2. Characterization of Air Quality Levels

2.1	SURFACE OZONE CONCENTRATIONS

This section provides an overview of ozone air quality in the continental U.S., covering to the extent practicable diurnal and seasonal time frames, rural and urban environments and trends. Materials were extracted and condensed from EPA's Criteria Document for Ozone ( EPA, 199?), the EPA Air Quality Trends Report (EPA, ??), and the EPA staff paper on ozone (EPA, 1996??).

2.1.1	Surface Ozone Concentrations

2.1.1.1 Ozone Background

Ozone is a naturally occurring, trace constituent of the atmosphere. Estimates of the natural component of O3 vary widely in the literature, and there is no standardized terminology regarding the concept of O3 background.

"Natural" O3 background is a multidimensional and complex concept. Background O3 concentrations vary by geographic location, altitude and season. For the purposes of this document, background ozone is defined as the ozone concentrations that would be observed in the U.S. in the absence of anthropogenic precursor emissions of VOCs and NOx in North America. During the summertime ozone season in the U.S., daily 1-hr. maximum background ozone concentrations are typically between 0.03 to 0.05 ppm. Part of this background is due to natural sources and part of it is due to long-range transport of anthropogenic precursor emissions.

The natural component of the background originates from three sources: stratospheric ozone which is transported down to the troposphere, ozone formed from the photochemically- initiated oxidation of biogenic and geogenic methane and carbon monoxide, and the photochemically-initiated oxidation of biogenic VOCs. The magnitude of this natural part cannot be precisely determined for two reasons. First, the part due to long-range transport of anthropogenic precursor emissions is not known. Second, NOx plays an important role in the oxidation of methane, carbon monoxide and the biogenic VOCs and it is not possible to determine the amount of ozone that would have been formed just due to natural NOx emissions. However, some estimates can be made.

On the basis of O3 data from isolated monitoring sites (EPA, 1993), an estimate of the O3 background concentration near sea level in the U.S. for an annual average is 0.020-0.035 ppm. This estimate includes a 0.005 to 0.015 ppm contribution (averaged over time) from stratospheric intrusions into the troposphere and a 0.01 ppm contribution from photochemically-initiated oxidation of methane and carbon monoxide. The remainder is due to the photochemically- initiated oxidation of biogenic VOCs and long-range transport.

Similarly, an estimate of the background O3 concentration near sea level in the U.S. for a 1-hour daily maximum during the summer is usually in the range of 0.03 to 0.05 ppm. At clean sites in the Western U.S., the maximum annual hourly values are in the range of 0.060 - 0.075 ppm. Such elevated O3 levels may be occurring at higher altitudes due to stratospheric O3 intrusion. An estimate for background 8-hour daily maximum O3 concentrations during the summer is also in the range of 0.03 to 0.05 ppm.]

2.1.1.2 Hourly Air Quality Distributions

Figure 2.1 presents histograms of the hourly O3 concentrations for the 3-month peak summer period at sites in varying geographic locations and source-receptor environments; an urban site in Chicago, a site downwind of Chicago, a site downwind of Atlanta, and a site at higher elevation in Albuquerque. All hourly concentrations are displayed. These charts show that there are distinct differences among these sites, with downwind sites exhibiting a greater frequency of higher concentrations.

Figure 2.1.2 shows the typical relationship between hourly ozone concentrations and the corresponding running 8-hour averages. The individual bars correspond to the hourly values and the line shows the corresponding 8-hour averages. Each 8-hour average is associated with the start hour of the 8-hour period. For example, the 8-hour average from 4 p.m. to midnight is plotted at 4 p.m. It should be noted that the next 8-hour period, from 5 p.m. to 1 a.m., is plotted at 5 p.m. but it contains hourly values from two different days. If the 8-hour daily maximum is selected from all 24 8-hour averages starting within the day, it is possible to have daily maximums from adjacent days that have some hourly values in common. This is unlikely for the typical urban diurnal pattern shown in Figure 2.2 but it may occur for sites with less pronounced diurnal patterns, such as rural, higher elevation sites.

Another point to note with 8-hour averages is that the 8-hour daily maximum for a particular day can actually be higher than the 1-hour maximum. Again, this would be uncommon but can happen because the daily maximum 8-hour average could contain up to seven hourly values from an adjacent day.

2.1.1.3 Air Quality Trends

The interpretation of recent O3 trends is difficult due to the large temporal variation that results from the confounding factors of meteorology and emissions changes. Peak O3 concentrations typically occur during hot, dry, stagnant summertime conditions. Thus, Summer 1988, as the third hottest summer on record since 1931, was highly conducive to O3 formation with peak O3 levels comparable to those recorded in the earlier peak year of 1983 (Heim, 1988; EPA, 1990). Meteorological conditions in 1991 and 1993 were also highly conducive to O3 formation, especially in the eastern half of the country, although the magnitude and frequency of exceedances were significantly less than those recorded in 1988 (Brown,1993; EPA, 1994). In contrast, the years 1989 and 1992 saw meteorological conditions that were generally not as conducive to O3 formation. These changes in meteorological conditions have led to large year-to year differences in peak O3 concentrations (Brown, 1992). In response to the National Academy of Sciences recommendations (NAS, 1991), EPA has developed a statistical model that adjusts for meteorological variability to detect the underlying O3 trend (Cox and Chu, 1994). Figure 2.3 presents the meteorologically adjusted, and unadjusted, ten year O3 trends in 43 metropolitan areas. The 99th percentile daily maximum 1-hour concentration declined 1 percent per year or 12 percent since 1984. The national trend in the composite mean of the annual second highest daily maximum 1-hour concentration at 509 sites is shown for comparison, which also declined by 12 percent between 1984 and 1993.

2.1.1.4 Spatial Distributions of Air Quality Concentrations

This section provides a brief overview of O3 concentrations vary across the country with respect to alternative standards. Figure 2.1.4 displays a map of those counties with 1-hour daily maximum, 1 expected exceedance O3 design values greater than 0.12 ppm based on 1991-93 air quality monitoring data. Figure 2.5 shows the spatial distribution of counties with average annual fifth highest 8-hour daily maximum 0.08 ppm design values greater than 0.08 ppm, based on 1991-93 data. Figure 2.6 depicts those counties with a 3-month SUM06 exposure index value greater than 25 ppm-hours in 1990. These Sum06 values are based only on the daylight hours, 8:00 am - 8:00 pm Local Standard Time (LST). In each of these maps, the county air quality status was determined by the peak design value site in each county. For the current standard, the design value is simply the fourth highest daily maximum 1-hour concentration measured during 1991-93, since if the fourth highest value is reduced to the level of the standard, there will be only three days above the level of the standard, or 1 exceedance per year. Because both the proposed primary and secondary standards are concentration based standards, the design value is identical to the standard test statistic.

2.1.1.5 Air Quality Status with Respect to Standards

Ambient air quality data from 1991-93 have been used to estimate the air quality status with respect to the current and the proposed primary and secondary standards. Table 2.1.1 presents the air quality status of the original 98 areas designated nonattainment for O3 under the Clean Air Act Amendments of 1990 with respect to these alternative standards.

Table 2.1.1	Number of original nonattainment areas not meeting selected standards based on 1991-93 air quality monitoring design values.

AIR QUALITY STANDARD	1991–93
Primary standard

1-hour, 1 exceedance, 0.12 ppm

39

8-hour, avg annual 2nd max, 0.08 ppm

, avg annual 2nd max, 0.08 ppm

Secondary standard	85

64

3-month Sum06 > 25 ppm-hours

(8:00 am-8:00 pm LST)	70
Number of Areas with monitors.	97

Air Quality Relationship Among Standards.	Although previous sections have shown that there are significant differences in air quality distributions among monitoring sites with differing environments, several patterns emerge from examining typical relationships. For example, the average ratio of 8-hour to 1-hour design values for a 1 exceedance per year standard is 0.86 based on 1991–93 data. This ratio has been fairly stable over time increasing slightly from 0.81 in 1980 to 0.86 in 1993. The ratio is also fairly consistent across EPA Regions, with the median ratio ranging from 0.80 to 0.88.

Design value comparisons based on 1991-93 data show that, on average, ozone design values for an 8-hour, 0.08 ppm average annual 5th highest daily maximum standard are 25% lower than the current 1-hour standard. On average, design values for an average annual 2nd highest daily maximum 8-hour standard are 18% lower than the current 1-hour standard.

In terms of exceedances, i.e., the number of days with daily maximum 8-hour average concentrations greater than 0.08 ppm, sites meeting an average annual 2nd highest daily maximum 0.08 ppm standard have, on average, 1.2 exceedances per year and 2.3 exceedances in the worst year of three. Sites meeting an average annual 5th highest daily maximum 0.08 ppm standard have, on average, 3.0 exceedances per year and 5.4 exceedances in the worst year of three. In the worst year of three, 95 % of sites meeting an average annual 2nd highest daily maximum 0.08 ppm standard have 7 or fewer exceedances, while 95 % of sites meeting an average annual 5th highest daily maximum 0.08 ppm standard have 12 or fewer exceedances.

2.1.2	Rural, Remote and Elevated Ozone Concentrations

[to be completed]

2.1.3	Ozone precursor concentartions

to-be-completed

[EKMA multi-city compilation; refer to special studies, NAS report and PAMS]

REFERENCES

Brown, W. O.; R .R. Heim (1992), Climate variations bulletin, Vol. 4, No. 8. Available from National Climatic Data Center, NOAA, Asheville, N.C.

Brown, W. O. (1993), Climate variations bulletin, Vol. 5, No. 8. Available from National Climatic Data Center, NOAA, Asheville, N.C.

Heim, R. H. (1988), Climate variations bulletin, August 1988. Available from National Climatic Data Center, NOAA, Asheville, N.C.

U. S. Environmental Protection Agency (1989), Volatility regulations for gasoline and alcohol blends sold in calendar years 1989 and beyond. Federal Register 54: 11868.

U. S. Environmental Protection Agency (1978), Section 107 states attainment status. Federal Register 43: 8962.

U. S. Environmental Protection Agency (1993), Air quality criteria for ozone and related photochemical oxidants, Vol. I, Chapter 4.3.3. EPA/600/AP-93/004a.

U. S. Environmental Protection Agency (1988), National Air Quality and Emissions Trends Report, 1990, EPA-450/4-90-002. U.S. Environmental Protection Agency.

U. S. Environmental Protection Agency (1994), National Air Quality and Emissions Trends Report, 1993, EPA-454/R-94/026. U.S. Environmental Protection Agency.