v>EPA
United States
Environmental Protection
Agency
Environmental Criteria and
Assessment Office
Research Triangle Park NC 27711
EPA-600/9-78-015
June 1978
Research and Development
Altitude as a Factor
in Air Pollution
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are
1 Environmental Health Effects Research
2 Environmental Protection Technology
3. Ecological Research
4 Environmental Monitoring
5. Socioeconomic Environmental Studies
6 Scientific and Technical Assessment Reports (STAR)
7 Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
EPA REVIEW NOTICE
This report has been reviewed by the Office of Research and Development. EPA, and approved for
publication. Mention of trade names or commercial products does not constitute endorsement or
recommendation lor use.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/9-78-015
July 1978
ALTITUDE AS A FACTOR IN
AIR POLLUTION
i L 606G4
- -tion
„, Kjorn 167Q
ENVIRONMENTAL CRITERIA AND ASSESSMENT OFFICE
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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PREFACE
This document has been prepared in response to a request by EPA Region VIII.
The major objective of the document is to provide a state-of-knowledge report on
the differential effects caused by higher altitudes upon air pollution and related
activities. Although preparation of this report has required a review and evaluation of
current scientific knowledge regarding altitude effects on air pollution, the document
does not jconstitute an in-depth scientific review.
A separate report to Congress is in preparation by Emissions Control Technology
Division (ECTD) at Ann Arbor for release October 1, 1978, in response to the Clean
Air Act Amendments of 1977. In contrast to the economic impact and technological
feasibility for separate high altitude standards to be discussed in the report to
Congress, the following document presents a general overview of altitude effects on
pollution.
The Agency acknowledges efforts and contributions of all persons who have
participated as authors and reviewers of this document.
in
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ABSTRACT
Air pollution is affected by change in altitude. Cities with surface elevations above
1,500 meters have atmospheric pressures which are approximately fifteen percent
(15%) below pressures at sea level. Consequently, mobile sources designed to operate
at pressures of one atmosphere perform less efficiently at high altitudes and emit
greater amounts of hydrocarbons and carbon monoxide than those designed to
operate at the lower atmospheric pressures. The net result is a photochemical smog
problem which is further enhanced by the increased solar radiation of higher altitudes.
The most significant effect of air pollution at high altitudes is upon human health.
This is due primarily to the inhalation of carbon monoxide at the reduced oxygen
concentrations of high altitudes. Particularly susceptible is the fetus exposed to
hypoxia and elevated carboxyhemoglobin levels. There is insufficient evidence to
establish significantly increased ecological effects at high altitudes. Reduction in
visibility is being observed in the vicinity of large metropolitan areas and near large
industrial complexes at high altitudes.
IV
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CONTENTS
Page
PREFACE iii
ABSTRACT iv
I. SUMMARY 1-1
II. ENVIRONMENTAL RELATIONSHIPS 2-1
Effects of Altitude on the Atmosphere 2-1
Introduction 2-1
General Climatology 2-1
Chemical Properties and Phenomena 2-2
Effects of Altitude on Aerosol Dynamics 2-3
Effects of Altitude on Soil 2-4
Effects of Altitude on Water 2-4
References 2-5
III. AIR POLLUTION MONITORING 3-1
Introduction 3-1
Altitude Effects on Concentrations Measured In PPM 3-1
Altitude Effects on Concentrations Reported in Mg/m3 3-2
Altitude Effects on Calibration 3-3
Altitude Effects on Automated Methods (Analyzers) 3-3
IV. STATIONARY SOURCES 4-1
Emissions 4-1
Measurements : 4-1
Control Technology for Stationary Sources 4-1
V. MOBILE SOURCES 5-1
Introduction 5-1
Theory 5-1
Measurements 5-2
Emissions 5-2
Control Devices 5-3
References 5-3
VI. AIR POLLUTION EFFECTS 6-1
Human Health 6-1
General Considerations 6-1
Hypobaric Pharmacology - Toxicology 6-1
Effects of Carbon Monoxide under Hypobaric Conditions 6-2
Effects of Altitude on Carboxyhemoglobin Formation Kinetics 6-5
Effects of Altitude and Carbon Monoxide on the Fetus 6-8
Ecology 6-8
Materials 6-9
Weather, Visibility, and Climate 6-9
References 6-10
APPENDIX A A-l
APPENDIX B B-l
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I. SUMMARY
Few air pollution studies have been performed in
which altitude (surface elevation above sea level) and
associated atmospheric properties (pressure,
temperature, density, solar radiation) have been
treated as independent variables. Notable exceptions
are the studies involving the health effects of carbon
monoxide (CO) under hypobaric conditions. The
results of these studies indicate that the effects of CO
exposure and of hypoxia induced by high altitude are
similar and suggest that when high altitude and CO
exposure are combined, the effects are additive. In
vivo, there is an interaction between these two factors
such that exposure to one may induce a physiological
response that influences the response of the body to the
other.
Significant alterations in alveolar ventilation occur
at high altitudes. This results in higher aero-toxicant
dosage, especially for particulate or gaseous pollutants
produced in closed spaces. Absorption and
translocation of aero-toxicants may be affected by
changes in altitude; however, experiments have not
been performed to confirm this.
Very few studies have been performed regarding the
effects of air pollution as a function of altitude on
ecosystems. The available data are inconclusive;
however, they suggest that at least certain ecosystem
components may be more sensitive to air pollutants at
altitudes above approximately 2,500 m.
Emissions from mobile sources are directly related
to changes in atmospheric density, hence changes in
altitude. Other factors being equal, most vehicles will
emit more hydrocarbons and carbon monoxide and
less oxides of nitrogen as the altitude increases.
Normally, the amount of hydrocarbons and CO in
diesels will increase with altitude. Changes in fuel-air
mixtures and fuel composition may partially
compensate for the change in altitude.
The efficiency of combustion will normally decrease
with an increase in altitude. This factor, coupled with
lower air viscosity at higher altitudes, may affect
particulate concentrations in the vicinity of stationary
sources located at higher altitudes. However, the effect
is thought to be small compared to other factors due to
the decrease in atmospheric density.
Altitude is a significant factor in ambient air
pollution monitoring. Changes in ambient
atmospheric pressure directly affect the volume of air
sampled. Most techniques and devices are designed for
operation at standard sea level pressure of 760 torr.
Failure to correct for a change in elevation to 1,525 m
(5,000 feet) may result in an error of approximately 15
percent in volume, hence concentration. Historically,
the National Air Surveillance Network (NASN) has
made no correction for differing atmospheric
pressures at the various sites in its network.
Calibration methods are affected by changes in
altitude and care must be exercised to minimize this
effect.
The two most common units of measurement of air
pollutants, micrograms per cubic meter (/ug/m3) and
parts per million (ppm), are both affected by changes
in altitude, but in different ways. Unless volumes of air
sampled are corrected to a reference pressure and
temperature, the calculated value of the mass of
pollutant per unit volume of air sampled will vary with
the altitude of the sampling site. Exposure to identical
parts per million concentration at differing pressures
does not expose the receptor to identical numbers of
moles of pollutant per unit volume of air, since the
conversion is a function of pressure. The implication of
this altitude effect on pollutant exposure data may not
be widely appreciated.
Important properties of the atmosphere (pressure,
temperature, density, solar radiation, wind, stability)
vary significantly with altitude. These properties,
coupled with the chemistry of the medium,
characterize the behavior of the atmospheric boundary
layer. It is logical to conclude that the behavior of
pollutants emitted to the atmosphere will be influenced
by variations in altitude. For example, reduction in
visibility is being observed in the vicinity of large
metropolitan areas and near large industrial
complexes. Also, differences in altitude may account
for a portion of the variance observed between
different urban atmospheres. Since altitude is only one
of many variables in a complex system, the question of
significance is one of degree.
1-1
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II. ENVIRONMENTAL RELATIONSHIPS
EFFECTS OF ALTITUDE ON THE
ATMOSPHERE
Introduction
The purpose of this document is to discuss the state
of knowledge of the effects of altitude (surface
elevation above sea level) on the production,
dispersion, and potential effects of air pollution. There
are four urban areas in the United States above 1,300
m and with populations greater than 200,000: Denver,
Salt Lake City, Albuquerque, and Colorado Springs.
The types and degree of air pollution vary
considerably across the United States. The main
factors causing the differences are source emissions,
chemical reactions in the free atmosphere, meteorology,
and topography. Altitude may also play a role in
determining the degree of pollution.
The meteorological and climatic conditions of a
given region are related to its geographical location
and local topography. Location determines the
large scale weather influences that dominate a region,
and topography alters air flow and thermal structure
of the atmosphere which may increase or restrict the
dispersion of air pollutants. Over the western United
States and in parts of the eastern United States,
climates change rapidly even within very short
distances due to the terrain and the variations in
altitude. A change in all climatic parameters is related
to changes in altitude. This is true, of course, for the
free atmosphere as well as for different land elevations.
In general, as the elevation of the land increases above
mean sea level, there is a decrease in atmospheric
pressure, a decrease in temperature, an increase of
wind speed and an increase of solar radiation. As
altitude increases there is a complex effect on the
distribution of cloudiness, precipitation, and
pollutants.
The effects of variations in ambient air temperature,
pressure, solar radiation and wind speed caused by
increased altitude on production and dispersion of
either primary or secondary pollutants may be either
direct or indirect.
General Climatology
The weather at a given place is the sum total of all
meteorological variables, such as solar radiation,
temperature, pressure, wind, precipitation, etc. Each
variable is subject to change with altitude.
The intensity of solar radiation on a horizontal
surface is generally greater at higher altitudes. This
results primarily because of the molecular effect of
three attenuators of solar energy: air molecules, water
vapor, and cloudiness. Cloudiness is by far the most
important factor, except in cases of high levels of
particulates, for example, volcanic eruptions.
Although cloudiness tends to decrease with increasing
altitude, some communities located at high altitudes
with a peculiar meteorological or topographic setting
may actually receive relatively small amounts of
sunshine because of significant cloud cover.
In terms of loss of ultraviolet radiation to the
atmosphere, one may consider total loss from the
outer limits of the atmosphere to sea level as 100
percent. At 1,525 to 2,500 m (5,000 to 7,675 feet), only 80
percent of that loss will have occurred. Therefore,
locations at altitudes of 1,500 to 2,500 m will receive
some 20 percent more ultraviolet radiation than
locations at sea level. Absorption of solar energy by
water vapor strongly affects the longer wavelength
infrared radiation but does not affect the ultraviolet
radiation. Since water vapor concentrations generally
decrease with increasing altitude, a site at 1,525 m
will usually receive some five percent more total solar
energy than a sea level locality.
From the central United States to the Rocky
Mountains, land elevation generally increases, while
atmospheric water vapor and cloudiness decrease.
Consequently, Denver annually receives about 10
percent more solar radiation than St. Louis or
Washington D.C., and about 20 percent more than
Chicago. This difference, however, is due to different
latitudes as well as to the higher altitude in Denver.
The normal variation of solar radiation with
altitude may be influenced by air pollutants, while the
intensity of solar radiation received at a given location
2-1
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may affect the chemical behavior of these same
pollutants.
Atmospheric pressure normally decreases
approximately logarithmically with height. At an
altitude of 1,458 m (4,781 feet), the pressure is
approximately 850 mb (646 torr), a decrease of
about 15 percent of the standard sea level value.
The diurnal variation at a given altitude is normally
on the order of one to three percent. It follows, of
course, that comparable decreases in density occur
with changes in altitude.
The average annual temperature depends to a
considerable degree on the altitude of a location and
its surrounding terrain. Higher altitudes generally
have cooler summer temperatures, and winter
temperatures can reach very low minima. Local
terrain conditions have a marked influence on the cold
season temperatures, frequently resulting in extreme
temperature differences between stations at the same
altitude. In all seasons, high altitude locations exhibit
a large daily temperature range.
In the free troposphere, the wind speed normally
increases with increasing height above sea level.
However, the correlation between higher land
elevations and higher wind speed is in part due to the
occurrence of rugged terrain at higher altitudes. The
dispersion of air pollutants is strongly dependent upon
local wind conditions.
On the windward side of mountainous terrain, the
amount of precipitation is considerably increased by
the forced ascent of the air. The leeward slopes, on the
other hand, are in the rain shadow and have much less
precipitation.
The total precipitation also decreases on each range
as one proceeds through a mountain system to the
leeward side. For example, more precipitation occurs
over the western than over the eastern Rocky
Mountains. The amount of precipitation at any one
location is, therefore, dependent in part upon its
exposure and altitude. Because precipitation is one of
the principal removal mechanisms for air pollutants,
changes in altitude may contribute to significant
pollution effects by this mechanism.
Chemical Properties and Phenomena
The photochemical reactions of pollutants to form
smog are a function of several factors. The major ones
are:
1. Concentration
2. Ultraviolet Radiation
3. Temperature
All three factors are vitally important to the kinetics
of the reactions.
The rate of a photochemical reaction is represented
by the general rate equation (1):
Rate = K [Af[B]n (1)
where [A] and [B] represent reactant concentrations,
m and n represent the stoichometry of the reaction,
and K is the reaction rate constant. Therefore, a
concentration increase results in a proportional effect
on the rate of the reaction when m and n are unity.
At higher altitudes, the atmospheric pressure of the
ambient air is lower than at low altitudes and this
results in lower partial pressures of oxygen. Unless
adjustments are made to increase air flow in engines,
the lower oxygen content will result in higher emission
rates of unreacted hydrocarbons from vehicular and
industrial sources, the major source of
photochemically reactive pollutants in urban areas.
Sunlight is necessary to initiate ambient
photochemical reactions. The simplified, general
mechanism for the formation of photochemical smog
is given by the reactions:
NO2 + hv
0 + 02
O3 + hv
-.*
M
NO + O (2)
03
02 + O* (3)
20H
R + H20
R02
RO + NO2
O + H2O -
OH + RH -
R + O2 -
RO2 + NO -
etc.
Solar radiation is supplied to these reactive
pollutants not only by direct sunlight but also by
reflection from the atmosphere and from the earth's
surface. The quantity of light radiation supplied to the
reactive pollutants is dependent on the solar spectral
irradiance incident on the atmosphere, the solar zenith
angle, the nature and amount of scattering, diffusion,
absorption of radiance by the atmosphere, and the
albedo of the surface under the region of interest. On
the other hand, the proportion of radiation absorbed
by the reactive pollutants is dependent upon
absorption coefficients, concentration of pollutants,
and effective path lengths. All of these factors must be
considered when evaluating the effect of light on
photochemical reactions. The mechanism of
photochemical smog formation has been discussed by
Demerjian et a/.(l) and Moortgat and Warneck.(2)
The exact reaction mechanisms proceeding from the
primary step of light absorption are not completely
understood. For the above reasons, it is difficult to
make exact statements about the photochemistry of
reactive pollutants even at sea level.
It is perhaps more convenient to limit the discussion
to the effect of altitude on two properties: wavelength
distribution and intensity. Altitude effects have been
2-2
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observed for both of these properties. Most of the
information available on these effects has been
determined by use of aircraft at high elevations above a
ground level reference point. Nader (3) reported a 14
percent increase in ultraviolet light intensity between
Los Angeles (350 feet surface elevation) and nearby
Mt. Wilson (5,700 feet surface elevation). These
measurements were made on a relatively clear day.
Included in the 14 percent was a 21 to 25 percent
increase in light intensity of wavelength distribution
between 3,050 and 3,450 A, and a 12 to 15 percent
increase in light intensity of wavelength distribution
between 3,450 and 3,950 A. These results would
suggest that altitude significantly effects ultraviolet
light intensity; however, a further evaluation of these
results is necessary since pollution in Los Angeles may
have been partly responsible for the observed
difference.
If increasing altitude results in increases in the
intensity of light at all wavelengths and the increase is
greatest for the lower wavelength region, then the
following effects will be expected:
1. Increased light intensity at higher altitudes will
result and, therefore, there will be an increase
in photochemical activity.
2. The rate of the primary step for the production
of photochemical smog, NO2 photolysis,
will be significantly increased due to the
increased intensity of shorter wavelengths of
light at higher altitudes. The increased lower
wavelength light may also have a great
effect on reactions of other secondary
pollutants such as the aldehydes and ketones.
Temperature is an important parameter in all
chemical reactions. Temperature is a measure of the
kinetic energy available to molecules; this energy
directly affects the collision frequency between
reactant molecules. Consequently, elevated
temperatures produce higher collision frequencies
between molecules resulting in a greater probability of
a reaction occurring. Also, since most reactions have
an activation energy, as the temperature increases,
more energy is supplied to overcome this energy
barrier, and the reaction rate increases.
Within the troposphere, ambient air temperatures
generally decrease with increasing altitude, except in
the case of temperature inversion. However, an
industrialized city contributes a considerable amount
of heat to its surrounding atmosphere. For this reason
the ambient temperatures of the atmosphere above
cities at higher altitudes may be higher than above
adjacent non-urban locales. In general, however, the
lower temperatures characteristic of higher altitudes
should contribute to slowing of photochemical
reactions.
To conclude, if emissions at high altitude are
comparable to those found at lower altitudes, the
greater solar intensity at high altitudes will tend to
increase photochemical smog production, while the
lower temperatures will tend to decrease it. Due to the
different effects of these parameters and the complex
interactions in photochemical reactions, it is not
possible to provide a general prediction of the altitude
effect on photochemical smog formation.
Effects of Altitude on Aerosol Dynamics1
The only mass transport parameters for air that
exhibit significant dependence on elevation are air
density (or molecular mean-free path) and
temperature. It is possible that the rates of aerosol
processes such as nucleation, condensation, dry and
wet deposition, coagulation, and gas-to-aerosol
conversion may be different at high elevation with
respect to sea level. Also, the performance of aerosol
instrumentation and the deposition efficiencies of
aerosols may be a function of elevation.
The process of heterogeneous nucleation is not yet
completely understood, and it is not possible to state
whether nucleation rates for the same mass
concentration of nucleating monomers would be
increased or decreased at higher elevations; this rate
also has a complex dependence on other factors, such
as the aerosol size distribution in the air. The rate of
condensation is not expected to be significantly
different at high elevations since the half-life for
molecular collisions with urban aerosols is on the
order of one second. Coagulation rates of fine aerosols
are pressure and temperature dependent; for the same
temperature, fine aerosols should coagulate 20 to 40
percent faster at high elevations. A decrease in
temperature decreases the coagulation rate.
The process of dry deposition for aerosols is not
completely understood. Generally, the deposition of
aerosols with diameters greater than 0.3 /urn is
discussed in terms of sedimentation and impaction.
Settling for aerosols with diameters less than 75 urn at
high elevations during cold periods (242° K) will be
greater (10 to 20 percent) than at low elevations during
warm (293° K) periods; however, there will be little
difference when the temperatures are equal. Impaction
efficiencies may be on the order of 20 percent greater at
high elevation. Most likely, dry deposition velocities of
coarse particles will be approximately equal for high
'For a more detailed discussion of this subject, see Appendix A
2-3
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and low elevations. Dry deposition of aerosols with
diameters less than 0.3 /urn is generally discussed in
terms of Brownian diffusion. The fine aerosol
deposition velocities will probably be about the same
for high and low elevations at a temperature of 293° K;
however, if the temperature is 242° K at the high
elevation site, then the fine aerosol deposition rate will
be lowered by about 20 percent.
The performance of aerosol samplers and monitors
calibrated at lower elevations may be affected when
used at high elevations. The devices that will be
affected are those that size particles by the air
resistance-to-motion principle. Such devices
include sedimentation chambers, elutriators,
impactors, cyclones, diffusion batteries, electrical
mobility analyzers, etc. The most important
parameter to be considered is the air viscosity for
devices sizing aerosols with diameters greater than
about 0.5 /urn. The air viscosity is independent of
pressure, but decreases as temperature decreases.
Thus, elutriators, impactors, and cyclones are
expected to increase in efficiency. The sizing of
small particles by diffusion batteries depends on
both the viscosity and the molecular mean-free
path, and is handled in a direct manner by
recalculating the aerosol diffusion coefficients.
However, the electrical mobility analyzer performance
is more complex due to its additional dependence
on the level of aerosol charging.
One of the important differences in high and low
elevations is the change in lung deposition efficiencies
of aerosols. Since the air is equilibrated to body
temperature before passing the pharynx, the air
viscosity is constant in the lungs at high and low
elevation. However, the pressure at high elevation is
decreased, resulting in an increase in the molecular
mean-free path. Thus, the particle diffusion
coefficients will be increased. Coarse aerosols will have
about the same deposition efficiencies in lungs at high
and low elevations, but fine particles may be deposited
with up to 20 percent greater efficiency at high
elevation.
EFFECTS OF ALTITUDE ON SOIL
Soils at higher altitudes in the eastern United States
are basically gray-brown podzolic soils. These are high
in organic matter, have low acidity, and contain
numerous microorganisms and earthworms. In the
Rockies and the other mountains of the West, they
vary from shallow soils of mountain, upland, and
valley to the gray desert types.
Studies dealing with effects of pollutants on soils at
high altitudes are limited in number. Therefore, it is
necessary to theorize. At high altitudes, radiation and
wind have easier access to the soil and rock surfaces
due to less vegetation. Climatic changes are extremely
important because of the precarious balance of the
soil, its organisms, and the vegetation growing in the
soil. Any pollutants which might bring about a change
in soil acidity, to cause vegetational changes or
influence mineral cycling, could adversely affect the
soil. Nutrient cycling and the decomposition of leaf
litter is of importance at surface elevations below the
timberline, which includes almost all of the mountains
of the eastern United States. Therefore, pollutant
effects upon these processes can appropriately be
studied in this area. Witkamp (4) has shown that
bacterial and fungal populations generally are
decreased at higher elevations. Temperature,
particularly, plays an important role in the numbers of
bacteria and fungi that are present and in the
respiration rate of soil microorganisms.
Inman el al. (5) and Abeles et al. (6) have shown that
soil microorganisms have the ability to take up CO and
hydrocarbons from auto exhaust. Many other
scientists have shown that hydrocarbons can be readily
metabolized by microorganisms. However, very few
studies have shown, at any altitude, how the changes in
microbial populations in the soil influence the soil
itself. There is a general need for soil studies at all
altitudes.
The Inman et al. study indicates that CO uptake
appears to be dependent upon altitude. The CO uptake
decreases from about 80 Mg/hr m2 at sea level to
approximately 5/ng/ hr m2 at 610 m (2,000 feet). How-
ever, Inman points out that altitude is but one of several
variables involved in the reactions. Other factors are
temperature, soil type, and the types of
microorganisms. These factors are difficult to
separate. A study is required that uses the same soil
with the same amount and types of microflora and
microfauna. This soil could then be transported to
different land elevations and the pollutant uptake
measured. Inman's results show that the maximum CO
uptake occurs at approximately 35°C. If higher land
elevations have lower temperatures, then the CO
uptake would be expected to decrease with all other
variables remaining constant.
EFFECTS OF ALTITUDE ON WATER
For the purposes of this report, it is assumed that
pollutant-to-water interactions take place mainly via
the following mechanisms:
1. Pollutant contribution to the atmosphere from
sea and land masses of water.
2-4
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2. Pollutant removal from the atmosphere by
scrubbing action of rain and by contact of the
atmosphere with water masses.
3. Change of pollutant form due to chemical
reaction with water.
Seiler and Junge (7) and Swinnerton el al. (8) have
stated that the ocean and rain water are sources of
carbon monoxide. Swinnerton found that rain water,
collected at an altitude of about 1,525 m (5,000 feet),
was supersaturated with carbon monoxide. As yet no
mechanism has been proposed and verified to account
for this phenomenon.
Organics with low boiling points that are immiscible
with water [i.e., CH2C12, (CH3-CH2)2-O] would
evaporate rapidly into the atmosphere if discharged
into a river, pond, or lake, and this would occur more
rapidly at higher altitudes.
For this reason, pollutant control strategies for
industrial activities operated at altitudes above
approximately 1,525 m (5,000 feet) should consider the
increased volatility of organic waste discharges to
determine impact on the atmosphere.
With regard to 2 and 3 above, the scrubbing of
pollutants from the atmosphere by water and the
change of pollutant form due to reaction with water
probably would not be significantly different between
1,525 m (5,000 feet) and sea level.
REFERENCES
1. Demerjian, K.. L. , J. A. Kerr, and J. G. Calvert. The
Mechanism of Photochemical Smog Formation. Adv.
Environ. Sci. Technol. 4:1-262, 1974.
2. Moortgat, G. K. and P. Warneck. Relative O('D) Quantum
Yields in the Near UV Photolysis of Ozone at 298° K. Z.
Naturforsch. Teil A 30:835-844, 1975.
3. Nader, J. S., (ed.). Pilot Study of Ultraviolet Radiation in Los
Angeles, October 1965. P.H.S. Publication No. 999-AP-38,
U. S. Government Printing Office, 1967, pp. 1 - 98.
4. Witkamp, M. Microbial Populations of Leaf Litter in Relation
to Environmental Conditions and Decomposition. Ecology
44:370-376, 1963.
5. Inman, R. E., R. B. Ingersoll, and E. A. Levy. Soil: A Natural
Sink for Carbon Monoxide. Science / 72:1229-1231, 1971.
6. Abeles, F. B., L. E. Craker, L. E. Florrence, and G. R.. Leather.
Fate of Air Pollutions: Removal of Ethylene, Sulfur •.Dioxide
and Nitrogen Dioxide by Soil. Science 772:914-916, .1971.
7. Seiler, W. and C. Junge. Carbon Monoxide in the Atmosphere.
J. Geophys. Res. 75:2217-2226, 1970.
8. Swinnerton, J. W , R. A. Lamontgne, and V. J.
Linnenbom. Carbon Monoxide in Rain Water. Science
/ 72:943-944, 1971.
2-5
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III. AIR POLLUTION MONITORING
INTRODUCTION
Gaseous air pollutants such as sulfur dioxide (SO:),
carbon monoxide (CO), ozone (Os), etc., are measured
by determining their concentrations in the ambient air.
Since both these pollutants and the ambient air are
gases, changes in pressure directly affect their volume.
According to Boyles law, if the temperature of a gas is
held constant, the volume occupied by the gas varies
inversely with the pressure (as pressure increases,
volume decreases). The major effect of altitude on air
pollution monitoring is that volume differences result
from the differences in pressure that prevail when
monitoring is carried out at different land elevations.
This pressure effect must be considered when
measuring pollutant concentrations. Normal atmo-
spheric pressure variations at any given sampling
location have very little effect on air pollutant
measurements. However, when comparing pollutant
concentrations measured at significantly different
altitudes, volume corrections are sometimes necessary
depending on the pollutant concentration units used.
The two units most commonly used in reporting
pollutant concentrations are micrograms per cubic
meter (/ug/ m3), a "mass of pollutant per volume of air
sample" measurement, and parts per million (ppm), a
"volume of pollutant per volume of air sample"
measurement. When /*g/m3 units are used, volume
corrections can be made such that results can be
reported at a reference pressure (760 torr). When ppm
units are used, volume corrections are not necessary
because the ppm unit is not dependent on pressure
effects.
The discussion on the following pages will show how
altitude (pressure) variation effects should be
considered when reporting air pollutant measurements
expressed either as ppm or ng/m3. Also a brief
discussion is given on how altitude affects calibration
and air monitoring instruments.
ALTITUDE EFFECTS ON CONCENTRATIONS
MEASURED IN PPM
The measurement of gaseous air pollutants in ppm
(volume per volume) units is widely used in the air
pollution scientific community. The use of this unit has
a number of advantages. It is dimensionless and is
numerically equivalent to such ratios as microliters per
liter, molecules per million molecules, and micromoles
per mole. It is readily understood by non-scientific
people and can be easily changed to "parts per billion"
to eliminate decimal fractions.
The ppm unit is used primarily in reporting
pollutant concentrations measured with continuous
analyzers, because it is a ratio of two volumes and is
therefore independent of pressure and temperature.
Both gases (pollutant and air sample) closely follow
the ideal gas law (Boyles law) where volume changes
proportionately in response to changes in pressure;
thus, their ratio remains unchanged. This means that
an air pollution analyzer can measure concentrations
in ppm under conditions of varying pressure without
error due to the resulting changes in volume. This does
not mean, however, that an analyzer can be calibrated
at low altitude and then carried to a higher altitude
without recalibration. In this case the sample air
metering device would be affected by the pressure
change from one altitude to another and recalibration
would be necessary at the new altitude.
The primary disadvantage of the use of ppm units is
the fact that while the ppm unit does not vary with
pressure, the mass of a gaseous pollutant per unit
volume does vary with pressure. For example, air
containing 1 ppm of a pollutant at a high altitude such
as Denver, Colorado, will still contain 1 ppm if that air
is brought down to sea level. However, the mass per
unit volume of air will increase with the decreasing
altitude.
This relationship can be demonstrated as follows:
recall that parts per million can be defined as
microliters (/nl) of gaseous pollutant per liter of air, or
_
ppm -
_ microliter (/il) of gaseous pollutant (g.p.)
- * - "*-* —
liter (1) of air
If gas behavior is assumed to be ideal, the ppm con-
centration unit can be converted into moles of gaseous
3-1
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pollutant per liter of air, a unit which is most useful in
considering health effects caused by exposure to air
pollution.
The derivation of moles of gaseous pollutant per
liter of air from the ppm unit follows:
moles of g.p.
1 of air =
1 mole of g.p.
24.45 1 of g.p.
760
of air
298
T
1(P1 of g.p.
Ml of g.p.
(5)
where P = atmospheric pressure at sampling
site, torr,
T = temperature at sampling site, °K
760 = standard pressure, torr,
298 = reference temperature, °K (298° K or
25° C is used by EPA as more appro-
priate to ambient air measurement
conditions than the conventional
273° K orO°C), and
24.45 = liters of gas/mole, at standard pres-
sure and reference temperature
g.p. = gaseous pollutants
which simplifies to:
moles of g.p.
1 of air
1.604 x 10"8 x |r
It is evident from equation 6 that constant parts per
million contain differing numbers of moles per unit
volume of air as pressure (and temperature) changes.
For this reason exposure to identical parts per million
concentration at differing pressures does not expose
the receptor to identical numbers of moles of pollutant
per unit volume of air. Although air pollutant
concentrations reported in ppm units are not directly
affected by altitude differences, the effective mass per
unit volume differences should be considered when
interpreting data taken from different altitudes. It is
important to note that if concentration results, in ppm,
are to be converted to moles per liter air or to pg/m*,
the pressure at which the sample was taken must be
known.
measurement methodology is based on a manual
method (i.e., the pararosaniline method for measuring
SO2). In the latter case, samples are generally collected
in liquid absorbing solutions which are taken to a
central laboratory for chemical analysis. In this case,
the pollutant is analyzed on a weight basis and,
consequently, results are computed in ng/m} units,
which can be converted to ppm units if the pressure
and temperature at which the sample was taken are
known.
When air pollutant concentrations are reported in
Ugl m3, the volume of air sampled must be corrected to
a reference pressure and temperature (temperature
effects are not considered in this discussion) if the
comparisons of data from different locations are to be
made. If not, the calculation of the mass of a pollutant
per unit volume of air sampled will be a function of
pressure, and concentrations obtained will vary with
the altitude of the sampling site.
For example, consider a situation in which 50 /ug/ m3
of paniculate matter is measured without correction
at a location having an atmospheric pressure of 625
torr (common in Denver). If a cubic meter of that air is
pressurized to 760 torr (reference conditions), it would
be compressed to about 80 percent of its original
volume, and the correct particulate concentration per
cubic meter would be correspondingly higher, as the
following calculation shows:
V2 =
PiVi
P2
where V2 = volume at reference conditions,
P2 = 760 torr,
Vi = 1 m3,
Pi = 625 torr, and
therefore, 3
v - 625 torr x 1 m _ . 3.
V2 - -,n . - 0.82 m
760 torr
thus, the particulate matter concentration would be
0.82m
ALTITUDE EFFECTS ON CONCENTRATIONS
REPORTED IN
The micrograms per cubic meter unit is used
primarily for pollutants which are not gases. However,
it is also used for gaseous pollutants when the
It is obvious that volume corrections should be
made when pollutant concentrations, in terms of
Mg/m3, are compared between locations of different
altitudes. For purposes of comparison, all
measurements reported in /zg/m3 should be corrected
3-2
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to the reference pressure of 760 torr, as specified in the
Code of Federal Regulation, Part 50.3 (40 CFR Part
50.3).
It should be noted here that historically the EPA
National Air Surveillance Network (NASN) has made no
corrections for the differing atmospheric pressures at
the various sites in its network. These corrections
would require measurement, or estimation, of the
barometric pressure during sampling at each site.
Pragmatically, for many sites, the elevation correction
would be comparable to or less than the intrinsic
variability of the earlier monitoring methods.
ALTITUDE EFFECTS ON CALIBRATION
Dynamic calibration of methods and instruments
for measuring gaseous air pollutants is commonly
done in one of two ways. The first of these employs a
known amount of a pollutant gas, generated at the alti-
tude of measurement and diluted with pollutant-free
air to the various concentrations needed for calibra-
tion. Permeation tubes and constant flow ozone
generators are two examples of devices using this
procedure to prepare calibration gases. The
second calibration method uses known mixtures
of the gaseous pollutant in a diluent gas, contained
in pressurized cylinders. These cylinders are
transported to the site of the measurement needed
for calibration. Altitude does not generally affect
either of these procedures. When permeation tubes
(or an ozone generator) are used, the calibration
gas is prepared at the altitude where it is being
used. When pressurized cylinders are used, the
concentration of pollutant in the cylinder is
assayed in ppm units, which is independent of
pressure.
When working in units of /ig/ m3 (mass per volume),
a correction must be applied in those calibration pro-
cedures using calibration gases that have been pre-
pared at one altitude and used at a different altitude.
For example, a cubic meter of a calibration gas that
contains 100 y.g of a pollutant at sea level atmos-
pheric pressure will expand with the decreasing
pressures at higher altitudes. Thus, a cubic meter
of the expanded gas is left with proportionately
less of the 100 ng of pollutant.
ALTITUDE EFFECTS ON AUTOMATED
METHODS (ANALYZERS)
Most air pollution analyzers can be operated at
almost any surface elevation in the continental United
States. The exceptions are the monitoring systems that
employ hydrogen flames, such as the flame ionization
detector for hydrocarbons or the flame photometric
detector for SCh. At altitudes in excess of 3,000 m
(10,000 feet), the partial pressure of oxygen is
sufficiently low that an auxiliary oxygen supply may be
necessary to support the combustion of these flames.
As stated above, the main influence of altitude on
measurements is that of pressure, which alters the mass
of a pollutant per unit volume. For example, 1 ppm of
NC>2 measured at 25°C and 760 torr pressure is
equivalent to 1,880 pg of NCh per cubic meter of air;
whereas, 1 ppm of NCh measured at 25° C and 625 torr
pressure is equivalent to 1,546 ^g of NO2 per cubic
meter of air. Since most commercial air monitoring
instruments record data in ppm, a knowledge of the
prevailing atmospheric pressure and temperature is
needed in order to convert correctly ppm readings to
Mg/ m3 at the condition of measurement. It should be
noted that factors given with the EPA reference
methods for measuring the criteria pollutants have
been defined such that any properly measured ppm
concentration can be converted correctly to /ig/m3,
under the reference conditions of 760 torr and 25°C.
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IV. STATIONARY SOURCES
EMISSIONS
There has been no documented evidence to date on
any altitudinal effect on the nature, quantity, or char-
acteristics of emissions from stationary sources. Based
upon current knowledge, one would not expect to see
measurable differences due to altitude, except, perhaps,
with losses by evaporation from dry cleaning plants
(organic solvents), petroleum products (volatile
matter), and surface-coating operations. Altitude
effects on evaporation loss from storage can be ex-
pected to be minimal.
In the case of paniculate matter, two competing
factors can be expected to affect the nature and quan-
tity of atmospheric particles. The two factors are less
complete combustion processes caused by the lower
atmospheric pressure (less available oxygen) and the
lower air viscosity. The first factor would be expected
to result in more (numerically) and larger particles; the
second factor would be expected to cause more rapid
fallout of particles, thus resulting in a smaller mean
mass diameter. Initial ambient air measurements at
high altitudes have shown the mean mass diameter to
be slightly larger than anticipated.
MEASUREMENTS
One must consider the two general types of
measurement systems separately. In the case of optical
(non-contact) methods using a fixed path length, the
lower atmospheric pressure at high altitudes means
fewer molecules are in the optical path to absorb the
light energy. This factor would be compensated for in
the calibration method. In the instance of manual
sampling systems, pressure may affect the gas
solubilities in collection media. However, variations
due to this effect are not expected to affect reproduci-
bility of the measurement method. No other
measurements are likely to be affected by slight
changes in pressure.
Pilot tube velocity measurements should be affected
by pressure differences due to altitude. The
measurements are directly proportional to the density
of the gas, and the primary effect of increased altitude
is a smaller pressure change for a given change in air
stream velocity. In the Denver area, this would result
in a sensitivity decrease of approximately 15 percent,
but the decrease could be greater at very low velocities.
Since the measurement systems for particles use
isokinetic sampling techniques and all measurements
are referenced back to standard atmospheric pressure
and temperature, no altitudinal effect is expected.
CONTROL TECHNOLOGY FOR STATIONARY
SOURCES
For essentially all combustion devices, the mass air
required to produce a given amount of heat release is
essentially constant. Because of the reduction in at-
mospheric pressure with increasing altitude, the
volume of air and exhaust gas per unit of energy will
increase proportionately. Thus, most atmospheric
pressure handling equipment must be sized 7 to 15
percent larger at 1,525 m (5,000 feet) land elevation
than at sea level and will cost 5 to 10 percent more.
Such gas handling equipment includes, for example,
scrubbers, baghouses, catalytic reactors, fans, and
compressors. The effect of atmospheric pressure is
insignificant compared to variations that would occur
at any altitude in pollutant concentration, gas temper-
ature, air dilution rates, and other source parameters.
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V. MOBILE SOURCES
INTRODUCTION
This section briefly describes the effect of changes in
altitude on regulated emissions from motor vehicles.
Regulated emissions are those for which emission
standards apply; these include HC, CO, NOX, and
particulate. Both exhaust engine emissions and evap-
orative emissions are discussed.
The references included at the end of this
section represent a selected few to provide an
introduction to the very substantial literature on the
general subject. No attempt has been made to cover all
aspects.
THEORY
The engine combustion process is affected when the
engine is forced to operate at differing altitudes.
Increase in altitude results in a decrease in atmospheric
pressure accompanied by a decrease in air density.
These density changes alter the mass of air available
for the combustion process. Consequently, changes in
altitude create changes in the air-fuel ratio at which the
engine must operate. Because combustion efficiency
and exhaust emissions are a function of air-fuel ratio,
altitude changes, unless compensated or corrected for,
affect both engine performance and emissions. For this
reason, vehicles are often "retuned" when they are to
be operated at higher altitudes.
Carbureted engines, which are essentially
volumetric devices, are those most susceptible to
altitude change. Since the fuel flow under fixed engine
operating conditions is only slightly affected by
pressure changes, the decreasing density of the
inducted air at increased altitude results in engine
operation with richer mixtures.
For a carburetor installation undergoing an altitude
change from sea level to 1,525 m (5,000 feet), the result
would be approximately seven percent enrichment of
air-fuel ratio. (1) This would result in decreased power,
decreased engine efficiency, and decreased NOX
emissions, and increased HC and CO emissions.
However, proper adjustment of the ignition timing for
vehicles to be operated at high altitudes would reduce
the magnitude of these effects.
Fuel injected engines behave somewhat differently
when subjected to changes in altitude. Although some
injection systems have been designed to compensate
for altitude effects, numerous noncompensating
systems are in use which cause a leaning of the air-fuel
ratio with increasing altitude. These systems
commonly utilize speed-density control systems
without back pressure sensing. It has been reported
that a change from sea level to 1,525 m (5,000 feet) of
altitude would result in a 2.1 percent leaning of the air-
fuel ratio of the mixture for noncompensating
injection systems.(l) This leaning would generally tend
to reduce engine regulated emissions slightly.
The theoretical effect of altitude changes on
evaporative emissions should also be considered.
Evaporative emissions are caused primarily by the
venting of fuel vapors from the carburetor float
chamber and vehicle fuel tank. These can constitute a
sizable source of HC emissions in the absence of
appropriately functioning control devices.
Low barometric pressures prevalent at high
altitudes effectively lower the distillation temperature
curve of gasoline causing it to become more volatile at
all temperatures. Gasoline losses increase significantly,
especially during vehicle hot-soak when temperatures
are elevated following engine shutdown.(2)
Diurnal evaporative losses show a strong
correlation with gasoline Reid Vapor Pressure, which
for high-altitude fuels, is generally lower than for
low-altitude fuels.(2-4) Although this fact would imply
that the altitude effect on volatility might be
compensated for, in actuality, high altitude
evaporative losses are greater according to surveillance
test data obtained in the Los Angeles and Denver
areas.(5)
Many additional factors such as an
altitude-temperature relationship, fuel characteristics,
fuel distribution efficiency, injection spray geometry
(diesel), fuel-air mixing efficiencies, etc., have a
5-1
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bearing on engine performance and emissions at high
altitudes. In the case of diesel engines, a fixed fuel
injection rate can be maintained over only a moderate
range of altitudes if excessive smoke and deposits are
to be avoided.
MEASUREMENTS
All differences in recommended measurement
procedures at high altitudes are practical and not
theoretical. All certification measurements are
corrected volumetrically to a mass basis at standard
sea level temperature and pressure.(6) All
measurements of physical and/or chemical moieties
must be corrected to a common physical base if the
results are to have any comparability. The base must
include an altitude correction if it is a relevant
parameter.
All affected measurements currently specified in the
Federal Register include an altitude (i.e., barometric
pressure) correction. Any measurement that does not
include this parameter should be considered suspect
unless made at sea level and at 25°C.
EMISSIONS
As theory has predicted, regulated exhaust
emissions from mobile sources are quantitatively
affected by altitude. Changes in air-fuel ratio brought
about by operation at varying altitudes are largely
responsible for variation in emission rates. When these
emissions are expressed as ppm, data regarding
carbureted vehicles, which comprise the bulk of mobile
source emitters currently in the United States, are most
susceptible to altitude effects. Those vehicles that are
not specifically designed for high altitude operation
will emit more hydrocarbons and carbon monoxide
and less nitrogen oxides with increases in altitude.
Smoke emitters, such as diesel engines, will emit
greater quantities of visible, carbonaceous material
with increasing altitude. The smoke capacity of a
turbocharged diesel engine almost triples between 183
and 1,830 m (600 and 6,000 feet) and then falls off at
higher altitudes. The same is true of smoke from diesel
locomotives.(7,8)
Only limited information is available on the
functional relationship between emissions and
altitude. From the information which is available, it
has been determined that a near linear relationship
exists between HC, CO, and NO* emissions and
altitude. A research paper published by Volkswagen
(9) contains data on a single vehicle which was tested at
six different altitudes. The linear regressions obtained
using these data are expressed as:
HC : E= 2.95+0.00072 A
CO : £ = 30.33+0.0074 A
NOX: E= 2.82-0.00027 A
where E is the emission in grams per kilometer and A is
the altitude in meters. The rates, expressed as a percent
change per 100 m, are 2.5,5.0, and -2.0 percent for HC,
CO, and NO,, respectively. Because of the linear
relationship cited above, interpolation may be used to
determine an emission rate at a given altitude when
emission rates at both a high and low altitude are
known. For such a case, the following expression may
be used:
EFh - EF,
EFu = EF, + (A. - A,)
(Ah - A,)
where EFu = user emission factor,
EFi = low altitude emission factor,
EFh = high altitude emission factor,
Au = user altitude,
Ai = low altitude level, and
AH = high altitude level.
This expression can be readily applied to emission
factor data listed for both high and low altitudes. The
average low altitude is assumed to be about 150 m (500
feet) and 1,677 m (5,500 feet) for the average high
altitude.(lO)
Evaporative emissions from gasoline powered
vehicles are categorized as either hot-soak losses or
diurnal losses. The hot-soak losses occur during the
vehicle's cooling-down period immediately following
engine shutdown. Diurnal losses occur while the
vehicle is not operating and is essentially in
temperature equilibrium with the environment.
In an emission surveillance study conducted on
in-use 1976 model vehicles, data were obtained on
evaporative emissions in both the Los Angeles and
Denver areas. The former city represents the sea level
case, while the latter represents the high altitude case at
about 1,585 m (5,200 feet). Both hot-soak and diurnal
evaporative losses were determined for 20 different
vehicles using the Sealed Housing for Evaporative
Determinations (SHED) technique. In both cases
Denver vehicles were found to have significantly
higher evaporative emissions. The mean diurnal loss in
Denver was 21.7 g as compared with 7.8 g in Los
Angeles, and the mean hot-soak loss in Denver was
10.5 g versus 5.4 g in Los Angeles. Assuming an
average of 3.3 trips per day and 47.3 km (29.4 miles)
5-2
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per day, the combined evaporative emission losses are
1.2 g per km (1.9 g per mile) in Denver versus 0.56 g per
km (0.9 g per mile) in Los Angeles.(5)
CONTROL DEVICES
Control of air-fuel ratio is the key to any system
which is designed to compensate for altitude
differences in mobile source emissions. Carburetors
have been designed with aneroid pressure sensors to
correct for altitude effects. The sensor utilizes feedback
to control the amount of inlet air which will bypass the
carburetor's venturi. Although carburetors of this type
have been on display, none are currently being used to
any extent by the major automobile manufacturers.
The three-way catalyst systems which are in
production utilize the feedback from an oxygen sensor
located in the exhaust gas stream to control air-fuel
ratio. This is possible because exhaust oxygen level can
be directly related to the engine operating air-fuel
ratio. The system is designed to control air-fuel ratios
within an extremely narrow range in order to achieve
80 percent catalyst efficiency. Control is within the
range of 0.02 to 0.08 units of air-fuel ratio for most
installations.(l 1) Foreign companies employing three-
way catalysts on their vehicles have used the feedback
system in conjunction with fuel injection systems.
Domestic companies, on the other hand, have recently
certified three-way catalyst vehicles that have
carbureted systems. The only shortcoming that the
three-way catalyst systems possess with regard to high
altitude operation is the limited range of response
designed into the system itself.
Some fuel injection techniques are sophisticated to
the point that control of air-fuel ratio is possible over a
wide range of altitudes. Engines having speed-density
control systems with back-pressure sensors essentially can
compensate for altitude changes. Another fuel injec-
tion approach has been to monitor the mass flow of air
into the engine by using volumetric flowmeters with
pressure and temperature sensors. A microprocessor
with flowmeter and sensor inputs is used to calculate
the air flow, and the fuel is metered accordingly.
REFERENCES
1. Bolt, J. A., S. P. Bergin, and F. J. Vesper. The Influence of the
Exhaust Back Pressure of a Piston Engine on Air Consumption
Performance, and Emissions. SAE Prepr. (730195), 1973.
2. Wade, D. T. Factors Influencing Vehicle Evaporative
Emissions. SAE Trans. 7(5:811-823, 1968.
3. Biller W. E., M. Manoff, J. Sachdev, W. C. Zegel, and D. T.
Wade. Mathematical Expressions Relating Evaporative
Emissions from Motor Vehicles Without Evaporative Loss
Control. SAE Prepr. (720700), 1972.
4. U.S. Department of the Interior, Bureau of Mines. Motor
Gasolines, Summer 1974. Petroleum Survey No. 88, 1975.
5. Rutherford, J. A. Automobile Exhaust Emission Surveillance-
Analysis of the FY 1975 Program, EPA-560/3-77-022,
USEPA, Ann Arbor, Mich., 1977.
6. Control of Air Pollution from New Motor Vehicles and New
Motor Vehicle Engines. Fed. Regist. 5(5:128, pp. 12652-12664.
7. Ephraim, M. Jr. Status Report on Locomotives as Sources of
Air Pollution. Internatl. Conf. on Transportation and the
Environment, Part I. SAE, New York, 1972, pp. 9-13.
8. Dennis, J. W. Turbocharged Diesel Engine Performance at
Altitudes. SAE Prepr. (710822), 1971.
9. Schurmarn, D. and H. Klingenberg. Considerations on a
Measuring Program to Investigate the Influence of the Ambient
Air Conditioning on Vehicle Exhaust Emissions. Research
Report No. MT-F5-77/13, Volkswagenwerk, A6, 1977.
10. Compilation of Air Pollution Emission Factors. Third Edition,
AP-42, USEPA, Research Triangle Park, N.C., 1977,
pp. 3.1.1.-1 to 3.3.3.-2.
11. Sittig, M. Automotive Pollution Control Catalysts and
Devices. Noyes Data Corporation, Park Ridge, New Jersey,
1977, pp. 195-214.
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VI. AIR POLLUTION EFFECTS
HUMAN HEALTH
General Considerations
Alterations of drug and toxic effects in man at high
altitudes may be anticipated because of significant
physiological alterations occurring at altitudes greater
than 2,290 m (7,500 feet). These physiological
alterations and most overt symptoms, including the
syndrome of mountain sickness, result almost entirely
from anoxia due to the diminishing oxygen content of
the rarified atmosphere.
In addition, significantly lowered total atmospheric
pressure can result in higher concentrations of toxic
gases or fumes when the emission occurs in enclosed
spaces.
For these reasons, and especially since the advent of
the Space Age, a wealth of information on the effect of
hypobaric environments on drug and toxic actions
might be presumed to exist. In fact, the opposite is
true.
A literature search of more than 500,000
publications in relevant subject areas uncovered very
few publications dealing directly with the differential
effects on human health caused by exposure to air
pollutants at high altitudes. This statement does not
include work on carbon monoxide, which will be
treated separately in this report. Nor does it include
high altitude problems of gas/vapor anesthetics which
are well understood and predictable. There is a
reduction of potency and duration of actions due to
lowered partial pressure in all but completely closed
systems. It may be that implications from these kinds
of observations account for the lack of experimental
interest in the influence of hypobaric environments on
the effects of other volatile noxious agents or
paniculate pollutants. Experts in oxygen-related
physiology or aero-toxicants generally agree that there
is a great lack of knowledge in this area of
pharmacology and toxicology.
Hypobaric Pharmacology-Toxicology
Despite the shortage of information, it is possible
to make some predictions based on pharmacological
principles, gas physics, and well-documented
physiological obeservations on man's response and
adaptation to high altitudes.
If it is true that no significant physiological
alterations occur much below 2,290 m (7,500 feet), it is
obvious that the following discussion will apply only to
a very small fraction of the American population.
Perhaps that is the reason why, with the exception of
teaching the principles of anesthetics, the medical
schools at Denver and Salt Lake City teach
pharmacology and toxicology much the same as
elsewhere.
In unenclosed spaces in free equilibrium with the
atmosphere, partial pressures of gaseous pollutants
decrease with increasing altitude. This results in
decreased alveolar partial pressure which reduces
systemic absorption. There probably is little
altitudinal effect on local injury potential. Responses
to particulate pollutants would not be affected by the
lower atmospheric density.
In enclosed spaces at high altitudes, a given quantity
of pollutant initially will reach a higher concentraion
in higher partial pressure relative to oxygen than is
reached at sea level; however, the absolute alveolar
partial pressure may or may not be increased.
Absorption as well as local injury potential may be
increased.
Significant alterations in alveolar ventilation occur
at high altitudes. There is an invariable increase in this
parameter, the maximum of which is a function of the
particular altitude. This maximum is reached within
100 hours for new arrivals at high altitudes. Prolonged
stay at these levels results in very gradual decline (over
years), but even natives have a higher ventilation rate
at rest. This results in higher aero-toxicant dosage,
especially for particulates and gaseous pollutants
produced in enclosed spaces.
Effects of altitude on absorption of aero-toxicants
can be summarized as follows:
1. Increased blood flow seen most prominently
after acute introduction to high altitude and
also in the early stages of adaptation will
6-1
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promote increased absorption of soluble
aero-toxicants.
2. Increased alveolar diffusing capacity in natives
and adapted sojourners will have similar
effects.
3. Reduced partial pressures of gaseous
pollutants will decrease absorption.
Effects of high altitude on physiological fate of
absorbed toxicants include the following:
1. Due to increased blood flow, there will be
increased absorption by peripheral tissues of
aero-toxicants initially absorbed by the lungs.
2. The decreased plasma volumes cause
increased plasma concentration of dissolved
toxicants which will also increase absorption
by peripheral tissues.
3. Reduced pressure gradients across membranes
will decrease translocation.
4. A redistribution of blood flow away from the
skin occurs at high altitude, an effect which
may favor translocation of toxicants to vital
organs.
While no specific information is available on
altitudinal alterations in metabolism of aero-toxicants,
Merritt and Medina (1) showed that hexobarbital
sleeping time decreased in mice which had been
acclimatized to a hypobaric atmosphere. A similar
decrease was shown for zoxazolamine paralysis time.
In both instances increased hepatic microsomal enzyme
activity could be demonstrated, resulting in more rapid
declines of blood levels of these drugs.
No definite trends can be deduced from sketchy
discussions in the literature. (1-6) When renal blood
flow decreases, there appears to be an increased
filtration rate. A tendency towards dehydration at high
altitudes implies that excretion is diminished and
blood and tissue levels of soluble toxicants or active
metabolites are prolonged.
An interesting example, experimentally
investigated, is the tenfold increase of amphetamine
toxicity in mice and rats under hypobaric conditions
(acute and acclimatized). This was found to be due to
increased brain levels of catecholamines.
Other examples of hypobaric effects cited by
Medina and Merritt (6), are increased toxicity of
digitalis in cats, of strychnine in rats and of reserpine in
mice.
Effects of Carbon Monoxide Under Hypobaric
Conditions2
The effects of CO exposure and of hypoxia induced
by high altitudes are similar. Much experimental data
suggest that when high altitudes and CO exposures are
combined, the effects are additive. In vivo, there is an
interaction between the two factors such that exposure
to one may induce a physiological response that
influences the response of the body to the other. For
example, Forbes et al. (7) have shown that during light
activity their subjects had an increased rate of CO
uptake at an altitude of 4,880 m (16,000 feet) compared
to that at sea level, becasue of the hyperventilation that
results from the decreased partial pressure of oxygen
(P02). When the ventilation rate was corrected to its
value at sea level, the rate of uptake of CO decreased to
a value within 10 percent of the sea level value.
Although the effects of hypoxia and CO appear to
be additive at very high altitudes, individually they
produce different physiological responses. This is
because they have different effects on blood PO2,on the
affinity of Oz for blood hemoglobin, on the extent of
oxyhemoglobin (O2Hb) saturation, and on
ventilation. The presence of carboxyhemoglobin
(COHb) increases the affinity of the remaining
hemoglobin for O2, but lowers the total O2Hb
satuaration. The ventilation rate appears to be influ-
enced by receptors in the carotid and aortic bodies that
are responsive to blood P02, and PCo2- (8) Hypoxia
results in a lowering of the P02 and increased
ventilation ensues; in CO exposure, the P02apparently
does not change sufficiently to induce increased
ventilation.
Several different physiological and psychomotor
tests have been used to determine the effects of altitude
with and without CO. Pitts et al. (9) have observed the
physiological responses to exercise when blood COHb
levels were increased by 6 percent and 13 percent in a
group of 10 men at simulated altitudes: 0; 2,130; 3,050;
and 4,575 m (sea level; 7,000; 10,000; and 15,000 feet).
The parameters measured were pulse rate, respiratory
rate, and minute ventilation. A previous study had
indicated that in subjects at rest, the pulse showed no
change with atmospheric pressure down to that
corresponding to an altitude of 5,000 m (16,500 feet).
After regulated exercise at atmospheric pressures
corresponding to altitudes from sea level up to 6,360 m
(21,000 feet), however, it showed a steep, almost linear
increase. Of the parameters measured in both these
studies, mean pulse rate during exercise and for the five
minutes immediately following exercise showed the
closest correlation with blood COHb and ambient Po2.
At sea level, an increase of 13 percent in COHb
increased the mean exercise pulse rate from 105 to 112
and the recovery pulse rate from 91 to 98. An increase
:For a more detailed discussion of this subject, see Appendix B
6-2
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of six percent COHb produced a significantly higher
exercise pulse rate compared to that produced by
simulated increase in altitude alone. Pitts calculated
that for every one percent increase in blood COHb in
normal subjects, up to 13 percent COHb, the increase
in exercise pulse rate is equal to that which would be
produced by a 100-m (335-foot) rise in altitude
throughout the range of 2,130 to 3,050 m (7,000 to
10,000 feet). It is likely that some of the subjects were
smokers, since the group mean control COHb level
ranged from 2.88 to 3.64 percent on different days.
Since the smokers would have been preconditioned in
part to the effects of CO, their responses to additional
small quantities of CO might be expected to be lower
than those of nonsmokers. This could have masked the
additive effect of CO and hypoxia. A more recent
study indicated that cigarette smokers may be partially
adapted to carbon monoxide. (10)
Another study compared the effects of CO exposure
and altitude. (11) Eight healthy male subjects were
divided randomly into two groups of four each; each
group followed a different daily schedule. The subjects
were briefly exposed daily for 10 days to
concentrations of five percent CO (57,500 mg/m3 or
50,000 ppm) at four-hour intervals between the hours
of 7 a.m. and 11 p.m. The doses were sufficient to give
an average COHb level of 15 percent, although values
ranging between 5 and 25 percent were recorded. A
variety of circulatory, ventilatory, and renal function
tests were performed during the course of the
experiment. The same experimental protocol was
carried out on the same eight subjects after they spent
10 days at a simulated altitude of 3,430 m(l 1,225 feet).
This condition gave roughly a degree of hypoxemia
equilalent to that given by 15 percent COHb. The data
obtained from the two studies are compared in Table 1.
Of great significance are the circulatory and
ventilatory responses to the two types of hypoxic
conditions. As expected, CO hypoxemia shifted the
O2Hb dissociation curve to the left; whereas, high
altitude caused a shift to the right, the latter occurring
during the first 24 hours. At increased simulated alti-
tude, both the cardiac output and the ventilatory rate
increased with the first 24 hours; whereas, CO had no
consistent effect on these parameters.
The lowered arterial P02 at simulated high altitude
most likely stimulated the chemoreceptors in the aortic
and carotid bodies, with the resultant regulatory
changes in ventilation. Mills and Edwards (12) have
shown that these chemoreceptors are also stimulated
during CO inhalation. These investigators have
suggested that the lack of ventilatory response during
CO hypoxemia is a result of depression of respiratory
centers in the central nervous system. The effects of the
two types of hypoxemia at the cellular level can be
estimated by comparing the P02 of the mixed venous
blood under both circumstances, since this reflects
tissue P02. The data in Table 1 show that the average
P02 of mixed venous blood in CO hypoxemia is lower
than in hypoxic (high altitude) hypoxemia. In both
types of hypoxemia, a lowered tissue P02 is expected;
as indicated by the mixed venous P02, this was more
pronounced in CO hypoxemia under the conditions
observed. At increased simulated altitude, the shift of
the O2Hb curve to the right and the circulatory and
ventilatory responses compensate for most of the
associated tissue hypoxia during the first 24 hours.
Such regulatory mechanisms do not appear to be
stimulated by CO hypoxemia, and hence, CO
hypoxemia may be considered to be more of a
physiological burden than comparable levels of
hypoxia hypoxemia.
In a study released by the National Asthma Center,
the effects of low-level carbon monoxide exposure at
1,610 m (5,300 feet) for healthy young men was
reported. (13) For COHb levels ranging from 0.96 to
1.15 percent, the effects of CO on exercise performance
are similar to, but not greater than, effects at sea level.
Other studies on the combined effects of CO and
altitude have used psychomotor tests. Variable results
have been reported, but some of these may be
explained by the use of different tests and others by the
lack of proper controls. The flicker fusion frequency
(FFF) and the critical flicker fusion (CFF), often
employed in these studies, have recently been criticized
because of their lack of reliability. (14)
McFarland et al (15) have used the increased
threshold of visual perception as an index of the effect
of both CO and high altitude. It is pertinent to note at
this point that McFarland et al. found that visual
perception was impaired in a single subject who had a
COHb level of about five percent at sea level. This is
equivalent to the impairment associated with low Po2
at an altitude of 2,130 m (7,000 feet).
In another experiment, a group of four trained
subjects was used to study the time course of recovery
from CO and altitude. (16) Data from a single subject
suggest that it takes longer to recover from a given
COHb level than from an equivalent lowering of P02
due to altitude. The difference could not be accounted
for wholly by the presence of COHb. It is possible that
the compensatory mechanisms normally activated by
lowered P02 were not activated when CO caused the
drop in O2Hb saturation. Alternatively, it is also
possible that CO exerts a specific toxic effect on the
6-3
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TABLE 1. AVERAGE DATA FOR EIGHT SUBJECTS (DIVIDED INTO TWO GROUPS OF FOUR)
TO COMPARE EFFECTS OF CO AND SIMULATED HIGH ALTITUDES
ON VARIOUS PHYSIOLOGICAL PARAMETERS (11)
Test
Effect of 15% COHb
Effect of 3,420 m simulated
altitude
O:Hb saturation curve
Affinity of Hb for Ch
Mixed venous oxygen
tension, by estimation
Shift to left
Increased (within 12 hr)
10 to 20% decrease throughout exposure
period
Shift to right
Decreased (within 24 hr)
20% increase on first day, 10% decrease on
second day, return to normal for rest of
experimental period
Ventilation
VESTPDa - rest
- work
VEBTPSb - rest
- work
Respiratory rate - rest
- work
No change
Slight increase on first day
No change
No systematic change
No change
No change
15 to 20% decrease
Slight decrease
30% increase
35% increase on first day, 50% increase by
tenth day
15 to 35% increase
15 to 35% increase
Circulation
Paco2' - rest
Pycoj" - rest
Cardiac output
Almost unchanged
Almost unchanged
Group 1 increased 27% on first day, followed
by return to normal; no change in Group 2
Continuous decrease from 90 to 75% of control
Continuous decrease from 90 to 75% of control
Group 1 increased 35 to 45% for the entire
10-day period Group 2 increased 25% by
fourth day and returned to normal by end
of stay
Mixed venous-arterial
CO2 difference, by day,
- work
Group 2 showed no change
Group 1 increased 20% on third and fifth
days, no other changes noted
Group 1 decreased 15 to 20% for the entire
10-day period; Group 2 decreased on
fourth day only
15 to 25% increase for the entire 10-day
period
Renal function
Glomerular
filtration
Renal plasma flow
Diuresis
Serum lipids
(cholesterol)
Hematocrit
Reticulocyte count
50% increase on first day, return to normal
on second day, remain within 20% of
control for rest of experimental period
40% increase on first day, return close to
pre-exposure value on second day,
remain < 15% below control value for rest
of experimental period
Increase of 400 to 500 ml
No significant change for first 4 days; 6%
increase in last 2 days (p<0 05)
No change
Threefold increase on third day; nearly
fourfold increase by sixth day
Very slight decrease but close to control
value
Very slight decrease, but close to control
value
Increase to more than twice control by
sixth day
6 to 9% increase in first 2 days
Very slight increase (43.5 to 47)
Twofold increase on third day; nearly
threefold by ninth day
'VESTPD (I/mm) - ventilation at 0°C, 760 torr, dry
"VEBTPS (I/mm) - ventilation at body temperature and ambient pressure saturated with water vapor
'P.tn, - alveolar carbon dioxide tension
dPyco1 - mixed venous carbon dioxide tension
6-4
-------
central nervous system that is unrelated to the COHb
level.
In both studies just described, the investigators
imply that the subjects exhibited similar responses, but
they do not supply supporting data. Other
investigators have commented on the great variability
in response between subjects when similar tests have
been used.
Lilienthal et al. (17) produced an impairment in
FFF in five subjects at simulated altitudes of 3,050 to
3,650 m (10,000 to 12,000 feet). Exposure to CO
(COHb increases of five to nine percent) combined
with decreased atmospheric pressure equivalent to an
altitude of 1,525 to 1,830 m (5,000 to 6,000 feet)
effected an impairment in FFF, although neither of
these stresses alone affected the FFF. The data indicate
that an increase in the COHb level of 8 to 10 percent
above resting values caused the tolerance for altitude
altitude to be lowered by 1,220 m (4,000 feet) or more.
It should be noted that impairment in FFF is not
necessarily consistent with a given COHb percentage.
For example, at simulated pressure equivalent to 1,525
m (5,000 feet), one subject showed a depressed FFF at
8.7 percent COHb; yet, in another experiment at the
same pressuie, his FFF remained constant at 10.5
percent COHb. There is a possibility that COHb
determinations in this experiment were inaccurate.
The subjects' resting COHb levels varied from 1.0 to 3.5
percent, indicating that there were smokers in the
group. Since smokers may have higher hemoglobin
values than nonsmokers (18), the assumed hemoglobin
value used in this experiment may not be valid.
By contrast, Vollmer et al. (19) have found that the
effects of CO and high altitude hypoxia are not
additive. Twenty subjects were used to study the effect
of CO and high altitude hypoxia [3,050 to 4,575 m
(10,000 and 15,000 feet)] on FFF, body sway, and size
of the red visual field all during light activity. There
was a significant impairment at simulated increased
altitude, compared with performance at sea level, with
and without exposure to CO. There was no significant
difference between the mean test scores during
hypoxia alone and the mean test scores following
administration of CO. The increases in COHb were
from 9 to 19 percent, with a final COHb ranging from
12 to 22 percent. This suggests that the resting COHb
was three percent, and indicates that some of the
subjects were smokers. Vollmer suggests that at 4,575
m (15,000 feet), any additional burden imposed by the
presence of small amounts of COHb is masked by
hypoxia compensatory mechanisms. Alternatively, he
considers that it is possible that 9 to 19 percent COHb
does not impose an important additional stress. At
hypoxia corresponding to 4,750 m (15,500 feet),
however, 4 of 17 subjects collapsed after being exposed
to CO. The tests used in this study appear to be
inadequate for predicting a serious cardiovascular
reaction; nor can their sensitivity be ranked very high.
Most of the above studies were conducted before
1950. In 1966, Denison (20) demonstrated the significant
effect hypoxia alone has on complex reaction times at a
simulated altitude of 1,525 m (5,000 feet) during light
work. At 1,525 m (5,000 feet), 8 of 10 subjects showed
slower reaction times than 9 of 10 matched controls (p
< 0.05). This effect of hypoxia was observed only
during the early stages of learning the complex
experimental task. Once the task had been learned, a
simulated altitude up to 2,440 m (8,000 feet) had no
effect. The effect of small amounts of CO on the
learning of a new task at increased altitude remains to
be determined.
Effects of Altitude on Carboxyhemoglobin Formation
Kinetics
Coburn, Forster, and Kane (21) published a detailed
theoretical analysis of the physiology and variables
that determine blood carboxyhemoglobin values in
man. Some of these are schematically depicted in
Figure 1.
The basic relationship they describe is:
A [COHb], - VcoB - PICO
= e (-tA/VbB) = e"
A [COHb]0 - VcoB - PICO
Defining terms:
[COHb]o = carboxyhemoglobin concentration
at t = o units: ml CO/ml blood
e.g. : 1 g of Hb combines stoichiometrically
with 1.34 ml of CO or 02. Assuming
15 g% Hb: 0.5% COHb = (0.005)
(1.34) (.15) = .005 ml CO/ml blood
[COHb]t = carboxyhemoglobin concentration at
t = minutes exposure
e.g. : 2.5% COHb = (0.025) (1.34) (.15) =
.005 ml CO/ml blood
A = PC02/M[02Hb]
PC02 = pulmonary capillary O2 tension at
sea level this ss 100 torr
M = relative affinity of Hb for CO and
02
[OjHb] = oxyhemoglobin in ml O2/ml
blood assuming 100% saturation
of arterial blood
6-5
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AMBIENT
PCO A
ALVEOLAR
PCO
EXOGENOUS
ENDOGENOUS
HEMOGLOBIN
CATABOLISM
OTHER HEME
0.3 ml/hr
0.1 ml/hr
CARBON MONOXIDE STORES
HEMOGLOBIN
8ml
MYO-
GLOBIN
. 1.5ml
OTHER
< 0.5 ml
0.2%/hr
METABOLISM
CO
Figure 1. Diagrammatic summary of current concepts regarding variables that influence body CO stores. (22)
e.g. : [ChHb] = (1.34) (.15) = .201 m!O2/
ml blood
B = 1 PL
DL VA
DL = pulmonary diffusing capacity for
ss 30 ml/min/torr
PL = sum of partial pressures of alveo-
lar gases excluding water Vapor
(47 torr at 37° C)
~ 760 -47 = 713 torr
VA = alveolar ventilation
=£4200 ml/min at rest
Vb = blood volume as 5,500 ml for 70
kilo man
PICO = Partial pressure of CO in inspired
air
k = Boltzmann's Constant
PicpxlO6
ppm - Barometric pressure
Vco = rate of endogeneous CO production.
This is related to heme metabolism
which can vary considerably, as a
function of red blood cell life span.
Any factor reducing RBC survival
elevates Vco. A reasonable average
value for healthy adults = 0.007 ml/min.
The most useful rearrangements of the basic relation-
ship are:
e" [A(COHb)0 - VcoB]
PICO =
ek - 1
VcoB - A(COHb),
whereby, exposure level needed to produce a given
change in COHb over a given time can be calculated:
[COHb], =
ICQ
e" [A(COHb)o - VcoB-Pico]
A
I" VcoB + P
L A
whereby, COHb resulting from a given exposure level
for a given duration of exposure can be calculated.
Careful comparison of these relationships with
available human exposure data confirms the general
validity of using a theoretical model to solve the
complicated problem of predicting COHb levels under
a variety of experimental and clinical conditions.
6-6
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Figure 2 shows a comparison of carboxyhemo-
globin levels predicted by three empirical formulas
and the theoretical model. During the first few hours
of exposure, the formulas of Hanks (23) and Forbes
(7, 24) agree very well with the model. Stewart's (25,
26) formula gives results which are higher than the
others from the beginning and this difference is
accentuated with time. Forbes' linear relationship is
only applicable to about the first four hours of
exposure. In order to examine the behavior of these
formulas with long exposure times, the curves in
Figure 3 were calculated. The inapplicability of both
Forbes' and Stewart's formulas to long exposure times
is dramatized. Results with Hanks' formula are
virtually indistinguishable from those of the
theoretical model for the first 10 hours, and with both
models COHb levels come into equilibrium at about the
same time with relatively little separation between the
two asymptotes. Figure 4 shows the comparison of
COHb equilibrium values for arbitrarily chosen
exposures calculated from the formula of the
California State Board of Public Health (22) and from
the theoretical model. The points lie very close to the
45 degree line of identity.
JD
8
a*
I I I I I I I
I I I I I I I I I
10
EXPOSURE, hours
Figure 2, Comparison of CHOb levels predicted by
three empirical formulas and a theoretical model.
It seems, therefore, that the California
mathematical formula agrees very closely with
available data when those data are properly
interpreted. In addition, of course, the theoretical
model allows computer simulation of CO exposure
kinetics with the ability to vary a large number of input
parameters. Great care must be exercised, however, in
choosing appropriate and internally consistent values
for these parameters as well as interpreting the results
of modeling in a context that is physiologically
reasonable.
The formula of Coburn et at. (21) becomes much
more versatile and general if the alveolar air equation
o
o
30 40 50 60 70 80 90 100
EXPOSURE, hours
Figure 3. Comparison of COHb levels predicted by
three empirical formulas and a theoretical model
over long exposure times.
and certain empirical relationships between altitude
and physiological variables (27) are included.
The alveolar equation is:
PA02 = FI02 (BP-47) - PAC02 [F102 + 1 - FI02]
where PA02 = alveolar oxygen tension (torr),
FIO = fraction of oxygen in inspired i
BP = barometric pressure (torr),
R
O
O
13
12
11
10
9
8
7
6
5
4
3
2
1
0
IT I \ I T I I I I T
I I I I I I I I I I I I I
8 9 10 11 12 13 14
% COHb (Theoretical)
Figure 4. Comparison of equilibrium values calculated
by the California State Department of Health's formula
and the theoretical model.
6-7
-------
PACOJ = alveolar COi tension (torr), and
R = respiratory exchange ratio =
Vco/Vo2.
With a negligible alveolar to arterial 02 gradient,
pulmonary capillary Oi tension approximately equals
alveolar 02 tension. The alveolar air equation,
therefore, can be used in the definition of factor A in
the Coburn formula.
Simulation of the effect of altitude can be
accomplished with the following empirical
relationship:
r»
-(-
P = 760 e
7924
where P = atmospheric pressure (torr)
a = altitude above sea level in meters.
The calculated atmospheric pressure at altitude can
thus be substituted for actual measured pressure in the
alveolar air equation and in the conversion from ppm
CO to PICO. It is important to remember that the
conversion from mass to volume units in aerometry
also involves atmospheric pressure. Thus, the details
of instrument calibration must be known before one
can decide whether or not it is appropriate to include
an atmospheric pressure correction factor in calcu-
lating Plco from mass units.
% RHb = 2.76 e
2133
where % RHb is reduced hemoglobin formed as a
result of exposure to altitude. This must be added
to the % RHb present in a given individual at sea
level and also taken into consideration when
calculating factor A in the Coburn equation.
Permutt and Farhi (28) have emphasized the
importance of considering the effect of CO exposure
on tissue oxygenation, and this effect can also be
calculated from known physiological variables. Thus,
reliable mathematical modeling of the effect of CO
exposure on tissue oxygenation is possible.
Effects of Altitude and Carbon Monoxide on the
Fetus
It has been stated that the fetus under unpolluted sea
level conditions lives at Po comparable to the summit
of Mt. Everest. (29) If this is the case, then decreases in
fetal blood oxygen content, such as caused by four to
five percent COHb, if prolonged, may cause brain
damage and mental retardation. (30-32) The
implications of these studies are great, particularly in
the case of the expectant mother living at high
altitudes who is exposed tg moderately high levels of
CO from auto exhaust or smoking.
ECOLOGY
From an ecological standpoint, a number of effects
of altitude on air pollution should be considered. The
pollutant conditions most frequently associated with
altitude are confinement or entrapment within an
inversion layer. The impact on the plant environment,
thus, may be accentuated either because of an
exposure of longer duration or a buildup of pollutant
concentrations.
Within the first few hundred feet above the
earth's surface the atmospheric composition is
influenced be vegetation. Plants growing at high-
er altitudes are susceptible to the same pollutants
as plants at lower levels. The pollutants reaching
the vegetation at higher altitudes are the result of
advection from air pollution sources at lower
levels. Ozone transport from Los Angeles has
been shown to be the cause of death of Jeffrey
(Pinus Jeffreyi) and Ponderosa (P. ponderosa)
pines in the San Bernardino and the Sierra Nevada
Mountains. (33-36) Transport of sulfur dioxide
could also cause vegetational injury at high
altitudes.
Vegetational injury from both sulfur dioxide and
ozone is dependent on pollutant entry into the leaves
through the stomata. Environmental conditions, such
as water stress, can result in closed stomata and
protection from injury. (5) The vegetation of high
altitude areas with low rainfall would probably be less
susceptible to injury. Plant life may also remove
pollutants from the atmosphere by presenting a
barrier to particulates and curtailing their dispersion,
or by adsorbing them. The latter processes may be
important in scavenging pollutant gases or in sound
absorption where noise is excessive. Pollutants
removed via these mechanisms may be deposited into
the soil. Although dependent on edaphic factors, such
deposition does not usually enrich the soil.
Altitude, as defined in this report, relates primarily
to ecosystems or their components located on plateaus
or in mountainous regions at land elevations from
1,525 m (5,000 feet) up to at least 3,050 m (10,000 feet)
or more. At these land elevations the distribution of
biotic communities is complicated due to the diversity
of physical conditions and due to the many species
unique to mountain biomes. (37) Growing conditions
for plants in the mountains become increasingly less
favorable with increasing land elevation. (34) Because
of the indigenous nature and lack of diversity of
species, any pollutant stress may trigger exaggerated
changes in these intricately balanced systems.
6-8
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For example, solar insolation increases (38) and the
light spectrum changes with increased altitudes. "The
alpine environment is more severe than the arctic
during the growing season...and higher ultraviolet
radiation adds to this severity." (39) Biological
ecosystem components at higher land elevations,
although receiving more ultraviolet radiation than
those at sea level, have adapted to such conditions.
(38,40,41) Pirschle (42) studied plants from high land
elevations in controlled chambers under varying light
conditions. Those from alpine areas subjected to
ultraviolet irradiation, although inhibited in their
elongation, suffered no other damage, while test plants
from sea level were killed. The amplitude of ultraviolet
radiation is tempered by ozone concentrations.
Biological aerosols, suspensions of microorganisms
in the air, and aeroallergens are ubiquitous in nature.
The altitudes at which these organisms may exist or
may concentrate and yet remain viable are shown in
Figure 5. (43) Most show seasonal fluctuations, while
some may occur at specific times only. Most exhibit
extreme dispersion which is beneficial in maintaining
the species since they are important parts of some of
our ecosystems. On the other hand, undesirable species
may become established in distant locations following
aerosol transport or may act detrimentally in spreading
disease or allergenic episodes within man's
environment. (44-45)
Temperature inversions limit normal upward
migration of microorganisms from the earth's surface,
resulting in large microorganismal populations below
the inversion.(46) Such confinement of disease-causing
or allergenic microorganisms can result in episodic
responses in the local human population.
MATERIALS
No known research has been published in which the
influence of elevation on air pollution damage to
materials has been studied.
Several studies (47-56) concerning textile materials,
however, have shown that the presence of sunlight
produces a synergistic damage effect (i.e., the overall
damage produced is greater than the sum of the
damage effects produced by sunlight and certain
pollutants individually). From a theoretical
standpoint, therefore, it is reasonable to assume that
because of increased ultraviolet radiation at higher
altitudes, reaction rates between pollutants and
materials would further accelerate. Of course the
deteriorating action of sunlight alone, which for some
materials is much more severe than that of pollutants
alone, would also increase and could conceivably have
greater significance at higher altitudes.
UJ
Q
D
30
27
24
21
18
15
12
'Note: It is not known what effects
pollutant intermixing
will have on such aerosols.
10"
10'6 io'5
10
ID'1
10''
CONC., jug/g air
Figure 5. Concentration of aerosols with altitude. (43)
The lack of research precludes any definitive
conclusions. However, a hypothetical conclusion is
that for a given level of air pollution, damage to
materials is probably greater (to an unknown extent)
at higher than at lower altitudes because of the
potentiating effect of the more intense insolation.
WEATHER, VISIBILITY, AND CLIMATE
In Chapter II, the impact of the increase in altitude
on weather parameters was discussed. It is now
appropriate to describe the effects of the variations of
weather factors on air pollution.
The greater amount of solar radiation experienced
at higher altitudes has the effect of enhancing the
formation of secondary pollutants such as
photochemical smog. The greater amount of
ultraviolet radiation combined with increases in
automobile traffic may account for the increased
photochemical smog noted at major metropolitan
areas in the Rockies. The greater amount of solar
6-9
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radiation available also has the effect of enhancing the
occurrence of convective air currents near the ground,
thereby promoting a well-developed mixing layer
daily. For example, at Denver throughout the year,
the mean afternoon mixing heights vary from about
915 m (3,000 feet) above ground level in January to
about 3,340 m (11,000 feet) in July. These mixing
height characteristics for Denver are typical of the
mountain and high land elevation areas of the West
and exceed the mean mixing heights experienced by
the rest of the nation.
Similarly, the loss of infrared radiation by the
earth's surface at night is enhanced at high altitudes.
Under clear skies the temperature profile at night is
drastically changed by the rapid radiational cooling of
the ground and the subsequent cooling of the layers of
air near the surface. This creates an inversion.
According to Hosier, (57) the occurrence of low-level
inversions is greatest in the Appalachian and Rocky
Mountain chains. Low-level inversions are present 30
to 50 percent of all hours in these areas. Under an
inversion the atmosphere is stable, vertical
interchange of air is inhibited, and pollutants tend to
remain near the ground.
In the warm season of the year, with much ground
heating, temperatures normally decrease rapidly with
increasing height at high altitudes, but in winter,
especially when the ground is covered by snow,
temperature inversions near the earth's surface are
frequent and persistent. Therefore, vertical dispersion
is favored during the summer and limited during
winter. Atmospheric conditions favoring pollution
episodes occur in the winter in the Rocky Mountains
and Great Basin areas of the western United States.
The rising air currents on the windward slopes of the
mountains tend to disperse pollution through deep
layers. Also the high frequency of "precipitation,
brought about by the forced ascent of air particularly
at high altitudes in the coastal ranges, tends to wash
out pollutants in the air mass. Snow is more efficient in
washing out pollution than rain. Washout of
pollutants, and damage to surface receptors, may
occur if the concentration of pollutants in the air mass
is high. Moreover, the pollution may find its way into
lakes and streams and contribute to the contamination
of these waters. Acidic particulate species may
adversely affect not only lakes and streams, but also
soil conditions in some areas.
In general, the visibility within air masses over the
higher altitudes in the western United States remains
good. However, in the vicinity of large metropolitan
areas and near large power plant complexes, reduction
in visibility is being observed. The major visibility
problems at the present time occur where large
metropolitan areas, with a multitude of pollutant
sources, are located in topographical depressions
which favor the occurrence of low wind speeds and
stable air near the earth's surface. The high frequency
of stagnating air masses in the late fall and winter favors
the occurrence of air pollution episodes in
metropolitan areas such as Denver. (58-61)
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6-12
-------
APPENDIX A
EFFECTS OF ALTITUDE ON
AEROSOL DYNAMICS
The atmospheric aerosol processes of interest that
might be affected by high altitude are: nucleation, con-
densation, dry and wet deposition, coagulation, and
gas-to-aerosol conversion. Aerosol instrumentation
response and performance might be changed for de-
vices calibrated at sea level and used at high land
elevations. Also, aerosol lung deposition efficiencies
at high altitudes might be changed with respect to
those at sea level. Modification of the rates of these
processes, instrumentation performance, and lung
deposition efficiencies will be discussed. However,
variations in source emission characteristics as a
function of altitude will not be addressed here, nor will
the performance of industrial devices for primary
aerosol control (such as electrostatic precipitators and
wet scrubbers).
The important mass transport parameters for air
that change with altitude and that might be expected
to influence aerosol dynamics are: air density (or
molecular mean-free path), air viscosity, the accelera-
tion due to gravity, and temperature. The change in
density, molecular mean-free path, and viscosity for
increasing altitude is shown in Table A-l. The
viscosity is essentially constant down to pressures
below 0.3 kg m~2(0.02 torr). Also, over the contiguous
48 states, the acceleration due to gravity varies within
the range of 9,790 to 9,809 m sec"2, which for our
purposes can be considered constant. Thus, the only
mass transport parameters for air exhibiting
significant dependence on altitude are density
(molecular mean-free path) and temperature.
Processes
Since an exact theory for nucleation is lacking for
heterogeneous mixtures, an equation for the
production rate of critical nuclei in the atmosphere as
a function of the collision frequency, temperature, and
the Gibbs free energy of adsorption cannot be stated.
The heterogeneous nucleation rate per unit of
substrate surface area decreases as pressure decreases.
However, the dependence of the rate on temperature
and the Gibbs free energy may be more important and
could result in increases or decreases at high altitude
cities compared to sea level for the same mass
concentration of nucleating monomers. The rate of
condensation of low-vapor pressure gases onto
existing aerosols (4) can be described by
F =
4?rR2
D
1 + cX/R
where F = mass flux per unit area to a spherical
particle,
= mass flux to a spherical particle of
radius R,
R = radius of sphere,
TABLE A-1. MASS TRANSPORT PARAMETERS (T = 293° K)
Elevation, meters
Sea level
1,524 m
3,048 m
(5,000 ft)
(10,000ft)
Standard
Pressure, kg
10.33
8.56
7.07
(760
(630
(520
m'1
torr)
torr)
torr)
Density
242" K
1.56
1.30
1.06
'. kg m'1
293° K
1.29
1.07
0.88
Molecular Mean-
Free Path' nm
242° K
54.0
64.8
79.2
293° K
65.3
78.8
95.4
Viscosity '
242° K
1.54
1.54
1.54
, kg m"' sec"'
293° K
1
1
1
.81
.81
.81
'From ret 1 "From ref 2 'From ret. 3
A-l
-------
D = gaseous diffusion coefficient of
condensing specie,
c = constant («* 1.5),
X = molecular mean-free path,
n,. = concentration of condensing specie far
from the particle's surface, and
n, - concentration of condensing specie at
the particle's surface.
The ratio of fluxes at sea level and at 1,524 m has been
calculated for T = 20° C (293° K), assuming that the
mass concentrations of the condensing specie are
equal. The ratios are presented in Table A-2 for var-
ious aerosol diameters.
TABLE A-2. RATIO OF FLUXES
(T= 20° C [293° K])
For polydisperse systems,
dn(ri,r2,t)/dt = -K(n,r2)n(ri,t)n(r2,t),
where K(ri,r2,) is the coagulation constant for particles
of radius r, with particles of radius rj, and is a function
of the mean thermal speeds, concentration gradient
thicknesses, and particle diffusion coefficients. With
the formula given by Fuchs (5), the ratio of
coagulation coefficients for various combinations of
atmospheric pressure and temperature can be
calculated, as shown in Table A-3.
TABLE A-3. RATIO OF
COAGULATION COEFFICIENTS
Diameters,
O.pVO.02
0.02^ 0.2
Diameter, ^m :
0(1,524 m)a
(sea level)"
0.01
1.00
0.1
1.00
1
1.17
KAb
Kcl
KAb
"A T = 293° K, / = 0.065 ^m, ,
1.03
0.92
i = 1.81 kg m "
1.36
0.97
•sec" '
1.20
0.97
'Pressure = 8.56 kg rrfj "Pressure = 10 33 km rrf2
The half-life for molecular collisions with urban
aerosols is on the order of a second; thus, condensa-
tion rates would be expected to be approximately the
same for sea level and high altitude cities for T = 20° C
(293° K). Lowering the temperature (at constant
pressure) from 20°C (293°K) to -31°C (242°K)
reduces the gaseous diffusion coefficient by about 30
percent, but also significantly lowers the vapor
pressure (ns) of the condensing species. The two effects
act in opposition and for various species the rate may
be increased or decreased. Because the half-life for
collisions is so short, the change in the condensation
rate due to pressure and temperature changes is
probably not significant, although a decrease in the
condensation rate may cause an increase in the
nucleation rate. For both sea level and high altitudes, a
lowering of the temperature depresses the equilibrium
vapor pressure and may result in large shifts of
condensible vapors from the gas phase to aerosols.
The process of coagulation changes the atmospheric
aerosol size distribution. The decrease in the aerosol
number concentration can be expressed by the basic
equation
dn/dt = -K0n2
for a monodisperse aerosol, where
n = aerosol number concentration, and
Ko = coagulation constant (monodisperse).
•B- T = 293° K, / = 0.079 (im, ^ = 1 81 kg m " 'sec" '
'C T = 242° K, / = 0.065 i*m. ju = 1 54 kg m ' 'sec '
At high altitude as compared to low altitude at the
same temperature, the coagulation rates for fine
aerosols will be faster, perhaps by as much as 20 to 40
percent. However, when the temperature at high
altitude is low (-31°C), the rates are essentially the
same.
The process of atmospheric dry deposition is not
completely understood. However, in general terms,
the deposition of aerosols with diameters greater than
about 0.3 nm is explained in terms of sedimentation
and impaction. For sedimentation, the terminal fall
velocity for viscous and non-viscous flow is (5)
v _
s
(PP -p.)g]
3p,Co J
where V, = terminal fall velocity,
R = radius,
PP = density of particle,
p, = density of air,
g = acceleration due to gravity, and
CD = drag coefficient.
Usually, for atmospheric aerosols, the particle density
(pp) is in the range 1 to 2.5 g/cm3, which is large in
comparison to the air density (0.00129 g/cm3 at sea
level). Thus, in the above equation, the term (pp-pa)
A-2
-------
can be replaced by pp for atmospheric applications.
For viscous flow, the drag coefficient is
CD = 24/Re
where Re = particle Reynolds number
= 2VR/H/i|,
V = fall velocity, and
77 = air viscosity.
Therefore, the terminal settling for viscous flow is
R
v-= I
which is independent of the air density. The values for
viscosity (17) and acceleration due to gravity (g) are
constant for changes in pressure, causing the
sedimentation rates for viscous flow to be the same for
sea level and high altitude sites (assuming constant
temperature). Sedimentation in nonviscous flow must
also be considered since the atmosphere is in turbulent
motion. The drag coefficient for non-viscous flow
must account for the air inertia terms in the equations
of motion, making it more complex. For a force of Ig
on the particle, the above equation can be written as
CDRe2 = 32
Davies' (6) empirical expression (not shown) for the
Reynolds number as a function of CoRe2 was used to
calculate the settling velocities for non-viscous flow at
sea level and 1,524 m at a temperature of 20° C
(293° K); the particle's density was taken to be 2.5
kg/ m3. The ratios of settling velocity are in Table A-4.
The settling velocity for particles with a geometric
diameter of lOOfxm is approximately four percent
greater at an altitude of 1,524 m than at sea level for the
same temperature; for smaller diameters, the ratio
decreases to a limit of unity. The settling velocity at
TABLE A-4. NON-VISCOUS FLOW SETTLING
VELOCITY RATIOS
(T = 293°K, p, = 2.5 kg m"3)
Diameter, ^m :
V. (1,524m)'
V. (sea level)"
V, (sea level)',
cm sec"'
: 1
1.00
'• 0.007
25
1.0
4.6
50
1.02
17.3
75
1.03
34.3
100
1.04
54.7
1,524 m for a pressure of 8.56 kg m 2 and a temperature
of -32° C (241.6° K) has been calculated and its ratio to
the settling velocity at sea level for a pressure of 10.33
kg m"2 and a temperature of 20° C (293 °K) is given in
Table 5.
TABLE A-5. NON-VISCOUS PLOW SETTLING
VELOCITY RATIOS (p, = 2.5 kg rrf3)
Diameter, /im
V. (1,524 m)'
V, (sea level)"
1 25 50. 75. 100
1.3 1.2 1.2 1.1 1.0
•Pressure = 8.56 kg m~!, p. = 107 kg m"!
'Pressure = 10.33 kg m"1, A = 1.29 Kg rtr1
•Pressure = 8.56 kg nf2, p. = 1 29 kg m"1, T = 242°K, M = 1.54 kg rrf' sec"1
"Pressure = 10.33 kg m"1, p. = 1.29 kg rrf', T = 293° K, v. = 1.81 kg m"' secM
Settling for aerosols with diameters less than 75 pm at
high altitudes during cold periods will be greater (10 to
20 percent) than at low altitudes during warm periods,
but at equal temperatures there is very little difference.
Impaction is also an important dry deposition
process for aerosols with diameters in the range 0.3 to
100 /*m. Impaction of aerosols onto environmental
surfaces can be thought of as a function of the stop
distance, which is the path length traversed by a
particle that is in motion and is diverted into still air.
The stop distance, L, is expressed as (5)
2V0r2 pp
9rj
where Vo = initial velocity of the particle upon entering
still air zone. Simplistically, one can visualize the
aerosol to be in turbulent motion in the atmosphere
and directed toward an environmental object that has
a thin viscous sublayer (thickness = X) of air at its
surface. If the aerosol has a stop distance (L) that is
greater than X, it will be impacted; if not, it will likely
be swept away by the turbulent air. In the above
equation, the viscosity is independent of pressure;
thus,the stop distance for high altitude and sea level is
constant for particles with the same radii, density, and
initial velocity. However, the thickness, X, of the
viscous sublayer (5) is approximately inversely
proportional to the fluid Reynolds number (Rer) as
X« Ref"' oc p,'1.
Thus, the ratio of thickness of the viscous sublayer for
1,524 m to sea level is
X (1,524 m) ^ p. (l,S24m)''
X (sea level) ~ p, (sea level)
« 1.20
The particle efficiency of penetration through the
viscous sublayer is inversely proportional to X; thus,
the dry deposition flux due to impaction is expected to
be approximately 20 percent less at an elevation of
1,524 m with respect to sea level at constant
temperature 20°C (293°K). However, for a
A-3
-------
temperature of-31 °C (242° K) at an altitude of 1,524 m
and for a temperature of 20° C (293 ° K) at sea level, the
ratio becomes unity; there should be no difference in
the dry deposition flux due to impaction. It is difficult
to resolve the contributions of settling and impaction
to dry deposition of aerosols in size range 0.3<
diameter < 100 yum. Most likely, dry deposition
velocities at high altitudes and sea level will be
approximately the same for particles in the range 0.3 <
diameter < lOOyum.
Settling and impaction are generally thought to be
unimportant for the dry deposition of atmospheric
aerosols with diameters less than approximately 0.3
nm. The dry deposition of these fine aerosols can be
visualized as follows: the fine aerosols are in a
turbulent eddy that moves toward an environmental
object that has a variable thin viscous sublayer
(thickness =5) of air at its surface. The fine aerosols are
transported to the edge of the laminar layer, and may
be deposited only by diffusing across the viscous
sublayer. The dry deposition flux of fine aerosol to the
surface is (5)
N-D-f-
= D (ns-ns) (6
where N = flux of aerosol to the surface,
n{ = aerosol concentration at the edge of
the viscous sublayer of thickness,*,
ns = aerosol concentration at a distance of
one molecular mean-free path of air
from the surface of the environmental
object, and
D = particle's Brownian diffusion
coefficient.
Since, ns = O, the equation for the flux becomes
N = Dns/8.
This equation must be integrated over the size distri-
bution to account for the dependence of the diffusion
coefficient on the particle radius. Both the diffusion
coefficient and thickness of the viscous sublayer vary
with altitude. The diffusion coefficient, D, is
D =kTB
B = [1 + Ap+Qpexpt-br/O^TTTjr)-'
where k = Boltzmann's constant
T = temperature, °K
B = particle mobility
A = 1.246
/ = molecular mean-free path of air
Q = 0.42
b = 0.87
The diffusion coefficient is independent of the density
of the aerosol, but dependent on changes in T, /, and M-
The ratios for diffusion coefficient for an altitude of
1,524 m and sea level are shown in Table A-6.
Since the viscous sublayer is inversely proportional
to the air density, the ratio of drv deposition for fine
aerosols is shown in Table A-7. Thus, for a
temperature of 20°C (293°K) at upper altitude and at
sea level, the dry deposition rate of fine aerosol would
be about the same. However, if the temperature at the
higher altitude is 242° K, then the fine aerosol
deposition rate will be about 20 percent lower than at
sea level.
Wet deposition of gases and aerosols is difficult to
compare for high and low altitudes, since in-cloud
scavenging (rainout and snowout) and below-cloud
scavenging (washout) are complex phenomena. The
removal efficiencies depend strongly on such
parameters as the raindrop or snowflake size
distribution, collision efficiencies, electrical charge,
crystal type, shape, fall speed, etc. Since these
processes are still not completely understood,
speculations regarding differences related to altitude
will not be made.
Aerosol Instrumentation
The performance of aerosol samplers and monitors
calibrated at lower altitudes may be affected when
TABLE A-6. RATIO OF DIFFUSION COEFFICIENT
Diameter, /*m :
D (1,524 m)1
D (sea level)'
D (1,524 m)c
0 (sea level)"
D (sea level)" ;
cm2sec
0.005
1.20
0.73
2.08x10"'
0,01
1.20
0.74
5.28 x 10"1
0.05
1.20
0.81
2.37 x 10~5
0.1
1.13
0.87
6.85 x 10"'
0.5
1.05
0.96
6.26 x 10'7
•T = 293'K /« 78.8 nm, p = 1.81 kg m ' sec '
'T = 293°K / = 85.3 nm, p = 1.81 kg m'1 sec '
T = 293CK / * 64.8 nm, p = 1 54 kg m"' sec'1
A-4
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TABLE A-7. RATIO OF FINE AEROSOL FLUX
Diameter, prn
N (1.524 m)'
N (sea level)"
N (1.524 m)c
N (sea level)"
0.005
1.00
0.73
0.01
1.00
0.74
0.05
0.98
0.81
0.1
0.94
0.87
0.5
0.87
0.96
•T = 293°K, ft = 1.07kgrrT'
1 = 293"^ p. = 129 kg rrT!
T = 242* K, p. = 1.30 kg m"J
used at high altitudes. The devices that will be affected
are those whose principle employs the resistance of the
air to the motion of the aerosol particles being sized.
Such devices include: sedimentation chambers,
horizontal and vertical elutriators, impactors, virtual
impactors, cyclones, diffusion batteries, electrical
mobility analyzers, etc. The parameters that can be
expected to modify the performance are viscosity (due
to temperature differences) and the mean-free path of
air. Sedimentation chambers and horizontal and
vertical elutriators are not now commonly used for
determining atmospheric size distributions or for
aerosol classification; however, they are of historic
interest. Their performance depends on the
sedimentation velocity and air flow profile (for
elutriators). The changes in sedimentation velocity as
a function of altitude can be treated directly and has
been discussed above. The flow profile within the
elutriator can be thought of as a function of the fluid
Reynolds number
Ref = ptvL/ ft
where L = characteristic dimension of conduit.
v = average fluid velocity.
If the dimension, L, is constant, for a constant
velocity, v, the Reynolds number depends on air
density and viscosity; however, difference between
the Reynolds number for high and low altitudes and
various temperatures is expected to be less than 10
percent. Thus, significant changes in performance due
to modification of the flow profile are not expected.
Cascade impactor is probably the most commonly
used research device for aerosol classification. The
efficiency of an impaction stage depends on the Stokes
number (7),
Stk = CppVd2/18ML
where C = Cunningham slip correction factor,
v = average velocity of air in jet, and
L = diameter (or width) of jet.
A stage is usually characterized by experimentally
determining the diameter (dso) for which 50 percent of
the particles are impacted. (For round jets, dso
corresponds to Stk = 0.2.) For aerosols with diameter
greater than 0.2 /tin, C does not change significantly
with altitude (pressure and temperature) to be of
concern. However, the viscosity is temperature-
dependent and a change from 20° C (293° K) to -31°C
(242° K) will lower the dso values by about eight
percent for a constant volume flowrate. Also, it can be
expected that the cut point diameter for virtual
impactors should exhibit a similar temperature
dependence. The dependence of flow profiles on the
fluid Reynolds number (which probably varies less
than 10 percent) is not expected to be important with
regard to performance.
There is not yet an exact theory for the operation of
cyclones. Fuchs (5) has made an estimate of the
efficiency (E) of cyclones, considering turbulent flow:
E = 1 - exp (-7rvrs/2h)
where v = average flow velocity,
s = number of turns of spiral,
h = width of inlet, and
T - relaxation time
= 2 r2pp/9M.
Although the equation cannot be considered to be
exact, it should be suitable for deducing the influence
of altitude factors on the performance of cyclones. For
a constant volume flow rate, the only parameter that
needs to be considered is the viscosity. Since viscosity
is independent of pressure, only temperature
variations are imporatant. From the above equation,
it is estimated that a cyclone with a 50 percent efficiency
for particles with a 4-jum diameter, operated at 20° C
(293° K), will have an efficiency of 56 percent if
operated at -31°C (242°K) with the same volume flow
rate (uncorrected). However, if the cyclone is operated
at constant air mass flow rate, its efficiency is
estimated to be 63 percent at -3I°C (242° K) for 4/im
diameter particles at 1,524 m, compared to 50 percent
at sea level.
The determination of aerosol size distributions for
particles with a diameter less than 0.2 /um has been
performed with diffusion batteries (5). The technique
is based on a measurement of the decrease in aerosol
number concentration as a function of length of the
A-5
-------
capillary conduit; the depletion of aerosol results from
diffusion deposition on the conduit walls. Thus, the
technique is dependent upon the aerosol diffusion
coefficient, which is a function of pressure and
temperature. The correction can be made to the
diffusion coefficient as described above in the section
on dry deposition.
Electrical mobility analyzers are coming into
frequent use for aerosol size measurement of particles
with diameters less than 0.5 /zm. There are two sources
of possible performance modification for this device:
the aerosol charging section and the aerosol drift tube.
Usually, the charging section is operated with a constant
corona current to yield an ion density*charging time
(Not) product of 106. The degree of aerosol charging
(number of particles charged and charge distribution)
depends on the ion mobility, which is proportional to
the molecular mean-free path. An instrument
calibrated at sea level and operated at constant corona
current at an altitude of 1,524 m will have its (Not)
decreased by 20 percent, due to the 20 percent increase
in ion mobility. The effect on aerosol charging is
nonlinear, and the charge distribution on aerosol is
very sensitive for small (Not) values of 106 or less. If a
(Not) value of 107 (at sea level) is employed, the
modification of charge distribution is less, but greater
multiple charging occurs, lowering the resolution of
the instrument. In the drift tube, sizing is inferred from
the quantity of charged aerosol that passes through the
tube as a function of applied electrical field. Thus, the
aerosol removal is a function of electrical mobility, Z:
Z = qB,
where q = number of electrons on particle, and
B = aerosol mechanical mobility (given in
discussion above on dry deposition/
diffusion).
B has a dependence on both the molecular mean-free
path and viscosity. The value of B at a constant
termperature, 20° C (293° K), will increase by 20
percent for 0.005 Mm and 5 percent for 0.5 pm
diameter particles in going from sea level to 1,524 m.
However, if the temperature is -31°C (242°K) at 1,524
m, B is depressed with respect to values at sea level
20°C (293°K). The performance of the mobility
analyzer is expected to be sensitive to altitude
parameters, but due to the complexity of the
calculations, no degree of deviation from performance
at sea level is anticipated.
Aspiration inlets for aerosol sampling are known
to have a variable efficiency for coarse particles (5).
The performance of inlets at this time cannot be
described exactly, but the same aerosol properties that
govern dry depostion of fine and coarse aerosol are
expected to govern inlet performance. It is expected
that inlet performance at sea level and at 1,524 m is
unity for particles with diameters less than 1 /*m. The
change in stop distance is used to estimate the change
in performance at high and low altitudes. Since the
stop distance is dependent upon the viscosity, but
independent of air density, one would expect the
coarse particle performance of inlets to depend only
upon temperature. A lowering of the temperature from
20° C (293° K) to -31° C (242° K) increases the
stop distance by about 20 percent, which implies
qualitatively that the efficiency of an aspiration inlet
will decrease as temperature decreases. Generally, it
would be expected that the efficiency of the high
volume sampler would be unity for fine aerosols at
high and low altitudes, but its efficiency (less than
unity) for coarse aerosol would be decreased as
temperature decreased.
It should be recognized that certain devices (such as
most single particle light-scattering instruments) that
do not measure a resistance-to-motion parameter have
the same calibration at both high and low altitudes.
Care must be taken with nephelometric measurements
in removing the air scatter, which is a function of air
density. Condensation nuclei counters would be
expected to have a slightly lower counting efficiency at
higher altitudes, due to the decreased thermal
conductivity of air at lower pressures.
REFERENCES
1. Fairbridge, R. W (ed.). The Encyclopedia of Atmospheric
Sciences and Astrogeology. Encyclopedia of Earth Sciences
Series, Vol. II. Van Nostrand Reinhold Publishing Corp., New
York, 1967.
2. Weast, R. C. Handbook of Chemistry and Physics, 57th
Edition. CRC Press, Cleveland, 1976.
3. Bretsznajder, S. Prediction of Transport and Other Physical
Properties of Fluids. Pergamon; Press, New York, 1971.
4. Hidy, G. M. and J. R. Brock. The Dynamics of Aerocollodial
Systems. Pergamon Press, New York, 1970.
5. Fuchs, N. A.The Mechanisms of Aerosols. Pergamon Press,
New York, 1964.
6. Strauss, W. Industrial Gas Cleaning. Pergamon Press, New
York, 1966.
7. Friedlander, S. K. Smoke, Dust and Haze. John Wiley and
Sons, New York, 1977, pp. 162-164.
A-6
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APPENDIX B
OTHER SYSTEMS - HIGH ALTITUDES
Precise data on the potential scope of the problems
due to carbon monoxide for high altitude residents
and visitors are not available. Approximately 2.2
million people live at altitudes above 1,524 m in the
United States. These figures do not present a complete
picture of potential numbers of individuals who may
be subjected to carbon monoxide at these altitudes
because the tourist population in these areas is high in
both summer and winter. Furthermore, proper tuning
of automobiles for high altitude travel is uncommon
and the influx of visitors with cars that emit carbon
monoxide and other contaminants may prove to be an
important factor in raising pollution exposure to an
unacceptable point.
Ambient air standards set at sea level are not
applicable for high altitude sites. The United States
Environmental Protection Agency's (EPA) primary
standards are expressed in milligrams per cubic meter
of air. At 1,540 m, each cubic meter contains
approximately 18 percent less air than at sea level.
Therefore, allowable concentrations of carbon
monoxide in air in a city like Denver will be 22 percent
higher than at sea level (i.e., a 10-mg/m3 maximal
permissible 8-hour average is equivalent to 10 mg/m3
at sea level but 11.8 mg/m3 at Denver's altitude).
Carbon monoxide exposure may aggravate the
oxygen deficiency at high altitudes. When high
altitude and carbon monoxide exposure are combined
(Table B-l), the effects are apparently additive. It
should be noted that each of these, decreased Po2 in the
air and increased carboxyhemoglobin, produce
different physiologic responses. They have different
effects on blood Po2, on the affinity of oxygen for
hemoglobin, on the extent of oxyhemoglobin
saturation (carbon monoxide hypoxemia shifts the
oxyhemoglobin dissociation curve to the left, and a
decrease in PAo2 shifts it to the right), and on
ventilatory drive.
The most supportive information on the additive
nature of CO hypoxia and hypoxic hypoxia originates
from psychophysiologic studies, and even these are
not as persuasive as one would desire. Blackmore (1)
analyzed the cause of aircraft accidents in Britain and
TABLE B-1. APPROXIMATE PHYSIOLOGICALLY
EQUIVALENT ALTITUDES AT EQUILIBRIUM
WITH AMBIENT CO LEVELS
Ambient
CO Concentration
mg/m1
0
28.6
57.3
114.5
ppm
0
25
50
100
Actual Altitude (meters)
0 (sea level)
1.524
3,048
(Physiologically Equivalent Altitudes with COHb)
0 (sea level)
1,829
3,048
3,749
1,524
2,530
3,658
4,663
3,048
3,962
4,694
5,486
found that carboxyhemoglobin levels provided
valuable information relative to altitude and CO
sources. The relatively high levels found could be
attributed to equipment failure, smoking, and fires.
No data are available on the effects of carbon
monoxide on native inhabitants of high altitude or on
the reactions of these natives when they are suddenly
removed to sea level and possible high ambient carbon
monoxide concentrations.
McFarland et al. (2) showed that changes in visual
threshold occurred with carboxyhemoglobin as low as
five percent or a simulated altitude of approximately
2,425 m. Halperin et al. (3) further noted that recovery
from the detrimental effects on visual function lagged
behind the elimination of carbon monoxide. However,
the data given were sparse and the variability among
the few subjects and the day-to-day variation were not
given. Vollmer et al. (4) studied the effects of carbon
monoxide at simulated altitudes of 3,070 and 4,555 m
and reported that there were no additive effects of
carbon monoxide and altitude. They suggested that
the effects of carbon monoxide were masked by some
compensatory mechanisms. The data presented were
not convincing. Lilienthal and Fugitt (5) indicated that
the combination of altitude (1,540 m) and a five to nine
percent carboxyhemoglobin induced a decrease in
flicker fusion frequency, although neither one alone
had an effect. They also reported that the presence of
8 to 10 percent carboxyhemoglobin was effective in
reducing altitude tolerance by some 1,215 m. Forbes et
al. (6) found that, during light activity at an altitude of
4,875 m, carbon monoxide uptake was increased,
probably owing to the hyperventilation caused by the
respiratory stimulus of decreased Po2 • Evidence that
B-l
-------
CO elimination was similiar at sea level and at
altitudes up to 10,000 m was obtained by several
investigators. (7,8) Increased ambient temperatures
up to 35°C and hard physical work increased the rate
of elimination. (4) Pitts and Pace (9) stated that every
one percent increase in carboxyhemoglobin (up to 13
percent) was equivalent to a 109-m rise in altitude if
the subjects were at altitudes of 2,100 to 3,070 m. Their
observations were based on changes in the heart rate
response to work. Subjects who may have been
smokers were not identified.
Two groups of investigators have presented data
comparing the physiologic responses of subjects to
altitude and carbon monoxide where the hypoxemia
due to altitude and the presence of
carboxyhemoglobin were approximately equivalent.
In one study, (10) the carboxyhemoglobin varied
around 12 percent (although the mode of presentation
of carbon monoxide was such that carboxyhemoglobin
ranged from 5 to 20 percent during the carbon
monoxide exposures) and the altitude study was
conducted at 3,977 m. The second study (8) compared
responses of altitudes of 4,000 m and a
carboxyhemoglobin content of 20 percent. In both
these studies, carboxyhemoglobin content was in
excess of that anticipated for typical ambient
pollution. However, it was suggested that the effects
attributable to carbon monoxide and to altitude were
equivalent.
It was recommended on theoretical grounds that the
ambient carbon monoxide in tunnels being
constructed at 3,859 m should not exceed 29 mg/m3.
(11) The maximal aerobic capacity is reduced
approximately 20 percent in individuals exposed to an
altitu ie of 3,085 m..Weiser et al. (12) reported that
max Vo2 was significantly impaired in subjects living
at 1,700 m when their COHb levels were five percent.
However, this decrement is similar to that seen in sea
level residents. No data are available for higher
altitudes. Brewer et al (13) conducted a study on
residents of Leadville, Colorado (3,085 m). The mean
COHb level in smokers at this altitude was higher than
that of smokers at sea level. This increased degree of
hypoxemia may have contributed to the elevated red
cell mass observed, since individuals who stopped
smoking demonstrated a reduction in their red cell
mass.
The most important information regarding carbon
monoxide exposures and altitudes, the preciseness of
their potentially additive effects., has not received
much attention, and what little information there is
has been obtained by assuming simple additive effects.
(14) It has not been verified by direct experiments.
Recently reported epidemiological data indicate a
correlation between complaints of cardiorespiratory
problems and ambient CO levels in Denver residents.
The results suggest there is a threshold effect at levels
lower than had been previously suggested. (15, 16).
REFERENCES
1. Blackmore, D. J. Aircraft Accident Toxicology: U.K. Exper-
ience 1967-1972. Aerosp. Med. 45:987-994, 1974.
2. McFarland, R. A., F. J. W. Roughton, M. H. Halperin, and
J. I. Niven. The Effects of Carbon Monoxide and Altitude on
Visual Thresholds. J. Aviat. Med. 75:381-394, 1944.
3. Halperin, M. H., R. A. McFarland, J. I. Niven, and F. J. W.
Roughton. The Time Course of the Effects of Carbon
Monoxide on Visual Thresholds. J. Physiol. (London) 146:583-
593, 1959.
4. Vollmer, E. P., B G. King, J. E. Birren, and M. B. Fisher.
The Effects of Carbon Monoxide on Three Types of Perform-
ance at Simulated Altitudes of 10,000 and 15,000 Feet. J. Exp.
Psychol. 56:244-251, 1946.
5. Lilienthal, J. L., Jr. and C. H. Fugitt. The Effect of Low
Concentrations of Carboxyhemoglobin on the "Altitude
Tolerance" of Man. Am. J. Physiol. /45:359-364, 1946.
6. Forbes, W. H., F. Sargent, and F. J. W. Roughton. The Rate of
Carbon Monoxide Uptake by Normal Men. Am. J. Physiol.
/
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