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Ozone-Induced Symptoms on Vegetation within the

Moosehorn National Wildlife Refuge in Maine

Author(s): Donald D. Davis

Source: Northeastern Naturalist, 14(3):403-414. 2007.

Published By: Eagle Hill Institute

DOI: http://

dx.doi.org/10.1656/1092-6194(2007)14[403:OSOVWT]2.0.CO;2

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2007 14(3):403–414 NORTHEASTERN NATURALIST

Ozone-Induced Symptoms on Vegetation within the

Moosehorn National Wildlife Refuge in Maine

Donald D. Davis1

Abstract - During 1998–2000 and 2002–2004, field surveys were conducted within

the Moosehorn National Wildlife Refuge, located in northeastern Maine, to determine

if ozone-induced symptoms occurred on refuge vegetation. Foliar symptoms were

observed on ozone-sensitive bioindicators during each survey year, but the incidence

(percentage) of plants exhibiting symptoms was generally low and varied among

species and years. Refuge plants that exhibited symptoms included Fraxinus spp.

(ash), Populus spp. (aspen), Corylus cornuta (beaked hazelnut), Prunus serotina

(black cherry), Prunus pensylvanica (pin cherry), Apocynum androsaemifolium

(spreading dogbane), and a viburnum tentatively identified as Viburnum nudum var.

cassinoides (withe-rod). Data from the nearest US EPA ozone-monitoring site, located

113 km southwest of the refuge in Acadia National Park, ME, revealed that ambient

SUM60 ozone levels during survey years ranged from approximately 17,900 ppb-hrs

in 2000 to more than 40,000 ppb-hrs in 1998. Therefore, the threshold level of SUM60

ozone capable of inducing symptoms on sensitive vegetation within this refuge and

Class-I Wilderness area is less than 18,000 ppb-hrs, and may be as low as 10,000 ppbhrs. The results of these surveys can be used by the US Fish and Wildlife Service when

making air-quality management decisions, including those related to the review of

Prevention of Significant Deterioration permits, and might serve as input into formulating more stringent National Ambient Air Quality Standards for ozone.

Introduction

The Moosehorn National Wildlife Refuge (NWR), located in northeastern Maine near the US–Canadian border, was established in 1937 as a

refuge and breeding ground for migratory birds and wildlife. The refuge

consists of two divisions: the 6507-ha Baring Division, located southwest

of Calais, and the 2697-ha Edmunds Division, adjacent to the ocean at

Cobscook Bay near Dennysville (Fig. 1). In 1975, approximately 1894 ha

of the Baring Division and 1125 ha of the Edmunds Division were

deemed wilderness areas by the US Congress and named the “Moosehorn

Wilderness”. In 1978, the Moosehorn Wilderness was designated a ClassI air-quality area, receiving further protection under the Clean Air Act as

amended in 1977 (US Congress 1977). At that time, the US Congress

gave the US Fish and Wildlife Service (USFWS), as well as other federal

land managers of Class-I areas, an affirmative responsibility to protect all

air-quality related values (AQRVs) in Class-I areas (US Congress 1977).

AQRVs include vegetation, wildlife, water, soils, visibility, and cultural

1

Department of Plant Pathology, Ecology Faculty, and Penn State Institutes of the

Environment, 211 Buckhout Laboratory, The Pennsylvania State University, University Park, PA 16802; ddd2@psu.edu.
404 Northeastern Naturalist Vol. 14, No. 3

resources. By federal law, AQRVs in Class-I areas must be protected

from deterioration. However, despite this special protection, significant

levels of ambient ozone still impinge on many Class-I air-quality areas in

the Northeast, adversely affecting AQRVs (Lefohn and Manning 1995,

Manning et al. 1996).

Refuge vegetation

Hardwood trees in the Moosehorn NWR include Populus tremuloides

Michx. (quaking apen), P. grandidentata Michx. (bigtooth aspen), Betula

papyrifera Marsh. (paper birch), B. populifolia Marsh. (gray birch), Acer

rubrum L. (red maple), Fagus grandifolia Ehrh. (American beech), Prunus

serotina Ehrh. (black cherry), and Prunus pensylvanica L. (pin cherry).

Coniferous trees include Abies balsamea L. (balsam fir), scattered Pinus

strobus L. (eastern white pine), and various Picea spp. (spruces). Pure

stands of Alnus rugosa (Du Roi) Spreng. (alder) occur within abandoned

farmlands, as well as along the edges of streams and beaver flowages.

Common understory plant species include Gaultheria procumbens L. (wintergreen), Pteridium aquilinum (L.) Kuhn (bracken fern), Carex spp.

(sedges), and Cornus canadensis L. (bunchberry). Wetlands include beaver

ponds and beaver meadows; marsh, shrub, and forested wet areas of various types; and natural ponds, streams, and lakes. Several Vaccinium spp.

(blueberry) fields are maintained as permanent forest openings. A longterm forest management plan, including cutting and burning of small

blocks of forest vegetation within the refuge, has been utilized since 1976

to increase the diversity of forest habitat, primarily to favor Scolopax

minor Gmelin (American woodcock). In the Cobscook Bay area, there are

open hayfields, as well as abandoned farms in various stages of succession.

Since the discovery that leaf “stipple” of Vitis spp. (grapes) was caused

by ozone (Richards et al. 1958), this characteristic symptom has been used to

evaluate ozone injury on broadleaved bioindicator plants. Stipples usually

appear as 1–2-mm diameter areas of pigmented, black or reddish-purple

tissue, restricted by the veinlets, on the adaxial surface of mature leaves (for

illustrations, see Skelly 2000). Immature leaves seldom exhibit stipple, and

premature defoliation of injured leaves may occur on sensitive species. To

the casual observer, these symptoms are similar to those induced by other

stresses (i.e., nutrient deficiency, early fall coloration, and heat stress).

However, the pigmented, adaxial stipple on sensitive plants is a reliable

diagnostic symptom that can be used by experienced observers to evaluate

and quantify ozone injury during field surveys (Skelly 2000).

Prior to the initial survey, the author examined unpublished refuge

flora lists and noted the presence of several bioindicator plants known to

be sensitive to ozone. The ozone-sensitivity of these potential bioindicator

plants has since been summarized (US DOI 2003). Potential bioindicator plants growing in the Moosehorn NWR included Sambucus canadensis

L. (American elder), bigtooth aspen, black cherry, pin cherry, Viburnum

spp. (viburnums), and Fraxius americana L. (white ash).
2007 D.D. Davis 405

Ambient ozone levels

Ground-level ozone is the most important plant-damaging air pollutant in

the US and Canada, and elevated levels of ozone occur annually throughout

much of eastern North America (Comrie 1994, Coulston et al. 2003). Elevated ozone concentrations are capable of injuring native plants in many

rural locations, including wilderness areas (Lefohn and Manning 1995,

Manning et al. 1996) and wildlife refuges (Davis and Orendovici 2006).

During these field studies, the EPA ozone monitor (AIRS site #23-009-

0102) nearest to the Moosehorn NWR was located within Acadia National

Park (NP), ME, approximately 113 km southwest (generally upwind) of the

refuge. Monitoring data have revealed that Acadia NP experiences some of

the highest concentrations of ozone on the east coast (Kohut et al. 2000). The

ozone, or its precursors, that affects the coast of Maine likely originates in

the megalopolis along the eastern seaboard to the southwest and upwind

from the refuge (Cleveland et al. 1976, Kohut et al. 2000). This ozone, or its

precursors, can travel downwind for hundreds of km during long-range

transport, as influenced by wind direction and weather fronts, and impinge

upon vegetation at the Moosehorn NWR.

Prior to the initial survey, the author examined the 1996 and 1997

ozone-monitoring data collected at Acadia NP and concluded that ambient

ozone had occurred at phytotoxic concentrations within this national park.

Assuming that these ozone levels also occurred in the Moosehorn NWR, and

the fact that ozone-sensitive plant species were listed as growing in the

refuge, the author hypothesized that ambient ozone was likely to injure

sensitive plants within the refuge.

Phytotoxic levels of ozone in the northeastern US would likely occur during

the vegetative growing season, from spring to fall. Therefore, the daily ozone

data from April 1 to September 31 during the survey years of 1998–2000 and

2002–2004 were examined. In this paper, ozone levels are expressed as

SUM60, the accumulation of ozone concentrations of 60 ppb or greater during

the growing season. The SUM60 ozone metric was of interest since it had been

correlated with ozone-induced symptoms on forest trees in the eastern US

(Hildebrand et al. 1996). However, the impact of ambient ozone levels can be

confounded by soil-moisture stress. Soil-moisture stress can induce stomatal

closure, limiting gas exchange and ozone uptake by vegetation, thereby

reducing or eliminating subsequent ozone injury (Showman 1991, Yuska et al.

2003). Soil-moisture stress may be the most important environmental factor

controlling the response of plant stomata during the growing season (Zierl

2001). Therefore, soil-moisture stress during survey years was evaluated using

the Palmer Drought Severity Index (PDSI; Palmer 1965).

The objectives of this study were to determine: 1) if ozone injury occurred on plants within the Moosehorn NWR, 2) the incidence (percentage)

of bioindicator plants exhibiting stipple, and 3) the relationship between

ozone injury and ozone levels or soil-moisture stress. To meet these objectives, surveys were conducted during 1998–2000 and 2002–2004.
406 Northeastern Naturalist Vol. 14, No. 3

Methods

Prior to the initial 1998 survey, the author examined maps of the

Moosehorn NWR to select tentative survey sites. Tentative sites were chosen within open areas, along roadsides, and the edge of fields, where

bioindicators were exposed to unrestricted air movement and direct sunlight,

criteria required for suitable sampling sites (Anderson et al. 1989). Twentyfive potential survey sites were visited in 1998, and 15 survey sites were

selected from these based on openness, accessibility, and presence of

bioindicators. During 1999 and succeeding survey years, some of these 15

sites were relocated slightly, othes eliminated, and new sites added as

species composition changed due to natural succession. However, the

Figure 1. Map showing location of 15 survey sites (open double circles) within the

two divisions (Baring and Edmunds) of the Moosehorn National Wildlife Refuge in

northeastern Maine. (Map courtesy of the US Fish and Wildlife Service).
2007 D.D. Davis 407

general location of these 15 sites formed the basis for the field survey. Eight

sites in the Baring Division and seven in the Edmonds Division were used

during most survey years (Fig. 1). Data were not taken at each site each year,

depending mainly on degres of insect injury to the foliage of the bioindicator

plants at the site. Plants with servere foliar injury from insects were not

rated. The primary bioindicator plants examined during the survey are listed

in Table 1.

The refuge was surveyed twice in 1998: during July 29–August 2 and

August 25–28. In 1999 and succeeding survey years, the refuge was visited

only once. In 1999, the refuge was surveyed during July 22–25, and it was

surveyed in 2000 from August 21–23. The refuge was not surveyed in 2001.

In 2002, surveys were conducted during the periods August 29–September

2, August 6–8 in 2003, and August 20–22 in 2004.

During each survey year, the number of plants that exhibited classic

ozone-induced stipple was recorded. Mainly saplings of the woody

Table 1. Summary of observations made during the 1998–2000 and 2002–2004 surveys at the

Moosehorn National Wildlife Refuge (the refuge was not surveyed in 2001). Numbers in table

refer to number of plants with ozone-induced stipple as compared to the total number of plants

evaluated for that species-genus; data also expressed as percentages.

Black Pin Spreading

Year (survey date) Ash Aspen cherry cherry dogbane Viburnum

1998 (July 29–Aug 2)

 Number plants examined 54 143 55 109 106

 Number plants injured 1 12 0 24 12

 Percentage 1.8% 8.4% 0.0% 22.0% 11.3%

1998 (Aug 25–28)

 Number plants examined 81 86 51 108

 Number plants injured 2 2 5 28

 Percentage 2.5% 2.3% 9.8% 25.9%

1999 (July 22–25)

 Number plants examined 77 396 111 196 178

 Number plants injured 2 9 5 4 10

 Percentage 2.6% 2.3% 4.5% 2.0% 5.6%

2000 (Aug 21–23)

 Number plants examined 150 154 86 176 120 10

 Number plants injured 6 12 0 13 24 2

 Percentage 4.0% 7.8% 0.0% 7.4% 20.0% 20.0%

2002 (Aug 29–Sept 1)

 Number plants examined 117 559 60 243 344 12

 Number plants injured 4 39 0 6 45 2

 Percentage 3.4% 5.4% 0.0% 2.5% 13.1% 16.7%

2003 (Aug 6–8)

 Number plants examined 163 166 43 282 307 46

 Number plants injured 1 1 0 2 2 6

 Percentage 0.6% 0.6% 0.0% 0.7% 0.6% 13.0%

2004 (Aug 20–22)

 Number plants examined 153 265 65 315 30 30

 Number plants injured 6 0 0 0 0 0

 Percentage 4.5% 0.0% 0.0% 0.0% 0.0% 0.0%

Average 2.8% 4.2% 2.1% 5.4% 8.6% 10.2%
408 Northeastern Naturalist Vol. 14, No. 3

bioindicators (ash, aspens, cherries, viburnum) were evaluated; some aspen

seedlings were rated. No attempt was made to evaluate mature, canopy trees.

Stipple was noted as present or absent for individual plants. Incidence was

calculated as (number of symptomatic plants)/(number of plants examined for

each species-genus) and expressed as a percentage. Factors that can influence

ozone injury incidence include plant-species sensitivity (US DOI 2003),

SUM60 ozone level (Hildebrand et al. 1996), and drought stress (Showman

1991, Yuska et al. 2003). We previously used binomial logistic regression to

successfully disclose the presence and strength of significant relationships

between incidence (presence or absence) of ozone induced-symptoms and

these factors in a similar, but larger, study within a NWR in New Jersey (Davis

and Orendovici 2006). Therefore, a binomial logistic regression analysis was

used herein to investigate significant relationships between incidence of

ozone injury and the following factors: plant species, ozone level, and drought

stress. Severity (as opposed to incidence) of injury was not evaluated. To

illustrate the ambient ozone levels that likely impinge upon the refuge, the

SUM60 ozone levels from Acadia NP were examined and graphed for each

survey year.

Results and Discussion

Symptom description and incidence

The classic dark, adaxial, ozone-induced stipple was the most common

foliar symptom observed on bioindicators within the Moosehorn NWR. During the first year of the survey (1998), the author observed that Apocynum

androsaemifolium L. (spreading dogbane) and a viburnum tentatively identified as Viburnum nudum L. var. cassinoides (L.) Torr. & Gray (withe-rod)

also exhibited adaxial stipple typical of that caused by ozone. Therefore,

spreading dogbane and withe-rod were added to the list of potential

bioindicators. In addition, stippling was also noted on Corylus cornuta Marsh.

(beaked hazelnut) shrubs in 2002 and 2003 (data not shown). However, the

stipple on hazelnut was very slight and difficult to evaluate in a consistent

manner; it was noted but not recorded as data.

Ash, aspen, cherry, and dogbane were the most common bioindicators in

the refuge. Based on the appearance of emerging seedlings, it was difficult to

distinguish between the two ash species, as well as the two aspen species.

Therefore, these plants were identified only as ash and aspen. Red foliage,

chlorotic stipple, and premature defoliation symptoms were noted occasionally on bioindicators, but these were not recorded as data. Although such

symptoms have been induced by ozone under controlled conditions, they also

may be caused by other factors such as high temperature, low soil moisture,

and early autumnal coloration (Orendovici et al. 2003).

Bioindicators within the refuge exhibited ozone-induced symptoms during most survey years (Table 1). However, the data most comparable for

statistical analyses were from the August surveys of 1998, 2000, 2002, and

2004. The ozone-sensitivity ranking of the bioindicators, based on mean
2007 D.D. Davis 409

percentage of individuals exhibiting stipple across these 4 most-comparable

years, was spreading dogbane (14.0%) > viburnum (7.7%) > pin cherry

(5.6%) > aspen (5.0%) > ash (3.6%) > black cherry (1.9%). Spreading

dogbane was judged to be the most sensitive bioindicator plant in the refuge.

The high sensitivity of spreading dogbane to ambient ozone has been confirmed in both open-top chamber studies and field surveys (Bergweiler and

Manning 1999, Eckert et al. 1999, Kohut et al. 2000). However, leaves of

spreading dogbane often became highly chlorotic and spotted, and began to

senesce by late summer, making it an unsuitable bioindicator later in the

growing season. Spreading dogbane would be more useful as an ozone

bioindicator in Maine early in the growing season, before the onset of

confounding symptoms that could mask ozone-induced stipple.

Viburnums, pin cherry, and aspen were the next most useful bioindicators,

all of which have been reported to be sensitive to ozone (US DOI 2003).

Although black cherry is considered to be very sensitive to ozone (Davis and

Skelly 1992, Davis et al. 1981), this species exhibited the lowest incidence

(1.9%) of any bioindicators. This low level of ozone injury was similar to the

incidence of ozone injury on black cherry during similar surveys within a NWR

in New Jersey, conducted within the same time frame (Davis and Orendovici

2006). For unknown reasons, black cherry might not be a useful bioindicator

for evaluating ozone injury along the coast of northeastern US. It is possible

that ozone-sensitive genotypes have been eliminated from the population.

Ambient ozone levels

During the survey years (1998–2000, 2002–2004), the August SUM60

ozone levels monitored at the EPA monitor in Acadia National Park were

highest in 1998 (ca. 40,000 ppb-hrs) and lowest in 2000 and 2004 (ca. 18,000

ppb-hrs) (Fig. 2). By late August, SUM60 ozone levels in 2002 were approximately 33,000 ppb-hrs, 29,000 ppb-hrs in 1999, and 26,000 ppb-hrs in 2003.

The overall pattern of ozone accumulation during the growing season was

similar from year to year, showing a gradual increase from May to September,

and a slight decrease thereafter. An exception to this trend occurred in 2002,

when the ambient ozone increased rapidly in early August.

In contrast to other wildlife refuges in eastern US, ozone levels monitored

in Acadia NP were slightly greater than those monitored within the more

pristine Seney NWR located in the remote upper peninsula of northern Michigan. SUM60 ozone monitored within the Seney refuge (EPA AIRS Site #26-

153-0001) was usually quite low, in the range of 5000–15,000 ppb-hrs by the

end of the growing season. In contrast to the relatively pristine Moosehorn and

Seney refuges, SUM60 ozone levels within the Forsythe NWR (EPA AIRS site

#34-001-0005) in New Jersey often exceeded 40,000 ppb-hrs by the end of the

summer, and has been reported as high as 70,000 ppb-hrs (Davis and

Orendovici 2006). Similarly, high SUM60 ozone levels, exceeding 70,000

ppb-hrs by late summer, have been reported in southeastern Missouri near the

Mingo NWR (as extrapolated from the nearest ozone monitor, EPA AIRS Site

#29-186-0005).
410 Northeastern Naturalist Vol. 14, No. 3

Relationship of incidence to species, ozone, and drought

As stated earlier, the most comparable incidence data were from the

August surveys of 4 years: 1998, 2000, 2002, and 2004. A binomial logistic

regression analysis (Davis and Orendovici 2006) was run on these 4 years’

data to determine if incidence of ozone injury was related to plant species,

ozone level, and drought stress. However, the Pearson goodness-of-fit

Figure 2. Sum of hourly ozone concentrations equaling or exceeding 60 ppb

(SUM60, ppb-hrs) recorded from May 1 to September 31, 1998–2000 (upper) and

2002–2004 (lower), at EPA AIRS Site #23-009-0102, located within the Acadia

National Park, 113 km southwest from the Moosehorn National Wildlife Refuge.
2007 D.D. Davis 411

analysis (Minitab 2003) revealed that logistic models could not be used to

predict the relationships, largely due to the limited number of years of

observations. The regression results (based on chi-square analysis) did reveal that significant differences (p = 0.05) in ozone incidence occurred

among species, ozone level, and drought stress (but could not be used for

predictive purposes). Therefore, data were analyzed using simple correlation

analysis (Minitab 2003) to evaluate the direction of relationships (not predictions) between incidence of stipple as compared to plant species/genus,

SUM60 ozone as of date of survey, and PDSI at the end of August (Fig. 3).

The incidence of ozone injury of aspen was significantly correlated with

that of spreading dogbane and viburnum, but not with the other species/

genera. This finding indicates that dogbane and viburnum may generally

respond in similar manner to ozone and/or environmental factors. The incidence of injury on black cherry was significantly correlated only with that of

pin cherry, indicating that the two Prunus species were responding to ozone or

the environment in a similar manner from year to year.

Figure 3. Palmer Drought Severity Index for coastal Maine, including Moosehorn

NWR, during 1895–2004. The horizontal line at “0” is considered normal moisture

levels. Areas above the line represent more than adequate moisture for normal plant

functioning, whereas areas below the line represent potential water stress. A drought

severity index of -3 is a severe drought, likely closing stomata and reducing ozone

uptake (data from http://www.ncdc.noaa.gov/oa/climate/onlineprod/drought/

xmgr.html).
412 Northeastern Naturalist Vol. 14, No. 3

Incidence of ozone injury for any of the six bioindicator species was not

significantly correlated (p = 0.05) with ambient SUM60 ozone levels. The

relationship between injury and level of ambient ozone is seldom simple and

direct, but is confounded by interacting environmental factors such as soilmoisture stress (Davis and Orendovici 2006). In fact, Manning (2003) stated

that use of cumulative ozone data as threshold values for predicting ozone

injury to plants must take into account biological and environmental factors

that affect ozone uptake via stomata. Soil-moisture stress can induce stomatal closure, which limits ozone uptake, thereby reducing or eliminating

subsequent ozone injury (Showman 1991, Yuska et al. 2003). Similarly,

Eckert et al. (1999) reported no relationship between ambient ozone levels in

Acadia NP and ozone injury on park vegetation, and also attributed this lack

of correlation to the confounding effects of moisture stress on stomatal

functioning. Along these lines, the incidence of ozone injury on aspen,

spreading dogbane, and viburnum, but not the other three bioindicators, at

Moosehorn NWR was negatively correlated (p = 0.05) with PDSI drought

stress. However, since the correlation was only significant for three of the

six bioindicators, the relationship between ozone injury and soil-moisture

stress apparently varies with plant species. In addition, the relationship

likely varies with the level of moisture stress. Plants operating at adequate

soil-moisture level, above some critical threshold, are likely to be more

responsive to varying amounts of ambient ozone. Plants operating under

moisture stress would not likely respond to varying ozone levels since

stomatal uptake of ozone would be limited.

The considerable distance between the Moosehorn NWR and the ozone

monitor at Acadia NP may also have obscured any relationship between

ambient ozone levels and incidence of plant injury that occurred within the

refuge. More long-term databases, involving field observations, are needed

to accurately relate ozone-induced injury with ambient levels of ozone.

Nevertheless, if the SUM60 ozone levels measured at Acadia National

Park, 113 km southwest from the Moosehorn NWR, were comparable to

those ozone levels reaching the refuge, then the threshold level for ozone

to induce symptoms on sensitive plants within the Moosehorn refuge was

likely near 10,000 ppb-hrs. This indicates that ozone injury might have

occurred on sensitive plants in the refuge by early June in 1998, and by late

June to early July in the other survey years (Fig. 2). Threshold ozone

concentrations could have exceeded phytotoxic levels as early as late June–

early July. If so, ozone might have injured sensitive species within the

Moosehorn refuge more than a month before most ozone injury surveys,

which are normally conducted in mid- to late-August. Results of the first

survey, conducted during July 29–August 2, 1998, supports this hypothesis,

since considerable ozone injury had already occurred within the Moosehorn

NWR by late July of that high-ozone year.

Given that high ozone levels occurred in 1998, it is possible that ozone

injury occurred in the refuge as early as May of that year. Plant species
2007 D.D. Davis 413

emerging and completing their life cycles early in the growing season, such

as late-spring or early-summer ephemerals, might be at risk in Maine in early

to mid-summer when SUM60 ozone levels exceed 10,000 ppb-hrs (Fig. 2).

Ozone injury on these ephemeral plants would likely go undetected during

August, when most ozone-injury surveys are conducted in the East, and

might represent an undetected threat to natural ecosystems (Davis

and Orendovici 2006). If the threshold value of ozone needed to cause injury

on such species is less than 10,000 ppb-hrs, then injury could occur even

earlier on sensitive plants in the refuge.

The findings of the present study revealed that bioindicators within the

Moosehorn NWR exhibited ozone-induced symptoms during each survey

year (1998–2000, 2002–2004). Eckert et al. (1999) reported that injury from

ambient ozone was observed on sensitive vegetation within the Acadia NP

during 1995–1997. Therefore, if ambient ozone levels at the Moosehorn

NWR are similar to those monitored at Acadia NP, it is likely that the

ambient ozone levels at the Moosehorn NWR are high enough in most years

to cause injury on sensitive refuge plants, including those growing in the

Class-I wilderness area. The US Fish and Wildlife Service can use the results

of these surveys when making air-quality management decisions, including

those related to review of Prevention of Significant Deterioration permits,

and such data can be used to strengthen our National Ambient Air Quality

Standards for ozone (US EPA 1996).

Acknowledgments

The author gratefully acknowledges receiving financial support from the US Fish

and Wildlife Service, Air Quality Branch, Denver, CO, and acknowledges receiving

ozone datasets from Mr. David Joseph, National Park Service, Denver, CO.

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