Air Quality

Concerns

Related to

Snowmobile

Usage in

National Parks

U.S. Department of the Interior • National Park Service

Air Resources Division • Denver, Colorado

February 2000

U.S. Department of the Interior • National Park Service

Air Resources Division • Denver, Colorado

Air Quality Concerns Related to

Snowmobile Usage in National Parks

February 20001

Background

National Park Service (NPS) regulations (36 CFR Part 7 and 36 CFR Part 13) allow

snowmobile use in more than 40 of the 378 units of the National Park System (see

Appendix A, Table 1). The amount of use, and associated impact, varies greatly among

units. For example, in the winter of 1998-1999, only about 10 snowmobile groups

entered Mount Rainier National Park (NP). On the other end of the spectrum, during the

same time period, more than 76,000 tourists visited Yellowstone NP on roughly 63,000

snowmobiles, with nearly 54,000 visitors travelling via a single corridor from the West

Yellowstone entrance in Montana. There are various environmental and health impacts

associated with the use of snowmobiles in national parks, including air quality impacts

from exhaust emissions. The Bluewater Network, a coalition of environmental

organizations, has petitioned the NPS to ban snowmobile use in units of the National

Park System. The NPS is currently considering the petition. The intent of this paper is to

present information regarding the potential air quality impacts associated with

snowmobile use in national parks. This information, in conjunction with other available

information, will enable the NPS to make informed decisions regarding the use of

snowmobiles in units of the National Park System.

This report summarizes available information and reports related to the air quality

impacts of snowmobiles. It is necessarily limited by the scope, methods, and content of

the studies cited. It provides a general overview of highly complex technical data.

Although it relies heavily on information obtained from studies conducted at Yellowstone

NP, it is not a complete compendium of the extensive studies that have been conducted

there and in other areas. The comparisons made using Yellowstone data could be viewed

as a high use scenario and apply only to Yellowstone. However, the results from

Yellowstone demonstrate the impacts on air quality that may result from heavy

snowmobile usage in an area. As the popularity of the sport grows, so will the potential

for air quality impacts on park resources, as will the need for park managers to carefully

consider these impacts in their decisions.

Snowmobile Emissions

Snowmobiles use two-stroke engines, in which induction of fresh fuel and expulsion of

exhaust take place simultaneously through openings in the cylinder. For a number of

reasons, two-stroke engines emit substantial amounts of air pollutants. First, up to onethird of the fuel delivered to the engine goes straight through without being burned.

Second, the lubricating oil is mixed with the fuel, and thus is expelled as part of the

 

1

 This is a revision of an October 1999 report. Appendix B discusses the changes between this report and

the previous version.
Air Quality Concerns Related to Snowmobile usage in National Parks, page 2 of 22

exhaust. Third, the combustion process results in high emissions of air pollutants. In

addition, snowmobiles and other gasoline-powered motor vehicles emit several

pollutants, such as benzene, 1,3-butadiene and gas-phase and particle-phase polycyclic

aromatic hydrocarbons (PAHs) that the U.S. Environmental Protection Agency (EPA)

classifies as known or probable human carcinogens. EPA estimates that mobile sources

account for as much as half of all cancers attributed to outdoor sources of air toxics (U.S.

EPA 1994a). Although “air toxics” can be defined simply as air pollutants that cause

adverse health effects, in the context of this report they mean toxic organic compounds.

Emissions of air toxics from snowmobile engines were found in significant amounts and

in proportions similar to those found in gasoline-fueled passenger cars in testing

performed by Southwest Research Institute (SwRI) (White and Carroll 1998). Numerous

air toxics are also present in diesel exhaust, which contains over 40 substances that are

listed by EPA as hazardous air pollutants and by the California Air Resources Board as

toxic air contaminants (California Air Resources Board 1998).

There are various sources of information on snowmobile emissions with the most recent

being those prepared for the State of Montana Department of Environmental Quality and

the International Snowmobile Manufacturers Association by SwRI (White and Carroll

1998) and for the Yellowstone Park Foundation (Carroll and White 1999). Table 1 below

lists these emission factors and compares them with factors previously published by EPA

and the California Air Resources Board. There is some variability in these factors due

primarily to the test procedure used on the engines, the fuels used, and the engine

lubrication oil. As also shown in Table 1 below, four-stroke engines emit substantially

less hydrocarbons (HC) and particulate matter (PM), about the same amount of carbon

monoxide (CO), but more nitrogen oxides (NOx) than 2-stroke engines. However, 4-

stroke engines are virtually non-existent in today’s snowmobile market. And although

there are efforts to develop a clean snowmobile, there are no current regulations driving

the development and widespread use of these engines. Similarly, the introduction of

lower-emitting, direct-injection 2-stroke engines in snowmobiles will likely not occur

quickly because there are no current regulations requiring the reduction in emissions from

snowmobiles.

When compared to various automobile emission estimates, a snowmobile operating for 4

hours, using a conventional 2-stroke engine, can emit between 10 and 70 times more CO

and between 45 and 250 times more HC than an automobile driven 100 miles (see

Appendix A, Table 4). A similar comparison of results from tests also conducted by the

University of Denver (DU) showed HC and CO emission factors from snowmobiles to be

about 80 and 7 times greater, respectively, than emission rates for the 1995-1996 Denver

automobile fleet (Bishop et al. 1999). However, when one takes into account the

difference in fuel efficiency between snowmobiles and automobiles, the ratios calculated

using the Bishop et al. factors are very similar to those cited above. There are many

reasons for the difference in these ratios, including the use of different testing methods

and procedures. The variability in these factors between and within tests underscores

some of the uncertainty in determining the actual emissions from the various source

categories. This is true for emission factors for automobiles as well as other mobile

sources.
Air Quality Concerns Related to Snowmobile usage in National Parks, page 3 of 22

As with automobiles, emissions of HC, CO and PM can increase if snowmobile engines

are not properly tuned for local conditions. The “SwRI 98 Polaris Rich” test (see Table 1

below) was conducted to simulate emissions from snowmobiles that are tuned for lower

elevations but used in higher elevations, such as Yellowstone. University of Denver

results also showed a wide range in CO emissions depending on engine manufacturer and

between fan-cooled and liquid-cooled engines. Unlike automobiles, where a relatively

small number of older cars emit at much higher rates than newer cars, snowmobile

emissions tended to be more clustered around the mean and were approximately normally

distributed.

Table 1. Snowmobile Emission Factors

Source of Emission Factor Emission Factor, grams/hp-hr

2-stroke engines HC NOx CO PM

1. EPA NEVES, 1991 228 1.7 321 4.8

2. EPA, 1999 154 0.5 390 2.4

3. CARB 109 1.7 169 4.8

4. SwRI 98 Polaris Base 151 0.4 416 1.3

5. SwRI 98 Polaris Rich 180 0.3 474 1.7

6. SwRI 98 Polaris Gasohol 127 0.4 377 1.0

7.SwRI 98 Polaris Gasohol/Bio Lube 104 0.4 332 1.7

8. SwRI 98 Polaris Aliphatic 201 0.5 424 2.0

9. SwRI 98 Arctic Cat Gasoline 156 0.5 363 3.5

10. SwRI 98 Arctic Cat Gasohol 164 0.5 342 3.4

11. SwRI 99 Polaris BASE 116 0.7 376 0.7

12. SwRI 99 Polaris BIO 120 0.7 391 1.4

4-stroke engines

13. EPA 4-stroke overhead engine 5.2 3.5 409 0.04

14. CARB 4-stroke <= 25 hp 4 1.8 240 0.09

15. CARB 4-stroke >25 hp 2 3 150 0.00

1. Nonroad Engine and Vehicle Emission Study, U.S. EPA Report No. 21A-2001, 1991

2., 13. Exhaust Emission Factors for Nonroad Engine Modeling—Spark Ignition, U.S. EPA Report No. NR-010b

(EPA420-R-99-009), 1999

3. California Air Resources Board (as reported by U.S. EPA in Exhaust Emission Factors -- Spark-Ignition,

 NR-010a (revision posted 6/19/98)—“compare.xls”)

4-10. Emissions from Snowmobile Engines using Bio-based Fuels and Lubricants, White and Carroll, 1998

11-12. “Characterization of Snowmobile Particulate Emissions,” Carroll and White, 1999

14-15. Emission Factors files (*.emf) used for Nonroad model at: http://www.epa.gov/oms/nonrdmdl.htm

In a similar study conducted by DU in the winter of 1999 to investigate the effects of

oxygenated (ethanol-blended) fuels on snowmobile emissions, the results showed that a

reduction in CO of 7 + 4% could be achieved in Yellowstone using oxygenated fuels.

However, oxygenated fuels did not appear to reduce HC emissions. This study also

attempted for the first time the real-time remote sensing of toluene, a component of

gasoline. Results from data collected at the West Entrance station over a 2-day period

showed a mean toluene concentration of 1976 ppm in snowmobile exhaust (Morris et al.

1999). The toluene to HC ratio measured in this study was consistent with toluene

content of non-oxygenated fuels.
Air Quality Concerns Related to Snowmobile usage in National Parks, page 4 of 22

Snowmobile emissions within a national park will vary based on numerous factors.

These factors include the number of machines entering the park, the number of hours

used, the type of engines and fuels used, engine speed, elevation, and ambient

temperature at which the machines are operated, and how well the engines are tuned. In

general, emissions increase with an increase in engine speed (rpm); direct-injection 2-

stroke and 4-stroke engines emit less HC and PM than conventional 2-stroke engines; and

oxygenated fuels can reduce the levels of certain pollutants. Carbon monoxide emissions

decrease with decreasing temperature because the mass of oxygen will increase for a

given volume of air at lower temperatures. Also, as the air/fuel ratio decreases from

stoichiometric the formation of CO increases due to the decrease of oxygen (Morris et al.

1999).

Yellowstone National Park: An Air Pollution Emissions Case Study

The use of snowmobiles at Yellowstone has grown substantially over the years. Since

1992 snowmobile use was highest in 1993 when more than 77,000 snowmobiles entered

the park. In 1999 (December 1998 – March 1999) 62,878 snowmobiles entered the park,

which is less than the average of 66,619 snowmobiles that entered the park between 1992

and 1999.

The snowmobile season lasts approximately three months at Yellowstone NP from midDecember through mid-March. Table 3 of Appendix A provides vehicle count statistics

for the period 1992-1999. Vehicle count and emission factor information can be used to

estimate total annual emissions for the different types of vehicles entering the park.

Although the number of snowmobiles is on average a factor of 16 lower than the number

of automobiles entering the park annually (1,075,295 cars vs. 66,619 snowmobiles),

snowmobile emissions can equal or exceed total annual emissions for CO and HC from

other mobile sources (i.e., cars, RVs, buses and snow coaches). Depending on which

emission factors and assumptions are used, estimates for total emissions from different

mobile sources can vary substantially. Figure 1 below shows how this variability in

emission factors influences estimates of total emissions for HC and CO from automobiles

and snowmobiles. From Figure 1 below, one also notes that total estimated HC

emissions based on SwRI and DU emission factors are similar but there is a factor of 2

difference in estimated CO emissions. Also one notes that EPA’s Mobile 5 emissions

model results in substantially higher HC and CO emissions than those using either EPA’s

1996 model year certification standards (EPA FTP) or the DU remote-sensing

measurements for the Denver automobile fleet. This report does not attempt to reconcile

the differences noted.

As shown in Table 2 below, the contribution from snowmobiles to total annual HC

emissions from all mobile sources can range from 68% to 90% at Yellowstone depending

on which emission factors are used to estimate emissions. Similarly, snowmobiles can

contribute from 35% to 68% of total CO annual emissions. These estimates are based on

the assumptions that each snowmobile is operated on average for 4 hours at 16

horsepower (hp) (or consumes 11 gallons of fuel per visit) and snow coaches travel an

average of 100 miles per visit. Cars, buses, and RVs are assumed to travel 120 miles

during each visit to the park. Snowmobiles are estimated to contribute about 2% and
Air Quality Concerns Related to Snowmobile usage in National Parks, page 5 of 22

39% of annual NOx and PM emissions, respectively, at the park. Although the numbers

of snow coaches and buses entering the park annually average 1,400 and 5,800,

respectively, emissions from these vehicles are estimated to contribute on average 1% or

less of CO and HC annual emissions (see Appendix A, Tables 5 and 6).

Figure 1. Comparison of estimated annual HC and CO emissions at Yellowstone National

Park based on various emission factors for automobiles and snowmobiles.

Using SwRI’s emission factors, as much as 765 tons of HC and 2,100 tons of CO can be

emitted in a single season (1993) at Yellowstone (see Appendix A, Tables 5 and 6). This

is equivalent to several major stationary sources being located within the park. On an

average day with 700 snowmobiles entering the park, approximately 7 and 19 tons of HC

and CO, respectively, can be emitted. On a peak day with 2,000 snowmobiles entering

the park, about 20 and 54 tons of HC and CO, respectively, can be emitted (see Appendix

A, Table 4). On an average July day automobiles can emit2, by the most conservative

estimate, as much as 2.5 tons of HC and 17.9 tons of CO.

 

2

 The maximum number of automobiles entering Yellowstone NP during any month between December

1991 and May 1999 was 285,637. An average on a July day was 9,214 automobiles. Mobile 5 emission

factors are used to estimate emissions.

 

0

500

1000

1500

2000

2500

HC CO

Auto (EPA Mobile 5) Auto (DU) Auto (EPA FTP)

Snowmobile (SwRI) Snowmobile (DU)
Air Quality Concerns Related to Snowmobile usage in National Parks, page 6 of 22

Table 2. Estimated Average Annual Emissions from Mobile Sources, Yellowstone

National Park, in tons

Hydrocarbons (HC)

Snowmobile Emission Factor /

Automobile Emission Factor

Autos RVs Snowmobiles Snowcoaches Buses Total

SwRI / Mobile 5 286 19 661 3 4 973

29% 2% 68% <1% <1%

SwRI / EPA FTP 58 19 661 3 4 745

8% 3% 89% <1% 1%

DU / DU 93 19 780 3 4 899

10% 2% 87% <1% <1%

DU / EPA FTP 58 19 780 3 4 864

7% 2% 90% <1% <1%

Carbon Monoxide (CO)

Autos RVs Snowmobiles Snowcoaches Buses Total

SwRI / Mobile 5 2089 337 1814 25 15 4281

49% 8% 42% 1% <1%

SwRI / EPA FTP 483 337 1814 25 15 2675

18% 13% 68% 1% 1%

DU / DU 1335 337 939 25 15 2652

52% 13% 35% 1% 1%

DU / EPA FTP 483 337 939 25 15 1800

27% 19% 52% 1% 1%

Other pollutants emitted from gasoline-powered vehicles include sulfur dioxide (SO2),

ammonia, and PAHs. Limited testing performed by SwRI showed SO2 emissions to be

low (0.04 g/hp-hr) due to low sulfur content in gasoline (White and Carroll 1998).

Ammonia emissions were also found to be low (0.075 g/hp-hr). Southwest Research

Institute also conducted hydrocarbon speciation tests, including four compounds

classified by EPA as air toxics (1,3-butadiene, benzene, formaldehyde, and acetaldehyde)

and numerous PAHs (White and Carroll 1998). The results of tests on a gasoline-fueled

Polaris engine showed that 1,3-butadiene, benzene, formaldehyde, and acetaldehyde were

present at levels of 0.14, 0.80, 0.64, and 0.10 percent, respectively, of total HC emissions.

Of the four compounds, benzene3 had the largest emission rate at 1.26 g/hp-hr. The

same study gives an emission rate for toluene of 31.78 g/hp-hr. White and Carroll also

reported that in terms of total mass, 61% of PAHs were emitted in the particle phase with

the remaining 39% in the vapor phase. Eighteen PAHs identified by SwRI had a

combined emission rate4 of 3643 μg/hp-hr (equivalently, for example, 58.3 mg/hr at 16

hp), with 22% (787 μg/hp-hr) of the PAHs being in the form of pyrene. This equates to 1

 

3

 Average of tests conducted on Polaris (1.69 g/kW-hr) and Arctic Cat (1.05 g/kW-hr) snowmobiles, as

reported in Table 12, White and Carroll 1998.

4

 Sum of Total PAHs appearing in Table 14, White and Carroll 1998, converted to g/hp-hr (1 g/kW-hr =

0.7457 g/hp-hr).
Air Quality Concerns Related to Snowmobile usage in National Parks, page 7 of 22

pound of PAHs, 355 pounds of benzene and 8,960 pounds of toluene being emitted on a

peak day (2,000 snowmobiles) at Yellowstone. Aromatic hydrocarbons, including PAHs,

are considered to be the most toxic component of petroleum products, and are also

associated with chronic and carcinogenic effects (Irwin et al. 1998).

Emission factors and calculations used for this section are found in Appendix A.

Air Quality and Personal Exposure Standards

Maximum allowable limits, or standards, for various air contaminants have been

established by the EPA, and subsequently by states, for concentrations in the ambient air.

Ambient air is defined as “that portion of the atmosphere, external to buildings, to which

the general public has access” (Code of Federal Regulations, Part 50, 1994). EPA has

established national ambient air quality standards (NAAQS) for six pollutants: CO, SO2,

nitrogen dioxide (NO2), ozone, particulate matter less than 10 microns in size (PM10), and

lead. (Note that EPA recently promulgated a standard for PM less than 2.5 microns in

size (PM2.5), but the standard has been challenged and its implementation is being

delayed). The NAAQS are set to protect the general public from the harmful effects of air

pollutants within an adequate margin of safety. These standards apply only in those areas

where the general public has access and do not apply in the workplace. Compliance with

these standards is determined through the use of EPA-designated reference or equivalent

analytical methods at appropriate sampling locations representative of human exposure.

Non-compliance with the NAAQS requires states to take appropriate measures to reduce

air pollution emissions sufficient to attain and maintain the NAAQS. Table 3 lists the

NAAQS for the various pollutants, as well as the Prevention of Significant Deterioration

(PSD) Class I and Class II increments. The relevance of the PSD increments will be

discussed in a later section.

In addition to EPA’s ambient standards, the Occupational Safety and Health

Administration (OSHA) has established Permissible Exposure Limits (PELs) for

numerous air contaminants to protect workers in the workplace. PELs are usually timeweighted average concentrations that must not be exceeded during any 8-hour workshift

of a 40-hour workweek. A short-term exposure limit (STEL), or ceiling value, is usually

measured over a 15-minute period and reflects the exposure limit that should not be

exceeded at any time during a workday. National Institute for Occupational Safety and

Health (NIOSH) also has published Recommended Exposure Levels (RELs) for timeweighted average exposures as well as short-term (15-minute) ceiling concentrations not

to be exceeded.

Occupational exposure standards for various pollutants present in snowmobile exhaust

are given in Table 4.
Air Quality Concerns Related to Snowmobile usage in National Parks, page 8 of 22

Table 3. National Ambient Air Quality Standards and PSD Increments, micrograms per

cubic meter (parts per million)

Pollutant Averaging time NAAQS

PSD Increments

(Class I/Class II)

Carbon Monoxide (CO) 1 hour1

8 hour1

40,000 (35)

10,000 (9)

None

None

Particulate Matter, less than 10

microns (PM10)

24 hour1

Annual Average

150

50

8/30

4/17

Particulate Matter, less than 2.5

microns (PM2.5)

24 hour1

Annual Average

65

15

None

None

Ozone (O3) 1 hour2

8 hour3

235 (0.12)

157 (0.08)

None

None

Sulfur Dioxide (SO2) 3 hour1, 4

24 hour1

Annual Average

1,300 (0.50)

365 (0.14)

80 (0.03)

25/512

5/91

2/20

Nitrogen Dioxide (NO2) Annual Average 100 (0.05) 2.5/25

Lead (Pb) Quarterly Average 1.5 None

1

 Not to be exceed more than once per year

2

 Standard attained when 3–year avg. number of expected exceedances is less than or equal to 1

3

 Standard attained when 3-year avg. of 4th highest daily maximum 8-hr concentration is less than 0.08

ppm. This standard is being challenged and its implementation is being delayed.

4

 Secondary standard set to protect public welfare effects

Table 4. Occupational Standards for Several Air Pollutants Present in Vehicle Exhaust

Pollutant Occupational Standards

 OSHA PEL NIOSH REL

Acetaldehyde 180 mg/m3 (100 ppm) 270 mg/m3 (150 ppm)

Benzene 3.26 mg/m3 (1 ppm) none

1,3-Butadiene 2,200 mg/m3 (1000 ppm) none

Benzo(a)pyrene none 0.1 mg/m3 (0.01 ppm)

Ethylbenzene 435 mg/m3 (100 ppm)

545 mg/m3 (125 ppm)(STEL)

same

Toluene 375 mg/m3 (100 ppm)

563 mg/m3 (150 ppm) (STEL)

same

Carbon monoxide 55 mg/m3 (50 ppm) 40 mg/m3(35 ppm)

OSHA PELs and NIOSH RELs are Time-Weighted 8-hour Averages; Short-Term Exposure Limits (STEL)

are 15-minute averages

Studies conducted at Yellowstone NP, as discussed previously and to be discussed in the

following sections, indicate that standards may have been exceeded at Yellowstone NP,
Air Quality Concerns Related to Snowmobile usage in National Parks, page 9 of 22

particularly on those days with peak snowmobile traffic. If this is the case, then

regulatory or NPS management action will be required to mitigate or eliminate these

impacts.

Potential Effects on Air Quality

Ambient and Personal Exposure Monitoring

Air quality studies at Yellowstone NP have shown that the accumulation of a large

number of snowmobiles in a small area, such as entrance stations and parking lots, can

result in short-term exposures to very high levels of CO, PM, and HC (including toxic

organics) (Air Resource Specialists, Inc. 1996, Kado et al. 1999). There is a direct

relationship between the number of snowmobiles and emissions and, consequently, air

quality levels. Meteorology also plays an important role in that cold, stable atmospheric

conditions with low wind speeds hinder the dispersion of air pollutants and allow

pollutants to accumulate in the immediate area of their release.

Prior to the establishment of an express lane in late 1995 at Yellowstone’s West Entrance

station, all snowmobiles entering the park were required to stop at one of the entrance

station kiosks to pay entrance fees and receive information about the park. This resulted

in long lines of snowmobiles and the accumulation of snowmobile emissions over a

relatively small area leading to high levels of CO near the kiosks, particularly on days

with poor dispersion conditions.

Grab sampling conducted at Yellowstone’s West Entrance station by the NPS during the

winter of 1995 showed that ambient CO levels could exceed 1-hour and 8-hour NAAQS

for CO (35 ppm and 9 ppm, respectively) in an area immediately adjacent to entrance

station booths. Maximum CO concentrations of 55 ppm and 15.9 ppm (1-hour and 8-

hour averages, respectively) were measured in the vicinity of the entrance station. High

concentrations of CO were highly localized and decreased rapidly with increasing

distance from the source of emissions. Monitoring results indicated that CO levels

decreased by a factor of 10 within 25 meters of the source of emissions. The results also

showed that an hourly traffic count of 450 snowmobiles would likely result in

concentrations above the 1-hour NAAQS (35 ppm CO) within about 3 meters of the

entrance station kiosks. Recent monitoring conducted by the State of Montana using

EPA-approved protocols at Yellowstone’s West Entrance Station (Ugorowski 1999)

showed a maximum 8-hour CO concentration of 8.9 ppm, which almost equaled the

national 8-hour standard of 9 ppm. During the same sampling period, a 1-hour

concentration of 18.2 ppm was recorded. By comparison, the State of Montana 1-hour

standard for CO is 23 ppm not to be exceeded more than once per year. The State of

Montana Department of Environmental Quality performed a preliminary air dispersion

modeling analysis to simulate various scenarios discussed in Yellowstone’s Winter Use

Draft Environmental Impact Statement. The results showed that the No Action

alternative (i.e., current snowmobile usage), as well as a derivative of this alternative that

substituted oxygenated fuels and low emissions lube oil, could result in exceedances of

the 1-hour CO NAAQS (Cain and Coefield 1999).
Air Quality Concerns Related to Snowmobile usage in National Parks, page 10 of 22

Hourly levels of PM are likely to be correlated with hourly CO levels, as the source of

emissions is the same. The majority of PM emissions from snowmobiles have been

found to be of a size that are easily respirable and able to be delivered into the lung,

typically in the ultra-fine range less than 100 nanometers (Carroll and White 1999). In

addition, there are a number of toxic compounds that adhere to the emitted PM. PM10

sampling conducted by the University of California at Davis in 1999 showed an apparent

gradient in concentration from the town of West Yellowstone, Montana to Old Faithful,

with peak PM levels occurring at the West Entrance station. PM10 concentrations

(nominally 8 hours in duration) were higher at Old Faithful than in downtown West

Yellowstone and in a residential site in West Yellowstone for one day of sampling

(February 20, 1999) (Kado et al. 1999).

A study conducted at Grand Teton NP (Snook and Davis 1997) showed that CO levels

when trailing a single snowmobile ranged from 0.51-23.1 ppm at distances of 25-125 feet

at speeds of 10-40 mph. This report also cites that the average snowmobile group size is

8, thus riders trailing a large number of snowmobiles are likely to experience

significantly higher CO exposures.

High levels of air pollutants are not limited to outdoor exposures. An improperly

ventilated entrance station kiosk at Yellowstone’s West Entrance resulted in CO levels

approaching the level of the 8-hour NAAQS for CO (9 ppm), although the levels

measured were well within the OSHA’s 8-hour time-weighted average of 50 ppm.

Retrofitting the air intake system at the kiosk significantly reduced employee exposure.

Thus, properly ventilated entrance station booths are an important consideration in

protecting employee health.

Additional monitoring at Yellowstone NP has included both fixed-site and personal

exposure monitoring for PM and volatile organic compounds (VOCs) (Kado et al. 1999).

The maximum time-weighted concentration of PM2.5 at the West Entrance on a busy

President’s day weekend (i.e., February 13 and February 14, 1999) was 78 μg/m3 based

on 4-hour morning and afternoon samples. Morning 4-hour concentrations for February

13 and 14 were 116 and 112 μg/m3, respectively. The morning concentration (nominally

8:30 a.m. to 12:30 p.m.) represented approximately 80% of the exposure and coincides

with the time that approximately 90% of the snowmobiles entered the park on those days

(Kado et al. 1999). The 24-hour PM2.5 NAAQS is 65 μg/m3, however, because of the

difference in averaging times, concentrations measured at Yellowstone NP during the

Kado et al. study are not directly comparable to the standard. The time-weighted

exposure throughout the entire workshift was at or near 60 μg/m3 at the West Entrance

station (Kado et al. 1999). This study also showed that concentrations measured at

Madison and Old Faithful were lower than those at the west entrance station likely due to

these areas being more open and because snowmobilers turn off their snowmobiles on

arrival at these areas.

Personal exposure measurements of PM less than 4.0 microns in size (PM4.0), called

PerPM, were conducted on park employees in a variety of job categories. Results

indicated exposure varied by job category, with a snowmobile mechanic having the
Air Quality Concerns Related to Snowmobile usage in National Parks, page 11 of 22

greatest PerPM exposure, followed by employees who worked the West Entrance station,

and then snowmobile patrol rangers. West Entrance station employees who worked in

the express lane (with more snowmobiles) had double the PM exposure of those who

worked in the regular lanes. PerPM concentrations were measured at levels up to 160

μg/m3 (Kado et al. 1999). Again, these concentrations cannot be compared directly to the

NAAQS, because of differences in particle size and averaging time. However, all PM

collected was thought to be less than PM2.5.

Personal exposure measurements to VOCs were conducted for Yellowstone NP

employees during the winter of 1999 University of California at Davis study. A number

of toxic compounds including benzene, toluene, and formaldehyde were monitored (Kado

et al. 1999). Of the VOCs measured, toluene had the highest relative concentration. The

fixed-site VOC monitoring showed the same types of gasoline-associated toxic

compounds (e.g., benzene and toluene) at each location, but concentrations were highest

at the West Entrance. West Entrance station employees had the greatest VOC exposure,

followed by snowmobile patrol rangers, followed by a snowmobile mechanic. The study

showed that the concentration of benzene for some employees could approach the RELs

as established by NIOSH. The study supported “the hypothesis that Park employees and

the surrounding environment are exposed to high levels of many toxic pollutants as a

result of snowmobile use within the Park...” (Kado et al. 1999).

These monitoring results indicate levels of individual pollutants, including carcinogens

such as benzene, that result from snowmobile exhaust can be high enough to be a threat

to human health. Moreover, it is unknown what synergistic effects may arise from the

cumulative, simultaneous exposure to various air pollutants. Therefore, it is important

that employee exposure to exhaust gases be minimized. It would seem that visitors

would likewise want to minimize their exposure to what are likely unhealthy levels of air

pollution.

Health Effects Associated with Vehicle Emissions

Vehicle exhaust, including that from snowmobiles, contains numerous toxic compounds.

The effects of these compounds are well documented and are discussed briefly in this

section.

Carbon monoxide interferes with the oxygen carrying capacity of blood, resulting in lack

of oxygen to the body’s organs and tissues. The health threat from lower levels of CO is

most serious for those who suffer from cardiovascular disease, such as angina pectoris.

At much higher levels of exposure, CO can be poisonous, and healthy individuals can be

affected. Visual impairment, reduced work capacity, reduced manual dexterity, poor

learning ability, and difficulty in performing complex tasks are also associated with

exposure to elevated CO levels (U.S. EPA 1997) as are nausea, headaches, dizziness, and

death (Radtke 1997).

Particulate matter includes fine (PM2.5) and coarse (PM10) particles. Both coarse and fine

particles can accumulate in the respiratory system and are associated with numerous

health effects. Exposure to coarse particles is primarily associated with the aggravation

of respiratory conditions such as asthma. Fine particles are most closely associated with
Air Quality Concerns Related to Snowmobile usage in National Parks, page 12 of 22

such health effects as decreased lung function, increased respiratory symptoms and

disease, and premature death. Sensitive groups include the elderly, individuals with

cardiopulmonary disease, and/or asthma, and children (U.S. EPA 1997).

The effects of most concern associated with short-term exposures (less than 3 hours) to

NO2 at ambient levels seen in the U.S. include changes in airway responsiveness and

pulmonary function in individuals with pre-existing respiratory illnesses and increases in

respiratory illnesses in children (5-12 years old) (U.S. EPA 1997).

Exposure to air toxics contained in automobile and snowmobile exhaust can result in

numerous respiratory and neurological effects. In addition, several of these air toxics

have been shown to be carcinogenic. Acute exposure to acetaldehyde results in irritation

of the eyes, skin, and respiratory tract in humans. Erythema, coughing, pulmonary

edema, and necrosis may also occur and, at extremely high concentrations, respiratory

paralysis and death are possible. EPA classifies acetaldehyde as a probable human

carcinogen of low carcinogenic hazard.

Acute exposure to 1,3-butadiene results in irritation of the eyes and nasal passages, and

causes neurological effects such as blurred vision, fatigue, headache, and vertigo. There

is also a possible association between 1,3-butadiene and cardiovascular diseases. EPA

has classified this compound as a probable human carcinogen of medium carcinogenic

hazard.

Acute (short-term) inhalation exposure to benzene may cause drowsiness, dizziness,

headaches, and unconsciousness in humans. These symptoms stop when exposure

ceases. Exposure to benzene liquid and vapor may irritate the skin, eyes, and upper

respiratory tract. Increased incidences of leukemia have been observed in humans

occupationally exposed to benzene. The EPA and the International Agency for Research

on Cancer (IARC) have classified benzene as a human carcinogen based on sufficient

evidence from epidemiological studies (U.S. EPA 1994a; IARC 1987).

Acute exposure to ethylbenzene in humans results in respiratory effects, such as throat

irritation and chest constriction, irritation of the eyes, and neurological effects such as

dizziness. Ethylbenzene is not classifiable as a human carcinogen due to the limited

amount of information available.

No information is available on the acute effects of Polycyclic Organic Matter, including

PAHs, in humans. Chronic exposure to benzo(a)pyrene in humans has resulted in

dermatitis, photosensitization in sunlight, irritation of the eyes, and cataracts. Cancer is

the major concern from exposure. Epidemiological studies have reported increased lung

cancer in humans exposed to emissions containing PAHs. EPA has classified

benzo(a)pyrene as a probable human carcinogen of medium carcinogenic hazard (U.S.

EPA 1994b). The toxicological effects of many PAHs can also be found in the NPS

Environmental Contaminants Encyclopedia (Irwin et al. 1998).
Air Quality Concerns Related to Snowmobile usage in National Parks, page 13 of 22

Toluene is an air toxic that can cause irritation of the eyes and upper respiratory tract,

dizziness, headaches, anesthesia, and respiratory arrest. Acute and chronic exposures can

affect the central nervous system (CNS) producing CNS depression and loss of memory

(Irwin et al. 1998).

Volatile organic compounds and NOx, in the presence of sunlight, combine to form

ozone. However, since snowmobile emissions occur during winter when photochemical

production and ambient concentrations are at their minimum, the ozone-forming potential

of these emissions is likely to be minimal, as are the health effects associated with

exposure to ozone.

The Federal Clean Air Act Amendments of 1990 provided the option of using

oxygenated fuels during the winter months in approximately 40 areas throughout the U.S.

that were not meeting the NAAQS for CO. Methyl tertiary butyl ether (MTBE) and

ethanol are two of several fuel additives used in oxygenated and reformulated gasoline to

reduce CO emissions. Ethanol is used year-round in all gasoline-powered NPS vehicles,

including snowmobiles, at Yellowstone NP and it is sold at West Yellowstone gas

stations in the winter. Some small amounts of MTBE have been found in a few

snowpack samples collected at Yellowstone NP immediately adjacent to roadways. This

is thought to be the result of snowmobiles that have been transported with gasoline from

locations that use MTBE. The extent to which either ethanol or MTBE additives are used

in snowmobiles entering other NPS units is unknown, as is the occurrence of impacts

associated with these additives.

MTBE in the air can cause headaches, dizziness, and eye irritation. A report prepared for

the Legislature and Governor of California (Keller et al. 1998) states that 10% ethanol by

volume in conventional gasoline can decrease emissions of CO (13% reduction), VOC

(16% reduction), and benzene (11% reduction). However, it can also increase exhaust

emissions of NOx and acetaldehyde by 5% and 159%, respectively. Ethanol is known to

have developmental and neurotoxic effects, although the risk of exposure to ambient

levels is not clear (Keller et al. 1998).

Based on the types of air pollutants associated with snowmobile exhaust, as well as the

quantities in which they are emitted, the cumulative health effects of what could be a

highly concentrated, multi-pollutant mixture in the vicinity of snowmobiles are likely to

be high. This is particularly relevant to individuals who spend a significant amount of

time in the vicinity of snowmobile exhaust, such as areas of traffic congestion (entrance

stations, parking lots) and poor emission dispersion.

Welfare Effects Associated with Snowmobile Emissions

In addition to health effects there are several public welfare concerns associated with

snowmobile emissions. Research conducted in 1996 at Yellowstone NP by the U.S.

Geological Survey showed that concentrations of ammonium and sulfate in snow

positively correlated with snowmachine (i.e., snowmobile, multi-passenger snow coach,

and other mechanized over-snow vehicle) use, as levels of these pollutants generally

declined a short distance from snowpacked roads (Ingersoll et al. 1997). Subsequent
Air Quality Concerns Related to Snowmobile usage in National Parks, page 14 of 22

ammonium and sulfate sampling by the U.S. Geological Survey in 1998 was consistent

with the 1996 data. In addition, in 1998, the snowpack was sampled for the presence of a

number of organics. Data showed concentrations of organics, such as benzene, correlated

well with snowmachine use (Ingersoll 1999). Of primary concern is the accumulation of

HC and toxic organics in the snowpack. These pollutants may be flushed into nearby

streams and water bodies during the spring snowmelt. The fate of these pollutants in the

snowpack is currently under investigation in a study funded by the Yellowstone Park

Foundation. Potential water quality concerns from snowmobile use in parks have also

been addressed in a separate report (Hagemann and VanMouwerik 1999).

Releasing air pollutants near the ground under stable meteorological conditions with light

wind speeds can also result in hazes accumulating along trails--potentially impairing

visibility and degrading park scenic vistas. Section 169A of the Clean Air Act

established as a national goal “the prevention of any future, and the remedying of any

existing, impairment in mandatory class I Federal areas which impairment results from

manmade air pollution.” Visible emissions from motor vehicles, including snowmobiles,

may interfere with the attainment of this goal and adversely affect the visitor’s park

experience. The use of gasohol has reportedly reduced the haze associated with

snowmobile use in congested areas (White and Carroll 1998).

Status of Efforts to Reduce Snowmobile Emissions

Proposed Regulations

The EPA recently accepted comment on a Notice of Proposed Finding related to Control

of Emissions from New Nonroad Spark-Ignition Engines Rated above 19 Kilowatts and

New Land-Based Recreational Spark-Ignition Engines (U.S. EPA 1999a). The proposal

addresses two issues: (1) that nonroad spark-ignition engines rated above 19 kilowatts

(25 horsepower), as well as all land-based recreational nonroad spark-ignition engines,

cause or contribute to air quality nonattainment in more than one ozone or CO

nonattainment area; and (2) that PM emissions from these engines cause or contribute to

air pollution that may reasonably be anticipated to endanger public health or welfare.

The EPA modeled contributions to emissions from mobile sources and determined that

recreational spark-ignition engines, which includes snowmobiles, contribute 15 percent,

0.2 percent, 9 percent, and 2 percent of VOC, NOx, CO and PM emissions, respectively

(U.S. EPA 1999b). These engines are not currently subject to federal emission standards;

this action begins the process leading to potential promulgation of such standards. The

NPS Air Resources Division’s comments on the Notice of Proposed Finding urged EPA

to develop emission standards for snowmobiles and similar engines and to base the

standards on the use of stringent emission control technology (National Park Service

1999). Unfortunately, even if standards were adopted, these standards would likely not

take effect for a substantial number of years. For example, recent EPA emission

standards applicable to marine engines (including personal watercraft whose 2-stroke

engines are very similar to those used in snowmobiles) will reduce emissions by 75% by

the year 2025. In California, emissions reductions will occur more rapidly with a 75%

reduction scheduled by 2001 and 90% reduction by 2008. These reductions will likely be

achieved mostly through the use of direct-injection 2-stroke engines rather than through

the use of 4-stroke engines. Requiring snowmobile manufacturers to use direct-injection
Air Quality Concerns Related to Snowmobile usage in National Parks, page 15 of 22

2-stroke or 4-stroke engines would significantly reduce emissions from snowmobiles for

most pollutants. However, there are no current regulations requiring their use nor has

NPS encouraged or required their use systemwide to minimize potential air quality

effects.

Two-stroke versus Four-stroke Engines

The California Air Resources Board reports that a personal watercraft with a 2-stroke

engine, which is similar to ones used in snowmobiles, operated for seven hours produces

more smog-forming emissions than a 1998 passenger car driven 100,000 miles

(California Air Resources Board 1999). As previously stated, four-stroke engines emit

substantially lower amounts of HC and PM than 2-stroke engines. Direct-injection 2-

stroke engines are estimated to reduce HC emissions by 70% to 75% when compared to

conventional 2-stroke engines (White 1999). The use of catalysts could further reduce

emissions from 2-stroke engines. From an emissions standpoint, 4-strokes engines are

preferred over 2-stroke engines. For example, HC emission factors for a 4-stroke engine

can be a factor of 40 lower than for a 2-stroke engine whereas CO emissions are about

the same. On the other hand, NOx emissions from a 4-stroke engine can be, by some

estimates, as much as 10 times greater than those from a 2-stroke engine (see Table 1

above). However, NOx emissions can be as much as two orders of magnitude lower than

HC and CO emissions. Although emission reductions between a direct-injection 2-stroke

engine and a 4-stroke engine may not be as dramatic as between a 4-stroke engine and a

conventional 2-stroke engine, they can nevertheless be substantial. Thus, requiring the

use of cleaner engine technologies, including hybrid electric or battery electric

snowmobiles, within NPS units is a sensible way of reducing emissions and, hence, the

air quality impacts from snowmobiles. As stated previously, there is no impetus

requiring the adoption of newer, cleaner technologies, although according to the Montana

Department of Environmental Quality, three organizations have built electric

snowmobiles (Montana Department of Environmental Quality 1999c).

Oxygenated Fuels

Areas that do not meet the NAAQS often use oxygenated fuels as a way of reducing CO

emissions. Common fuel additives include MTBE and ethanol. The Montana

Department of Environmental Quality reports that measured ambient levels of CO in

Missoula have been reduced by 24.3 percent on average since the introduction of

oxygenated fuels (8% ethanol) (Montana Department of Environmental Quality 1999a).

Tests performed by SwRI (refer to Table 1 above) indicate substantial reductions in

emissions of total HC (16%), CO (9%) and PM (24%) over conventional gasoline.

Montana also reports that emissions of NOx and two known carcinogens (formaldehyde

and acetaldehyde) increase with the use of gasohol (10% ethanol).

Yellowstone NP has converted its entire unleaded gasoline-fueled winter fleet (including

106 snowmobiles) to gasohol since June 1998 in efforts to minimize emissions.

According to park officials, several gas stations in West Yellowstone began offering

ethanol-blended fuel and biodegradable or low-smoke oils in the winter of 1998-1999

(Yellowstone National Park 1999). Use of low-smoke lubricants can substantially reduce

PM emissions as well (Montana Department of Environmental Quality 1999b).
Air Quality Concerns Related to Snowmobile usage in National Parks, page 16 of 22

As mentioned previously, the winter 1999 study by the DU evaluated the effects of

oxygenated fuels (estimated at 8% ethanol) on snowmobile emissions using real-time

remote sensing (Morris et al. 1999). Oxygenated fuels were found to emit 7 ± 4% less

CO than engines using non-oxygenated fuels, with no apparent difference in the amount

of HC emitted.

Management Considerations and Options

In making decisions regarding snowmobile use in an NPS unit, managers must take into

consideration varying amounts of information regarding the potential effects that such

use might have on park natural resources and the health of its visitors and employees.

With respect to air quality impacts, the following aspects should be considered.

NPS Management Policies

NPS air resource management policies seek to perpetuate the best possible air quality in

parks because of its critical importance to visitor enjoyment, human health, scenic vistas,

and the preservation of natural systems and cultural resources (National Park Service

1988). These policies require managers to assume an aggressive role in promoting and

pursuing measures to safeguard air quality related values from the adverse impacts of air

pollution. Air pollution sources within parks must comply with all federal, state and local

regulations. NPS Management Policies are clear that “[I]n cases of doubt as to the

impacts of existing or potential air pollution on park resources, the Park Service will err

on the side of protecting air quality and related values for future generations.”

Existing Air Quality

NPS areas that do not meet the NAAQS or whose resources are already being adversely

affected by current ambient levels require a greater degree of consideration and scrutiny

by NPS managers. Areas that do not meet the NAAQS for any pollutant are designated

as nonattainment areas. Section 176 of the Clean Air Act states:

No department, agency, or instrumentality of the Federal Government shall

engage in, support in any way or provide financial assistance for, license or

permit, or approve, any activity which does not conform to an [State]

implementation plan...[T]he assurance of conformity to such a plan shall be an

affirmative responsibility of the head of such department, agency or

instrumentality.

Essentially, federal agencies must ensure that any action taken does not interfere with a

State’s plan to attain and maintain the NAAQS in designated nonattainment areas. In

making decisions regarding the use of snowmobiles within a designated nonattainment

area, park managers should discuss their plans with the appropriate State air pollution

control agency to determine the applicability of conformity requirements.

Notwithstanding an area’s attainment designation under the Clean Air Act, NPS areas

that have documented adverse effects due to air pollution must seek to mitigate or

eliminate these impacts. When a new industrial or other major stationary source of
Air Quality Concerns Related to Snowmobile usage in National Parks, page 17 of 22

pollution proposes to construct near an NPS unit where there are known adverse effects

occurring, the NPS routinely requests the State permitting authority to find ways to

offset proposed emissions increases that might contribute to existing adverse effects on

park resources with contemporaneous emission reductions from a different source in the

same area. At a minimum, NPS requests the most stringent control technologies be

applied to these sources. These same policy objectives should guide management of

pollution sources operating within park boundaries.

Clean Air Act Area Classifications

The Clean Air Act includes provisions to protect air quality and prevent air quality from

deteriorating in areas where the air is relatively clean. One of the purposes of this

“Prevention of Significant Deterioration (PSD)” program is “to preserve, protect, and

enhance [emphasis added] the air quality in national parks, national wilderness areas,

national monuments, national seashores, and other areas of special national or regional

natural, recreational, scenic or historic value.” (42 U.S.C. 7401 et seq.). Under the Clean

Air Act, the NPS, as a federal land manager, has an affirmative responsibility to protect

air quality related values, including visibility, from the adverse effects of air pollution in

areas that are designated as “Class I.” Class I areas include international parks, national

wilderness areas and national memorial parks in excess of 5,000 acres, and national parks

in excess of 6,000 acres that were in existence as of August 7, 1977 when the Clean Air

Act was amended. There are 48 Class I areas that are part of the National Park System

(see Appendix A, Table 1 for a list of the NPS Class I areas in which snowmobile use is

permitted). Congress intended that Class I areas be afforded the greatest degree of air

quality protection and specified that only very small amounts of air quality deterioration

be permitted. Congress also established a national goal of remedying existing man-made

visibility impairment and preventing future impairment in Class I areas.

NPS areas that are not designated as Class I are Class II, and the Clean Air Act allows

only moderate increases beyond baseline air quality concentrations in these areas. For

Class II areas, the NPS Organic Act, the Wilderness Act of 1964, and other

environmental statutes, policies, and regulations may provide a stronger basis for

protection than the Clean Air Act. NPS management policies direct park managers to

assume an aggressive role in protecting air quality and related values regardless of the

unit’s Clean Air Act designation.

Preventing Significant Air Quality Deterioration

One of the primary means for carrying out NPS responsibilities under the Clean Air Act

and otherwise preventing air quality deterioration is through the review of permit

applications for the construction or modification of stationary sources. Although the PSD

permitting requirements of the Clean Air Act apply only to stationary sources, it is clear

that the protection and enhancement of air quality in areas whose air quality is better than

the NAAQS is consistent with the purposes of PSD and NPS management policies.

NPS managers must also avoid engaging in or permitting activities that cause or

contribute to increases in pollutant concentrations that might exceed the maximum

allowable increases for the area. The Clean Air Act and PSD regulations specify that
Air Quality Concerns Related to Snowmobile usage in National Parks, page 18 of 22

only small increases in certain pollutants (i.e., PM, SO2, and NO2) above a “baseline”

concentration are allowed in Class I areas, while moderate increases are allowed in Class

II areas. The allowable increases, called increments, are generally a fraction of the

NAAQS (see Table 3 above). The baseline date for a particular pollutant is triggered

when the first complete PSD permit application in an area is submitted to the permitting

authority. After the baseline date is established, increases in emissions from all sources--

including mobile and minor sources--must be counted to assess whether the allowable

increments have been exceeded. Determining how much increment has been consumed

is a modeling, as opposed to a monitoring, exercise. Once modeling indicates the

increment has been totally consumed, there may be restrictions on new sources of air

pollution in the area. If emissions from a new major stationary source are predicted to

cause a violation of a Class I increment, the source may only be permitted if the federal

land manager certifies that there will be no adverse effect on air quality related values.

Otherwise, if an increment violation is discovered, states must develop additional

pollution control measures to remedy the violations. The latter is true for Class II areas

as well.

In parks where there has been a significant increase in snowmobile use since the late

1970s, increment consumption may be an issue. For example, PM baseline dates were

established in Yellowstone NP in 1979 by both the states of Montana and Wyoming.

Visitor use surveys at the park indicate the number of snowmobiles entering the park has

increased substantially over the years since the baseline date was established. Increased

emissions associated with increase usage must be counted as consuming the allowable

increment for PM. It is possible that emissions increases from snowmobiles, as well as

from other mobile sources, have consumed the entire 24-hour PM increment in the park.

Air Quality Levels and Employee and Visitor Health

There are two issues related to the potential effects of snowmobile emissions on human

health. The first issue pertains to the NPS’s responsibility to provide safe working

conditions. Park employees in a variety of job categories can be affected by snowmobile

emissions. The second issue is that NPS should also ensure that park visitors are not

exposed to potentially harmful levels of air pollution. A previous section regarding the

potential effects on air quality from snowmobile emissions described the potential

impacts on employee and visitor health that may arise from exposure to the various

constituents of snowmobile exhaust. An important consideration in decisions regarding

snowmobile use, therefore, is the need to minimize employee and visitor exposure to high

levels of air pollutants. This can be done by reducing either the duration or the level of

exposure. Today’s engine technologies (e.g., 4-stroke engines) have not been used

extensively in snowmobiles and there does not appear to be any real regulatory pressure

to produce a much cleaner snowmobile engine for widespread use. Therefore, it appears

that some of the quickest ways of reducing emissions in parks are to minimize the total

number of snowmobiles entering parks or to regulate the number entering parks at any

given time.
Air Quality Concerns Related to Snowmobile usage in National Parks, page 19 of 22

Management Options

As discussed above, emissions from 2-stroke snowmobile engines can be substantial.

These emissions are highly toxic and acute exposures to high concentrations of these

pollutants can affect the health of park visitors and employees. They can potentially

affect visibility and other air quality related values within the park. Therefore, existing or

potential air quality concerns should be factored into decisions regarding the use of

snowmobiles in units of the National Park System. While the NPS continues to

encourage states and the EPA to limit air pollution generated outside NPS units, the NPS

should demonstrate environmental leadership by reducing emissions generated within

NPS areas, by encouraging the snowmobile industry to produce cleaner engines, and by

promoting the use of cleaner snowmobiles by park visitors.

Management options to reduce emissions or the impacts on ambient air quality from

snowmobiles include: a complete ban on snowmobile use; the use of alternative, cleaner

modes of transportation; limiting the number and/or staggering the entrance of

recreational snowmobiles that are allowed in an NPS unit each day; limiting entrance to

visitors with lower-emitting snowmobiles (e.g., direct-injection 2-stroke or 4-stroke

engines only); use of oxygenated fuels (preferably ethanol blended fuels); and/or

ensuring that the accumulation of snowmobiles in small areas (e.g., entrance stations)

does not occur. The NPS should ensure that park fleets include newer, cleaner

snowmobiles and that these fleets use oxygenated fuels and low-emissions lubricants.

Finally, the NPS should also take other actions to minimize emissions from park fleets

including adjusting snowmobiles for local conditions and elevations, adjusting clutches,

using proper jets, keeping engines tuned, minimizing idling, and using biodegradable

oils.
Air Quality Concerns Related to Snowmobile usage in National Parks, page 20 of 22

References

Air Resource Specialists, Inc. 1996. Carbon Monoxide and Particulate Matter Levels at

Yellowstone National Park, West Entrance Station: Results of an Ambient Air Quality

Study, Winter 1995. Draft Report prepared for the National Park Service.

Bishop, G.A., D.H Stedman, M. Hektner, and J.D. Ray. 1999. An In-Use Snowmobile

Emission Survey in Yellowstone National Park. Environmental Science & Technology,

33: 3924-3926.

Cain, C.J. and J. Coefield. 1999. Preliminary Air Dispersion Modeling Analysis of

Yellowstone National Park West Entrance Wintertime Carbon Monoxide Emissions.

State of Montana Department of Environmental Quality.

California Air Resources Board. 1998. The Report on Diesel Exhaust, Findings of the

Scientific Review Board. Adopted at meeting April 22, 1998. (see

http://arbis.arb.ca.gov/regact/dieseltac/dieseltac.htm.)

California Air Resources Board. 1999. Fact Sheet -- New Regulations for Gasoline

Engines, http://www.arb.ca.gov/msprog/marine/marine.htm.

Carroll, J.N. and J.J White. 1999. Characterization of Snowmobile Particulate Emissions.

Final Letter Report. Prepared for the Yellowstone Park Foundation, Inc., by Southwest

Research Institute. Report number SwRI 08-2457. 20 p.

Code of Federal Regulations 1994. Title 40, Protection of the Environment, Part 50 (40 CFR

50). U.S. Government Printing Office, Washington, DC.

Hagemann, M. and M. VanMouwerik. 1999. Potential water quality concerns related to

snowmobile usage. Internal memo, National Park Service, Water Resources Division.

Ingersoll, G., J. Turk, C. McClure, S. Lawlor, D. Clow, and A. Mast. 1997. Snowpack

chemistry as an indicator of pollutant emission levels from motorized winter vehicles in

Yellowstone National Park. Presented at the 65th Annual Western Snow Conference, May

4-8, Banff, Canada. pp. 103-113.

Ingersoll, G. 1999. Effects of snowmobile use on snowpack chemistry in Yellowstone

National Park, 1998. U.S. Geological Survey. Water Resources Investigations Report 99-

4148.

International Agency for Research on Cancer (IARC). 1987. IARC Monographs on the

Evaluation of Carcinogenic Risks to Humans: Overall Evaluation of Carcinogenicity: An

Updating of IARC Monographs Volumes 1 to 42. Supplement 7. World Health

Organization. Lyon, France.
Air Quality Concerns Related to Snowmobile usage in National Parks, page 21 of 22

Irwin, R.J., M. VanMouwerik, L. Stevens, M.D. Seese, and W. Basham. 1998.

Environmental Contaminants Encyclopedia. National Park Service, Water Resources

Division, Fort Collins, Colorado. (see http://www.nature.nps.gov/toxic/index.html)

Kado, N.Y., P.A. Kuzmicky, and R.A. Okamoto. 1999. Measurement and Exposure to

Airborne Particulate Matter and Toxic Air Pollutants Emitted from Snowmobiles at

Yellowstone National Park. Interim Final Report prepared for the Yellowstone Park

Foundation, Inc., by University of California, Davis.

Keller, A. et al. 1998. Health and Environmental Assessment of MTBE: Report to the

Governor and Legislature of the State of California as Sponsored by SB 521, Volume I,

Summary and Recommendations. University of California, Davis. 63 p.

Montana Department of Environmental Quality. 1999a. The Snowmobile Dilemma, or, Who

Spilled What in the Refrigerator vs. Who’s Cleaning It Up? by Howard E. Haines, Montana

Department of Environmental Quality.

http://www.deq.mt.gov/ppa/p2/snowmobl/snowmobl.htm.

Montana Department of Environmental Quality. 1999b. Emissions from Snowmobile

Engines using Bio-based Fuels and Lubricants by J.J. White and J.N. Carroll, Southwest

Research Institute, and H.E. Haines, Montana Department of Environmental Quality.

http://www.deq.mt.gov/ppa/p2/biofuels/biofuels.htm

Montana Department of Environmental Quality. 1999c. November 15, 1999, letter from

Howard Haines, Bioenergy Engineering Specialist, Montana Department of

Environmental Quality, to John Sacklin, Yellowstone National Park.

Morris J.A., G.A. Bishop, and D.H. Stedman. 1999. Real-time Remote Sensing of

Snowmobile Emissions at Yellowstone National Park: An Oxygenated Fuel Study, 1999.

Prepared for Western Regional Biomass Energy Program, Lincoln, NE. University of

Denver, August 1999.

National Park Service. 1988. Management Policies. U.S. Department of the Interior,

National Park Service. Washington, D.C.

National Park Service. 1999b. June 4, 1999, letter from Christine Shaver, Chief Air

Resources Division, to U.S. Environmental Protection Agency Public Docket A-98-01.

Radtke, T. 1997. Industrial Hygiene Consultation Report. Report No. 970101. U.S.

Department of the Interior, Office of Managing Risk and Public Safety. Lakewood,

Colorado. 59 p.

Snook, L.M. and W.T. Davis. 1997. An investigation of driver exposure to carbon

monoxide while travelling in the wake of a snowmobile. Paper 97-RP143.02. Presented

at the Air and Waste Management Association’s 90th Annual Meeting and Exhibition,

June 8-13, Toronto, Ontario, Canada.
Air Quality Concerns Related to Snowmobile usage in National Parks, page 22 of 22

Ugorowski, J. 1999. Montana Department of Environmental Quality. Comments

presented May 10, 1999.

U.S. EPA. 1991. Nonroad Engine and Vehicle Emission Study—Report. EPA 21A-2001.

U.S. EPA, Office of Air and Radiation. Washington, D.C.

U.S. EPA. 1994a. Air Toxics from Motor Vehicles. EPA 400-F-92-004. U.S. EPA, Office

of Mobile Sources. Ann Arbor, Michigan. 3 p.

U.S. EPA. 1994b. Health Effects Notebook for Hazardous Air Pollutants. Review Draft.

Air Risk Information Center, Research Triangle Park, North Carolina. (see

http://www.epa.gov/ttn/uatw/hapindex.html)

U.S. EPA. 1997. National Air Quality and Emissions Trends Report, 1997. EPA 454/R98-016. U.S. EPA Office of Air Quality Planning and Standards. Research Triangle Park,

NC. 185 p.

U.S. EPA. 1999a. Regulatory Announcement. Proposed Finding on Emission Standards

for New Large Spark-Ignition Nonroad Engines. EPA420-F-99-004. U.S. EPA, Office of

Mobile Sources. Ann Arbor, Michigan. 3 p.

U.S. EPA. 1999b. Regulatory Update. EPA’s Nonroad Engine Emissions Control

Programs. EPA420-F-99-001. U.S. EPA, Office of Mobile Sources. Ann Arbor,

Michigan. 8 p.

U.S. EPA. 1999c. Exhaust Emission Factors for Nonroad Engine Modeling—Spark

Ignition. Report No. NR-010b, revised March 30, 1999. EPA420-R-99-009. U.S. EPA

Office of Mobile Sources. Ann Arbor, Michigan. 22 p.

White, J.J. and J.N. Carroll. 1998. Emissions from Snowmobile Engines Using Bio-based

Fuels and Lubricants. Prepared for the Montana Department of Environmental Quality, by

Southwest Research Institute. Report number SwRI 7383. 53 p.

White, J.J. 1999. Personal communication.

Yellowstone National Park. 1999. June 1, 1999, letter from Superintendent Michael Finley

to U.S. Environmental Protection Agency Public Docket A-98-01.

Relevant World Wide Web Sites

http://www.epa.gov/oms/nonroad.htm

http://www.nature.nps.gov/toxic/index.html
Air Quality Concerns Related to Snowmobile Usage in National Parks

Appendix A

A-1

Table 1. Snowmobile Use in NPS Units

NPS Unit(1) State(s) Class(2) Winter 1998-1999 Use 1995-1999 Trend

Acadia NP ME I Limited None

Aniakchak NM and NPres. AK II No data(4) No data

Appalachian National Scenic Trail Many II Limited(3) None

Bering Land Bridge NPres. AK II 6000 Unknown

Bighorn Canyon NRA MT, WY II None None

Black Canyon of the Gunnison NP CO I Limited None

Blue Ridge Parkway NC, VA II Limited None

Cape Krusenstern NM AK II 6750 Unknown

Cedar Breaks NM UT II No data No data

Crater Lake NP OR I No data No data

Curecanti NRA CO II Limited None

Delaware Water Gap NRA NJ, PA II Limited None

Denali NP and NPres. AK I Significant(5) Increasing

Dinosaur NM CO, UT II Limited No data

Gates of the Arctic NP and NPres. AK II Limited Increasing

Glacier Bay NP and NPres. AK II None None

Grand Portage NM MN II Limited None

Grand Teton NP WY I 3511(6) None

Herbert Hoover NHS IA II Limited None

John D. Rockefeller Jr. Memorial Parkway WY II 17160(6) Decreasing

Katmai NP and NPres. AK II Limited Unknown

Kenai Fjords NP AK II 807 Increasing

Klondike Gold Rush NHP AK II Limited None

Kobuk Valley NP AK II 7500 Unknown

Lake Chelan NRA WA II Limited None

Lake Clark NP and NPres. AK II Limited Increasing

Mount Rainier NP WA I Limited None

Noatak NPres. AK II 2250 Unknown

North Cascades NP WA I Limited None

Olympic NP WA I None None

Perry’s Victory and Int’l Peace Memorial MI II Limited None
Air Quality Concerns Related to Snowmobile Usage in National Parks

Appendix A

A-2

Pictured Rocks NS MI II 26190 Slight increase

Rocky Mountain NP CO I 27288 Slight increase

Ross Lake NRA WA II Limited None

Saint Croix NSR WI II Limited None

Sequoia and Kings Canyon NPs CA I Limited None

Theodore Roosevelt NP ND I Limited No data

Voyageurs NP MN I 35410 Increasing

Wrangell-St. Elias NP and NPres. AK II Limited Unknown

Yellowstone NP ID, MT, WY I 63000 None

Yukon-Charley Rivers NPres. AK II Limited Increasing

Zion NP UT I Limited None

(1)These NPS areas outside of Alaska are open to recreational snowmobile operation in accordance with special regulations found at 36

CFR Part 7.

Alaska NPS units listed permit snowmobile operation by local rural residents engaged in subsistence uses under 36 CFR Part 13.

(2)Clean Air Act designation—Class I areas are allowed less air quality deterioration than Class II areas

(3)Limited use means less than 200 snowmobiles a year enter the NPS unit or use is restricted to a small area of the park, e.g., access to

inholdings, trails on private land cross NPS property for a short distance, use on access road leading to NPS unit, etc.

(4)ARD did not receive snowmobile information for the NPS unit

(5)Snowmobile numbers for Denali NP not provided

(6)Unknown if numbers for Grand Teton NP and John D. Rockefeller Jr. Memorial Parkway represent total number of snowmobiles or

total number of passengers
Air Quality Concerns Related to Snowmobile Usage in National Parks

Appendix A

A-3

Table 2. Baseline Snowmobile Emissions using Gasoline, in grams/hp-hr

based on tests conducted by Southwest Research Institute (SwRI)

HC CO NOx PM

SWRI 98 Polaris 151 420 0.44 1.35

SWRI 98 Arctic Cat 156 363 0.50 3.45

SWRI 99 Polaris 116 376 0.69 0.70

Average 141 386 0.54 1.83

Note: The Arctic Cat test is for a liquid cooled engine, whereas the Polaris tests are for an

air cooled engine. The average reflected above is a straight average, although in the Bishop

et al. study only 28% of the snowmobiles entering Yellowstone at the West Entrance station

were liquid cooled.

Resulting emissions per snowmobile using above average and assuming 4 hours of operation at 16 hp.

Emissions (g)/4h @ 16 hp 9006 24719 27 92

Emissions (lbs)/4h @ 16 hp 19.84 54.45 0.06 0.20

Table 3. Vehicle Counts, Yellowstone NP

Year Autos RVs Snowmobiles Snowcoaches Buses Grand Total

1992 1,100,171 96,594 59,569 1,112 4,788 1,256,333

1993 1,019,325 68,197 77,108 1,704 4,159 1,164,630

1994 1,055,122 80,004 74,181 1,564 5,317 1,209,307

1995 1,102,611 81,408 74,859 1,579 5,202 1,258,877

1996 1,051,054 65,291 64,798 1,118 6,937 1,181,142

1997 1,044,550 56,261 59,446 1,293 6,566 1,160,256

1998 1,154,234 56,830 60,110 1,326 6,741 1,271,174

1999 incomplete 64,345 62,878 1,396 6,455 Incomplete

Average 1,075,295 71,116 66,619 1,387 5,771 1,214,531

Vehicle count data obtained from NPS Public Use Statistics Office, Denver, CO

Notes: Data summed from December of previous year to November of year indicated.

 Average number of autos based on 7-year average, 1992-1998; all others based on 8-year average, 1992-1999
Air Quality Concerns Related to Snowmobile Usage in National Parks

Appendix A

A-4

Table 4. Emission Factors Used in Calculations

Vehicle Type, Units HC CO NOx PM

Auto, g/mi 2.01 14.70 1.87 0.08 Note (1)

Auto, lbs per 100 miles 0.44 3.24 0.41 0.02

Auto, g/mi 0.41 3.40 0.40 0.08 Note (2)

Auto, lbs per 100 miles 0.09 0.75 0.09 0.02

Auto, g/mi 0.59 8.46 Note (3)

Auto, lbs per 100 miles 0.13 1.86

RV, g/mi 2.05 35.90 3.24 0.12 Note (4)

RV, lbs per 100 miles 0.45 7.91 0.71 0.03

Snowmobile, lbs per 4h visit 19.84 54.45 0.06 0.20 From Table 1, based on average of 5 SwRI tests

Snowmobile, g/kg 331 460 Note (5)

Snowmobile, lbs 22.27 30.95 Based on 11.2 gallons of fuel; 6 lbs per gallon

Coaches, g/mi 18.44 166.33 2.90 0.12 Note (6)

Coaches, lbs per 100 miles 4.06 36.64 0.64 0.03

Diesel Buses, g/mi 5.57 20.21 14.63 1.20 Note (7)

Diesel Buses, lbs per 100 miles 1.23 4.45 3.22 0.26

Snowmobiles Peak Day, tons 19.8 54.4 0.06 0.20 Based on 2,000 snowmobiles, SwRI factors

Autos, Avg Day on Peak Month 2.5 17.9 2.3 0.10 Based on 9,214 automobiles (Mobile 5 factors)

 Snowmobiles Avg Day, tons 6.9 19.1 0.02 0.07 Based on 700 snowmobiles, SwRI factors

Autos, Avg Day 1.4 9.9 1.3 0.05 Based on 5,120 automobiles (Mobile 5 factors)

Notes

(1): Auto Factors obtained using Mobile 5 Emissions model assuming 2/3 auto, 1/3 LD truck mix; PM factor is certification standard

(2): Factors are US EPA certification standards, Tier 1 (EPA420-B-98-001)

(3): Factors derived from University of Denver remote sensing tests for 95-96 Denver fleet; assuming 20 mpg fuel efficiency & 6 lbs per gallon of fuel (62 g CO/kg; 4.3 g HC/kg)

(4): RV factors are from Table 4.1B.2 of Appendix H, AP-42, Volume II for Heavy-Duty Gasoline-powered vehicles at High Altitude, 1990-1997 model years,

 50K miles, adjusted for avg fuel and including tampering; PM factor is certification standard for LD trucks > 5,750 lbs.

(5): University of Denver (DU) remote sensing tests (Bishop et. al.)

(6): HC,CO, and NOx factors from Tables 1.25, 1.32, 1.39, US EPA AP-42, Volume II, Appendix J for 1995 calendar year; PM factor is same as for RVs; Mobile 5 model results show

 the HC and CO factors to be conservative (i.e., greater than 80% to 150%, respectively).

(7): HC,CO, and NOx factors from Tables 1.25, 1.32, 1.39, US EPA AP-42, Volume II, Appendix J for 1995 calendar year; PM factor is certification standard from EPA420-F-97-014
Air Quality Concerns Related to Snowmobile Usage in National Parks

Appendix A

A-5

Table 5. Total HC Emissions (TPY), Yellowstone NP

Year Coaches Autos RVs Snowmobiles Buses

1992 2.26 292.79 26.13 590.85 3.52

1993 3.46 271.27 18.45 764.82 3.06

1994 3.18 280.80 21.64 735.79 3.91

1995 3.21 293.44 22.02 742.51 3.83

1996 2.27 279.71 17.66 642.72 5.11

1997 2.63 277.98 15.22 589.63 4.83

1998 2.69 307.17 15.37 596.22 4.96

1999 2.84 N/A 17.41 623.68 4.75 Total Emissions, tons

Average Emissions, all years 2.82 286.17 19.24 660.78 4.25 973.25

Snow Coach HC Emissions (tons) = (1,387 coaches)*(100 mi/coach)*(18.44 g/mi)/(454 g/lb)/(2000 lb/ton)= 2.82 tons

Auto HC Emissions (tons) = (1,075,295 vehicles)*(120 mi/vehicle)*(2.01 g/mi)/(454 g/lb)/(2000 lb/ton) = 286.17 tons

RV HC Emissions (tons) = (71,116 RVs)*(120 mi/RV)*(2.05 g/mi)/(454 g/lb)/(2000 lb/ton) = 19.24 tons

Snowmobile HC Emissions (tons) = (66,619 sleds)*(19.84 lb/sled)/(2000 lb/ton) = 660.78 tons(based on 4 hrs of operation @ 16 hp)

Bus HC Emissions (tons) = (5,771 vehicles)*(120 mi/vehicle)*(5.57 g/mi)/(454 g/lb)/(2000 lb/ton) = 4.25 tons

Table 6. CO Emissions (TPY), Yellowstone NP

Year Coaches Autos RVs Snowmobiles Buses

1992 20.37 2,137.10 458.26 1,621.71 12.79

1993 31.21 1,980.06 323.54 2,099.19 11.11

1994 28.65 2,049.60 379.56 2,019.51 14.20

1995 28.92 2,141.84 386.22 2,037.97 13.89

1996 20.48 2,041.69 309.76 1,764.07 18.53

1997 23.69 2,029.06 266.92 1,618.36 17.54

1998 24.29 2,242.12 269.61 1,636.44 18.00

1999 25.57 N/A 305.27 1,711.80 17.24 Total Emissions, tons

Average Emissions, all years 25.40 2,088.78 337.39 1,813.63 15.41 4,280.62

Snow Coach CO Emissions (tons) = (1,387 coaches)*(100 mi/coach)*(166.33 g/mi)/(454 g/lb)/(2000 lb/ton) = 25.40 tons

Auto CO Emissions (tons) = (1,075,295 vehicles)*(120 mi/vehicle)*(14.70 g/mi)/(454 g/lb)/(2000 lb/ton) = 2,088.78 tons

RV CO Emissions (tons) = (71,116 RVs)*(120 mi/RV)*(35.90 g/mi)/(454 g/lb)/(2000 lb/ton) = 337.39 tons

Snowmobile CO Emissions (tons) = (66,619 sleds)*(54.45 lb/sled)/(2000 lb/ton) = 1,813.63 tons (based on 4 hrs of operation @ 16 hp)

Bus CO Emissions (tons) = (5,771 vehicles)*(120 mi/vehicle)*(20.21 g/mi)/(454 g/lb)/(2000 lb/ton) = 15.41 tons
Air Quality Concerns Related to Snowmobile Usage in National Parks

Appendix A

A-6

Table 7. NOx Emissions (TPY), Yellowstone NP

Year Coaches Autos RVs Snowmobiles Buses

1992 0.36 272.14 41.36 1.77 9.26

1993 0.54 252.14 29.20 2.29 8.04

1994 0.50 261.00 34.26 2.21 10.28

1995 0.50 272.74 34.86 2.23 10.06

1996 0.36 259.99 27.96 1.93 13.41

1997 0.41 258.38 24.09 1.77 12.70

1998 0.42 285.51 24.33 1.79 13.03

1999 0.45 N/A 27.55 1.87 12.48 Total Emissions, tons

Average Emissions, all

years

0.44 265.99 30.45 1.98 11.16 309.58

Snow Coach NOx Emissions (tons) = (1,387 coaches)*(100 mi/coach)*(2.9 g/mi)/(454 g/lb)/(2000 lb/ton) = 0.44 tons

Auto NOx Emissions (tons) = (1,075,295 vehicles)*(120 mi/vehicle)*(1.87 g/mi)/(454 g/lb)/(2000 lb/ton) = 265.99 tons

RV NOx Emissions (tons) = (71,116 RVs)*(120 mi/RV)*(3.24 g/mi)/(454 g/lb)/(2000 lb/ton) = 30.45 tons

Snowmobile NOx Emissions (tons) = (66,619 sleds)*(0.06 lb/sled)/(2000 lb/ton) = 1.98 tons (based on 4 hrs of operation @ 16 hp)

Bus NOx Emissions (tons) = (5,771 vehicles)*(120 mi/vehicle)*(14.63 g/mi)/(454 g/lb)/(2000 lb/ton) = 11.16 tons

Table 8. PM Emissions (TPY), Yellowstone NP

Year Coaches Autos RVs Snowmobiles Buses

1992 0.01 11.63 1.53 6.02 0.76

1993 0.02 10.78 1.08 7.79 0.66

1994 0.02 11.16 1.27 7.49 0.84

1995 0.02 11.66 1.29 7.56 0.82

1996 0.01 11.11 1.04 6.54 1.10

1997 0.01 11.04 0.89 6.00 1.04

1998 0.01 12.20 0.90 6.07 1.07

1999 0.01 N/A 1.02 6.35 1.02 Total Emissions, tons

Average Emissions, all years 0.01 11.37 1.13 6.73 0.92 20.14

Snow Coach PM Emissions (tons) = (1,387 coaches)*(75 mi/coach)*(0.12 g/mi)/(454 g/lb)/(2000 lb/ton) = 0.01 tons

Auto PM Emissions (tons) = (1,075,295 vehicles)*(120 mi/vehicle)*(0.08 g/mi)/(454 g/lb)/(2000 lb/ton) = 11.37 tons

RV PM Emissions (tons) = (71,116 RVs)*(120 mi/RV)*(0.12 g/mi)/(454 g/lb)/(2000 lb/ton) = 1.13 tons

Snowmobile PM Emissions (tons) = (66,619 sleds)*(0.2 lb/sled)/(2000 lb/ton) = 6.73 tons (based on 4 hrs of operation @ 16 hp)

Bus PM Emissions (tons) = (5,771 vehicles)*(120 mi/vehicle)*(1.2 g/mi)/(454 g/lb)/(2000 lb/ton) = 0.92 tons
B-1

APPENDIX B

Explanation of Changes Made to Original Report (October 1999)

on “Air Quality Concerns Related to Snowmobile Usage in National Parks”

1. Snowmobile Use Assumptions. In the original report the estimates for annual emissions

from snowmobiles were based on the assumption of 4 hours of operation at 25 hp. These

estimates were considered reasonable based on travel distances to popular destinations at the

park from the different entrance stations and from discussions with Southwest Research

Institute. For example, the round-trip distance to Old Faithful from the west, south, and north

(Mammoth) entrance stations is 60, 80, and 106 miles, respectively. Assuming a travel speed

of 25 mph, and allowing for some incidental use while in the park, 4 hours seemed

reasonable. Reviewers pointed out that the total emissions being projected for each 4-hour

visit based on this assumption would require the use of at least 18 gallons of fuel, based on a

fuel density of 6 lbs. per gallon. This was considered to be too high by a factor of 2. It is

believed that on average, between 9 and 11 gallons of fuel are used per snowmobile during a

visit to Yellowstone. Although there are no statistics available indicating what the actual fuel

consumption for snowmobiles is at Yellowstone, the above range of fuel consumption is

considered reasonable based on total gasoline sales in the park, the number of snowmobiles

entering the park, and the estimated total miles traveled based on visitor use surveys

contained in Yellowstone’s Winter Use DEIS. Also, a snowmobile tank has an 11-13 gallon

capacity, and outfitters report that rentals are returned with 1 to 2 gallons of fuel left in the

tank.

2. Total Snowmobile Emissions. Based on 1. above, assuming 4 hours of operation at 16 hp

would bring the estimate of emissions more in line with what is likely to occur. This resulted

in the total amount of emissions attributable to snowmobiles on an annual basis being

reduced by approximately one-third. The original report stated that snowmobiles emitted as

much as 1,195 tons of HC and 3,270 tons of CO annually. The revised report changes these

estimates to 765 tons of HC and 2,100 tons of CO.

3. Snowmobile Emission Factors. The emission rate used in the revised report is based on the

average of five tests conducted by Southwest Research Institute using conventional gasolines

rather than the average of all emission factors from all studies. This resulted in some minor

changes in the snowmobile emission factors used, as follows:

Changes to Snowmobile Emission Factors, Original Report vs. Revised Report, g/hp-hr.

Pollutant Original Report Revised Report

 HC 151 141

 CO 393 385

 NOx 0.7 0.54

 PM 2.7 1.83

The changes resulted in further lowering the amount of emissions attributable to snowmobiles

on a per-trip and on an annual basis.

4. Automobile Emission Factors. In the original report, automobile emissions were based on

U.S. EPA Tier-1 certification standards. Reviewers pointed out that these emissions were too
B-2

low based on actual in-use measurements, and suggested that estimates obtained using EPA’s

Mobile 5 mobile source model be used. This resulted in a substantial increase in the factors

used for HC, CO and NOx, as presented below. Mobile 5 does not provide an estimate of PM

emissions; therefore, EPA certification standards were used instead (Note: the original report

erroneously cited the Tier 0 standard rather than the Tier 1 standard for PM). For Mobile 5,

the vehicle mix used was: 0.67 light-duty vehicles and 0.33 light-duty trucks.

Changes to Automobile Emission Factors, Original Report v. Revised Report, g/mi.

Pollutant Original Report Revised Report

 HC 0.41 2.01

 CO 3.40 14.70

 NOx 1.0 1.87

 PM ` 0.20 0.08

The result of the above changes produced changes to the emissions attributable to

automobiles by as much as a factor of 5 for HC and CO.

5. Snow Coach Emissions. In the original report, emissions for snow coaches were based on

counts obtained from NPS’s Public Use Statistics Office, which combined snow coaches and

diesel buses in the count. Revised numbers for snow coaches and buses obtained from

Yellowstone are now used. The snow coach emission factors used in the original report were

based on the results of 2 measurements made by the University of Denver as part of their

real-time remote sensing study in Yellowstone for a Bombardier and Ford Econoline coaches.

The revised report uses emission factors obtained from EPA (AP-42, Volume II, Appendix J)

for Light-Duty Trucks as follows:

Changes to Snow Coach Emission Factors, Original Report v. Revised Report, g/mi.

Pollutant Original Report Revised Report

 HC 1.4 18.44

 CO 37.8 166.33

In spite of the large difference in emission factors used, the contribution of snow coaches to

total HC and CO annual emissions remains insignificant compared to those for snowmobiles

and automobiles.

6. Diesel Bus Emissions. The original report contained no estimates for buses (buses were

included in the snow coach count as explained in 5. above). An estimate for diesel bus

emissions has been made using factors from EPA (AP-42, Volume II, Appendix J) for

Heavy-Duty Diesel Vehicles.

7. Comparison of Snowmobile vs. Other Mobile Source Emissions. In the original report,

NPS compared the emissions from snowmobiles to EPA certification standards only. In the

revised report, emissions have been compared using all available information cited in the

report. For example, snowmobile emissions were calculated based on test results from

Southwest Research Institute and the University of Denver. Automobile emissions were

calculated using test results from the University of Denver (Denver fleet), EPA’s Mobile 5

emissions model, and EPA’s certification standards. The revised report provides the range in

the comparison of emissions between snowmobiles and automobiles based on the following
B-3

six comparisons (first entry represents source of snowmobile emissions estimate; second

entry the source of auto emissions estimates).

1. Southwest Research Institute (SWRI) – EPA Mobile 5

2. SwRI – EPA Certification Standards

3. SwRI – University of Denver (DU)

4. DU – DU

5. DU – EPA Mobile 5

6. DU – EPA Certification Standards

The results of these comparisons (expressed as the ratio of snowmobile emission factor to

automobile emission factor) are as follows:

Comparison HC CO

1. 45 17

2. 220 73

3. 153 29

4. 172 17

5. 50 10

6. 247 41

As a result of the above comparisons, the statement that snowmobiles emit about 100 times

more CO and 300 times more HC than automobiles has been changed. The report now

reflects the range of estimates calculated as “between 10 and 70 times more CO and between

45 and 250 times more HC” based on all emission factors.

8. Calculation of PAH Emissions. Reviewers pointed out that incorrect units had been used to

calculate emissions of polycyclic aromatic hydrocarbons (i.e., grams were reported as

pounds). The revised report corrects this error and recalculates the amount of emissions for

2000 snowmobiles (a peak day at Yellowstone) based on the assumption that snowmobiles

are operated at 16 horsepower for 4 hours. In addition, the total emissions for benzene and

toluene are given as well. The text now reads: “This equates to 1 pound of PAHs, 355

pounds of benzene and 8,960 pounds of toluene being emitted on a peak day (2,000

snowmobiles) at Yellowstone.” The revised report makes the comparison on the basis of

2,000 snowmobiles (a peak day) rather than on a 1000 snowmobile basis as in the original

report.

9. Comparison of Air Quality Levels to National Ambient Air Quality Standards. The

original report misstated the 24-hour PM2.5 national ambient air quality standard. The report

has been revised to indicate that the standard is 65 ug/m3, not 60 ug/m3. The revised report

also clarifies that concentrations measured during 4-hour periods cannot be directly compared

to a 24-hour standard. Language in the original report that attempted to provide some

perspective on the concentrations measured at Yellowstone NP by referencing the maximum

24-hour PM2.5 concentrations in a Los Angeles suburb has been removed. This contextual

statement was misinterpreted, resulting in inappropriate comparisons.

10. Miscellaneous Editorial Changes. The report has been reorganized extensively to achieve

better readability. Some redundancy has been removed. It also incorporates many
B-4

suggestions provided by the numerous reviewers (both internal and external to NPS).

Additional information regarding health effects associated with some of the toxic compounds

present in vehicle exhaust has been added. Tables of National Ambient Air Quality

Standards, PSD increments and several occupational exposure standards have been added.

Information on the effects of ozone and MTBE has been abbreviated, as it is believed that

these pollutants do not appear to be emitted or present in significant amounts. A table has

been added to the appendix indicating those parks where snowmobiling is allowed, and where

available, the number of snowmobiles (or visitors) entering these NPS units.