Tag Archives: W126

EPA’s New Ozone Rule: Part 18

Posted on January 17, 2013 | 2 comments

In our last post, we saw how EPA’s CASAC reacted strongly to its decision to make the secondary standard of ground-level ozone identical to the primary standard. That influenced EPA to reconsider its decision as reported in the document National Ambient Air Quality Standards for Ozone, Final Preamble, 2011 (p. 215):

In reconsidering the 2008 final rule in the 2010 proposal, the Administrator agreed with the conclusions drawn in the 2006 Criteria Document, 2007 Staff Paper and by CASAC that the scientific evidence available in the 2008 rulemaking continues to demonstrate the cumulative nature of O3 – induced plant effects and the need to give greater weight to higher concentrations. Thus, the Administrator concluded that a cumulative exposure index that differentially weights O3 concentrations represents a reasonable policy choice for a secondary standard to protect against the effects of O3 on vegetation during the growing season. The Administrator further agreed with both the 2007 Staff Paper and CASAC that the most appropriate cumulative, concentration-weighted form to consider is the sigmoidally weighted W126 form.

As EPA noted before, the amount of protection the primary standard would give to vegetation is uncertain, but the hint is that EPA is now prepared to err on the side of regulation. In this excerpt (p. 216), EPA argues that we can’t be sure that the primary standard can protect vegetation as well as the W126 standard. A comparison is hard to make because the results of such a comparison would likely differ from year to year, and because we don’t have enough data in the areas where the secondary standard might do the most good, in rural areas. (The paragraph sign [¶] indicates a paragraph break that I introduced that wasn’t there in the original text. The “8-hour average standard” is the primary standard, which averages ozone readings taken during an eight-hour period.):

The Administrator noted that… EPA proposed a second option of revising the then-current 8-hour average secondary standard by making it identical to the proposed 8-hour primary standard. The 2007 Staff Paper analyzed the degree of overlap expected between alternative 8-hour and cumulative seasonal secondary standards using recent air quality monitoring data. Based on the results, the 2007 Staff Paper concluded that the degree to which the current 8-hour standard form and level would overlap with areas of concern for vegetation expressed in terms of the 12-hour W126 standard is inconsistent from year to year and would depend greatly on the level of the 12-hour W126 and 8-hour standards selected and the distribution of hourly O3 concentrations within the annual and/or 3-year average period.

¶ The 2007 Staff Paper also recognized that meeting the then current or alternative levels of the 8-hour average standard could result in air quality improvements that would potentially benefit vegetation in some areas, but urged caution be used in evaluating the likely vegetation impacts associated with a given level of air quality expressed in terms of the 8-hour average form in the absence of parallel W126 information. This caution was due to the concern that the analysis in the 2007 Staff Paper may not be an accurate reflection of the true situation in non-monitored, rural counties due to the lack of more complete monitor coverage in many rural areas. Further, of the counties that did not show overlap between the two standard forms, most were located in rural/remote high elevation areas which have O3 air quality patterns that are typically different from those associated with urban and near urban sites at lower elevations. Because the majority of such areas are currently not monitored, there are likely to be additional areas that have similar air quality distributions that would lead to the same disconnect between forms. Thus, the 2007 Staff Paper concluded that it remains problematic to determine the appropriate level of protection for vegetation using an 8-hour average form. [emphasis mine — MHK]

Now here is the real rationale behind the secondary rule: cumulative exposure hurts plants more than it hurts humans. But why that should be? That question I can’t answer. The document continues (p. 217):

The Administrator also noted in the 2010 proposal that CASAC recognized that an important difference between the effects of acute exposures to O3 on human health and the effects of O3 exposures on welfare [of vegetation — MHK] is that vegetation effects are more dependent on the cumulative exposure to, and uptake of, O3 over the course of the entire growing season (Henderson, 2006c). The CASAC O3 Panel members were unanimous in concluding the protection of natural terrestrial ecosystems and managed agricultural crops requires a secondary O3 standard that is substantially different from the primary O3 standard in form, averaging time, and level (Henderson, 2007).

That concludes the EPA’s rationale in the document. Again, it seems to me that the decision was based on a judgement call. You may agree with me that there is less of a moral imperative to safeguard property and crops than there is safeguarding human life, so when evaluating the secondary standard, it makes even more sense to compare gains and losses. True, a secondary standard might improve agricultural crops, but is it worth the additional cost to industry to maintain that standard? That question is especially hard to answer when we don’t know exactly how much benefit the secondary standard would bring us above and beyond the primary standard. It’s a very tricky question. More about this in my final comments on the subject. In the meantime, let’s discuss how EPA standards are implemented.

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EPA’s New Ozone Rule: Part 16

Posted on January 17, 2013 | 1 comment

We are continuing our discussion in our last post about why the EPA felt it necessary to formulate a new secondary standard for ground-level ozone concentration. As we noted before, initially the EPA felt it adequate for the secondary standard to be identical to the primary standard, but then it reconsidered its position.

The EPA performed an evaluation comparing primary and secondary standards and found that high cumulative exposures were widespread. Below is a summary of what they found, taken from the document National Ambient Air Quality Standards for Ozone, Final Preamble, 2011. Note point #4 where EPA explains why it thinks the primary standard is insufficient (p. 201):

…The following key observations were drawn from comparing predicted changes in interpolated air quality under each alternative standard form and level scenario analyzed:

  1. The results of the exposure assessment indicate that then-current air quality levels could result in significant impacts to vegetation in some areas. For example, [bulleted list is my formatting — MHK]
    • For the base year (2001), a large portion of California had 12-hr W126 O3 levels above 31 ppm-hours, which has been associated with approximately up to 14% biomass loss in 50% of tree seedling cases studies.
    • Broader multi-state regions in the East (NC, TN, KY, IN, OH, PA, NJ, NY, DE, MD, VA) and West (CA, NV, AZ, OK, TX) are predicted to have levels of air quality above the W126 level of 21 ppm-hours, which is approximately equal to the secondary standard proposed in 1996 and is associated with biomass loss levels no greater than approximately 9% in 50% of tree seedling cases studied, and biomass loss levels greater than approximately 9% in the other 50%.
    • Much of the East and Arizona and California have 12- hour W126 O3 levels above 13 ppm-hours which has been associated with biomass loss levels no greater than approximately 7% biomass loss in 75% of tree seedling cases studied and biomass loss levels greater than approximately 7% in the remaining 25% of cases studied.
  2. When 2001 air quality was rolled back to meet the then current 8-hour secondary standard, the overall 3-month 12-hour W126 O3 levels were somewhat improved, but not substantially. Under this scenario, there were still many areas in California with 12-hour W126 O3 levels above 31 ppm hours. A broad multi-state region in the East (NC, TN, KY, IN, OH, PA, MD) and West (CA, NV, AZ, OK, TX) were still predicted to have O3 levels above the W126 level of 21 ppm-hours.
  3. Exposures generated for just meeting a 0.070 ppm, 4th-highest maximum 8-hour average alternative standard (the lower end of the proposed range for the primary O3 standard) showed substantially improved O3 air quality when compared to just meeting the then-current 0.08 ppm, 8-hour standard. Most areas were predicted to have O3 levels below the W126 level of 21 ppm-hr, although some areas in the East (KY, TN, MI, AR, MO, IL) and West (CA, NV, AZ, UT, NM, CO, OK, TX) were still predicted to have O3 levels above the W126 level of 13 ppm-hours.
  4. While these results suggested that meeting a proposed 0.070 ppm, 8-hour secondary standard would provide substantially improved protection in some areas, the Staff Paper recognized that other areas could continue to have elevated seasonal exposures, including forested park lands and other natural areas, and Class I areas which are federally mandated to preserve certain air quality related values. This is especially important in the high elevation forests in the western U.S. where there are few O3 monitors and where air quality patterns can result in relatively low 8-hour averages while still experiencing relatively high cumulative exposures.

Now the EPA will explain where in particular the lack of a separate secondary standard is a problem. It seems that ozone levels in high-elevation rural areas remain fairly constant during the day, so that the ozone concentration may be below the primary standard and yet deliver a large cumulative exposure. This is where attention to a cumulative-based secondary standard might be particularly useful. Note that the 8-hour average form refers to the primary standard, which depends on the average of ozone measurements taken during an eight-hour time period (p. 202):

To further characterize O3 air quality in terms of the 8-hour and alternative secondary standard forms, an analysis was performed in the 2007 Staff Paper to evaluate the extent to which county-level O3 air quality measured in terms of various levels of the 8-hour average form overlapped with that measured in terms of various levels of the 12-hour W126 cumulative, seasonal form. This analysis was limited by the lack of monitoring in rural areas where important vegetation and ecosystems are located, especially at higher elevation sites. This is because O3 air quality distributions at high elevation sites often do not reflect the typical urban and near-urban pattern of low morning and evening O3 concentrations with a high mid-day peak, but instead maintain relatively flat patterns with many concentrations in the mid-range (e.g., 0.05-0.09 ppm) for extended periods. These conditions can lead to relatively low daily maximum 8-hour averages concurrently with high cumulative values so that there is potentially less overlap between an 8-hour average and a cumulative, seasonal form at these sites. The 2007 Staff Paper concluded that it is reasonable to anticipate that additional unmonitored rural high elevation areas important for vegetation may not be adequately protected even with a lower level of the 8-hour form.

Then the EPA seems to reverse its position. Since we can’t be confident that the primary standard will be adequate, especially in rural areas and remote areas where data on ozone might be sparse, we may need to establish a secondary standard. Whereas before the EPA wanted to err on the side of less regulation, now they want to err on the side of more regulation (p. 203):

It continues to remain uncertain as to the extent to which air quality improvements designed to reduce 8-hour O3 average concentrations would reduce O3 exposures measured by a seasonal, cumulative W126 index. The 2007 Staff Paper indicated this to be an important consideration because:

  1. The biological database stresses the importance of cumulative, seasonal exposures in determining plant response;
  2. Plants have not been specifically tested for the importance of daily maximum 8-hour O3 concentrations in relation to plant response;
  3. The effects of attainment of a 8-hour standard in upwind urban areas on rural air quality distributions cannot be characterized with confidence due to the lack of monitoring data in rural and remote areas.

These factors remain important considerations in the Administrator’s reconsideration of whether the current 8-hour form can appropriately provide requisite protection for vegetation.

Question on point #3: If we can’t be sure of the effects of attainment of an 8-hour standard on rural areas because we don’t have enough monitoring data, how would we be any more sure of the effects of attainment of the secondary standard?

The EPA’s own CASAC (Clean Air Scientific Advisory Committee) was also very unhappy with the decision to make the secondary standard equal to the primary standard. We will see what they have to say in the next post.

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