Constructed Climates

Figure 4.12: Maximum daily ozone levels increase with daily maximum temperature, as shown at left with data from Atlanta, Georgia, and New York City (data collected May–October 1988–1990, after Bernard et al. 2001), and Los Angeles, California (data collected every day in 1985, after Pomerantz et al. 1999). The dotted line represents the 120 ppb one-hour averaging regulatory standard in place before the 1997 80 ppb 8-hour standards, shown as the dashed line. The right-hand plot compares 1-hour and 8-hour ozone averages measured in Southern California during June to September 1993 (after Chock et al. 1999). The data indicate that ozone levels measured as 120 ppb 1-hour averages correspond to those measured as about 100 ppb 8-hour averages.

The graph at left in Figure 4.12demonstrates the dependence between maximum daily ozone levels and maximum daily temperature in several cities, here showing 1988-1990 data for Atlanta, Georgia, and New York City, [61] and for Los Angeles, California, every day in 1985. Looking back at Figure 4.11, ozone levels in rural Tennesssee in 1991, even in the heat of summer, the highest rural levels topped out at 90 ppbV; in these three cities the highest ozone levels are around 150-200 ppbV. Ozone levels in cities are much higher.

Granted, ozone levels fluctuate, in part, beyond human control due to weather variation, but also within human control of fossil-fuel emissions (see Figure 4.8). A moving-average method allows occasional short bursts of high-ozone levels by averaging them with background levels, thus reducing measured variability. Horizontal lines indicate the Environmental Protection Agency’s (EPA) nonattainment ozone levels, dashed being the post-1997 8-hour level, and dotted the pre-1997 one-hour level. Effective May 27, 2007, the EPA set the allowable levels to 75 ppb measured as an 8-hour average. There are 24 possible 8-hour moving averages in a given day; the day is given the highest of these possible values. A city violates the standard if, averaged over the prior three years, the day with the fourth highest average exceeds this level.[62]

As mentioned before, though, complicated atmospheric chemistry underlies setting air quality standards, and the root cause of poor urban air comes from burning fossil fuels in automobiles. Imagine the statistical analyses and atmospheric chemistry that had to have gone into setting these 8-hour ozone standards: Why not the third highest average or the fifth highest average over the prior two years or four years? Those are detailed scientific (and political) issues well beyond this synthesis, yet we know that ozone differs between green and urban areas. Presumably trees (or the lack of fossil-fuel burning-cars in expansive green areas) might help reduce urban ozone problems.

Statistical measures of distributions also connect with one another, and so they do for ozone standards. The plot at right compares 1- and 8-hour averaging in Southern California. Similarly, ranges cited for 1-hour and 8-hour ozone measurements in Atlanta show, respectively, 15–129 ppb vs. 13–108 ppb in Fulton County and 18–163 vs. 15–125 ppb in DeKalb County.[63] Average values in the two standards can be set to comparable values; the real question becomes how the two measures deal with the exceedingly rare high ozone days. Similar studies carried out ozone research in Atlanta, finding that the really bad ozone days result from specific weather patterns that “cook,” and recook, the same air mass over several days.[64] Bad weather events like these result in ozone violations no matter how they’re averaged, and garner the greatest concern by regulators.

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[61]The dependence of ozone levels and maximum daily temperature in various cities was discussed by Bernard et al. (2001) and Pomerantz et al. (1999). Both sources cite other sources for the data, and neither source states whether or not the measurements are one-hour averages, but by the context, I assume that to be the case.

[62]This information on the primary standards for ozone comes from a page on the EPA’s epa.gov website, air/criteria, current as of September 15, 2008. These limits are set “to protect public health, including the health of ‘sensitive’ populations such as asthmatics, children, and the elderly.”

Eight-hour standard set in 2008: 0.075 ppm. “To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within an area over each year must not exceed 0.075 ppm (effective May 27, 2008).”

Eight-hour standard set in 1997: 0.08 ppm. “(a) To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within an area over each year must not exceed 0.08 ppm. (b) The 1997 standard—and the implementation rules for that standard—will remain in place for implementation purposes as EPA undertakes rulemaking to address the transition from the 1997 ozone standard to the 2008 ozone standard.”

One-hour standards being revoked: 0.12 ppm. This standard only applies to limited areas. “(a) The standard is attained when the expected number of days per calendar year with maximum hourly average concentrations above 0.12 ppm is < 1. (b) As of June 15, 2005 EPA revoked the 1-hour ozone standard in all areas except the 8-hour ozone nonattainment Early Action Compact (EAC) Areas.”
Originally, the one-hour standard was set at 0.08 ppm in 1971, revised to 0.12 ppm in 1979. Whether regulatory or not, the one-hour level still has violations. A quick search found the Texas Commission on Environmental Quality reporting many 2008 summer days exceeding 125 ppb.

[63]White et al. (1994) compare the 1-hour and 8-hour ozone measurements for Atlanta, Georgia.

[64]St. John and Chameides (1997) discuss how really bad ozone days in Atlanta, Georgia, result from specific weather patterns that cook, and recook, the same air mass over several days. They also compare 1-hour and 8-hour averaging.