Ozone Concerns

Some Years Up, Others Down, but Ozone Concerns are Here to Stay

Background

Ozone (O3)—one of the original six criteria air pollutants with national standards established by the 1970 Clean Air Act Amendments—is the major component of smog. While naturally occurring O3 in the upper atmosphere (i.e. the stratosphere) helps shield the earth's surface from excessive ultraviolet radiation, high concentrations of O3 at ground level can be detrimental to both health and the environment. In short, O3 is "good up high and bad nearby."

Exposure to high levels of O3 can harm lung tissue, impair lung function, and sensitize the respiratory system. Recent evidence suggests that elevated O3 exposure not only affects those with existing health problems—such as asthma or heart disease—but also may be detrimental to healthy people as well. In addition to health concerns, high levels of O3 exposure can injure sensitive ecosystems and can cause premature aging of textiles, rubber, plastics, and coatings.

A "secondary" air pollutant, O3 is formed through a complex series of chemical reactions between reactive volatile organic compounds (VOCs) and nitrogen oxides (NOX) in the presence of sunlight. Given the multiplicity of human-caused VOC and NOX sources in urban and industrial areas, it is not surprising to find peak ground-level O3 concentrations downwind of these source regions. While changes in human-caused VOC and NOX emissions clearly do influence regional O3 levels, the primary source of day-to-day and year-to-year variation in O3 exposure is the weather. Days and years with hot, dry, and stagnant weather exhibit elevated O3 exposures, whereas those with cooler, wetter, and windier weather exhibit diminished O3 exposures.

Initial efforts to manage O3 pollution focused on lowering human-caused VOC emissions. While this strategy is effective in some areas, such as Los Angeles, it does not work well in the eastern United States. The more extensive forests and vegetative cover in the East provide a significant natural VOC source that is a much greater factor than in most Western States. Thanks to O3 research, it is now understood that combined VOC and NOX control strategies are needed to effectively manage O3 in the eastern United States.

As the effects of O3 exposure become better understood, O3 standards have changed considerably over the past 30 years. In 1971, the original O3 standard was set at a 1-hour, 0.08 parts-per-million (ppm) level not to be exceeded more than once per year. In 1979, the standard was revised to a 1-hour, 0.12 ppm level not to be exceeded more than once per year when averaged over a 3-consecutive-year period. In 1997, the O3 standard was again revised to an 8-hour, 0.08 ppm level, not to be exceeded by the 3-consecutive-year average of the 4th highest, daily 8-hour maximum for each year. While this latter "8-hour standard" is "on-the-books," O3 attainment status continues to be determined by the 1979 1-hour standard.

Assessment Database

In this report, seasonal (April-October) O3 data sets are examined from long-term rural Southeastern air quality monitoring stations (Figure 1). Rural monitoring data are used because they better reflect broad patterns over time and space than do their urban/suburban monitor counterparts, which are subject to rapid, dynamic changes in emissions associated with urban growth. The number of rural Southeastern O3 monitoring stations has tripled from 26 in 1988 to 79 in 1995-2000.

Time Trends

To estimate ozone-season time trends, three different O3 exposure indices were examined from those rural monitoring stations with complete records from 1988 through 2000. Three of these ten complete-record stations were in Kentucky, four in South Carolina, and three in Tennessee. The trend analysis was computed using three different commonly used ozone indices. The selected indices included: (1) 8-Hour Days—a cumulative seasonal sum of days with a maximum 8-hour ozone concentration exceeding 0.08 ppm; (2) W126—a cumulative seasonal sum of weighted hourly O3 concentrations; (3) SUM06—a cumulative maximum 3-month sum of hourly ozone concentrations greater than 0.06 ppm.

The 8-Hour Day index was chosen because of its relevance to the 8-hour standard. The seasonal W126 and 3-month Sum06 indices were selected because they have, at various times, been considered as possible secondary (i.e., based on environmental effects) O3 standards. The bottom line is that, notwithstanding computational differences, all these indices reflect very similar patterns (Figure 2). All three reflect a pattern of high O3 exposure in 1988, 1998, and 1999 and low exposure in 1989, 1991, and 1992. While there is substantial year-to-year variation, no overall O3 trend—up or down—is evident for this period.

Spatial Variation

Just as O3 exposure can vary significantly from year-to-year, so can its spatial distribution.To demonstrate this, consider the distribution of the maximum, 3-month Sum06 O3 exposures for two very different seasons—a high O3 season of 1999 and a low O3 season of 1992.

The 1999 map (Figure 3) indicates that rural SUM06 O3 exposures above 30 parts-per-million hours (ppm hrs) were commonplace across the inland Southeast. The 1992 map (Figure 4), on the other hand, shows a very different picture with only a small region near southwest Tennessee exceeding the 30 ppm hrs level.

Comparison to Current National Standards

Only four of our rural monitoring stations exceeded the 1-hour, 0.12 ppm O3 standard at some time during the 13-year period of interest—Oldham County Kentucky (northeast of Louisville), Jefferson County Tennessee (east of Knoxville), Brazoria County Texas (south of Houston), and Gregg County Texas (east of Dallas). The highest 1-hour value—0.177 ppm—was measured at the Brazoria Texas station in 1999.

In sharp contrast, more than two-thirds of our rural O3 stations (68%) exhibited one or more consecutive three-year periods exceeding the 8-hour O3 standard. The percentage of reporting stations exceeding the 8-hour standard ranged from a low of 5 of 39 stations (13%) during 1991-93 to a high of 13 of 13 stations (100%) during 1988-90. The highest three-year average of the fourth highest 8-hour maximum value—0.105 ppm—was measured at the Hancock County Kentucky station from 1988-1990. The lowest three-year average of the fourth highest 8-hour maximum value—0.047 ppm—was measured in Burke County North Carolina from 1991-1993.

Meteorological Factors

As discussed previously, day-to-day and year-to-year weather has a profound effect on the production, accumulation and transport of O3 and O3 precursors. Important meteorological factors include temperature, precipitation, solar radiation, wind speed and direction, and atmospheric stagnation. We examined the O3-season patterns for the first three of these factors during the relatively high O3 seasons of 1988, 1990, 1993, 1998, and 1999 and the low O3 seasons in 1989 and 1992.

For the five high-O3 seasons, the weather exhibited above average temperatures, high solar radiation and below average precipitation. The year 1988, demonstrably the worst O3 year on record, was characterized by persistent, wide-scale drought conditions across much of the Eastern United States.

In contrast, the two low-O3 seasons, 1989 and 1992, were characterized by below average temperatures, lower solar radiation and above average precipitation. In 1992, lower solar radiation was, in part, the result of the explosive eruption of Mount Pinatubo in the Philippines in late 1991, which resulted in an extensive layer of fine particles in the stratosphere over the Northern Hemisphere.

Summary

While there is substantial year-to-year variation in rural O3 levels, largely influenced by the weather, no trends are evident.
During low-O3 seasons, elevated rural O3 exposures appear downwind of large urban areas. In high-O3 seasons, however, elevated rural O3 exposures are found across most of the inland Southeast.
Between 1988 and 2000, only four rural O3 stations exceeded the level of the 1-hour 0.12 ppm O3 standard. These stations were all downwind of large urban/industrial areas.
For this same period, in contrast, 54 rural O3 stations exceeded the level of the 8-hour 0.80 ppm O3 standard.
No doubt about it, the 8-hour standard will be far more difficult to achieve than the 1-hour standard.

Information Contacts

William J. Parkhurst
(256) 386-2793
wjparkhurst@tva.gov

Frances P. Weatherford
256 386-2344
fpweatherford@tva.gov

Cassandra L. Wylie
865-632-1645
clwylie@tva.gov

Last updated on 3-11-2002.
Inquiries and comments should be sent to wjparkhurst@tva.gov.