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(December 2000)


The Federal Land Managers' Air Quality Related Values Work Group (FLAG) was formed to develop a more consistent approach for the Federal Land Managers (FLMs) to evaluate air pollution effects on their resources. Of particular importance is the New Source Review (NSR) program, especially in the review of Prevention of Significant Deterioration (PSD) of air quality permit applications. The goals of FLAG have been to provide consistent policies and processes both for identifying air quality related values (AQRVs) and for evaluating the effects of air pollution on AQRVs, primarily those in Federal Class I air quality areas, but in some instances, in Class II areas. Federal Class I areas are defined in the Clean Air Act as national parks over 6,000 acres and wilderness areas and memorial parks over 5,000 acres, established as of 1977. All other FLM areas are designated Class II.

FLAG members include representatives from the three FLMs that administer the nation's Federal Class I areas: the U.S. Department of Agriculture Forest Service (USDA/FS), the National Park Service (NPS), and the U.S. Fish and Wildlife Service (FWS). (Subsequently in this report, these three agencies collectively will be referred to as "FLMs." Class I and Class II air quality areas are called "FLM areas" in this report.)

This report describes the work accomplished in Phase I of the FLAG effort. That work includes identifying policies and processes common to the FLMs (herein called "commonalities") and developing new policies and processes using readily available information. This report provides State permitting authorities and potential permit applicants a consistent and predictable process for assessing the impacts of new and existing sources on AQRVs, including a process to identify those AQRVs and potential adverse impacts. The report also discusses non-new source review considerations and managing emissions in Federal areas. In Phase II, FLAG will address unresolved issues including those that will require research and the collection of new data. Below is an extraction from the FLAG Ozone Chapter in the report. Additional information concerning visibility and deposition can be found at


D. 3. Ozone

a. Introduction

Ozone is a toxic air pollutant that is formed on warm, sunny days when its precursor's nitrogen oxides (NOx) and volatile organic compounds (VOC) react in the presence of sunlight. Because ozone is a regional pollutant, precursor sources both near and far from FLM areas can contribute to ozone formation.

High ozone exposure can harm human health (U.S. EPA, 1996). Ozone is also phytotoxic, causing considerable damage to vegetation throughout the world. Some plant species are more sensitive to ozone than are humans (U.S. EPA, 1996). The primary National Ambient Air Quality Standard (NAAQS) for ozone is designed to protect human health, and the secondary NAAQS is set to achieve protection of the public welfare, including vegetation. The primary and secondary standards for ozone are the same. The new 8-hour, 0.08 ppm NAAQS for ozone is expected to be more protective of vegetation than the 1-hour, 0.12 ppm NAAQS. Attaining and maintaining compliance with the NAAQS is the responsibility of states and EPA rather than the FLMs. FLAG guidelines are not for regulatory purposes, but provide guidance for the FLM to identify ozone impacts on lands they manage.

FLAG recognizes that specific relationships between precursor emissions and ambient ozone concentrations at a FLM area are difficult to quantify. Further, it is difficult to quantify the specific relationship between ambient ozone at an FLM area and vegetation response. Therefore, FLAG has chosen to focus on the effects of ozone on vegetation and the levels of ozone generally known to be phytotoxic in FLM areas as indicators of concern regarding ozone impacts on AQRVs.

The objectives of this chapter are to document information currently known about vegetation response to ozone exposure, and to describe FLM procedures for responding to new source review (NSR) permit applications. If the FLMs have evidence that ozone is adversely impacting an area they manage, they will work to restrict further emissions of ozone precursors until those adverse impacts are mitigated.

b. Ozone Effects on Vegetation

Most ozone effects research has focused on agricultural crops because of the large economic losses that have been documented. Nevertheless, research has identified many native plants in natural ecosystems that are sensitive to ozone (U.S. EPA, 1996). Some of these ozone-sensitive plant species have been used as "bioindicators" of ozone to document phytotoxicity of ozone in the field due to ambient ozone. A listing of key literature describing known ozone effects on native vegetation is provided in Appendix H.

The definitions for ozone injury and damage used by FLMs are based on the classical definitions (for example, see Guderian 1977). Injury is all physical or biological responses to pollutants, such as change in metabolism, reduced photosynthesis, leaf necrosis, premature leaf drop, and chlorosis. Damage is reduction in the intended use or value of the biological or physical resource; for example, economic production, ecological structure and function, aesthetic value, and biological or genetic diversity that may be altered through the impact of pollutants.

Ozone enters plants through leaf stomata. It oxidizes plant tissue, causing changes in biochemical and physiological processes. These biochemical and physiological changes occur within the leaf long before visible necrotic symptoms appear (Guderian et al.1985). Plants must expend energy to detoxify ozone and repair injured tissue that could otherwise be used for growth or for maintenance of plant health. The injured plant cells eventually die if detoxification and repair cannot keep up with ozone uptake. The mesophyll cells under the upper epidermis of leaves are the most sensitive to ozone, and those are the first cells to die. The adjacent epidermal cells then die, forming a small black or brown interveinal necrotic lesion that becomes visible on the upper surface of the leaf. These visible lesions most frequently begin to develop on leaves that have just become fully matured, with older leaves on a stem showing increased amounts of injury. These lesions, termed oxidant stipple, are quite specific indicators that the plant has been exposed to ozone. Other plant symptoms that can result from exposure to ozone, with or without the presence of oxidant stipple, include chlorosis, premature senescence, and reduced growth. However, these symptoms are non-specific for ozone since other stressors can also cause them to occur. Further, these non-specific symptoms are difficult to quantify in natural ecosystems, although limited data are available from exposure response experiments to estimate growth losses from specific ozone exposures. In general, the only indicator that a FLM has to document that ozone has impacted vegetation is visible symptoms of injury such as oxidant stipple.

In addition to affecting individual plants, ozone can also affect entire ecosystems. Research shows that plants growing in areas with high exposure to ambient ozone may undergo natural selection for ozone tolerance (U.S. EPA, 1996). The final result could be the elimination of the most ozone-sensitive genotypes from the area. Regardless of the amount of ozone exposure, the magnitude of plant response may vary depending on the geographic area because of changes in meteorological and climatic conditions, and differences in plant conditions in space and time. Factors of most importance that influence plant response to ozone are the species/genotype, soil moisture, and nitrogen availability. Other factors influencing plant response to ozone include nutrient status, atmospheric humidity, temperature, solar radiation, phenological stage of development, day length, regional climatic differences, other pollutant interactions, and population/ecosystem interactions (U.S. EPA, 1996).

Ozone-induced physiological changes and/or growth reductions in plants may exist long before necrotic lesions appear on foliage; however, it is very difficult to attribute these effects directly to ozone. Similarly, changes in growth, ecosystem form or function, or biological or genetic diversity caused by ozone are difficult to document in natural ecosystems. Limited data are available regarding injury and growth response to specific ozone exposures. Given the difficulty in determining ozone-induced physiological or growth changes in natural ecosystems, FLMs will utilize as indicators of ozone effects on vegetation (1) symptoms that are clearly ozone induced such as oxidant stipple, and (2) ozone exposures that have been shown to be phytotoxic.

c. Recommended Metric to Determine Phytotoxic Ozone Concentrations

Various metrics have been used to relate ozone exposure to plant response. Biologically relevant ozone metrics for plants cannot be directly related to, nor can they be calculated from, the 8-hour NAAQS for ozone. The NAAQS ozone metric does not directly account for peak concentrations, nor does it accumulate exposure, important parameters in any biologically relevant ozone metric. Biologically relevant metrics considered by FLAG include the W126, SUM06, AOT40, and ozone flux. The W126 is an index that uses a sigmoidal weighted function to weight each hourly ozone concentration. The W126 index is determined by summing all the sigmoidal weighted concentrations for a specified time period (Lefohn and Runeckles, 1987). The W126 index was described and used in EPA's Air Quality Criteria for Ozone and Related Photochemical Oxidants, Vol. II (U.S. EPA, 1996) to characterize ozone trends. FLMs will use the W126 metric to determine phytotoxic ozone concentrations in FLM areas. The W126 is preferred to other cumulative metrics for a couple of reasons. First, the W126 preferentially weights the peak exposures, whereas other metrics, such as the AOT40 or the SUM06, do not. Second, the W126 accumulates ozone exposures at lower concentrations than does the AOT40 or SUM06. The AOT40 and SUM06 only accumulate concentrations above their particular threshold, e.g., 40 ppb for AOT40 and 60 ppb for SUM06. Phytotoxic effects have been shown to occur at exposure concentrations below 60 ppb (U.S. EPA 1996). The AOT40 metric is commonly used in Europe. Some European scientists recently have concluded that the AOT40 metric is useful for exceedance mapping but not for assessment of biomass loss (Kaerenlampi and Skaerby 1996). Therefore, FLAG does not recommend the AOT40 for FLM assessments.

The SUM06, W126, and AOT40 are ambient ozone exposure parameters, whereas flux is an ozone dose parameter for internal uptake. Flux is determined from ambient ozone concentration at the leaf surface and stomatal conductance. Ozone uptake relates more closely to plant response than does ambient ozone exposure. However, detoxification of ozone once it enters the plant is also an important component of plant response, and measuring uptake alone will not necessarily reflect the potential plant response. A benefit of flux is that it might allow differential weighting of daytime versus nighttime exposure (with daytime being weighted more heavily in most cases).

Science has not advanced sufficiently for FLAG to recommend use of flux as a metric for plant response to ozone at this time. However, research on the use of flux as an ozone metric is continuing (Massman et al. 2000) and it will be examined for possible future use.

To use the W126 metric, the daily and seasonal time periods of measurement must also be determined. Although most ozone uptake occurs during the day, many plant species can have nighttime stomatal conductance resulting in ozone uptake (Musselman and Minnick 2000). Nighttime uptake is a function of many variables, including species, region (e.g., desert, deciduous forest, etc.), season, and elevation. In addition, many FLM areas, particularly those in mountainous regions, have high nighttime ozone exposures. Further, plants may be more sensitive to ozone at night (Musselman and Minnick 2000). Therefore, FLAG endorses use of a 24-hour time period for the W126 metric.

Plant sensitivity and exposure to ozone will change throughout the growing season. Use of a rolling 90-day cumulative value for the W126 metric would account for changes in exposure over the season. However, some vegetation exposure/response and ozone monitoring data are currently available using 7-month (April through October) seasonal cumulative W126 values. In order to take advantage of this existing information, FLAG will use the April-October time period for the W126 metric.

FLAG recommends that peak concentrations (hourly ozone values equal to or greater than 100 ppb or N100) be included as a parameter of measurement in conjunction with the W126 parameter. Experimental evidence confirms that peak concentrations are important (U.S. EPA 1996). Accounting for peak concentrations also provides important information regarding the timing of events and helps determine if a response is due to chronic or acute exposure. Also, the quantitative exposure/growth response information used by FLAG for determination of critical exposure ozone levels was generated from experimentation based on fumigation treatments containing numerous occurrences of high hourly average concentrations. FLAG recognizes that oxidant stipple injury can occur at zero N100 for sensitive plant species; but the N100 should not be used alone as an indicator of sensitivity of vegetation to ozone.

W126 and N100 values for injury and growth loss for selected eastern U.S. vegetation are presented in Table O-1 and Table O-2. Data for Table O-1 were derived under favorable environmental conditions, and report the lowest exposure level treatment where visible ozone symptoms were first observed. Thus, threshold exposure levels for ozone symptom response could be lower than those exposures reported here. Data from Table O-2 were calculated from exposure response relationships for a 10 percent growth loss when plants are grown under favorable environmental conditions. It is recognized that data for other eastern U.S. plant species, and for plant species growing in the Western U.S., are not currently available. However, some additional exposure/response data for other species are available from which these values can be calculated. It is important to note that the critical level for injury or growth loss to vegetation from ozone is highly dependent on plant species and environmental conditions when the plants are exposed. The results obtained in Tables O-1 and O-2 could vary under different combinations of environmental conditions. Additional research under varying environmental conditions and ozone exposures should be conducted.

Table O-1. W126 (ppm-h) and N100 (number of hourly average concentrations greater than or equal to 0.1 ppm) exposure levels that result in foliar necrotic symptoms for selected plant species (from Lefohn 1998.)


W126 (ppm-h)


Table mountain pine 20.0 2
Sweetgum 5.6 3
Sycamore 31.2 89
Winged sumac 3.3 5
Black cherry 11.5 10
Tall milkweed  0.3 0
Black-eyed Susan 12.8 50
Dwarf dandelion 0.3 0
Yellow buckeye 4.7 3
Virginia pine 30.0 50
Cutleaf coneflower 5.5 3

Table O-2. W126 (ppm-h) and N100 (number of hourly average concentrations greater than or equal to 0.1 ppm) exposure levels that resulted in a 10 percent growth loss for selected plant species (from Lefohn 1998.)


W126 (ppm-h)


Aspen 259 6.4 4
Aspen wild 71.4 243
Black cherry 6.5 1
Red maple 85.4 245
Whorled-wood aster 8.2 10
Yellow poplar 14.4 4
Eastern white pine 30.2 66
Sugar maple 44.7 131
Sycamore 15.4 27
Winged sumac 9.7 4

Ambient W126 and N100 values are available for many Class I areas in the eastern U.S., with values soon to be available for additional FLM areas. A table showing representative high and low W126 and N100 values for selected FLM areas is appended to this report (Appendix 3.B). Unfortunately, ambient ozone data are lacking for many western U.S. FLM areas, and large differences in terrain and elevation may limit the use of nearby data.

d. Identification of Ozone Sensitive AQRVs or Sensitive Receptors

FLMs have determined that given the high ecological, aesthetic, and intrinsic value of federal lands, all native species are significant and warrant protection. Ideally, protection efforts would focus on the identification and protection of the most sensitive species in an area. Unfortunately, AQRV identification is limited by incomplete species inventories and/or lack of exposure/response data for most species of native vegetation. Sensitive species identification will improve as more information becomes available. In the meantime, FLAG is providing a preliminary list of sensitive plant species for each Class I area, i.e., those species that have been observed to exhibit ozone symptoms at ambient ozone exposures (Appendix 3.A). Those ambient levels have not necessarily occurred at the specific Class I area where the plants occur. AQRV lists will be available in the Air Synthesis and NRIS-AIR databases (See Section B.4.f. of this report) and will be updated as necessary.

e. Review Process for Sources that Could Affect Ozone Levels or Vegetation in FLM Areas

As mentioned above, NOx and VOC are ozone precursors. States and the EPA have based ozone control strategies in various parts of the country on the determination of which precursor is most likely to influence the formation of ozone. Information suggests that in areas where ozone formation is driven by VOC emissions, i.e., VOC-limited areas, VOC to NOx ratios are less than 4:1. In VOC-limited areas, minimizing or reducing VOC emissions is the most effective means of limiting or lowering ozone concentrations. Conversely, in NOx-limited areas, where VOC to NOx ratios are greater than 15:1, controlling NOx emissions is most effective. It is generally thought that most rural areas of the U.S. are NOx-limited, most or all of the time, with the possible exception of the rural areas of southern California. The FLMs do not have current data to show that all areas are NOx limited, nor do they consider VOCs to be unimportant as ozone precursors. However, until there is enough information available for FLAG to determine whether ozone formation in each FLM area is primarily limited by NOx or VOC emissions, we will assume all FLM areas are NOx-limited and will focus on control of NOx emissions. Where FLMs have information indicating a specific area is VOC limited, they will shift ozone protection strategy to focus on VOC rather than NOx emissions.

Source/receptor modeling is required in most NSR permit applications for particulate matter, sulfur dioxide, and nitrogen dioxide. FLAG is aware of attempts by EPA and others to develop dispersion models that can relate emissions from a single source to changes in ozone concentrations. We recognize that there is currently no model available that can provide this kind of single source attribution information for ozone. Nevertheless, because of existing and suspected ozone concerns in a number of FLM areas (e.g., evidence of phytotoxic effects and high ambient concentrations), we will consider ozone effects when reviewing NSR permit applications. However, because single source attribution modeling is possible for both visibility and deposition, FLMs will be more concerned about ozone if modeling indicates NOx emissions are likely to cause an adverse impact on visibility, soils, and/or surface waters.

The FLMs recognize that oxidant stipple can occur at hourly ozone concentrations that can be considered natural background levels (Singh et al. 1978). Many of the high hourly background concentrations can be attributed to stratospheric intrusions or stratospheric mixing in the upper troposphere (Singh et al. 1978); but stratospheric intrusions rarely occur in the middle and southern latitudes after May (Singh et al. 1980, Wooldridge et al. 1997), and thus do not coincide with the major portion of the growing season. However, oxidant stipple has been observed on foliage in the spring when these intrusions can occur. In general, oxidant stipple observed on foliage from June through September cannot be attributed to natural background ozone from stratospheric sources. Low levels of ambient ozone may occasionally occur in the troposphere from non-anthropogenic and non-stratospheric sources.

The occurrence of oxidant stipple necrosis on plant foliage may indicate further ozone induced physiological and growth impacts. Point sources emit precursors that could produce ozone at the FLM area, and increased ozone could induce further injury or damage to vegetation. However, we assume that restriction on increases in ozone precursors will prevent additional ambient ozone and subsequent increases in injury or damage to vegetation in FLM managed areas. It is important that ambient ozone monitoring be conducted by the State or Local air pollution control agency or by the FLM to determine the seasonal ozone exposure.

FLM actions or specific requests on a permit application will be based on the existing air pollution situation at the FLM area(s) that may be affected by the source. Some FLMs may rely on growth loss rather than foliar necrosis to make an adverse impact. Each FLM will determine if actions are warranted to limit emissions that might lead to increased ambient ozone, based on the expected impact of ozone in their particular area.

FLM response will depend on whether or not:

1. ozone vegetation effects have been documented in the area (as evidenced by foliar injury or damage to vegetation);
2. ozone exposure levels occurring in the area are high enough that they could affect vegetation (i.e., ozone exposures are at levels shown to be phytotoxic).

Figure O-1 outlines the general FLM process for responding to NSR permit applications based on ozone exposure and vegetation effects at the receptor site. Management decisions regarding acceptance of an existing or future ozone exposure will be area-specific and may differ significantly between agencies, or even regionally within agencies. Each FLM will determine if injury and/or damage are necessary to warrant action, based on the expected impact in the area they manage. The decisions are based on the FLM interpretation of regulations, past experience in the NSR arena, availability of ozone effect exposure/response information for species that occur in the area, and other factors. The FLM will negotiate with the NSR permit applicant and the permitting authority regarding the options listed in Figure O-1.

Note: "Ozone exposure currently recognized as "phytotoxic" is determined based on data from exposure response studies and ambient ozone at the site. The FLM may ask the applicant to calculate the ozone exposure values if these data are not already available. "Ozone damage to vegetation" is determined from field observations at the impacted site."

f. Further Guidance to FLMs

As mentioned above, limited information about ozone exposure/response relationships in plants and lack of an ozone source/receptor model make it difficult to protect FLM areas from the effects of ozone from new sources. However, there are other area-specific gaps in information that also limit protection efforts. It is important for local land managers to attempt to collect the missing information. This section provides guidance specifically to FLMs on what types of data should be collected and how the data could be collected.

Identifying and Monitoring Ozone-sensitive AQRVs

Many FLM areas need more details regarding plant species presence, location, and abundance. FLAG recommends FLMs gather this information, where needed, and refine their lists of area-specific ozone-sensitive plants. FLMs are currently developing lists of sensitive species to crosscheck with the plant species list for their area to determine potential sensitivity to ozone. In the future, the FLMs will place ozone sensitive plant species lists in the NRIS-AIR or Air Synthesis databases for comparison to plant species lists for each wilderness area or national park.

FLAG recommends that once local FLMs have developed lists of potentially sensitive AQRVs specific for their site, they conduct surveys to detect the presence of ozone-induced foliar injury on the selected species. The USDA/FS Forest Health Monitoring (FHM) Program has developed foliar injury survey protocols and QA/QC procedures that can be used to collect this information. Another resource is the foliar injury training module developed by the NPS Air Resources Division and Pennsylvania State University. This module helps field staff identify and quantify ozone injury symptoms on plant foliage. Field crews must obtain proper training and field experience in identifying foliar injury symptoms before surveys can be conducted.

Ideally, to verify ozone-induced foliar injury symptoms in the field, exposure/response fumigation studies should be conducted on these species, using concentrations that reflect current ambient exposure. Plants should also be tested at higher exposures, simulating increased levels of ambient ozone that might occur in the future. Due to the expense of constructing and operating such systems, it would be most appropriate for agencies to join resources and develop regional fumigation facilities. At a minimum, such facilities should be constructed both in the eastern and western U.S., since ambient conditions at an eastern facility might not be appropriate for western species and vice versa.

Ambient Ozone Monitoring

Many FLM areas do not currently have either on-site or nearby ambient ozone monitoring data. FLAG recommends that local FLMs make every effort to collect this information and that they use quality-assured ambient ozone monitoring protocols developed by the EPA and the state air quality agency. Continuous (active) monitoring is preferred since this type of data is necessary to determine compliance with the ozone NAAQS. Continuous monitoring is also necessary to determine the temporal dynamics of ozone exposure for vegetation, and is necessary to calculate the W126 and N100 parameters. Unfortunately, continuous monitoring is expensive and requires electric power that is often not available in or near remote FLM areas. When installing a continuous monitor is not an option, FLAG recommends use of passive monitors. Passive monitors give total exposure loading values (SUM00) for a specified period of time. The data are useful for indicating year-to-year changes in total ozone exposure at an individual site, and for indicating where continuous monitors should be installed. However, FLMs recognize the limitation of passive samplers in relating ozone exposure to plant response.

g. Ozone Air Pollution Web Sites

U.S. EPA ozone information:

NPS ozone information:

Ozone effects research, USDA ARS, North Carolina:

Ozone effects research, England:

Ozone effects research, Switzerland:

Ozone exposure metrics for vegetation:


To view the complete set of Appendices, please visit:

H. Bibliography

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Brace, S., D.L. Peterson and D. Horner. 1998. Diagnosing Ozone Injury in Vascular Plants of the Pacific Northwest. PNW-GTR-xxx (In Press). U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station.

Chappelka, A.H., L.J. Samuelson, J.M. Skelly, A.S. Lefohn and D. Nesdill. 1995. Effects of Ozone on Vegetation in Southern Appalachians: an Annotated Bibliography. Auburn University, School of Forestry. Auburn, Alabama.

Davis, D.D. 1995. Evaluation of Ambient Ozone Injury on the Foliage of Vegetation in the Edwin B. Forsythe National Wildlife Refuge, Brigantine, New Jersey. Final Report.

Davis, D.D. 1998. Evaluation of Ambient Ozone Injury on the Foliage of Vegetation in the Cape Romain National Wildlife Refuge, South Carolina. 1997 Observations.

Davis, D.D. 1998. Evaluation of Ozone Injury on Vegetation in the Okefenokee National Wildlife Refuge, Georgia. 1997 Observations.

Eckert, R., R. Kohut, J. Laurence, P. King, T. Lee, B. Rock, D. Moss and A. Theisen. 1991. Studies to Assess the Effects of Ozone on Native Vegetation of Acadia National Park. Annual Report. University of New Hampshire. Durham, New Hampshire.

Guderian, R., D.T. Tingey, and R. Rabe. 1985. Effects of photochemical oxidants on plants. Part 2. In: Guderian, R., ed. Air Pollution by Photochemical Oxidants. Springer-Verlag. New York. p. 129-296.

Guderian, R. 1977. Air Pollution. Phytotoxicity of Acidic Gases and Its Significance in Air Pollution Control. Springer-Verlag. New York.

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Heck, W.W., C.S. Furiness, C.K. Simms and E.B. Cowling, eds. 1997. Report from Workshop on Research Needs to Assess the Effects of Ozone on Crop, Forest and Natural Ecosystems. May 18, 1997. Southern Oxidants Study, North Carolina State University. Raleigh, North Carolina.

Kaerenlampi, L. and L. Skaerby, eds. 1996. Critical Levels for Ozone in Europe: Testing and Finalizing the Concepts. UN-ECE Workshop Report. University of Kuopio, Department of Ecology and Environmental Science.

Lefohn, A.S. 1998. The Identification of Ozone Exposures that Result in Vegetation Visible Injury and Growth Loss for Specific Species Grown in the Southern Appalachian Mountain Region. Southern Appalachian Mountains Initiative Report.

Lefohn, A.S. and V.C. Runeckles. 1987. Establishing a standard to protect vegetation - ozone exposure/dose considerations. Atmospheric Environment 21(3):561-568.

Massman, W.J., R.C. Musselman, and A.S. Lefohn. 2000. A conceptual ozone dose-response model to develop a standard to protect vegetation. Atmospheric Environment 34:745-759.

Miller, P., R. Guthrey and S. Schilling. 1995 Draft. Third Progress Report of FOREST. Comparisons of 1991-1995 Ozone Injury Index at Sierra Nevada and San Bernardino Mountains Sites. 9 pp.

Miller, P.R., K.W. Stolte, D.M. Duriscoe and J. Pronos. 1996. Evaluating Ozone Air Pollution Effects on Pines in the Western United States. General Technical Report PSW-GTR-155. U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. Albany, California.

Miller, P., R. Guthrey, S. Schilling and J. Carroll. 1996. Ozone injury responses of ponderosa and Jeffrey pine in the Sierra Nevada and San Bernardino Mountains in California. Paper presented at International Symposium: Air Pollution and Climate Change Effects on Forest Ecosystems, Feb 5-9, 1996. U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. Riverside, California.

Multi-stakeholder NOx/VOC Science Program. 1997. Report of the Vegetation Objective Working Group: Canadian 1996 NOx/VOC Science Assessment. Atmospheric Environment Service, Environment Canada. Toronto, Ontario, Canada.

Musselman, R.C. and T.J. Minnick. 2000. Nocturnal stomatal conductance and ambient air quality standards for ozone. Atmospheric Environment 34:719-733.

Neufeld, H.S. and J.R. Renfro. 1993a. Sensitivity of Black Cherry Seedlings (Prunus serotina Ehrh.) to Ozone in Great Smoky Mountains National Park. The 1989 Seedling Set. Natural Resources Report NPS/NRTR-93/112. U.S. Department of the Interior, National Park Service, Air Quality Division. Denver, Colorado. 26 pp.

Neufeld, H.S. and J.R. Renfro. 1993b. Sensitivity of Sycamore Seedlings (Platanus occidentalis) to Ozone in Great Smoky Mountains National Park. Data from 1989. Natural Resources Report NPS/NRTR-93/131. U.S. Department of the Interior, National Park Service, Air Quality Division. 39 pp.

Singh, H.B.; Ludwig, F.L.; Johnson, W.B. 1978. Tropospheric ozone: concentrations and variabilities in clean remote atmospheres. Atmospheric Environment 12:2185-2196.

Singh, H.B.; Viezee, W.; Johnson, W.B.; Ludwig, F.L. 1980. The impact of stratospheric ozone on trpospheric air quality. Journal Air Pollution Control Association 30:1009-1017.

Skaerby, L. and P.E. Karlsson. 1996. Critical levels for ozone to protect forest trees - best available knowledge from the nordic countries and the rest of Europe. In: Kaerenlampi, L. and L. Skaerby, eds. Critical Levels for Ozone in Europe: Testing and Finalizing the Concepts. UN-ECE Workshop Report. University of Kuopio, Department of Ecology and Environmental Science. p. 72-85.

U.S. EPA. 1992. Summary of Selected New Information on Effects of Ozone on Health and Vegetation: Supplement to 1986 Air Quality Criteria for Ozone and Other Photochemical Oxidants. EPA/600/8-88/105F. Research Triangle Park, NC: US Environmental Protection Agency, Office of Health and Environmental Assessment.

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U.S. EPA. 1996b. Air Quality Criteria for Ozone and Related Photochemical Oxidants. Vol. 2. EPA/600/P-93/004bF. U.S. EPA, Office of Research and Development. Research Triangle Park, North Carolina.

Wooldridge, G; Zeller, K.; Musselman, R. 1997. Ozone concentration characteristics at a high-elevation forest site. Theoretical and Applied Climatology 56:153-164.

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