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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. Although the EPA Administrator recommended the W126 as the secondary ozone standard, based on advice from the White House (Washington Post, April 8, 2008; Page D02), the EPA Administrator made the secondary ozone standard the same as the primary 8-hour average standard (0.075 ppm). 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 website 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, 2010 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 and deferred to the normal cycle of evaluating the current state of science associated with ozone and its effects on human health and vegetation.

It is important to note that in the United States the daylength during the summer months at all locations is greater than 12 hours. In some locations, the daylength is greater than 16 hours. The figure below illustrates the daylength 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 daylength is less than 12 hours is after the third week in September. The daylength 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 daylength 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 (2007) acknowledged 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. Heath et al. (2009) discuss the importance of nighttime ozone exposures associated with changes in the detoxification potential as a function of the time of day. Lee et al. (2022) note that in some circumstances, there may not always be concordance between diurnal patterns of O3 concentrations and those for stomatal conductance. The authors note that high-elevation sites often have O3 concentrations that remain high at night and even peak at these times and consequently, a 12-h O3 exposure metric may not produce optimal predictions at high elevations and for some species with meaningful nocturnal stomatal conductance and flux. Goumenaki et al. (2021) noted that ascorbic acid (AA) content and/or redox state was subject to day/night control. The investigators reported that plants exposed to equivalent O3 fluxes administered during daytime versus nighttime exhibited a significant decline in biomass in both cases, and the losses were greater in plants subjected to equivalent O3 flux at night. Thus, nighttime O3 exposures appeared to be important. The change in diurnal detoxification is very important when considering the use of flux-based indices (see Musselman et al., 2006; Lefohn et al., 2018). In addition, it appears that cumulative O3 exposures over a 24-hour period may be important.

In addition to the concern whether the accumulation period should be 24 hours versus 12 hours for the W126 exposure index for assessing vegetation effects, it is important to address the exposure regime patterns used in the crop and forest seedling experiments in the 1980s and 1990s. The experimental exposure protocols used to introduce enhanced ozone concentrations into the chambers resulted in numerous hourly average concentrations greater than or equal to 100 ppb for some of the crops and tree seedling species (but not all) in the experiments. While frequent occurrences under ambient conditions of hourly average concentrations greater than or equal to 100 ppb were prevelant in the 1980s and 1990s, this is not the case today. Thus, some of the exposure regimes used in the NCLAN and forest seedling experiments do not match the ambient exposure regimes currently experienced in the United States. Lefohn and Foley (1992) and Lefohn et al. (1997) noted the frequent occurrences of hourly average concentrations greater than or equal to 100 ppb in the NCLAN and forest seedling experiments and suggested that an additional exposure metric that described the number of hourly average concentrations greater than or equal to 100 ppb (i.e., N100 index) be coupled with the W126 metric for assessing vegetation effects if the exposure-response relationships were based on some of the NCLAN and forest seedling experiments performed in the 1980s and 1990s. Lee et al. (2022) reported in their analysis of tree seedling data that the most sensitive
species in their analysis experienced a biomass loss of 5% at a W126 of 2.5–9.2 ppm-hrs and that the N100 values ranged from 0 to 7 at those exposures. The seedling O3 exposure studies for western and eastern tree species in the Lee et al. (2022) analysis were conducted from 1988 to 1995 at the U.S. Environmental Protection Agency research laboratory in Corvallis, Oregon, Michigan Technological University’s Ford Forestry Center in Alberta, Michigan and by researchers from Appalachian State University at Great Smoky Mountains National Park near Gatlinburg, Tennessee. The requirement for an N100 index has been discussed in Musselman et al. (2006) and Davis and Orendovici (2006). Additional discussion of the N100 metric coupled with the W126 index is provided by
clicking here. During the reconsideration of the O3 standard that took place during the 2022-2023 period, the EPA suggested that if a secondary standard in the form of the W126 index were to be considered by the Administrator, it should also be accompanied by an additional metric that focuses on control of the high hourly O3 concentrations. In August 2023, the EPA Administrator decided to initiate a new review of the ozone NAAQS, which meant that the entire ozone rulemaking process would begin once again and take several years.


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.

Goumenaki, E., González-Fernández, I., Barnes, J. 2021. Ozone uptake at night is more damaging to plants than equivalent day-time flux. Planta 253(3):75. doi: 10.1007/s00425-021-03580-w.

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.

Heath R. L.; Lefohn A. S.; Musselman R. C. (2009) Temporal processes that contribute to nonlinearity in vegetation responses to ozone exposure and dose. Atmospheric Environment. 43:2919-2928.

Lee, E.H., Andersen, C.P., Beedlow, P.A., Tingey, D.T., Koike, S., Dubois, J.-J., Kaylor, S.D., Novak, K., Rice, R.B., Neufeld, H.S., Herrick, J.D. (2022). Ozone exposure-response relationships parametrized for sixteen tree species with varying sensitivity in the United States, Atmospheric Environment (2022), doi:

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.

Lefohn, A.S.; Jackson, W.; Shadwick, D.S.; Knudsen, H.P. (1997) Effect of surface ozone exposures on vegetation grown in the southern Appalachian Mountains: Identification of possible areas of concern. Atmospheric Environment 31(11): 1695-1708.

Lefohn, A.S., Malley, C.S., Smith, L., Wells, B., Hazucha, M., Simon, H., Naik, V., Mills, G., Schultz, M.G., Paoletti, E., De Marco, A., Xu, X., Zhang, L., Wang, T., Neufeld, H.S., Musselman, R.C., Tarasick, T., Brauer, M., Feng, Z., Tang, T., Kobayashi, K., Sicard, P., Solberg, S., and Gerosa. G. (2018). Tropospheric ozone assessment report: global ozone metrics for climate change, human health, and crop/ecosystem research. Elem Sci Anth. 2018;6(1):28. DOI:

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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