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:
- 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.
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:
- The biological database stresses the importance of cumulative, seasonal exposures in determining plant response;
- Plants have not been specifically tested for the importance of daily maximum 8-hour O3 concentrations in relation to plant response;
- 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.