Return to Home Page

Relationship Between Surface Ozone Concentrations and Other Pollutants
(Continued)

Co-occurrence of Ozone with Nitrogen Oxides

Ozone occurs frequently at concentrations equal to or greater than 0.05 ppm at many rural and remote monitoring sites in the United States (U.S. EPA, 2006) as well as at policy-relevant background monitoring locations. Therefore, for many rural locations in the United States, the co-occurrence patterns observed by Lefohn and Tingey (1984) for O3 and NO2 were defined by the presence or absence of NO2. Lefohn and Tingey (1984) reported that most of the sites analyzed 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 1 summarizes the simultaneous co-occurrence patterns reported by Lefohn and Tingey (1984). The authors noted that several urban monitoring sites in the South Coast Air Basin experienced more than 450 co-occurrences. For more moderate areas of the country, Lefohn et al. (1987) reported that even with a threshold of 0.03 ppm O3, the number of co-occurrences with NO2 was small.

Using 2001 data from the EPA AQS database, patterns that showed air pollutant pairs of O3/NO2 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. The data were characterized over the EPA-defined O3 season. In 2001, there were 341 monitoring sites that co-monitored O3 and NO2. 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 1. The co-occurrence pattern for ozone and nitrogen dioxide (Source: Lefohn and Tingey, 1984).

Figure 2 illustrates the results of the analysis. 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 2. The co-occurrence pattern for ozone and nitrogen 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.

U.S. Environmental Protection Agency (2006) Air Quality Criteria for Ozone and Related Photochemical Oxidants. Research Triangle Park, NC: Office of Research and Development; report no. EPA/600/R-05/004af.

Home Page | News | Corporation | Maps | Publications | Table of Contents | Multimedia Center

Copyright © 1995-2017 A.S.L. & Associates. All rights reserved.