Current Vegetation W126 / N100

“NOTE: When the data completeness for the year is less than 75%, as it was in 2021 due to equipment failure, it is recommended that the W126 calculation utilitze the method provided by the Environmental Protection Agency in the Federal Register, Vol 75, No. 11, Tuesday, January 19, 2010, page 3051. The narrative provides that all blank data cells be filled with the minimum value measured within the year. In 2021 the minimum was 8 ppb.”

The W126 metric is a sigmoidally weighted index that preferentially weights the higher concentrations more than the mid- and lower-levels.

The US EPA considers the W126 the most biologically relevant cumulative, seasonal form appropriate to consider as a secondary ozone standard in the context of the Agency’s 2008 and 2015 ozone rulemaking (U.S. EPA, 2008; US EPA, 2010: US EPA, 2013; US EPA, 2014a; US EPA, 2014b; US Federal Register, 2015) to protect sensitive natural vegetation and ecosystems in specially designated areas. The Agency used the W126 index to determine the secondary standard for ozone in its 2015 revision to the national ozone standard, concluding that vegetation would be protected by a standard that generally limits cumulative seasonal exposures to a 3-year average W126 value of 17 ppm-hours (ppm-hrs) or lower.  But rather than setting the secondary standard using a W126 value, EPA determined that an 8-hour standard level of 0.070 ppm (the primary standard) was equivalent and would provide the same protective levels as the W126 value of 17 ppm-hrs.

FCPC believes that the W126 index is most suitable to its Class I area.  Based on the review of the available scientific literature, FCPC chose to use the W126 exposure index accumulated over a 24-h period for a 3-month period as one of two indices to protect vegetation. FCPC determined that using the 24-hour W126 index, rather than the 12-hour W126 suggested by EPA, provides greater protection of FCPC’s vegetation resources.

Due to the importance of peak concentrations affecting the results in the experiments used to develop exposure-response relationships for assessing vegetation, FCPC decided to use the N100 metric (i.e., number of hourly average concentrations equal to or greater than 100 ppb [ 0.10 ppm] for the same 3-month period) as the second index to be used in combination with the W126 exposure index for assessing vegetation impact.

The threshold for FCPC’s W126 is calculated by averaging the highest 24-hour daily values during the months of June, July and August over 3 years.  Similarly, the design value for FCPC’s N100 is determined by averaging the total number of hours with ozone values of 100 ppb or greater during the same months over 3 years.

For information on how the W126 ozone exposure index was developed, visit asl-associates.com

For EPA guidance on computing W126 values, see EPA’s 2010 proposed rule on National Ambient Air Quality Standards for Ozone, at page 3051.

EPA’s National Ambient Air Quality Standards for Ozone, 80 FR, 65292, Monday, October 26, 2015
https://www.gpo.gov/fdsys/pkg/FR-2015-10-26/pdf/2015-26594.pdf

US EPA. 2013. Integrated Science Assessment of Ozone and Related Photochemical Oxidants (Final Report). EPA/600/R-10/076F. Research Triangle Park, NC: Environmental Protection Agency.  Available at: http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_isa.html.

EPA. 2014a. Welfare Risk and Exposure Assessment for Ozone. Final Report. EPA-452/P-14-005a. Research Triangle Park, NC: Environmental Protection Agency.  Available at https://www.epa.gov/naaqs/ozone-o3-standards-risk-and-exposure-assessments-current-review.

US EPA. 2014b. Policy Assessment for the Review of the Ozone National Ambient Air Quality Standards. Final Report. EPA-452/R-14-006. Research Triangle Park, NC: Environmental Protection Agency. Available at: https://www3.epa.gov/ttn/naaqs/standards/ozone/data/20140829pa.pdf (accessed on 18 October 2017).

US Federal Register. 2015. National Ambient Air Quality Standards for Ozone, 40 CFR Part 50, 51, 52, 53, and 58, pp 65292-65468.