Return to Home Page

Relationship Between Surface Ozone Concentrations and Other Pollutants

Introduction

In 2003, Dr. Lefohn provided a summary to the U.S. EPA for its Ozone Criteria Document (U.S. EPA, 2006) on the subject of the co-occurrence of ozone with other air pollutants. The information is provided here for those who wish to know more about the subject. The summary provides an update to the original results reported by Lefohn and Tingey (1984) and Lefohn et al. (1987). These two peer-reviewed research papers provided the basis for investigating the relevance of square-wave mixture exposures for assessing human health and vegetation effects.

There have been several attempts to characterize gaseous air pollutant mixtures (Lefohn and Tingey, 1984; Lefohn et al., 1987). Lefohn et al. (1987) discussed the various patterns of pollutant exposures. Pollutant combinations can occur at or above a threshold concentration either together or temporally separated from one another. Patterns that show air pollutant pairs appearing at the same hour of the day at concentrations equal to or greater than a minimum hourly mean value were defined as simultaneous-only daily co-occurrences. When pollutant pairs occurred at or above a minimum concentration during the 24-h period, without occurring during the same hour, a "sequential-only" co-occurrence was defined. During a 24-h period, if the pollutant pair occurred at or above the minimum level at the same hour of the day and at different hours during the period, the co-occurrence pattern was defined as "complex-sequential".

For characterizing the different types of co-occurrence patterns for O3/NO2, O3/SO2, and NO2/SO2, Lefohn and Tingey (1984), including a large number of air quality urban monitoring sites along with rural sites, used a 0.05 ppm threshold to identify the number of hourly simultaneous-only co-occurrences for the period May through September. Data used in the analysis included hourly averaged (1) EPA SAROAD data for 1981, (2) EPRI-SURE and -ERAQS data for 1978 and 1979, and (3) TVA data from 1979 to 1982. Lefohn and Tingey (1984) concluded, for the pollutant combinations, that (1) the co-occurrence of two-pollutant mixtures lasted only a few hours per episode and (2) the time interval between episodes was generally large (weeks, sometimes months).

Lefohn et al. (1987), using a 0.03 ppm threshold, grouped air quality data from rural and relatively remote monitoring sites (as characterized in the EPA database) within a 24-h period starting at 0000 hours and ending at 2359 hours. Data were analyzed for the May to September period. Data used in the analysis included hourly averaged (1) EPA SAROAD data from 1978 to 1982, (2) EPRI-SURE and -ERAQS data for 1978 and 1979, and (3) TVA data from 1979 to 1982. Patterns that showed air pollutant pairs appearing at the same hour of the day at concentrations equal to or greater than a minimum hourly mean value were defined as simultaneous-only daily co-occurrences. When pollutant pairs occurred at or above a minimum concentration during the 24-h period, without occurring during the same hour, a "sequential-only" co-occurrence was defined. During a 24-h period, if the pollutant pair occurred at or above the minimum level at the same hour of the day and at different hours during the period, the co-occurrence pattern was defined as "complex-sequential". A co-occurrence was not indicated if one pollutant exceeded the minimum concentration just before midnight and the other pollutant exceeded the minimum concentration just after midnight. As will be discussed below, studies of the joint occurrence of gaseous NO2/O3 and SO2/O3 reached two conclusions: (1) hourly simultaneous and daily simultaneous-only co-occurrences are fairly rare and (2) when co-occurrences are present, complex-sequential and sequential-only co-occurrence patterns predominate. The authors reported that year-to-year variability was found to be insignificant; most of the monitoring sites experienced co-occurrences of any type less than 12% of the 153 days.

Since 1999, monitoring stations across the United States have been routinely measuring the 24-h average concentrations for PM2.5. Because of the availability of the PM2.5 data, daily co-occurrence of PM2.5 and O3 over a 24-h period was characterized. Because PM2.5 data are mostly summarized as 24-h average concentrations in the AQS data base, a daily co-occurrence of O3 and PM2.5 was subjectively defined as when an hourly average O3 concentration greater than or equal to 0.05 ppm and a PM2.5 24-h concentration equal to or greater than 40 ug/m3 occurred over the same 24-h period.

To investigate the co-occurrence patterns associated with ozone and other air pollutants, please click on the links below.

Co-occurrence of Ozone with Nitrogen Oxides

Co-occurrence of Ozone with Sulfur Dioxide

Co-occurrence of Ozone and Daily PM2.5

 

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.