampler (10)(11)(12). Thompson and co-workers (13) have shown that this sampling frequency is sufficient to characterize the annual distribution of particle strong acidity, sulfate, and particle mass concentrations.
In South Brunswick, New Jersey, HEADS samplers were operated by investigators from the Environmental and Occupational Health Sciences Institute, University of Medicine and Dentistry of 1. Site New Jersey-Robert Wood Johnson Medical School during the summer before administering health questionnaires but not in other months. In Egbert, Ontario, the sampling site was operated by Atmospheric Jnited Environment, Canada. Delays in establishing a laboratory in Egbert resulted in only a -le partial year of sampling for particle mass )lte and particle strong acidity, but a full year ate of ozone measurements. 3) The HEADS  identical impactor stages which provide a sharp cut at 10 pm aerodynamic diameter (15). Total mass of particles collected in both cases were determined gravimetrically.
In two sites with expected low particle strong acidity concentrations (Aberdeen, South Dakota, and Yorkton, Saskatchewan), samples were collected using the Harvard Impactor sampler with an aluminum honeycomb ammonia denuder (14) to measure particle strong acidity, sulfate, and ammonium. Collocated sampling has shown good agreement in particle strong acidity measurements of this sampler compared to the HEADS sampler (16).

Results
The particle and ozone measurements are summarized in Tables 1 and 2, respectively, and the rest of the measured gaseous pollutants are shown in Table 3. As previously mentioned, the communities were grouped into four categories. The Sulfate Belt signifies those sites that are in the Appalachian region including Ohio, Pennsylvania, Virginia, West Virginia, Tennessee, and Kentucky. Many of the country's largest sulfur emission sources are located in these states (17), and previous studies have shown that the region to the west of the Appalachian Plateau and the Allegheny Mountains experiences the greatest amount of acid deposition and the highest sulfate particle concentrations (18).
The Transport Region includes communities that experience the effects of sulfate particle pollution transported from source regions, but mean annual concentrations of sulfate and acid particles are uniformly lower than in the Sulfate Belt. These communities are north, northeast, and east of the states with substantial sulfur emissions and experience less frequent episodes of high particle strong acidity events. The West Coast group consists of three California communities with a range of air pollution: Livermore and Simi Valley have higher ozone and nitric acid concentrations, while Monterey was chosen to be a "clean" coastal site. The Background group consists of four continental communities: Springdale, Arkansas; Aberdeen, South Dakota; Yorkton, Saskatchewan; and Penticton, British Columbia.

Particulate Matter
A summary of the particulate measurements PM1O, PM2.1, sulfate, and particle strong acidity concentrations is shown in   Table 4 shows the day-to-day correlations between selected pollutants within each community. Correlations were calculated separately for summer days (May-September). When the warm season correlations differed from the annual correlation by more than 0.05, the values are shown in parentheses. The regional averages were calculated using the simple mean of individual community correlations. Annual and summer correlations between PM10 and PM2.1, particle mass and sulfate con-Volume 104, Number 5, May 1996 * Environmental Health Perspectives Articles -Acid aerosol and air pollution exposures Particulate ammonium showed a more uniform pattern (Fig. 2) than nitrates over the eastern United States and Canada, with annual average concentrations between 1.5 and 2.5 pg/m3. The higher concentrations were in western Pennsylvania and southern Ontario. Penticton, British Columbia, Monterey and Livermore, California, and Pembroke, Ontario had annual concentrations less than 1 pig/m3. Summertime values in these four communities were equal to or less than the annual concentrations. In most cases, summer ammonium concentrations were 40-50% greater than the annual mean.
Ion balance ratios ([H+ + NH4+1/ [2SO42-+ NO3-]) indicate that particulate sulfate and nitrates are largely present in the form of ammonium salts. Adding these components together gives insight into the composition of fine particulate mass. Together ammonium sulfates and nitrates composed more than 50% of the PM2.1 mass in 14 communities and more than 60% in 6 communities (Fig. 3). In those communities with the lowest percentage of sulfate and nitrate compounds composing PM2.1 mass, in Morehead, Kentucky; Parson, West Virginia; and Pembroke, Ontario, more than half of the homes reported using wood as the primary heating fuel. Two other towns, Newtown, Connecticut, and Penticton, British Columbia, had wood burning in 34% and 40% of homes, respectively. Penticton, BC, and Monterey, California, both had low annual mean PM21, about 9 jig/m3 with 80% and 62%, respectively, composed of compounds other than sulfate and nitrate particles. The most likely contributors to fine particulate mass, other than ammonium sulfate and nitrate, are wood burning and motor vehicles. Analysis of elemental composition (bromine, potassium, etc.) and carbon (elemental and organic) is needed to resolve PM2.1 source contribution.

Partide Strong Acidity
Particle strong acidity consists primarily of sulfuric acid (H2SO4) or partially neutralized ammonium bisulfate salts. Particle strong acidity is reported as hydrogen ion concentrations in nmol/m3; the equivalent sulfuric acid concentration in pg/m3 can be determined by dividing by 20.4.
The annual mean concentrations of particle strong acidity (Table 1) ranged from below the limit of detection in Yorkton, Saskatchewan, to approximately 50 nmol/m3 in Uniontown, Pennsylvania, and Parsons, West Virginia. The summer or warm season (May-September) mean concentrations ( Table 5) (Fig. 3). In seven communities there were days when particle strong acidity concentrations exceeded 500 nmol/m3 (25 pg/m3). Transport events were responsible for highly variable exposures in the northeast, with occasional high particle strong acidity exposure days, but a fairly low annual mean.
The correlation between sulfate and PMIO daily concentrations was higher in the Sulfate Belt and Transport Region than in the other communities (Table 4). Among the eastern communities in the high sulfate region, the daily sulfate to particle strong acidity correlations averaged 0.9 annually and for the summer months.
Strong correlations were also observed for communities affected by transported acid sulfates (r = 0.88). These findings suggest that daily variations in particle mass, sulfate, and acidity measurements are all highly correlated in the Sulfate Belt and Transport Regions of North America.

Gaseous Pollutants
Annual mean ozone concentrations ranged from 16 ppb in Yorkton, Saskatchewan, to 35 ppb in Simi Valley, California; there was a slightly greater than twofold difference across the range of communities ( Table 2). The interquartile range of annual mean concentrations was 7 ppb. There was a threefold range of 8-hr daytime ozone concentrations ( Table 2). The United States and Canadian health standards for ozone are based on acute 1-hr exposures. The Canadian 1-hr ozone standard, 100 ppb, was exceeded in 18 communities; the United States standard, 0.12 ppm, was exceeded in 10 communities ( Table 2). Simi Valley, just north of Los Angeles, had the second highest mean for the daily maximum hourly and the second highest single hourly value recorded, 170 ppb, behind Newtown, Connecticut. The same was true for the daytime (1000-1800 hr) averaged concentrations but not for the 24-hr mean concentrations. Annual average ozone concentrations in Hendersonville, Tennessee (20 km northeast of Nashville) and the more rural locations of Morehead, Kentucky; Dunnville, Ontario; and State College, Pennsylvania, approached those in Simi Valley. In fact, the maximum 24-hr average ozone concentration in five of the eastern communities exceeded the maximum 77 ppb 24-hr mean observed in Simi Valley. Ozone concentrations in Simi Valley displayed the greatest diurnal variation, particularly during the summer months. The correlation between daily particle strong acidity concentrations and maximum 1-hr ozone levels did not show any distinct patterns ( Table 4) Table 1 for site codes. the east. The daily correlation of ozone with particle strong acidity was low in Hendersonville (r = 0.26) and in Morehead (r = 0.29), while for the other communities in the Sulfate Belt daily correlations of ozone with particle strong acidity were higher than 0.60. Parkhurst et al. (20) have reported that ozone in Hendersonville is substantially and consistently higher than levels measured in Nashville and higher than other regional Tennessee Valley Authority monitoring sites. These observations suggest that the southwest winds transporting ozone precursors from Nashville lead to elevated ozone concentrations. However, high concentrations of particle strong acidity are not associated with these meteorological conditions. Ammonia is associated with natural, industrial, and agricultural sources. The highest mean ammonia concentrations (Table 3) were observed in Springdale, Arkansas, a community with a high density of chicken farms and processing plants. The farming communities of Leamington and Dunnville, Ontario; Blacksburg, Virginia; and Penticton, British Columbia, also had annual mean ammonia concentrations exceeding 1 ppb, as did Penn Hills, Pennsylvania, an eastern suburb of the industrial city of Pittsburgh (population 2.2 million).  Table 1 for site codes.
For the eastern sites, nitric acid ranged between 0.7 and 1.5 ppb (29-61 nmol/m3; Table 3). The nitric acid annual mean concentration in Simi Valley, California, was more than double the mean concentration for almost all other sites. Ten percent of days in Simi Valley had concentrations exceeding 4.3 ppb, with the highest single day of 12.2 ppb. Annual nitric acid concentrations exceeded 1 ppb only in Uniontown and Penn Hills, Pennsylvania. At most sites the summer concentrations were equal to or greater than the annual mean. However, Edgerton et al. (18) observed that winter nitric acid concentrations were higher than other seasons during 1989 across the National Dry Deposition Network.
The highest annual nitrous acid concentration ( concentrations were about one-third the nitric acid concentrations except at these three sites. There was more gas-phase acidity (nitric and nitrous acid) than particlephase strong acidity at every site except Parsons, West Virginia. There was eight times more gas-phase acidity than particlephase acidity in Livermore and Simi Valley. But, even downwind of Pittsburgh, the Penn Hills site had substantially more gasphase acidity than particle-phase acid (102 nmol/m3 versus 30 nmol/m3). Seasonal Patterns in Particle Strong Acidity Monthly mean concentrations of particle strong acidity in the eastern communities were elevated during May-September, whereas in the western sites of Simi Valley and Monterey, California, and Penticton, British Columbia, there was little seasonal variability (Fig. 4). Particle strong acidity occurs during meteorological conditions that favor the conversion of sulfur dioxide to acid sulfates without the presence of excess ammonia. Although these meteorological conditions occur more frequently during summer months, they can occur in other seasons. In the more southern towns of Hendersonville, Tennessee, and Morehead, Kentucky, elevated particle strong acidity was observed during the winter months. Particle strong acidity in Uniontown, Pennsylvania, was elevated during December, April, and May, as well as during the summer months (Fig. 4). The elevated monthly mean for December was the result of a single event when the 24-hr concentration reached 133 nmol/m3, possibly due to the impact of a nearby coal-fired power plant.  Table 1 for site codes.
The cumulative monthly percent contribution to the total annual exposure was calculated for each community. With the exception of Livermore, California, 10-20% of the acid occurred during the first 4 months of the year. Less than 20% occurred during the last three months. In the Sulfate Belt and Transport Regions, between 60 and 80% of the annual particle strong acidity exposure occurs between the beginning of May and the end of September. In contrast, in the western communities the particle strong acidity exposures are evenly distributed throughout the year.
The particle strong acidity events data from Oak Ridge, Tennessee; Parsons, West Virginia; Penn Hills, Pennsylvania; and Pembroke, Ontario-the four communities on an axis through the highest particle strong acidity concentrations-were closely examined (21). Days were ranked by particle acidity, and the cumulative annual exposure was normalized to 100%. For comparison, the number of days required to reach 75% of the annual particle strong acidity exposure is an indication of the episodic nature of particle strong acidity events. The community farthest south, Oak Ridge, reached 75% of the annual exposure in 40% of the days. The communities to the northeast received 75% of the annual particle strong acidity exposures in less than 4 months (33%) of the year. In Penn Hills, proximate to Pittsburgh, only 22% of the days were required to reach 75% of the annual exposure. Although particle strong acidity events can occur throughout the year, particle strong acidity pollution is episodic and more likely during spring and summer months.

Interrelationships of Annual Means
The annual mean concentrations of these pollutants (Tables 1 and 2) provide esti-mates of long-term exposures for the residents of these 24 communities. The cityspecific annual mean concentration of particle strong acidity (n = 24) was moderately correlated with PMIO (r = 0.47), but strongly correlated with sulfate (r = 0.90) and PM2.1 (r = 0.82). Three exposure parameters were considered for the annual mean ozone concentration: the average maximum 1-hr mean, the average daytime 8-hr mean, and the average daily 24-hour mean. All three ozone parameters were highly correlated across the 22 communities with Pearson correlation coefficients ranging from 0.74 to 0.98. Particle strong acidity was only weakly correlated with the three ozone parameters; the strongest correlation was with the average daytime 1-hr maximum ozone concentration (r = 0.37).

Discussion
Neither sulfur dioxide nor PM10 exceeded the U.S. National Ambient Air Quality Standards during the monitoring period in any of the study communities. Sulfur dioxide concentrations reached 56 ppb on the highest single day, which is 40% of the 24hr standard. The highest annual mean was 13 ppb, about one-third of the standard.
Inhalable particulate matter (PM10) annual concentrations were all less than two-thirds of the United States standards of 50 pg/m3 annual mean. There were no recorded exceedences of the 150 pg/m3 maximum daily standard. Across the Sulfate Belt and Transport Regions, fine particles (PM21) were 60% to more than 80% of the mean PM1O mass concentration. Both particle mass measures (PM21 and PM1O) and the sulfate concentrations were highly correlated. Similarly, high correlations were observed in these two regions during the warm season between particle strong acidity and the particle measurements (PM2.1, PM1O, and sulfate).
Annual and daytime mean ozone concentrations varied by only a factor of two or three, respectively, across sites. Although the site selection did provide a contrast among clean communities (low particles, acidity, and ozone) and communities with higher air pollution, it proved difficult to find sites with high particle strong acidity but low ozone. The West Coast communities of Simi Valley and Livermore, California, had higher ozone concentrations with low particle sulfate and hence low particle strong acidity. However, these two communities had substantial gas-phase acidity.
There was a 10-fold difference in atmospheric sulfate and a larger range of acid particle concentrations. The higher annual, seasonal (spring and summer), and daily particle strong acidity concentrations Volume 104, Number

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Articles e Acid aerosol and air pollution exposures occurred in communities along the western edge of the Appalachian Plateau and Allegheny Mountains. Days occurred when the atmospheric particle strong acidity exceeded 300 nmol/m3 (15 jig/m3 sulfuric acid equivalent) throughout the Sulfate Belt and into the downwind Transport Region in Canada and the eastern United States. Annual mean community exposures are determined, in part, by the number of particle strong acidity events. Days with elevated particle strong acidity occurred over highly populated urban and rural areas of the eastern United States.