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.
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.