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The Importance of Nighttime Ozone Exposures for Vegetation Effects

In 2007, the EPA Administrator proposed the use of a 3-month, 12-hour W126 exposure index as a possible secondary O3 standard. On September 16, 2009, the EPA announced it would reconsider the 2008 national ambient air quality standards (NAAQS) for ground-level ozone for both human health and environmental effects. The Agency planned to propose any needed revisions to the ozone standards by December 2009 and issue a final decision by August 2010. On January 7, 2010, the EPA announced on its web site its proposal to strengthen the national ambient air quality standards for ground-level ozone. The EPA's proposal decreased the 8-hour “primary” ozone standard level, designed to protect public health, to a level within the range of 0.060-0.070 parts per million (ppm). EPA proposed to establish a distinct cumulative, seasonal “secondary” standard, referred to as the W126 index, which was designed to protect sensitive vegetation and ecosystems, including forests, parks, wildlife refuges, and wilderness areas. EPA proposed to set the level of the W126 secondary standard within the range of 7-15 ppm-hours. The accumulation period of the proposed W126 standard was 12 hours. On August 20, the Agency announced that it would delay its final announcement to on or around the end of October. In early November, the EPA announced that it would reach a final decision on the ozone standards by December 31, 2010. On December 8, the EPA announced that it would delay its final decision on the ozone standards until July 2011. EPA announced on July 26 that it would not make a decision on the ozone standards by its previously announced deadline of July 29. On September 2, 2011, President Obama requested that the EPA withdraw its proposed revisions to the ozone standards.

It is important to note that in the United States, the day length during the summer months at all locations is greater than 12 hours. In some locations, the day length is greater than 16 hours. The figure below illustrates the day length at latitudes that cover the area from Montana to southern Florida during the period from April 1 to October 29. Note that the only time during this period that the day length is less than 12 hours is after the third week in September. The day length during June in Montana is greater than 16 hours and in southern Florida is 13.5 hours.

European scientists use a time window that is dependent upon location and time of year. For low-elevation sites, the 0600-2059h window is often used. What is the scientific justification for using a cumulative 12 hours, while the actual day length during the summer months is greater than that value?

An extensive review of the literature reported that a large number of species had varying degrees of nocturnal stomatal conductance (Musselman and Minnick, 2000). Although EPA acknowledges that uptake of O3 during the nighttime may be important, the Agency states on page 8-17,

"…staff concludes that it remains unclear to what extent nocturnal uptake contributes to the vegetation effects of yield loss, biomass loss or visible foliar injury. Due to the many species- and site-specific variables that influence the potential for and significance of nocturnal uptake, staff concludes that additional research needs to be done before considering whether this component of vegetation exposure should be addressed with a different averaging time."

Nocturnal O3 flux depends on the level of turbulence that intermittently occurs at night. Massman (2004) suggested that nocturnal stomatal O3 uptake accounted for about 15% of the cumulative daily effective O3 dose that was related to predicted injury. Similarly, Grulke et al. (2004) showed that the stomatal conductance at night for Ponderosa pine in the San Bernardino National Forest (CA) ranged from one tenth to one fourth that of maximum daytime gas exchange.

By selecting a 12-hour window as originally suggested by the Agency, the EPA may have contributed to the the problem reported of the inconsistency of the W126 index. Adding to the accumulation period issue, the W126 will be inconsistent when predicting ozone vegetation effects due to the problem of ignoring the high hourly average concentrations that were contained within the vegetation experiments used to establish the exposure-response relationships. The requirement for an additional metric that describes the number of hourly average concentrations greater than or equal to 100 ppb (i.e., N100 index) has been discussed in the peer-reviewed literature. Davis and Orendovici (2006) reported that the incidence of vegetation symptoms in the field was most related to the W126 index coupled with the N100 index. The requirement for an N100 index is discussed further by Musselman et al. (2006) and Lefohn and Foley (1992) and an introduction to the N100 is provided by clicking here.


Davis, D.D.; Orendovici, T. (2006). Incidence of ozone symptoms on vegetation within a National Wildlife Refuge in New Jersey, USA. Environmental Pollution. 143:555-564.

Grulke, N. E.; Alonso, R.; Nguyen, T.; Cascio, C.; Dobrowolski, W. (2004) Stomata open at night in pole-sized and mature ponderosa pine: implications for O3 exposure metrics. Tree Physiology 24, 1001-1010.

Lefohn, A. S.; Foley, J. K. (1992) NCLAN results and their application to the standard-setting process: protecting vegetation from surface ozone exposures. J. Air Waste Manage. Assoc. 42: 1046-1052.

Massman, W. J. (2004) Toward an ozone standard to protect vegetation based on effective dose: a review of deposition resistance and a possible metric. Atmospheric Environment. 38: 2323-2337.

Musselman, R. C.; Minnick, T. J. (2000) Nocturnal stomatal conductance and ambient air quality standards for ozone. Atmos. Environ. 34: 719-733.

Musselman R. C.; Lefohn A. S.; Massman W. J.; Heath, R. L. (2006) A critical review and analysis of the use of exposure- and flux-based ozone indices for predicting vegetation effects. Atmospheric Environment. 40:1869-1888.

U.S. Environmental Protection Agency (2007) Review of the National Ambient Air Quality Standards for Ozone: Policy Assessment of Scientific and Technical Information OAQPS Staff Paper. Research Triangle Park, NC: Office of Air Quality and Planning and Standards, EPA-452/R-07-003. January.

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