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.
