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Relationship Between Surface Ozone Concentrations and Other Pollutants
(Continued)

Co-occurrence of Ozone with Sulfur Dioxide

Because elevated SO2 concentrations are mostly associated with industrial activities (U.S. Environmental Protection Agency, 1992), co-occurrence observations are usually associated with monitors located near these types of sources. Lefohn and Tingey (1984) reported that, for the rural and nonrural monitoring sites investigated, most sites experienced fewer than 10 co-occurrences of SO2 and O3. Lefohn et al. (1987) reported that even with a threshold of 0.03 ppm O3, the number of co-occurrences with SO2 was small. Figure 3 illustrates the simultaneous co-occurrence results reported by Lefohn and Tingey (1984).

Figure 3. The co-occurrence pattern for ozone and sulfur dioxide (Source: Lefohn and Tingey, 1984).

Meagher et al. (1987) reported that several documented O3 episodes at specific rural locations appeared to be associated with elevated SO2 levels. The investigators defined the co-occurrence of O3 and SO2 to be when hourly mean concentrations were equal to or greater than 0.10 and 0.01 ppm, respectively.

The above discussion was based on the co-occurrence patterns associated with the presence or absence of hourly average concentrations of pollutant pairs. Taylor et al. (1992) have discussed the joint occurrence of O3, nitrogen, and sulfur in forested areas using cumulative exposures of O3 with data on dry deposition of sulfur and nitrogen. The authors concluded in their study that the forest landscapes with the highest loadings of sulfur and nitrogen via dry deposition tended to be the same forests with the highest average O3 concentrations and largest cumulative exposure. Although the authors concluded that the joint occurrences of multiple pollutants in forest landscapes were important, nothing was mentioned about the hourly co-occurrences of O3 and SO2 or O3 and NO2.

Using 2001 data from the EPA AQS database, patterns that showed air pollutant pairs of O3/SO2 appearing at the same hour of the day at concentrations equal to or greater 0.05 ppm were characterized. The data were not segregated by location settings categories (i.e., rural, suburban, and urban and center city) or land use types (i.e., agricultural, commercial, desert, forest, industrial, mobile, or residential). Data capture was not a consideration in the analysis. In 2001, there were 246 monitoring sites that co-monitored O3 and SO2. As discussed previously, because of possible missing hourly average concentration data during periods when co-monitoring may have occurred, no attempt was made to characterize the number of co-occurrences in the 0 category. Thus, co-occurrence patterns were identified for those monitoring sites that experienced one or more co-occurrences. Figure 4 shows the results from this analysis for the simultaneous co-occurrence of O3 and SO2. Similar to the analysis summarized by Lefohn and Tingey (1984), most of the co-located monitoring sites analyzed, using the 2001 data, experienced fewer than 10 co-occurrences (when both pollutants were present at an hourly average concentration greater than or equal to 0.05 ppm).

Figure 4. The co-occurrence pattern for ozone and sulfur dioxide using 2001 data from AQS.

 

References

Lefohn, A. S.; Tingey, D. T. (1984) The co-occurrence of potentially phytotoxic concentrations of various gaseous air pollutants. Atmos. Environ. 18: 2521-2526.

Lefohn, A. S.; Davis, C. E.; Jones, C. K.; Tingey, D. T.; Hogsett, W. E. (1987) Co-occurrence patterns of gaseous air pollutant pairs at different minimum concentrations in the United States. Atmos. Environ. 21: 2435-2444.

Meagher, J. F.; Lee, N. T.; Valente, R. J.; Parkhurst, W. J. (1987) Rural ozone in the southeastern United States. Atmos. Environ. 21: 605-615.

U.S. Environmental Protection Agency. (1992) National Air Quality and Emissions Trends Report, 1991. Research Triangle Park, NC: U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards; report no. EPA/450-R-92-001. Available from: NTIS, Springfield, VA; PB93-143998.

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