Introduction

to Visibility

Introduction

 

to Visibility

William C.M

 

alm

William C. Malm

Colorado State

University

CIRA

ISSN 0737-5352-40
INTRODUCTION

TO

VISIBILITY

By

William C. Malm

Air Resources Division

National Park Service

Cooperative Institute for Research in the Atmosphere

(CIRA)

NPS Visibility Program

Colorado State University

Fort Collins, CO 80523

Under Cooperative Agreement

CA2350-97-001: T097-04, T098-06

May, 1999
Table of Contents

List of Figures .................................................................................................................................... ii

List of Tables ................................................................................................................................... vii

Introduction ........................................................................................................................................ 1 

Section 1: On the Nature of Light .................................................................................................. 3

Section 2: Interaction of Light and Particles .................................................................................. 7

Section 3: Vision Through the Atmosphere ................................................................................. 11

Section 4: Transport and Transformation of Atmospheric Particulates and Gases Affecting

Visibility ...................................................................................................................... 19

4.1 Meteorology ................................................................................................................ 19

4.2 Atmospheric Chemistry .............................................................................................. 20

4.3 Transport and Transformation ..................................................................................... 24

Section 5: Visibility Measurements ............................................................................................... 27

5.1 Measurements of Scattering and Extinction ............................................................... 27

5.2 Measurements of Particles in the Atmosphere ........................................................... 28

Section 6: Particle Concentration and Visibility Trends .............................................................. 31

6.1 Natural Conditions ...................................................................................................... 31

6.2 Current Conditions ...................................................................................................... 33

6.2.1 Seasonal Patterns of Particle Concentrations ................................................... 33

6.2.2 Spatial Trends in Visibility ............................................................................... 35

6.3 Long-Term Trends ....................................................................................................... 38

6.3.1 Eastern United States ........................................................................................ 38

6.3.2 Western United States ....................................................................................... 39

6.4 Historical Relationships Between SO2 Emissions and Visibility ............................... 40

Section 7: Identification of Sources Contributing to Visibility Impairment ................................ 43

7.1 Back Trajectory Receptor Models .............................................................................. 45

7.2 Trajectory Apportionment Model ............................................................................... 49

Section 8: Human Perception of Visual Air Quality .................................................................... 53

8.1 Perception Thresholds of Layered Haze (Plume Blight) ............................................ 54

8.2 Perceived Visual Air Quality (PVAQ) ........................................................................ 55

Glossary of Terms............................................................................................................................... 65

ii
iii

List of Figures

Figure Page

I The ability of an observer to clearly see and appreciate varied scenic elements

(a) Navajo Mountain, as seen from Bryce Canyon National Park ................................ 2

(b) The La Sal Mountains, as seen from the Colorado River ....................................... 2

(c) Highly textured foreground canyon walls against the backdrop of the La Sal 

Mountains ................................................................................................................ 2

(d) Bryce Canyon as seen from Sunset Point ............................................................... 2

1.1 Water waves illustrate the concept of wavelengths ............................................................. 3

1.2 Wavelengths of electromagnetic radiation .......................................................................... 3

1.3 Light can exhibit either wave-like or particle-like characteristics

(a) Blue photon .............................................................................................................. 4

(b) Green photon ........................................................................................................... 4

(c) Red photon ............................................................................................................... 4

1.4 Why some objects appear white while others appear colored ............................................ 4

1.5 Important factors involved in viewing a scenic vista .......................................................... 5

2.1 Large particle light scattering and absorption

(a) Diffraction ................................................................................................................ 7

(b) Refraction ................................................................................................................ 7

(c) Phase Shift ............................................................................................................... 7

(d) Absorption ............................................................................................................... 7

2.2 Light scattering by small particles ....................................................................................... 8

2.3 Light scattering by large particles ....................................................................................... 8

2.4 The relative efficiency with which particles of various sizes scatter light ......................... 8

2.5 The relative amount of mass typically found in a given particle size range and the 

relative amount of particle scattering associated with that mass ........................................ 9

2.6 A beam of light striking small particles .............................................................................. 9

2.7 Light striking particles near the wavelength of the light .................................................... 9

2.8 Light passing through a concentration of nitrogen dioxide .............................................. 10

2.9 Photographs of smoke from a cigarette

(a) Small particles illuminated by white light ........................................................... 10

(b) Large particles illuminated by white light ........................................................... 10

3.1 Diagram of the human eye ................................................................................................. 11

3.2 The effect of regional or uniform haze on a Glacier National Park vista

(a) at 7.6 μg/m3 .......................................................................................................... 11

(b) at 12.0 μg/m3 ........................................................................................................ 11

(c) at 21.7 μg/m3 ........................................................................................................ 11

(d) at 65.3 μg/m3 ........................................................................................................ 11

3.3 Effects of uniform haze on the Chuska Mountains as seen from Mesa Verde National Park 12
Figure Page

3.4 Uniform haze at Bryce Canyon National Park .................................................................. 12

3.5 Navajo Mountain showing the appearance of layered haze .............................................. 13 

3.6 Navajo Mountain with a suspended haze layer ................................................................. 13

3.7 Classic example of "plume blight" .................................................................................... 13

3.8 An example of one kind of point source that emits pollutants into the atmosphere ........ 13

3.9 Smoke trapped by an inversion layer in the Grand Canyon ............................................. 14 

3.10 Example of power plant emissions trapped in an air inversion layer in the Grand Canyon 14

3.11 Effects of an inversion layer in the Grand Canyon ........................................................... 15

3.12 Effects of layered haze trapped in front of the Chuska Mountains ................................... 15 

3.13 Forest fire plume ................................................................................................................ 15

3.14 Example of how light-absorbing particles affect the ability to see a vista ....................... 15 

3.15 The effect of illumination on the appearance of plumes ................................................... 16 

3.16 The brown discoloration resulting from an atmosphere containing nitrogen dioxide (NO2) 16 

3.17 Effects of illumination under an inversion layer 

(a) Looking to the east at the La Sal Mountains at 9:00 a.m, haze appears white .... 17 

(b) Looking to the east at the La Sal Mountains later in the day, haze appears dark 17

3.18 Photographs showing the effect of a progressively shifting sun angle on the 

appearance of a vista between 6:00 a.m. and noon ........................................................... 18

4.1 Warm air rises through the earth's atmosphere, while cool air sinks ................................ 19

4.2 Three forms of air pollution that contribute to visual degrdation of a scenic vista

(a) Uniform haze ......................................................................................................... 21

(b) Coherent plume ..................................................................................................... 21

(c) Layered haze .......................................................................................................... 21

4.3 Five atoms that play a significant role in determining air quality .................................... 23

4.4 Sulfur dioxide gas converts in the atmosphere to ammonium sulfate particles ................ 23

4.5 Relative size of beach sand, a grain of flour, and a secondary fine particle ..................... 24

4.6 Particles arranged by their typical mass/size distribution in the atmosphere ................... 24

4.7 Sulfur dioxide emissions ................................................................................................... 25

4.8 Nitrogen oxide and hydrocarbon (VOC) gas emissions .................................................... 25

4.9 Five particle types that make up fine particle mass .......................................................... 26

5.1 Diagram of a camera ......................................................................................................... 27

5.2 Placement of a light source and detector as used in a transmissometer ........................... 28

5.3 Placement of a detector for the measurement of the number of photons scattered by a

concentration of particles and gas ..................................................................................... 29 

5.4 Diagram of a cyclone-type particle monitor ...................................................................... 29

6.1 Location of the monitoring sites used in IMPROVE ........................................................ 31

6.2 Natural visibility in the East and West .............................................................................. 33

6.3 Sulfate trends at Shenandoah, Mount Rainier, Rocky Mountain, Grand Canyon,

Acadia, and Big Bend National Parks ............................................................................... 34

iv
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Figure Page

6.4 Summary of seasonal trends in fine mass concentration for four geographic regions

of the United States ........................................................................................................... 35

6.5 Comparison of extinction, deciview, and visual range ...................................................... 35

6.6 Average reconstructed light extinction coefficient ............................................................ 36

6.7 Average visibility ............................................................................................................... 36

6.8 Extinction and percent contribution attributed to coarse mass and fine soil .................... 36

6.9 Extinction and percent contribution attributed to sulfates ................................................ 37

6.10 Extinction and percent contribution attributed to organic carbon ..................................... 38

6.11 Extinction and percent contribution attributed to nitrates ................................................. 38

6.12 Extinction and percent contribution attributed to light-absorbing carbon ........................ 39

6.13 Trends in median visual range over the eastern United States from 1948 through 1982 . 39

6.14 Historical trends in percent of hours of reduced visibility at Phoenix and Tucson

compared to trends in SO2 emissions from Arizona copper smelters ............................... 40 

6.15 Scatter plot of de-seasonalized sulfur concentrations vs. smelter emissions at 

Hopi Point and Chiricahua National Monument ............................................................... 41

6.16 Comparison of sulfur emission trends and deciviews 

(a) For the southeastern United States during the winter months ............................. 41

(b) For the southeastern United States during the summer months .......................... 41

(c) For the northeastern United States during the winter months ............................. 41

(d) For the northeastern United States during the summer months ........................... 41

7.1 Emissions rates for sulfur dioxide, nitrogen oxides, and volatile organic carbon gases .. 44 

7.2 Geographical distribution of sulfur dioxide, nitrogen oxides, and volatile organic 

carbon gas emissions ......................................................................................................... 44

7.3 (a) Extreme fine sulfur concentration source contribution, Mount Rainier National Park . 46

(b) Extreme fine sulfur concentration conditional probability, Mount Rainier National Park . 46

7.4 (a) Extreme fine sulfur concentration source contribution, Glacier National Park ......... 47

(b) Extreme fine sulfur concentration conditional probability, Glacier National Park .... 47

7.5 (a) Extreme fine sulfur concentration source contribution Grand Canyon National Park 47

(b) Extreme fine sulfur concentration conditional probability, Grand Canyon National Park 47

7.6 Extreme fine sulfur concentration source contribution, Rocky Mountain National Park . 47

7.7 Extreme fine sulfur concentration source contribution, Chiricahua National Monument 48

7.8 Extreme fine sulfur concentration source contribution, Big Bend National Park ............ 48

7.9 Extreme fine sulfur concentration source contribution, Shenandoah National Park ........ 48

7.10 (a) Low fine sulfur concentration conditional probability, Grand Canyon National Park 49

(b) Low fine sulfur concentration conditional probability, Mount Rainier National Park 49

7.11 Plots of constant source contribution function lines 

(a) southern California ................................................................................................ 50

(b) southern Arizona ................................................................................................... 50

(c) Monterrey, Mexico ................................................................................................ 50

(d) Navajo Generating Station .................................................................................... 50

(e) northern Utah, Salt Lake City, and surrounding area ............................................. 50
Figure Page

7.12 Fraction of sulfate from various source regions

(a) Mount Rainier National Park .................................................................................. 52

(b) Grand Canyon National Park ................................................................................ 52

(c) Big Bend National Park ......................................................................................... 52

(d) Acadia National Park ............................................................................................ 52

(e) Shenandoah National Park .................................................................................... 52

(f) Great Smoky Mountains National Park ................................................................. 52

8.1 Configuration of the laboratory setup used to conduct the visual sensitivity experiments 54

8.2 Predicted probability of detection curves for one subject used in the full length plume 

study .............................................................................................................................. 55

8.3 Threshold modulation contrast plotted as a function of plume width for full length,

oval, and circular plumes ................................................................................................... 55

8.4 Sample slides used to gauge public perception of visual air quality

(a) and (b) 50 km distant La Sal Mountains as seen from Canyonlands National Park 56

(c) 96 km distant Chuska Mountains as seen from Mesa Verde National Park ......... 56

(d) Forest fire plume as seen from Grand Canyon National Park .............................. 56

(e) and (f) Desert View as seen from Hopi Point ....................................................... 57

(g) From Hopi Point but in the opposite direction of Desert View ............................ 57

(h) 50 km distant San Francisco Peaks as seen from Grand Canyon National Park .. 57

8.5 Appearance of one Canyonlands National Park vista under various air quality levels

(a) Sky-mountain contrast is -0.39 .............................................................................. 58

(b) Sky-mountain contrast is -0.26 ............................................................................. 58

(c) Sky-mountain contrast is -0.23 ............................................................................... 58

8.6 Mt. Trumbull as seen from Grand Canyon National Park

(a) Sky-mountain contrast is -0.32 .............................................................................. 59

(b) Sky-mountain contrast is -0.29 ............................................................................. 59

(c) Sky-mountain contrast is -0.15 .............................................................................. 59

8.7 Plot of judgments of perceived air quality (PVAQ) .......................................................... 59 

8.8 (a) and (b) Mt. Trumbull as viewed from Hopi Point under two different lighting 

conditions .................................................................................................................... 60 

8.9 (a) and (b) The effect of changes in sun angle in the La Sal Mountains .......................... 60 

8.10 Perceived visual air quality plotted as a function of sun angle ......................................... 61 

8.11 Perceived visual air quality plotted against particulate concentration .............................. 62 

8.12 Relationships between perceived visual air quality and vista distance ............................ 62

8.13 Three ways in which air pollutants can manifest themselves as layered haze

(a) White plume .......................................................................................................... 63

(b) Dark plume ............................................................................................................ 63 

(c) Dark haze layer ...................................................................................................... 63

8.14 Summarization of the results of the layered haze perception studies ............................... 63

vi
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List of Tables

Table Page

4.1 Definitions of terms that describe airborne particulate matter .......................................... 21

4.2 Typical size ranges of a number of aerosols commonly found in the atmosphere ........... 22

6.1 Estimated natural background particulate concentrations and extinction ......................... 32
viii
Introduction 1

Visibility, as it relates to management of

the many visual resources found in

national parks, is a complex and difficult concept

to define. Should visibility be explained in

strictly technical terms that concern themselves

with exact measurements of illumination, threshold contrast, and precisely measured distances?

Or is visibility more closely allied with value

judgments of an observer viewing a scenic vista?

Historically, “visibility” has been defined as

“the greatest distance at which an observer can

just see a black object viewed against the horizon

sky.” An object is usually referred to as at threshold contrast when the difference between the

brightness of the sky and the brightness of the

object is reduced to such a degree that an observer

can just barely see the object. Much effort has

been expended in establishing the threshold contrast for various targets under a variety of illumination and atmospheric conditions. An important

result of this work is that threshold contrast for

the eye, adapted to daylight, changes very little

with background brightness, but it is strongly

dependent upon the size of the target and the time

spent looking for the target.

Nevertheless, visibility is more than being

able to see a black object at a distance for which

the contrast reaches a threshold value. Coming

upon a mountain such as one of those shown in

Figures Ia and Ib, an observer does not ask, “How

far do I have to back away before the vista disappears?” Rather, the observer will comment on the

color of the mountain, on whether geological features can be seen and appreciated, or on the

amount of snow cover resulting from a recent

storm system. Approaching landscape features

such as those shown in Figures Ic and Id, the

observer may comment on the contrast detail of

nearby geological structures or on shadows cast

by overhead clouds.

Visibility is more closely associated with conditions that allow appreciation of the inherent

beauty of landscape features. It is important to

recognize and appreciate the form, contrast detail,

and color of near and distant features. Because

visibility includes psychophysical processes and

concurrent value judgments of visual impacts, as

well as the physical interaction of light with particles in the atmosphere, it is of interest to understand the psychological process involved in viewing a scenic resource, the value that an observer

places on visibility, and to be able to establish a

link between the physical and psychological

processes.

Whether we define visibility in terms of visual

range or in terms of some parameter more closely

related to how visitors perceive a visual resource,

the preservation or improvement of visibility

requires an understanding of what constituents in

the atmosphere impair visibility as well as the origins of those constituents.

Scientists know that introduction of particulate matter and certain gases into the atmosphere

interferes with the ability of an observer to see

landscape features. Monitoring, modeling, and

controlling sources of visibility-reducing particulate matter and gases depend on scientific and

technical understanding of how these pollutants

interact with light, transform from a gas into parINTRODUCTION NTRODUCTION ....................................................
ticles that impair visibility, and are dispersed

across land masses and into local canyons and valleys.

Scientific understanding of some of these

issues is more complete than of others. The goal

of this publication is to assist the reader in developing basic knowledge of those concepts for

which there is an understanding and to indicate

the areas that need further research.

2 Introduction to Visibility

Fig. I. Photographs (a) through (d) show that, from a visual resource point of view, visibility is not how far a

person can see, but rather the ability of an observer to clearly see and appreciate the many and varied scenic

elements in each vista.

(a) The farthest scenic feature is the 130 km distant

Navajo Mountain, as seen from Bryce Canyon

National Park.

(b) The La Sal Mountains, as seen from the Colorado

River, are a dominant form on the distant horizon.

(c) This view in Canyonlands National Park shows the

highly textured foreground canyon walls against the

backdrop of the La Sal Mountains. The La Sals are 50

km away from the observation.

(d) Bryce Canyon as seen from Sunset Point. Notice

the highly textured and brightly colored foreground

features.
Section 1: On the Nature of Light 3

One of our principal contacts with the

world around us is through light. Not

only are we personally dependent on light to carry

visual information, but also much of what we

know about the stars and the solar system is

derived from light waves registering on our eyes

and on optical instruments.

Light can be thought of as waves, and to a certain extent they are analogous to water and sound

waves. Figure 1.1 is a schematic representation

of water waves with the distance from crest to

crest denoted as one wavelength.

Similar oscillations of

electric and magnetic

fields are called electromagnetic radiation.

Ordinary light is a form of

electromagnetic radiation,

as are x-rays, ultraviolet,

infrared, radar, and radio

waves. All of these travel

at approximately 300,000

km/sec (186,000 mi/sec)

and only differ from one

another in wavelength.

Figure 1.2 is a schematic representation of the

electromagnetic spectrum with the visible portion

shown in color to emphasize the portion of the

spectrum to which the human eye is sensitive.

The visible spectrum is white light separated into

its component wavelengths or colors. The wavelength of light, typically measured in terms of

millionths of a meter (microns, or μm), extends

from about 0.4 to 0.7 microns.

Waves of all kinds, including light waves,

carry energy. Electromagnetic energy is unique in

that energy is carried in small, discrete parcels

called photons. Schematic representations of a

blue, green, and red photon are shown in Figure

1.3. Blue, green, and red photons have wavelengths of around 0.45, 0.55, and 0.65 microns,

respectively. The color properties of light depend

on its behavior both as waves and as particles.

Colors, created from white light by passing it

through a prism, are a result of the wave-like

SECTION SECTION 1 ................................................................

ON THE NA ON THE NATURE OF LIGHT TURE OF LIGHT

Fig. 1.2 Vibrations of electric and magnetic fields are referred to as electromagnetic radiation. This diagram shows the wavelengths of various types of electromagnetic radiation including visible light. The wavelength of the visible spectrum

varies from 0.4 microns (blue) to 0.7 microns (red). One micron equals one millionth of a meter.

Fig. 1.1 Water waves illustrate the concept of wavelengths. A wavelength is defined as the distance from

one crest to the next.
4 Introduction to Visibility

mostly red light while absorbing all others, so the

apple, to an eye-brain system, appears to be red. 

For all practical purposes, in visibility, it is

most convenient to think of light as being made of

small colored particles. The following sections of

this document will discuss more specifically how

these “light” particles interact with atmospheric

particulate matter and gases. 

Visibility involves more than specifying how

light is absorbed and scattered by the atmosphere.

Visibility is a psychophysical process of perceiving the environment through the use of the eyebrain system. 

Important factors involved in seeing an object

are outlined in Figure 1.5 and summarized here.

- Illumination of the overall scene by the sun,

including illumination resulting from sunlight

scattered by clouds and atmosphere as well as

reflections by ground and vegetation.

nature of light. A prism separates the colors of

light by bending (refracting) each color to a different degree. Colors in a rainbow are the result

of water droplets, acting like small prisms, dispersed through the atmosphere. Each water

droplet refracts light into the component colors of

the visible spectrum.

More commonly, the colors of light are separated in other ways. When light strikes an object,

certain color photons are captured by molecules in

that object. Different types of molecules capture

photons of different colors. The only colors we

see are those photons that the surface reflects. For

instance, chlorophyll in leaves captures photons of

red and blue light and allows green photons to

bounce back, thus providing the green appearance

of leaves. Nitrogen dioxide, a gas emitted into the

atmosphere by combustion sources, captures blue

photons. Consequently, nitrogen dioxide gas

tends to look reddish brown. Figure 1.4 is an

example of an eggshell reflecting all wavelengths

of light. The eye perceives the eggshell to be

white. An apple, on the other hand, reflects

Fig. 1.3 At times light can exhibit either wave-like or particle-like characteristics. Light can be thought of as consisting of bundles of vibrating electric and magnetic waves.

These bundles of energy are called photons, and the wavelengths of radiant energy making up the photon determine

its “color.” (a), (b), and (c) schematically show a blue,

green and red photon, respectively.

Fig. 1.4 Why some objects appear white while others

appear colored. White light, which is composed of all

“colors” of photons, strikes an object. If the object is

white, photons of every color are reflected. However, if

some photons are absorbed while others are reflected,

the object will appear to be colored; a red apple, for

instance, reflects red photons and absorbs all others.
- Target characteristics that include color, texture, form, and brightness.

- Optical characteristics of intervening atmosphere:

i. image-forming information (radiation)

originating from landscape features is

scattered and absorbed (attenuated) as it

passes through the atmosphere toward

the observer, and

ii. sunlight, ground reflected light, and light

reflected by other objects are scattered

by the intervening atmosphere into the

sight path.

- Psychophysical response of the eye-brain system to incoming radiation.

It is important to understand the significance of

the light that is scattered in the sight path toward

the observer. The amount of light scattered by the

atmosphere and particles between the object and

observer can be so bright and dominant that the

light reflected by the landscape features becomes

insignificant. This is somewhat analogous to

viewing a candle in a brightly lit room and in a

room that would otherwise be in total darkness. In

the first case, the candle can hardly be seen, while

in the other it becomes the dominant feature in the

room.

Section 1: On the Nature of Light 5

Fig. 1.5 Important factors involved in seeing a scenic vista are outlined. Image-forming information from an

object is reduced (scattered and absorbed) as it passes through the atmosphere to the human observer. Air light

is also added to the sight path by scattering processes. Sunlight, light from clouds, and ground-reflected light all

impinge on and scatter from particulates located in the sight path. Some of this scattered light remains in the sight

path, and at times it can become so bright that the image essentially disappears. A final important factor in seeing and appreciating a scenic vista are the characteristics of the human observer.
6 Introduction to Visibility
Aphoton (light “particle”) is said to be

scattered when it is received by a particle

and re-radiated at the same wavelength in any

direction. Visibility degradation results from light

scattering and absorption by atmospheric particles

and gases that are nearly the same size as the

wavelength of the light. Particles somewhat

larger than the wavelength of light can scatter

light as a result of a combination of the first three

phenomena shown schematically in Figures 2.1a,

2.1b, and 2.1c. Figure 2.1a shows diffraction, a

phenomenon whereby radiation is bent to “fill in

the shadow” behind the particle. Figure 2.1b

depicts light being bent (refracted) as it passes

through the particle. A third effect resulting from

slowing a photon is a little difficult to understand.

Consider two photons approaching a particle,

each vibrating “in phase” with one another. One

passes by the particle, retaining its original speed,

while the other, passing through the particle, has

its speed altered. When this photon emerges from

the particle, it will be vibrating “out of phase”

with its neighbor photon; when it vibrates up, its

neighbor will vibrate down. As a consequence,

they interfere with each other’s ability to propagate in certain directions (Figure 2.1c).

Figure 2.1d indicates how a photon can be

absorbed by the particle. The radiant energy of

the photon is transferred to internal molecular

energy or heat energy. In the absorption process,

the photon is not redistributed into space; the photon ceases to exist.

The efficiency with which a particle can scatter light and the direction in which the incident

light is redistributed are dependent on all four of

these effects. Photons can be scattered equally in

all directions (isotropic scattering), but in most

instances photons are scattered in a forward direction.

Figures 2.2 and 2.3 show the distribution of

scattered light for particles that are respectively

much smaller and much larger than the wavelength of light. If the particles are small (such as

the air molecules themselves) the amount of light

Section 2: Interaction of Light and Particles 7

SECTION SECTION 2 ...................................................................................................

INTERACTION OF LIGHT AND P INTERACTION OF LIGHT AND PARTICLES

Fig. 2.1 Large particle light scattering and absorption. Diffraction (a) and refraction (b) combine to

"bend" light to "fill in the shadow" behind the particle.

Diffraction, an edge effect, causes photons passing

very close to a particle to bend into the shadow area;

refraction is a result of the light wavefront slowing

down as it enters the particle. While the photon is

within the particle, its wavelength is also shortened.

Thus, when it emerges from the particle, it may vibrate

out of phase with adjacent photons and interfere with

their ability to propagate in a pre-prescribed direction. This effect (phase shift) is shown in (c). As a

fourth possibility, the photon may be absorbed by the

particle (d). In this case, the internal energy of the

particle is increased. The particle may rotate faster or

its molecules may vibrate with greater amplitude.
scattered in the forward and backward directions

are nearly the same. This type of scattering is

referred to as Rayleigh scattering. As the particle

increases in size, more light tends to scatter in the

forward direction until for large particles nearly

100% of the incident photons end up being scattered in the forward direction.

The fact that light scatters preferentially in

different directions as a function of particle size is

extremely important in determining the effects

that atmospheric particulates have on a visual

resource. The angular relationship between the

sun and observer in conjunction with the size of

particulates determines how much of the sunlight

is redistributed into the observer’s eye. 

The effect of particulates on visibility is further complicated by the fact that particulates of

different sizes are able to scatter light with varying degrees of efficiency. It is of interest to investigate the efficiency with which an individual particle can scatter light. The efficiency factor is

expressed as a ratio of a particle’s effective cross

section to its actual cross section. Figure 2.4

shows how this efficiency varies as a function of

particle size. Very small particles and molecules

are very inefficient at scattering light. As a particle increases in size, it becomes a more efficient

light scatterer until, at a size that is close to the

wavelength of the incident light, it can scatter

more light than a particle five times its size. Even

particles that are very large scatter light as if they

were twice as big as they actually measure. These

particles remove twice the amount of light intercepted by its geometric cross-sectional area.

Figure 2.5 shows the relative amounts of small

and large particles found in the atmosphere. The

blue line is a typical mass size distribution of particles. The y-axis is the amount of mass in a given

8 Introduction to Visibility

Fig. 2.3 If the particle is large (greater than 10

microns), most of the incident light is scattered in the

forward direction.

Fig. 2.4 The relative efficiency with which particles of

various sizes scatter light. The green line corresponds

to the scattering efficiency of molecules. The orange

and red lines show the efficiency with which fine and

coarse particles scatter light. Note that fine particles

(0.1 microns to 1.0 microns) can be more efficient at

scattering light than are either molecules or coarse

particles.

Fig. 2.2 Light interacts with a particle through the

processes shown in Fig. 2.1. If the particle is very

small (the size of a molecule) the net result of the interaction process is to redistribute incident light in a way

shown in the above diagram. Equal numbers of photons are scattered in the forward and backward directions and about one-half of the number of forward

scattered photons are directed to the sides (90 degree

scattering).
size range; the x-axis is particle size measured in

microns. Notice the two-humped, or bi-modal

curve. Those particles less than about 2.5 microns

are referred to as fine particles and particles larger

than 2.5 microns are called coarse particles.

The orange curve is the corresponding amount

of light scattering that can be associated with each

size range. Even though there is less mass concentrated in the fine mode, it is the fine particulates that are the most responsible for scattering

light. This is because fine particles are more efficient light scatterers than large particles, and

because there are more of them, even though their

total mass is less than the coarse mode.

Consequently, it is the origin and transport of fine

particles that is of greatest concern when assessing visibility impacts. 

It is this scattering phenomenon that is responsible for the colors of haze in the sky. The sky is

blue because blue photons, with their shorter

wavelengths, are nearer the size of the molecules

that make up the atmosphere than are their green

and red counterparts. Thus blue photons are scattered more efficiently by air molecules than red

photons, and as a consequence, the sky looks blue.

Figure 2.6 schematically shows what happens

when the red, blue, and green photons of white

light strike small particles. Only the blue photons

are scattered because scattering efficiency is

greatest when the size relationship of photon

wavelength to particle is close to 1:1. The red and

green photons pass on through the particles. To an

observer standing to the side of the particle concentration, the haze would appear to be blue.

Figure 2.7 shows what happens when the particles

are about the same size as the incoming radiation.

All photons are scattered equally, and the haze

appears to be white or gray.

Section 2: Interaction of Light and Particles 9

Fig. 2.7 When particles are near or larger than the

wavelength of the incident light, photons of all colors

are scattered out of the beam path.

Fig. 2.6 As a beam of white light (consisting of all

"colored" photons) passes through a haze made up of

small particles, it is predominantly the blue photons,

which are scattered in various directions.

Fig. 2.5 The blue line shows the relative amount of

mass typically found in a given particle size range. The

orange line shows the relative amount of particle scattering associated with that mass. Note that even though

mass is associated with coarse particles, it is the fine

particles that are primarily responsible for scattering

light.
Figure 2.8 is a similar diagram of white light

passing through a concentration of nitrogen dioxide (NO2) molecules. Blue photons are absorbed,

so a person viewing a NO2 haze would see it as

being reddish brown (i.e. without blue) rather than

white.

Figures 2.9a and 2.9b further exemplify the

relationship of particle size and the color of scattered light. Figure 2.9a shows a lighted cigarette

held in a strong beam of white light. Notice that

the smoke appears to have a bluish tinge to it.

One can conclude that these particles must be

quite small because they are scattering more blue

than green or red photons. Figure 2.9b is smoke

from the same cigarette. However, the smoke in

Figure 2.9b has been held in the mouth for a few

seconds. The inside of a person’s mouth is humid,

and smoke particles have a high affinity for water

vapor. These hygroscopic particles tend to grow

to sizes that are near the wavelengths of light and

thus scatter all wavelengths of light equally.

Scattered photons having wavelengths that extend

over the whole visible spectrum are, of course,

perceived to be white or gray.

10 Introduction to Visibility

(a) (b)

Fig. 2.9 (a) This photograph shows the color of small particles that have been illuminated by white light. Because

the smoke appears blue it can be concluded that the scattering particles must be quite small, less than the wavelength

of visible light. (b) A photograph of similar particles after they have been allowed to grow in a humid environment.

Note that as a result of equal scattering of all photon colors, these larger particles appear white instead of blue.

Fig. 2.8 An atmosphere containing nitrogen dioxide (NO2) will tend to deplete the number of blue photons through

the absorption process. As a result, white light will tend to look reddish or brownish in color after passing through

a nitrogen dioxide haze.
The eye, shown in Figure 3.1, is much like a

camera in that it has a lens, an aperture to

control the amount of light entering the eye (iris),

and a detector, called the retina. The eye, whether

it is looking at a vista or a candle in the room,

detects relative differences in brightness rather

than the overall brightness level. That is to say, the

eye measures contrast between adjacent objects or

between an object and its background. Contrast of

an object is simply the percent difference between

object luminance and its background luminance.

Section 3: Vision Through the Atmosphere 11

SECTION SECTION 3 .........................................................................................

VISION THROUGH THE A VISION THROUGH THE ATMOSPHERE TMOSPHERE

Fig. 3.1 The human eye operates much like a photographic camera. It has a lens to focus an image on a very

sensitive detector called the retina. Also, the amount of

light entering the eye is controlled by an aperture called

the iris. The iris is the colored portion of the eye.

(a) (b)

(c) (d)

Fig. 3.2 The effect of regional or uniform haze on a Glacier National Park vista. The view is of the Garden Wall from across

Lake McDonald. Atmospheric particulate concentrations associated with photographs (a), (b), (c), and (d) correspond to

7.6, 12.0, 21.7, and 65.3 μg/m3.
The camera can be an effective tool in capturing the visual impact that pollutants have on a

visual resource. In the following paragraphs, pictures are presented that show the visual impact

that haze has on scenic vistas under different

lighting and air quality conditions. 

Figure 3.2a,b,c, and d show the effect that different levels of uniform haze have on Glacier

National Park in Montana. These photographs

were taken near Apgar on the southwestern end of

Lake McDonald. Sky-mountain contrasts are

-0.18, -0.14, -0.04, and greater than -0.02, while

the associated atmospheric fine particulate concentrations in each case are 7.6, 12.0, 21.7, and

65.3 μg/m3, respectively. Figures 3.3 and 3.4

show similar hazes of vistas at Mesa Verde and

Bryce Canyon National Parks. The Chuska

Mountains in Figure 3.3 are 95 km away, with the

contrast at -0.26. Navajo Mountain is 130 km distant (Figure 3.4) and in this photograph the skymountain contrast is -0.08. This photograph

should be compared with Figure Ia, a photograph

of Navajo Mountain taken on a day in which the

particulate concentration in the atmosphere was

near zero.

Under stagnant air mass conditions, aerosols

can be “trapped” and produce a visibility condition usually referred to as layered haze. Figure

3.5 shows Navajo Mountain viewed from Bryce

Canyon National Park with a bright layer of haze

that extends from the ground to about halfway up

the mountain. Figure 3.6 is a similar example of

layered haze but with the top portion of the mountain obscured. Figure 3.7 is a classic example of

plume blight. In plume blight instances, specific

sources such as those shown in Figure 3.8 emit

pollutants into a stable atmosphere. The pollutants are then transported in some direction with

little or no vertical mixing.

12 Introduction to Visibility

Fig. 3.3 Effects of uniform haze on the Chuska Mountains as seen from Mesa Verde National Park. The atmospheric particulate concentration on the day this photograph was taken corresponded to 1 μg/m3.

Fig. 3.4 Uniform haze degrades visual air quality at

Bryce Canyon National Park. The 130 km distant

landscape feature is Navajo Mountain. Atmospheric

particulate concentration on the day this photograph

was taken is 3 μg/m3.
Figures 3.9, 3.10, 3.11, and 3.12 show other

layered haze conditions that frequently occur at

Grand Canyon and Mesa Verde National Parks.

At the Grand Canyon layered hazes are usually

associated with smoke and nearby coal-fired

power plants, while at Mesa Verde, much of the

pollution comes from urban areas and the Four

Corners and San Juan Power Plants.

Figures 3.13 and 3.14 show the appearance of

plumes containing carbon. In both of these cases

the pollutants are being emitted from forest fires.

However, Figure 3.13 shows the appearance of a

specific forest fire plume, while Figure 3.14

shows the effect of viewing a vista through a concentration of particles containing carbon. In this

instance, the vista is the north wall of the Grand

Canyon as seen from the top of San Francisco

Peaks in northern Arizona. Notice the overall

“graying” and reduction of contrast of the distant

Section 3: Vision Through the Atmosphere 13

Fig. 3.5 Navajo Mountain as seen from Bryce Canyon

National Park, showing the appearance of layered

haze. The pollutants are trapped in a stable air mass

that extends from the ground to about halfway up the

mountain side.

Fig. 3.6 Photograph of Navajo Mountain similar to

Figure 3.5 but with a suspended haze layer that

obscures the top portion of the mountain.

Fig. 3.7 Classic example of “plume blight.” The thin,

dark plume on Navajo Mountain results from a point

source emitting particulate matter into a stable atmosphere.

Fig. 3.8 An example of one kind of point source that

emits pollutants into the atmosphere.
scenic features. Remember that carbon absorbs

all wavelengths of light and scatters very little.

Thus the scene will always tend to be darkened.

Figure 3.15 shows the effects of illumination

on the appearance of power plant plumes. The

two plumes on the left

are particulate plumes,

while the two plumes on

the right consist of water

droplets. The plume on

the far right, which is

illuminated by direct

sunlight, appears to be

white. The second identical water droplet

plume, which is shaded,

appears dark. The

amount of illumination

can have a significant

effect on how particulate

concentrations appear.

Figure 3.16 demonstrates how the effect of

nitrogen dioxide gas

(NO2), in combination with varied background

illumination, can combine to yield a very brown

atmospheric discoloration. If a volume of atmosphere containing NO2 is shaded and if light passes

through this shaded portion of the atmosphere, the

light reaching the eye will be deficient in photons

14 Introduction to Visibility

Fig. 3.10 An example of power plant emissions trapped in an air inversion layer in the Grand Canyon.

Fig. 3.9 Smoke trapped by an inversion layer in the Grand Canyon. During the

winter months inversions are quite common in almost all parts of the United

States.
Section 3: Vision Through the Atmosphere 15

Fig. 3.11 Effects of an inversion layer in the Grand Canyon. In this case, a cloud has formed within the canyon walls.

Fig. 3.12 Effects of layered haze trapped in front of the Chuska Mountains as viewed from Mesa Verde National

Park. This condition occurs 30 to 40% of the time during winter months.

Fig. 3.13 Forest fire plume exemplifying the appearance of carbon particles and demonstrating the effect

of lighting. Where the plume is illuminated it appears

gray, but identical particles in the shadow of the

plume appear dark or almost black.

Fig. 3.14 Example of how light-absorbing particles

(in this case carbon) affect the ability to see a vista.

Carbon absorbs all wavelengths of light and generally

causes a “graying” of the overall scene. Shown here

is the north wall of the Grand Canyon as seen from the

top of the San Francisco Peaks in northern Arizona.
in the blue part of the spectrum. As a consequence, the light will appear brown or reddish in

color. However, if light is allowed to shine on, but

not through, that same portion of the atmosphere, scattered light reaches the

observer’s eye and the light

can appear to be gray in

nature. Both of these conditions are shown in Figure

3.16. On the right side of the

photo the mixture of NO2

and particulates is shaded by

clouds. The same atmosphere, illuminated because

the cloud cover has disappeared, appears almost gray

in the middle portion of the

photograph.

Figure 3.17 is an easterly

view of the La Sal

Mountains in southeastern

Utah as seen from an elevated point that is some 100

kilometers distant. The photograph shown in

Figure 3.17a was taken at 9:00 a.m., while the

photograph shown in 3.17b was taken later in the

16 Introduction to Visibility

Fig. 3.16 The brown discoloration resulting from an atmosphere containing nitrogen dioxide (NO2) being shaded

by clouds but viewed against a clear blue sky. Light scattered by particulate matter in that atmosphere can dominate light absorbed by NO2, causing a gray or blue appearing haze (left side of photograph).

Fig. 3.15 The effect of illumination on the appearance of plumes. The two

plumes on the right are identical in terms of their chemical make-up, in that

they are primarily water droplets. However, the far right plume is directly

illuminated by the sun and the plume second from the right is shaded. The

first plume appears white and the second appears almost black. The two

plumes on the left are fly-ash plumes.
day. These photographs show how these views, or

vistas, appear when obscured by a layer of haze.

In the first view the haze layer appears white, but

the same air mass viewed later in the day has a

dark gray appearance. This effect is entirely due

to the geometry involved with the observer and

the sun. In the first view the sun is low in the eastern sky. Consequently, the photons reaching the

observer have been scattered

in the forward direction.

Because the haze appears

white, we can conclude that

the particles must be quite

large in comparison to the

wavelength of light. The

assumption that particles are

large is further reinforced by

their appearance when the

sun is behind the observer as

shown in 3.17b. In order for

scattered photons to reach

the observer, they would

have to be back scattered

from the particles. Because

the haze appears dark, we

can conclude that there is

very little back scattering,

which is consistent with the

large particle hypothesis.

The angle at which the

sun illuminates a vista or

landscape feature (sun angle)

plays another important role.

Figures 3.18a-d exemplify

this effect. The view is from

Island in the Sky,

Canyonlands National Park,

looking out over

Canyonlands with its many

colorful features toward the

50 km distant La Sal

Mountains. Figure 3.18a

shows how the canyon

appears when it is in total

shadow (6:00 a.m.). Figures

3.18a, b, and c show a progressively higher sun

angle until in Figure 3.18d the scene is entirely

illuminated. In each case, the air quality is the

same. The only change is in the angle at which

the sun illuminated the vista. There are primarily

two reasons for the apparent change in visual air

quality. First, at higher sun angles, there is less

scattering of light by the intervening atmosphere

Section 3: Vision Through the Atmosphere 17

Fig. 3.17 Photographs showing how the vistas appeared on a day when pollutants were trapped under an inversion layer. In (a) the haze appears white;

in (b) the identical haze is dark or gray. Because most of the light energy is

scattered in the forward direction (white haze), it can be concluded that the

particles must be quite large in comparison to the wavelength of light.

(a)

(b)
in the direction of the observer. Second, the vista

reflects more light; consequently, more imageforming information (reflected photons from the

vista) reaches the eye. The contrast detail and

scene are enhanced.

18 Introduction to Visibility

Fig. 3.18 Four photographs showing the effect of a progressively shifting sun angle on the appearance of a vista as

seen from Island in the Sky, Canyonlands National Park. In each photograph, the air quality is the same. In (a) (6:00

a.m.) the sun angle-observer-vista geometry results in a large amount of scattered air light (forward scattering) added

to the sight path, but minimal amount of imaging light reflected from the vista. (d) (12 noon) shows just the opposite

case. Scattered light is minimized and reflected imaging light is at a maximum.

(c) (d)

(a) (b)
Understanding how air moves across the

oceans and land masses is key to understanding how pollutants are transported and transformed as they move from their source to locations

where they impair visibility.

4.1 Meteorology

Meteorological factors, such as wind, cloud

cover, rain, and temperature are interesting in that

they are affected by pollution, and they in turn

affect pollution. The rate at which pollutants are

converted to other pollutants—sulfur dioxide gas

to sulfate particles or nitrogen oxides and hydrocarbons to ozone—is determined by the availability of sunlight and the presence or absence of

clouds. The vertical temperature profile of the

atmosphere determines whether the pollutants are

mixed and diluted throughout the atmosphere or

whether they are “clamped” under a lid (inversion)

and become trapped and thus accumulate in the

communities that produce the pollution. 

Figure 4.1 schematically illustrates the temperature change above the earth’s surface. The red

depicts warm air, while the shading to blue is

meant to show the decrease in temperature as the

distance above ground increases. The sun heats

the earth’s surface, and the surface in turn heats the

air that comes in contact with it. The warm air

rises, while at the same time cooler air sinks and

the cycle goes on. When these processes are in

equilibrium, there is about a 5.5oF change per 1000

feet change in elevation. For instance, at Grand

Canyon National Park, where the rim is 5000 feet

higher than the Colorado River, one would expect

about a 25-30oF difference between the top and

bottom of the canyon. A temperature of 80oF at

the top translates into 105-110oF on the river.

The rate at which temperature changes above

the earth’s surface determines the stability of the

atmosphere. Consider air masses labeled A and B

in Figure 4.1. Air mass A is warmer (redder) than

its surrounding air and will therefore rise through

the atmosphere, while air mass B, which is cooler

than its surroundings, will sink. As air mass A

rises, it will expand and therefore cool. Even

though air mass A cools, as long as it stays warmer

than its surroundings, it will continue to rise. If

this happens, the atmosphere is said to be unstable.

Section 4: Transport and Transformation of Particulates and Gases 19

SECTION SECTION 4 ......................................................................................................

TRANSPOR TRANSPORT AND TRANSFORMA T AND TRANSFORMATION OF TION OF

ATMOSPHERIC P TMOSPHERIC PARTICULATES AND GASES TES AND GASES

AFFECTING VISIBILITY AFFECTING VISIBILITY

Fig. 4.1 Warm air rises through the earth’s atmosphere, while cool air sinks. Atmospheric resistance to

these vertical disturbances (stability) depends on the

temperature distribution of the atmosphere.
Conversely, if its cooling process causes air mass

A to become cooler than its surroundings, then it

will stop rising and either sink or stay at some

height above the earth’s surface. When this happens, the atmosphere is said to be stable. 

The inset in Figure 4.1 shows an example of

where a layer of warmer air has developed at some

height above the surface. This layer is schematically depicted by the red ridge line labeled warm

air on the inset. Air mass A on the inset is just

below the warm layer. It is depicted to be warmer

(redder) than its immediate surroundings but

cooler (bluer) than the layer. Thus, the air mass

will rise until it comes in contact with the layer but

will not rise above it. This phenomenon is known

as an inversion. Pollutants become trapped below

this layer and can only escape after sunlight penetrates the inversion and heats the earth’s surface

sufficiently to break up the inversion.

The heating of the earth’s surface and the

resultant vertical temperature profile determine

whether pollutants are dispersed or mixed vertically. A second and important process for mixing

of the earth’s atmosphere is wind and the resultant

mechanical mixing when wind passes over surface

structures such as tall buildings or mountainous

terrain. Some of the cleanest and clearest air is

found on the windiest days.

Pollutants emitted that are well mixed will

appear as a uniform haze. This condition is shown

schematically in Figure 4.2a. When pollutants are

emitted into a stable atmosphere, usually one of

two things will happen, depending on whether

there is surface wind or not. If a wind is present,

the emitted pollutants usually form a plume, as

indicated in Figure 4.2b. If there are no surface

winds or if pollutants are emitted into a stagnant

air mass over periods of days, a condition schematically shown in Figure 4.2c can occur. A layer of

haze forms near the ground and continues to build

as long as the stagnation condition persists.

Layered hazes are usually associated with emissions that are local in nature as opposed to pollutants that are transported over hundreds of kilometers.

4.2 Atmospheric Chemistry

Particulates and gases in the atmosphere can

originate from natural or man-made sources.

Table 4.1 includes the terms that are usually used

to describe airborne particles; Table 4.2 shows the

size range of typical atmospheric aerosols.

The ability to see and appreciate a visual

resource is limited, in the unpolluted atmosphere,

by light scattering of the molecules that make up

the atmosphere. These molecules are primarily

nitrogen and oxygen along with some trace gases

such as argon and hydrogen. Other forms of natural aerosol that limit our ability to see are condensed water vapor (water droplets), wind-blown

dust, and organic aerosols such as pollen and

smoke from wild fires.

Aerosols, whether they are man-made or natural, are said to be primary or secondary in nature.

Primary refers to gases or particles emitted from a

source directly, while secondary refers to airborne

dispersions of gases and particles formed by

atmospheric reactions of precursor or primary

emissions. Examples of primary particles are

smoke from forest and prescribed fires, soot from

diesels, fly ash from the burning of coal, and windblown dust. Primary gaseous emissions of concern are sulfur dioxides emitted from coal burning,

nitrogen oxides that are the result of any type of

combustion such as coal-fired power plants and

automobiles, and hydrocarbons, usually associated

with automobiles but are also emitted by vegetation, especially conifers. 

These gases can be converted into secondary

particles through complex chemical reactions.

Furthermore, primary gases can combine to form

other secondary gases. Atoms and molecules of

special interest along with their relative sizes are

shown in Figure 4.3. Five atoms, in order of their

size, that play significant roles in determining air

20 Introduction to Visibility
Section 4: Transport and Transformation of Particulates and Gases 21

(a) (b)

(c)

Fig. 4.2 The three ways that air pollution can visually degrade a scenic vista. When there is sufficient sunlight to cause

the atmosphere to become turbulent, pollutants emitted into the atmosphere become well mixed and appear as a uniform haze. This condition is shown in (a). On the other hand, during cold winter months the atmosphere becomes stagnant. Pollutants emitted during these periods will appear either as a coherent plume (b) or as a layered haze (c).

Term Definition

Particulate matter Any material, except uncombined water, that exists in the solid or liquid state in the 

atmosphere or gas stream at standard condition.

Aerosol A dispersion of microscopic solid or liquid particles in gaseous media.

Dust Solid particles larger than colloidal size capable of temporary suspension in air.

Fly ash Finely divided particles of ash entrained in flue gas. Particles may contain unburned fuel.

Fog Visible aerosol.

Fume Particles formed by condensation, sublimation, or chemical reaction, predominantly 

smaller than 1 micron (tobacco smoke).

Mist Dispersion of small liquid droplets of sufficient size to fall from the air.

Particle Discrete mass of solid or liquid matter.

Smoke Small gasborne particles resulting from combustion.

Soot An agglomeration of carbon particles.

Table 4.1. Definitions of terms that describe airborne particulate matter.
quality are hydrogen (H), oxygen (O), nitrogen

(N), carbon (C), and sulfur (S). Sulfur dioxide

(SO2) is ultimately converted to sulfates, such as

ammonium sulfate ((NH4)2SO4), nitrogen oxides

(NOx) convert to nitrates such as nitric acid or

ammonium nitrate (NH4NO3), hydrocarbons convert to larger organic or hydrocarbon molecules,

and hydrocarbon gases interfere with a naturally

occurring cycle between hydrocarbon and NO2 to

yield ozone (O3).

The gas-to-particle conversion process takes

place by essentially three processes: condensation, nucleation, and coagulation. Condensation

involves gaseous vapors condensing on or combining with existing small nuclei, usually called

condensation nuclei. Small condensation nuclei

may have their origin in sea salts or combustion

processes. Gases may also interact and combine

with droplets of their own kind and form larger

aerosols. This process is called homogeneous

nucleation. Heterogeneous nucleation occurs

when gases nucleate on particles of a different

nature than themselves. Once aerosols are formed,

they can grow in size by a process called coagulation, in which particles essentially bump into each

other and “stick” together.

Figure 4.4 schematically shows the conversion

of sulfur dioxide to sulfate, the growth of sulfate

22 Introduction to Visibility

Table 4.2 Typical size ranges of a number of aerosols commonly found in the atmosphere.
molecules into sulfate particles and the very important

process of water absorption

by the sulfate particle.

Some inorganic salts, such

as ammonium sulfate and

nitrate, undergo sudden

phase transitions from solid

particles to solution droplets

when the relative humidity

(RH) rises above a threshold

level. Thus, under higher

RH (>70%) levels, these

salts become disproportionately responsible for visibility impairment as compared

with other particles that do

not uptake water molecules.

The size of most secondary particles ranges

between 0.1 and 1.0

microns. For reference, the

relative size of beach sand,

a grain of flour, and a secondary

particle is shown in Figure 4.5. 

Figure 4.6 shows a typical

mass size distribution for particles found in the atmosphere.

Those particles less than about

2.5 microns are usually secondary in nature and are referred

to as fine particles. Fine particles

tend to be man-made, while particles larger than 2.5 microns,

referred to as coarse particles,

tend to have a natural origin. It is

the fine particles that cause most

of the visibility impairment and

have the greatest adverse health

effects. The formation mechanisms are also schematically

shown in Figure 4.6.

Section 4: Transport and Transformation of Particulates and Gases 23

Fig. 4.3 The top row shows five atoms, in order of size, that play a significant

role in determining air quality. They are hydrogen, oxygen, nitrogen, carbon,

and sulfur. Through complex sets of chemical reactions, gases are formed that,

in some cases, react to form visibility reducing particles. Sulfur dioxide reacts

to form ammonium sulfate, nitrogen oxide forms ammonium nitrate, oxygen is

converted to ozone, and carbon, hydrogen, and oxygen form hydrocarbon particles.

Fig. 4.4 Sulfur dioxide gas converts in the atmosphere to ammonium sulfate particles. These particles are hygroscopic, meaning they grow

rapidly in the presence of water to reach a size that is disproportionately

responsible for visibility impairment.
Near a source (within 0-100 km), such as an

urban center, power plant, or other industrial facilities, haze is usually a mixture of gases and secondary and primary aerosols. After these pollutants have been transported hundreds of kilometers, gaseous emissions have either deposited to

aquatic or terrestrial surfaces or converted to secondary aerosols. Thus, in remote areas of the

United States, man-made components of haze are

usually composed of secondary particles.

However, in some parts of the forested United

States, fire emissions can contribute significantly

to primary carbon particles.

4.3 Transport and Transformation

These concepts are summarized in Figures 4.7

and 4.8. Emissions are transported (or accumulated depending on inversion characteristics),

transformed into other gaseous or particle species,

and deposited to the terrestrial ecosystem. In

Figure 4.7, SO2 emissions and (NH4)2SO4 are

characterized as red and green dots, respectively.

In Section A, SO2 is emitted

and immediately dispersed

downwind. SO2 begins to convert to SO4 and both SO2 and

SO4 are deposited to the terrestrial ecosystem (this includes

water and ground surfaces as

well as plants and animals) as

the material is carried by air

movement. This process of

depositing the material to the

ground is known as dry deposition. Once SO2 enters the cloud

environment, the conversion of

SO2 to SO4 begins in earnest.

The cloud droplets act as tiny

reactors and the chemistry of

SO2 to SO4 conversion goes on

very rapidly as long as the

chemical components necessary for conversion are present.

The cloud can evaporate leaving behind SO4 particles that

affect visibility or the SO4 can

deposit out of the cloud as acid

24 Introduction to Visibility

Fig. 4.6 The particles are arranged by their typical mass/size distribution

in the atmosphere. Coarse particles tend to have natural origins and

deposit out close to the source. Fine particles are usually man-made, can

transport great distances, and cause the greatest visibility impairment.

Fig. 4.5 Relative size of beach sand, a grain of flour,

and a secondary fine particle.
rain (wet deposition).

Figure 3.10 shows a case

where SO2 was transported

into the Grand Canyon

inside clouds. After the

clouds evaporate, only the

(NH4)2SO4 particles are left

and the walls and depths of

the Grand Canyon have disappeared.

Section B of Figure 4.7

shows a similar process of

SO2 to SO4 conversion but

under stable meteorological

conditions. Again, SO2 is

shown to enter the cloud

reactor where it is converted

to SO4. Clouds evaporate

leaving behind sulfate particles in the form of regional

haze. Also, phytoplankton,

shown under the magnifying

glass, emit natural sulfur as

dimethyl sulfide that is converted to SO2.

Figure 4.8 shows the

interrelationships between

NOx, hydrocarbons, and

organic particle emissions.

These reactions are very

complex and Figure 4.8 is

meant to show only some of

the main features of the

process. Gaseous hydrocarbons (labeled as volatile

organic carbon or VOCs)

and NOx emissions are in

the form of red and purple

dots. The subscript x is

meant to suggest that the

emissions may be in the

form of NO or NO2. NOx is

converted into nitric acid

Section 4: Transport and Transformation of Particulates and Gases 25

Fig. 4.7 Sulfur dioxide emissions can deposit directly to the earth’s surface or

biological system or they can cause rain water to acidify. Sulfur dioxide can

also chemically convert to sulfate particles and deposit to the earth’s surface.

Together these modes of deposition are known as “acid rain.” The sulfate particles that remain in the atmosphere cause visibility impairment.

Fig. 4.8 Nitrogen oxide and hydrocarbon (VOC) gases emitted into the atmosphere cause the formation of ozone and other photochemical oxidizing agents

that cause eyes to burn and stunt or kill vegetation. Nitrogen dioxide can cause

acidification of cloud water and form nitric acid vapor or can change into

nitrate particles. The deposition of these species is a large part of the acid rain

problem. Furthermore, particulate nitrate can cause visibility impairment.

Hydrocarbon gases can convert into carbon particles and carbon particulate

can be emitted directly from natural sources such as fire or from the diesel

engine. These carbon particles contribute significantly to visibility impairment.
vapor and NH4NO3 molecules that in turn form

NH4NO3 particles with similar characteristics as

(NH4)2SO4. Nitrates are represented as brown

dots. The reactor shown in Figure 4.8 represents

those reactions discussed above and those that

involve NOx and VOCs and the production of O3.

O3 is shown as green dots. Also produced in the

reactor are secondary organic particles shown as

yellow dots. Secondary organic particles and

nitrates contribute to regional haze, while O3 is

important in the production of secondary particles

and gases and has adverse effects on biological

and terrestrial systems. Section A of Figure 4.8 is

meant to show an unstable “windy” meteorological scheme and to explicitly show the deposition

process. NOx and VOCs are emitted, which in turn

convert to nitrate and organic particles and O3 gas.

All of these species deposit to the terrestrial

ecosystem as they come in contact with it.

Secondly, the gases and particles can enter clouds

where they continue to react and eventually rain

out as acid deposition. Nitrates are important contributors to acid rain although there can be some

weak organic acids.

Figure 4.8 further shows hydrocarbon emissions from forests and the emissions from firerelated activity. Fire emissions not only include

NOx and VOC gases, but also primary organic particles in the form of uncombusted material.

Figure 4.9 schematically shows the five particle types that make up nearly all of the fine particle mass found in the atmosphere. They are, in

order of their relative contribution to visibility

impairment, sulfates, organics, soil, elemental carbon, and nitrates. In some parts of the United

States, the relative order of importance can

change. For instance, in areas in southern

California, nitrates can cause most of the visibility

impairment. 

26 Introduction to Visibility

Fig. 4.9 The five particle types that make up the fine

particle mass: sulfates, organics, elemental carbon,

soil, and nitrates.
Section 5: Visibility Measurements 27

Developing the links between visibility and

particles that scatter and absorb light

requires extensive monitoring programs in which

both atmospheric optical variables and particulate

concentrations that cause visibility impairment are

measured. Since visibility cannot be defined by a

single parameter, it follows that a well-defined

monitoring methodology does not exist. However,

monitoring methods can be subdivided into three

classes: view, optical, and aerosol monitoring. 

Visibility, in the most general sense, reduces to

understanding the effect that various types of

aerosol and lighting conditions have on the

appearance of landscape features. Many visibility

indices have been proposed to quantify the appearance of a scene; however, a photograph relating

the effects particles have on the appearance of

landscape features is the most simple and direct

form of communicating visibility impairment.

Therefore, a systematic photography program

(view monitoring) that records the appearance of

the scene under a variety of lighting conditions

and aerosol concentrations is a key part of most

visibility monitoring programs. The camera in its

simplest form is shown in Figure 5.1. It consists

of a lens to focus the image on a strip of photographic film, and an aperture and shutter to control

the amount of light entering the camera. 

However, because it is difficult to extract

quantitative information from color slides or pictures, some direct measure of a fundamental optical property of the atmosphere is desirable.

Therefore, most visibility programs include some

measure of either atmospheric extinction or scattering.

5.1 Measurements of Scattering and

Extinction

The scattering coefficient is a measure of the

ability of particles to scatter photons out of a beam

of light, while the absorption coefficient is a measure of how many photons are absorbed. Each

parameter is expressed as a number proportional

to the amount of photons scattered or absorbed per

distance. The sum of scattering and absorption is

referred to as extinction or attenuation.

Figure 5.2 is a schematic diagram showing a

beam of light made up of photons with varying

wavelengths that is incident on a concentration of

particles and absorbing gas. Knowing the number

of photons incident on a concentration of particles

and measuring the number of photons successfully

passing through the particulate concentration, it is

possible to calculate the number of photons scattered and absorbed. The instrument that measures

extinction (sum of scattering and absorption) is

known as a transmissometer.

SECTION SECTION 5 .....................................................................

VISIBILITY MEASUREMENTS VISIBILITY MEASUREMENTS

Fig. 5.1 A camera is the simplest way to monitor visibility. Much like the eye, it has a lens to focus the

image onto the film, and an aperture and shutter to

control the amount of light entering the camera.
The light source is usually an incandescent

lamp, and the receiver is a telescope fitted with an

appropriate detector. The light source and detector can be placed 1-10 kilometers apart, and the

measurement is usually referred to as long-path

measurement. 

A similar light source-detector configuration

can be used to measure just the scattering ability of

particles and gases. If the detector is placed parallel to the incident photons, only those photons that

are scattered will be detected. This type of instrument is called a nephelometer (Figure 5.3). If the

detector is so aligned as to measure scattering in

only one direction it is referred to as a polar nephelometer. On the other hand, if all photons scattered in forward, side, and back directions are

allowed to hit the detector, the instrument is

referred to as an integrating (summing) nephelometer. The instrument is usually constructed in

such a way as to have the sampling chamber and

light source confined to a small volume so that the

instrument makes a “point” or localized measurement of scattering. 

Most monitoring programs use combinations of

transmissometers and integrating nephelometers to

measure extinction and scattering. Historically,

the National Weather Service (NWS) program

estimated visual range (from which atmospheric

extinction can be approximated) by viewing a

series of landscape features at a variety of distances and recording the most distant feature that

can be seen. While this program has been discontinued, the database, which goes back to the 1940s,

is still useful for tracking visibility changes that

have occurred over decades.

5.2 Measurements of Particles in

the Atmosphere 

Finally, particle measurements are generally

made in conjunction with optical measurements to

help infer the cause of visibility impairment, and to

estimate the source of visibility reducing aerosols.

Size and composition are the two dimensions of

particle characterization most often used in visibility monitoring programs. Particles between 0.1 to

28 Introduction to Visibility

Fig. 5.2 Placement of a light source and detector as shown here is known as a transmissometer. As photons pass

through a concentration of particles and gases, they are either scattered out of the light path or they are absorbed.

Thus a detector placed as indicated measures only those photons that are transmitted the length of the light path.

Because this instrument is sensitive to both scattering and absorption, it can be calibrated to measure the extinction coefficient.
1.0 microns are most

effective on a per mass

basis in reducing visibility and tend to be associated with man-made

emissions. Figure 5.4

shows a diagram of a

cyclone-type particle

monitor that separates all

those particles less than a

specified size (usually

2.5 μm) and collects

them on a filter substrate

for additional analysis.

The air is caused to spin

in much the same way as

a merry-go-round. The

heavier particles, those

larger than 2.5 μm, fall

off the merry-go-round

and impact on the side of

the sampler to be discarded to the bottom of

the sampler. Those particles staying in the air

stream pass through a

filter where they are

extracted for further

analysis. Particles are

speciated into sulfates,

nitrates, organic material,

elemental carbon (soot),

and soil. The speciation

of particles helps determine the chemical-optical characteristics and

the ability of the particle

to absorb water (RH

effects) and is important

to separate out the origin

of the aerosol.

Section 5: Visibility Measurements 29

Fig. 5.3 Placement of a detector for the measurement of the number of photons

scattered by a concentration of particles and gas.

Fig. 5.4 Diagram of a cyclone-type particle monitor. The air inside the cyclone

spins in such a way as to cause larger particles to deposit on the inside of the

monitor and fall to the bottom, while smaller particles continue in the air stream

and are collected on a filter substrate for analysis.
30 Introduction to Visibility
There are and have been a number of particle and visibility monitoring programs

implemented in the United States, most notably

the Interagency Monitoring of Protected Visual

Environments (IMPROVE) and the National

Weather Service (NWS) program. The

IMPROVE program, by design, has its focus on

nonurban environments, while the NWS program

was carried out at airports across the United

States. In the next sections, a brief summary from

results of these programs will be presented.

Figure 6.1 shows the locations of the monitoring

sites used in the IMPROVE program.

6.1 Natural Conditions

Best estimates of average natural background

particle concentrations for east and west of the

Mississippi River have been developed.

Neglecting effects of rain and clouds, it is possiSection 6: Particle Concentration and Visibility Trends 31

SECTION SECTION 6 ...............................................................................

PARTICLE CONCENTRA TICLE CONCENTRATION AND TION AND 

VISIBILITY TRENDS VISIBILITY TRENDS

Fig. 6.1 Location of the monitoring sites used in IMPROVE. (1999)
ble to use particle concentrations, listed in Table

6.1, to estimate natural visibility conditions. The

extinction efficiencies (efficiency with which the

particles scatter and absorb light) assumed for

each particle species, also listed in Table 6.1, are

multiplied by the particle concentration to estimate dry particle extinction. The extinction efficiencies used for sulfates, nitrates, and organics

are based on theoretical calculations assuming

typical particle size distributions and chemical

characterics as well as a literature review. 

Additionally, it is also necessary to estimate

the effect of high relative humidity on scattering

by water soluble particles, such as sulfates and

nitrates. Therefore, in the East, soluble particles’

extinctions are multiplied by an additional factor

of 4.1, while in the West the factor used was 2.2.

These factors correspond to effective relative

humidity of 87% and 70%, respectively. Lower

and upper bounds on natural visibility are calculated by first assuming only sulfates and nitrates

are hygroscopic, and secondly assuming that 0.5

of the organics are also water soluble. The

assumption that 0 to 0.5 of the organics are water

soluble reflects the uncertain nature of the chemical characteristics of organic particles.

From these extinctions, and due to the scattering associated with clear sky (Rayleigh atmosphere), it is possible to estimate natural median

visibilities. These calculations are summarized

graphically in Figure 6.2. In the East visual

ranges are estimated to be 60 to 80 miles, while in

the West they are between 110-115 miles. Under

natural conditions, carbon-based particles are

responsible for most of the non-Rayleigh particleassociated visibility reduction, with all other particle species contributing significantly less.

Scattering by air molecules is the largest contributing factor to the reduction of visual range, at

about 40-60%. 

It is expected that coastlines and highly vegetated areas may be lower than these averages,

while some elevated areas could exceed these

background estimates. Furthermore, it was not

32 Introduction to Visibility

Average Mass Dry Extinction % Contribution to 

Concentrations Efficiencies Visibility Reduction

 EAST WEST EAST WEST 

FINE PARTICLES

Sulfates ((NH4)2 SO4) 0.23 μg/m3 0.115 μg/m3 3.0 m2/gm 9-12% 5-5%

Organics 1.5 μg/m3 0.5 μg/m3 3.0 m2/gm 19-38% 10-15%

Elemental Carbon 0.02 μg/m3 0.02 μg/m3 10.0 m2/gm 0.5-1% 1-1%

Nitrates (NH4NO3) 0.1 μg/m3 0.1 μg/m3 3.0 m2/gm 4-5% 4-4%

Soil Dust 0.5 μg/m3 0.5 μg/m3 1.25 m2/gm 2-3% 4-4%

COARSE PARTICLES 3.0 μg/m3 3.0 μg/m3 0.6 m2/gm 6-8% 11-12%

CLEAN AIR NA NA NA 33-43% 61-64% 

VISUAL RANGE NA NA NA 100-130 km 182-193 km

Table 6.1. Estimated natural background particulate concentrations and extinction. Assuming the dry

extinction efficiencies listed in the table, and sulfates, nitrates, and half the organics are hygroscopic,

average natural visual ranges, along with the percent contribution of each aerosol species to natural visibility impairment was estimated.
attempted to estimate a frequency distribution of

background particle concentrations and therefore

a distribution of visibilities cannot be calculated.

It is expected that natural or background visibility,

on some days, approached the Rayleigh limit

because current monitoring data show that even

today there are some time periods where the

atmosphere is essentially free of visibility reducing particles.

6.2 Current Conditions

Current conditions of the particle concentrations that affect visibility degradation will be

explored first and their effect on visibility will be

presented second.

6.2.1 Seasonal Patterns of Particle 

Concentrations

Sulfate and carbon species are the single

largest contributors to visibility reduction at all

monitoring sites. In the East, sulfate species make

up over 70% of the measured visibility impacts,

while in the West they are responsible for about

30%. Figure 6.3 shows time

plots of sulfur concentrations

measured at Shenandoah,

Mount Rainier, Rocky

Mountain, Grand Canyon,

Acadia, and Big Bend

National Parks. If the sulfur is

in the form of ammonium sulfate (NH4)2SO4, then the sulfur concentrations are multiplied by 4.125 to estimate

ammonium sulfate mass.

These sites were selected to

show typical temporal variability of sulfate concentrations that are measured across

the nonurban United States.

Shenandoah shows the

strongest seasonal trends with

summer months having the

highest concentrations, and

winter the lowest. Similar trends for sulfur concentrations are observed at Grand Canyon, Rocky

Mountain and Mount Rainier but not at Acadia

and Big Bend. Many of the monitoring sites show

seasonal trends; however, the strength of the seasonal signal diminishes as overall sulfur concentrations decrease. Elevated summer sulfate levels

are a result of SO2 to SO4 conversion that is associated with a photochemical process involving the

increase in summertime sunlight. In the East, similar trends are observed for organics, while in

other parts of the United States the seasonal trend

is much less pronounced and in most cases nonexistent. Nitrates, on the other hand, at almost all

locations tend to be the highest during the winter

and lowest during the summer months. 

The average major seasonal trends are presented as seasonal fine mass budgets for four large

geographic areas in Figure 6.4. The Northwest

summary is a combined average of the Cascade

Mountains, central Rocky Mountains, Great

Basin, northern Great Plains, northern Rocky

Mountains, Sierra Nevada, and Sierra-Humboldt

regions. The Southwest summary combines the

Section 6: Particle Concentration and Visibility Trends 33

Fig. 6.2 Natural visibility in the East is estimated to be between 60 and 80

miles, while in the West it is between 110-115 miles. Under natural conditions,

sulfur is only 10 and 5% of the visibility impairment in the East and West,

respectively. Currently, sulfur is 60 to 90% of the visibility reduction in the East

and about 30% in the West. Dust is the sum of soil dust and coarse particles.
Colorado Plateau and Sonoran Desert regions,

while Pinnacles National Monument, and San

Gorgonio Wilderness Area make up the California

summary. The Appalachian Mountains region is

presented as the eastern summary.

The single outstanding feature of all four geographic areas is a similar seasonal trend in the

total fine mass concentration represented as the

sum of the aerosol species and in the concentration of each individual species. The highest fine

mass concentration occurs in the summer, while

winter has the lowest. Concentrations of sulfates

and organics have a similar trend in all four areas.

Nitrates tend to be higher in winter and spring

than in summer and fall. Trends in soil are variable, while elemental carbon (EC) shows little

variation from season to season. Sulfates are by

far the single largest contributer to fine mass in

the eastern United States, while in the Northwest

organics contribute most to fine mass. Nitrates

edge out organics and sulfates in southern

California, while in the Southwest sulfates, organics, and soil all contribute about equally to fine

mass.

34 Introduction to Visibility

Fig. 6.3 Sulfate trends at Shenandoah, Mount Rainier, Rocky Mountain, Grand Canyon, Acadia, and Big Bend

National Parks. Sulfate is usually highest during the summer months and lowest in winter. However, at some

national parks, such as Acadia and Big Bend, seasonal trends are not very pronounced with more amounts of sulfur found throughout the year.
6.2.2 Spatial Trends in Visibility

While there is no one definition of

visibility that meets all the criteria of

“seeing” of landscape features, a number

of visibility indices have evolved.

Extinction, in the form of inverse megameters (Mm-1), is proportional to the

amount of light lost as it travels over a

million meters and is most useful for

relating visibility directly to particle

species concentrations, while deciviews

are related to extinction but scaled in

such a way that it is perceptually correct.

For example, a one deciview change on

a 20 deciview day will be perceived to

be the same as on a 5 deciview day. This

is not the case for extinction or visual

range. For reference, Figure 6.5 compares extinction in Mm-1, deciviews

(dv), which are unitless, and visual range

in km. For instance, 10 Mm-1 corresponds to about 400 km visual range and

0.0 dv, while 1000 Mm-1 is about 4 km

visual range and 46 dv.

Since light extinction of sulfates and

nitrates, on a relative basis, is larger than

that of other fine particles due to associated water, and since light-absorbing

carbon absorbs light very efficiently on a

per mass basis, extinction budgets are

different from mass budgets. However,

for the sake of brevity, because spatial

trends in particle mass concentration and

visibility are similar, only visibility data

will be presented here. 

Section 6: Particle Concentration and Visibility Trends 35

Fig. 6.4 Summary of seasonal trends in fine mass concentration

for four geographic regions of the United States. The height of

the bar is the fine mass in μg/m3 and the shaded patterns are proportional to the contribution of various particle species.

Fig. 6.5 Comparison of extinction (Mm-1), deciview (dv), and visual range (km).
Figure 6.6 shows isopleths of the aerosol-only

light extinction coefficient for the United States

and Figure 6.7 shows the same plot but in terms

of deciviews. Because there are only a few sites

in the eastern United States, isopleths cannot be

drawn with a high degree of accuracy. There is a

strong east-west dichotomy. The highest light

extinction (more than 120 Mm-1) occurs in the

eastern United States, while the lowest light

extinction (less than 15 Mm-1) occurs in the Great

Basin, central Rocky Mountains, and nonurban

southwest. Extinctions are also relatively high

near the Los Angeles and San Francisco metropolitan areas of California, and to a lesser extent,

in the Pacific Northwest. Notably, the monitoring

site in southern California is not in the Los

Angeles basin but in the San Gorgonio

Wilderness Area, well outside the population centers of southern California.

Figures 6.8 through 6.12 are composite figures that present isopleths of extinction associated with the aerosol species on the first half of

the graph and the fraction of total extinction on

the second half. Figure 6.8 shows the contribution of coarse mass plus fine soil to total extinction and in most cases would be considered

mostly natural, primarily wind-blown dust.

36 Introduction to Visibility

(a)

(b)

Fig. 6.8 Map (a) shows extinction, expressed in terms

of inverse megameters, attributed to coarse mass and

fine soil, while (b) shows the percent contribution of

coarse mass and fine soil to total extinction.

Fig. 6.7 Average visibility, expressed as deciviews,

calculated from aerosol concentrations measured in

the IMPROVE monitoring program.

Fig. 6.6 Average reconstructed light extinction coefficient (Mm-1) calculated from the aerosol concentrations measured during IMPROVE.
Extinction associated with coarse mass/fine soil

does not show strong spatial trends. It tends to be

highest in the desert southwest and lowest in

forested areas. The fraction that coarse mass/fine

soil contributes to total extinction indicates large

spatial trends primarily because in some parts of

the country, especially the eastern United States,

the overall extinction is so high that coarse

mass/fine soil is almost negligible. However, in

the west, coarse mass/fine soil is 15-20% of estimated extinction.

Figure 6.9 shows isopleths of ammonium sulfate extinction and the fraction of extinction

attributed to ammonium sulfate. Note that the

ammonium sulfate extinction in the eastern

United States is about a factor of 25 to 30 higher

than in the Great Basin area and a factor of 10

higher than the desert Southwest, central Rocky

Mountains, and Sierra Mountains. There is also a

gradient from the San Francisco Bay area and

from the Pacific Northwest to the central West.

Generally, the lowest ammonium sulfate extinction occurs in northern California and Nevada,

southern Oregon, Idaho, and Wyoming. Sulfates

account for roughly 60-75% of the extinction in

the eastern United States, with the fractional contribution of ammonium sulfate to extinction

decreasing the farther west one goes. The geographic region where sulfates contribute least to

visibility impairment, about 25%, occurs in an

area extending from western California, Nevada,

southern Oregon, and Idaho. Sulfates account for

approximately 30-40% of the visibility impairment in much of the remaining western United

States.

Figure 6.10 shows isopleths of the organic

carbon light extinction throughout the United

States. Note that extinction fraction caused by

organic carbon is largest in the Great Basin area of

the United States, and is lowest in the eastern

United States. In most areas of the western

United States, organics contribute about 20-40%

of the extinction. In the eastern United States they

contribute between 10 and 15% of estimated

extinction.

Figure 6.11 shows the nitrate light extinction.

There is a small overall gradient from the east to

west. The strongest gradient is from the urban

areas of California, especially the Los Angeles

metropolitan area to the California desert. Nitrate

extinction is 20 times higher at monitoring sites in

southern and coastal areas of California than in

the Great Basin, Colorado Plateau, and Rocky

Mountains. In southern California its contribution

to extinction is highest at about 40%, and interestingly, only a few hundred kilometers away to the

Section 6: Particle Concentration and Visibility Trends 37

(a)

(b)

Fig. 6.9 Map (a) shows extinction, expressed in terms

of inverse megameters, attributed to sulfates, while (b)

shows the percent contribution of sulfates to total

extinction.
north and west, its fractional contribution is about

1/6 as much. In the East, nitrates contribute

between 3 and 10% of the extinction.

Figure 6.12 shows isopleths of the extinction

caused by elemental and other light-absorbing

carbon (LAC). It is highest in southern California

and in the southeastern United States, and lowest

in the nonurban west. LAC contributes about 7 to

15% to extinction in the western United States,

and about 4-6% in the eastern United States.

6.3 Long-Term Trends

6.3.1 Eastern United States

Shown in Figure 6.13 are isopleths of eastern

United States median visual range, derived from

airport data, for 1948-1982. Winter includes

January, February, and March, spring includes

April, May, and June, and so on. First, the current

eastern seasonal visibility trends are quite evident.

Summer corresponds to the lowest visibility,

while the winter months have better visibility. 

Second, long-term trends are also quite evident. In the winter season, there has been some

improvement in visibility in New England and the

38 Introduction to Visibility

(a)

(b)

Fig. 6.11 Map (a) shows extinction, expressed in

terms of inverse megameters, attributed to nitrates,

while (b) shows the percent contribution of nitrates to

total extinction.

(a)

(b)

Fig. 6.10 Map (a) shows extinction, expressed in

terms of inverse megameters, attributed to organic

carbon, while (b) shows the percent contribution of

organic carbon to total extinction.
north central United States from 1948-1952 and

1960-1964. However, since 1970 the winter season has shown decreased visibility, especially in

the Southeast. The spring season shows a degradation of visibility in the entire eastern United

States, especially along the gulf coast and the

south and central east coast. The most dramatic

changes, however, are evident during the summer

months. A region of modest visibility in the eastern United States during

1948-1952 steadily

expanded and became worse

until the entire eastern

United States and southeastern Canada were significantly degraded. The fall

season shows significant

improvement in the north

central industrial areas from

1970-1974 and 1978-1982.

6.3.2 Western United

States

Airport visibility data

have also been examined for

trends in visual air quality in

the Rocky Mountains southwest over the time period

from 1948 to 1976. Results suggest that in the late

1940’s to the early/mid 1950’s, visibility trends

were mixed, with some geographic areas showing

a slight improvement, and a lesser number of areas

showing a slight deterioration. From the early/mid

1950’s (1953-1955) to the early 1970’s (1970-

1972), most areas indicated a drop in visibility of

approximately 10 to 30%. From the early 1970’s

(1970-1972) to the middle 1970’s (1974-1976),

Section 6: Particle Concentration and Visibility Trends 39

(a) (b)

Fig. 6.12 Map (a) shows extinction, expressed in terms of inverse megameters, attributed to light-absorbing carbon, while (b) shows the percent contribution of light-absorbing carbon to total extinction.

Fig. 6.13 Trends in median visual range over the eastern United States from

1948 through 1982.
visibility generally tended to increase by about 5-

10%, especially at those sites in or near Arizona. 

Airport data at 67 sites in California were also

examined. Plots of long-term trends in median

visibility (for all data with no sorting for meteorology) at the study sites reveal that visibility

trends in California tend to split into two general

sub-periods, divided at approximately 1966.

Before 1966, nearly all locations exhibited deteriorating visibility, with especially large visibility

decreases occurring in and near central California

(Central Valley). After 1966, nearly all locations

have displayed improving visibility.

Other trend analyses using California airport

data centered on “adverse” and “superior” visibility. “Adverse” visibility refers to those days that

have concentrations of visibility-reducing particles

sufficient to reduce prevailing visibility to less

than 10 miles when the relative humidity is less

than 70%. Visibilities that are 30 miles or more

are termed “superior.” The trend of the composite

superior visibility for two pristine stations, Mount

Shasta and Bishop, California, shows a gradual

(about 1% per decade) decrease in average superior visibility occurrences. 

6.4 Historical Relationships Between

SO2 Emissions and Visibility

Dominant relationships between smelter SO2

emissions and visibility have been shown to exist

in the southwestern United States. Figure 6.14

shows time plots from 1949 to 1976 of the percent

of hours in one year that Phoenix and Tucson,

Arizona, have visibility below 40 and 60 miles,

respectively, and Arizona smelter SO2 emissions

in tons per day. The correlation between these

two variables is 0.81 at Phoenix and 0.88 at

Tucson. 

Trends between smelter SO2 emissions and

elemental sulfur concentrations (presumably

ammonium sulfate) were examined in seven

national park units: Chiricahua National

Monument, Tonto National Monument, Grand

Canyon National Park, Chaco Culture National

Historic Park, Mesa Verde National Park, Bryce

Canyon National Park, and Canyonlands National

Park.

Figure 6.15 shows a scatter plot of 1980-1981

smelter sulfur dioxide emissions and sulfur concentrations for Grand Canyon and Chiricahua.

There is a clear relationship between sulfur dioxide emissions and sulfur concentrations at both

parks with the relationship being strongest at

Chiricahua, which is located in the smelter country of southern Arizona and weaker at Grand

Canyon, which is 300 km distant.

Furthermore, SO2 emission data and visibility

trend data were compiled for the eastern United

States. The emissions are expressed as million

tons of sulfur/year, and visibility is expressed as

deciviews. Figures 6.16a and 6.16b show the sulfur emission trends for the southeastern United

States for winter and summer months. Figures

6.16c and 6.16d show the same trends for the

40 Introduction to Visibility

Fig. 6.14 Historical trends in percent of hours of

reduced visibility at Phoenix and Tucson, compared to

trends in SO2 emissions from Arizona copper smelters.
northeastern United States. The collinearity

between SO2 emissions and visibility reduction is

impressive. However, it is apparent that the

deciview change per incremental change in SO2

emissions is significantly higher in the Southeast

than Northeast. Possible explanations may be that

the Southeast has higher SO2 oxidation rates,

higher humidity, and more stagnant air masses, or

other emissions may be collinear with SO2 emissions.

These data show that trends in sulfur dioxide

emissions provide a plausible explanation for

variability observed in regional visibility and sulfate concentration variations. 

Section 6: Particle Concentration and Visibility Trends 41

Fig. 6.15 Scatter plot of de-seasonalized sulfur concentrations (μg/m3) vs. smelter emissions (tons/day)

for monthly data at Hopi Point in Grand Canyon

National Park and Chiricahua National Monument.

(a) (b)

(c) (d)

Fig. 6.16 (a) Comparison of sulfur emission trends and deciviews for the southeastern United States during the

winter months. (b) Comparison of sulfur emission trends and deciviews for the southeastern United States during the summer months. (c) Comparison of sulfur emission trends and deciviews for the northeastern United

States during the winter months. (d) Comparison of sulfur emission trends and deciviews for the northeastern

United States during the summer months.
42 Introduction to Visibility
Because the goal of identifying the particles

affecting visibility is to reduce their concentration and thereby improve the seeing of landscape features, it becomes necessary to identify the

sources emitting the precursor pollutants that form

visibility reducing particles. There are generally

two ways to go about this. One can formulate a

model that mimics all those processes outlined in

Figures 4.7 and 4.8. The model must predict transport of gases such as sulfur dioxide, nitrogen dioxide, and reactive hydrocarbons, convert them into

secondary particles, deposit them as wet and dry

deposition, and form estimates of size and composition of concentrations that affect visibility. Since

the model will only be as accurate as the emission

estimates that are input into the model, it is crucial

to develop an accurate emission inventory. These

types of models are referred to as deterministic or

first-principle source-oriented models. They tend

to capture only broad-scale temporal and spatial

characteristics of haze formation and are computer

intensive.

Diagnostic receptor-oriented models have

evolved as a clear alternative to source-oriented

dispersion models. Receptor models start with the

measurement of specific features of the aerosol at

the receptor, and use these features to develop estimates of aerosol contributions of specific source

types and/or source location. 

In the most general sense, geographic regions

with high emissions will have high particle loadings. For instance, high sulfur dioxide emissions

will be associated with high ambient sulfate concentrations and sulfate deposition, and conversely

low emissions will correlate with low ambient

concentrations. In North America, about 27% of

emitted sulfur dioxide is dry deposited, 34% wet

deposited, and 39% remains in the atmosphere and

is eventually exported from the continent primarily to the Atlantic Ocean. Figure 7.1 shows the

emission rates, in millions of tons per year, for sulfur dioxide, nitrogen oxides, and volatile hydrocarbon gases for five major source categories for

the United States, and for comparison, Canada.

Notice that the single largest source of sulfur dioxide is the electric utility industry (coal-fired power

plants), while sources of nitrogen oxides are nearly

evenly split between the electric utility industry

and transportation. Most hydrocarbon gases are

emitted by transportation sources. 

The geographical distribution of those emissions is summarized in Figure 7.2 for ten different

regions of the country. It is worth noting that the

worst visibility occurs in the East. It is associated

primarily with sulfate particles since 84% of the

sulfur dioxide emissions are in the East. Nitrogen

oxide and hydrocarbon emissions are more evenly

split between the East and West with about a 60-40

split, with the East having the highest emissions.

The exercising of models helps to further identify or fine tune source-receptor relationships. A

discussion or review of the many modeling activities that have been carried out over just the last ten

or so years is beyond the scope of this discussion.

However, some general and insightful relationships have been established using statistical treatments of back trajectory models.

Section 7: Identification of Sources Contributing to Visibility Impairment 43

SECTION SECTION 7 ..........................................................................................................

IDENTIFICA IDENTIFICATION OF SOURCES TION OF SOURCES 

CONTRIBUTING T CONTRIBUTING TO VISIBILITY IMP O VISIBILITY IMPAIRMENT AIRMENT
44 Introduction to Visibility

Fig. 7.2 Geographical distribution of sulfur dioxide (SO2), nitrogen oxides (NOx), and volatile organic carbon

(VOC) gas emissions.

Fig. 7.1 Emission rates, in millions of tons per year, for sulfur dioxide (SO2),

nitrogen oxides (NOx), and volatile organic carbon (VOC) gases for five

major source categories for the United States and Canada.
7.1 Back Trajectory Receptor Models

Two receptor modeling approaches that incorporate statistical treatments of air mass back trajectories serve to identify geographic regions with

high emissions, and allow for apportionment estimates of secondary aerosols to general source

areas and, therefore, in some cases, the source

itself.

Back trajectory techniques have been applied

quite successfully in all parts of the United States.

A back trajectory is just a trace of where an air

mass has been in some past-time increment. In

the summary of back trajectory results presented

here, a one layer model that relies solely on

national weather service soundings was used. As

such, it is not sensitive to terrain induced flow

such as drainage of emissions down canyons and

valleys. Therefore, this model tends to be more

accurate when the atmosphere is well mixed in the

vertical direction. These conditions are most

often found during summer months. Furthermore,

most trajectory analyses have focused on sulfurrelated species, not because the analysis can not

be carried out for other particle types, but because

most sulfur particles are man-made and because

they usually constitute a major fraction of the visibility reducing fine particle mass. Therefore, the

following presentation will be directed toward

presenting some of the information learned about

the origins of fine particle sulfur.

Figures 7.3 through 7.11 are isopleth plots of

the source contribution function (SCF) and, in

some cases, conditional probabilities (CP) for

“extreme” sulfur concentrations at Mount Rainier,

Glacier, Grand Canyon, and Rocky Mountain

National Parks, Chiricahua National Monument,

and Big Bend and Shenandoah National Parks.

Extreme refers to those concentrations that are

greater than one standard deviation above the

mean. The methodologies calculating these functions will be briefly reviewed here.

The geographic domain of interest is subdivided into one-by-one-degree grid cells. Then the

number of back trajectory endpoints residing over

each grid cell is counted for the sampling period

and characteristic of interest. (A back trajectory

endpoint is the geographic location of a back trajectory at some prespecified time interval.)

Although aerosol samples with any characteristic

could be used, the discussion here will be associated with aerosol samples that correspond to

extreme sulfur concentrations. The spatial distribution of the number of endpoints in each grid cell

is referred to as the residence time surface. If an

SO2 source area is responsible for high sulfate

concentration at a receptor site, one might hypothesize that more trajectory endpoints will be found

over that source area when sulfur concentrations

are highest at the monitoring site. However,

because all back trajectories originate at the receptor site, the number of endpoints will always

increase as one moves toward the receptor location. This central tendency can be normalized out

in a number of different ways.

The source contribution function surface

reflects one method of normalization. It is calculated by dividing the residence time surface corresponding to extreme sulfur concentrations by the

residence time surface corresponding to back trajectories departing from the receptor site in any

direction with equal probability but with some

average wind speed. Therefore, each isopleth line

on the source contribution function contour plots

in Figures 7.3 through 7.10 corresponds to those

grid areas that are associated with equal probability of contributing extreme sulfur concentrations

at the receptor site. The number associated with

each SCF isopleth line is a relative probability that

each grid area along the isopleth line will contribute to extreme sulfur contributions at the

receptor site relative to any other contour line.

For instance, grid areas along the line labeled 100

are 5 times as likely to contribute to high sulfur as

along the contour line labeled 20. Since the contour lines are relative probabilities, they are

dimensionless. The source contribution plots give

an estimation of the location of those sources most

likely to contribute to extreme sulfur concentration at a receptor site.

Section 7: Identification of Sources Contributing to Visibility Impairment 45
A second way of normalizing the residence

time surface gives an indication not of the most

likely origin of sources contributing to extreme

sulfur concentrations, but rather the likelihood of

an area to contribute to extreme sulfur concentrations given the condition that the air mass had

actually passed over the source area. This conditional probability surface is obtained by dividing

the residence time surface corresponding to

extreme sulfur concentrations by the residence

time surface for all sampling periods. This is

referred to as the conditional probability surface.

The numbers associated with each conditional

probability contour in Figures 7.3 through 7.10

indicate the probability of air masses, having

passed over a grid cell, to contribute to extreme

sulfur concentrations at the receptor site. 

It is possible for a region of the country to be

associated with a low source contribution function, while at the same time having a high conditional probability. For instance, Figures 7.3a and

7.3b are the source contribution function and conditional probability surfaces for Mount Rainier

National Park. Figure 7.3a shows that sources

associated with extreme sulfur concentrations at

Mount Rainier originate from a source region to

the northwest. However, the front range of the

Canadian Rocky Mountains (Alberta) has the

highest probability of contributing to extreme sulfur concentration if the air mass actually passes

over that region and arrives at Mount Rainier. 

Figures 7.4a and 7.4b indicate the extreme

sulfur source contribution and conditional probability surfaces for Glacier National Park. Most of

the elevated sulfur days in Glacier are associated

with source areas in eastern Washington and

southwestern Idaho. More interesting, however,

is the conditional probability surface which, like

the conditional probability surface for Mount

Rainier, identifies the front range of the Canadian

Rocky Mountains as a source area associated with

extreme sulfur at Glacier. Canadian sources north

of the Dakotas, and possible sources in

Minnesota, also contribute to extreme sulfate concentrations in Glacier. 

The source contribution surfaces for Grand

Canyon and Rocky Mountain National Parks are

shown in Figures 7.5a and 7.6. Both parks show

southern California, a source area hundreds of

kilometers distant, to be the most probable source

area associated with extreme sulfate concentrations at these parks. On the other hand, Figure

7.7, the source contribution function surface for

Chiricahua National Monument, shows the nearby

southern Arizona copper smelter region to be the

most probable source of extreme sulfate, with

southern California and an area along the Texas46 Introduction to Visibility

Fig. 7.3 (a) Extreme fine sulfur concentration source contribution, Mount Rainier National Park. (b) Extreme fine

sulfur concentration conditional probability, Mount Rainier National Park.

(a) (b)
Mexico border being the second most probable

sources of sulfates. The Big Bend National Park

source contribution function surface (Figure 7.8)

seems to confirm the sulfate source area along the

Texas-Mexico border. This area has the highest

probability of contributing extreme sulfur to Big

Bend. The conditional probability surfaces for all

four parks are similar. Figure 7.5b, the conditional probability surface for Grand Canyon

National Park, shows that air masses arriving

from the east have the greatest probability of

being associated with extreme sulfur concentrations. 

Section 7: Identification of Sources Contributing to Visibility Impairment 47

Fig. 7.5 (a) Extreme fine sulfur concentration source contribution, Grand Canyon National Park. (b) Extreme

fine sulfur concentration probability, Grand Canyon National Park.

(a) (b)

Fig. 7.4 (a) Extreme fine sulfur concentration source contribution, Glacier National Park. (b) Extreme fine sulfur concentration probability, Glacier National Park.

(a) (b)

Fig. 7.6 Extreme fine sulfur concentration source contribution, Rocky Mountain National Park.
Figure 7.9 shows that the air masses passing

over the Ohio River Valley are most likely to contribute to extreme sulfur concentrations at

Shenandoah National Park. The air masses passing over the source areas around Detroit and

Chicago are the second most likely to contribute

sulfur laden air to Shenandoah. The conditional

probability surface for Shenandoah is similar to

Figure 7.9. However, additional source areas

show up along the Northeast coast. The source

area in the Ohio River Valley has a large number

of coal-fired power plants that emit sulfur dioxide. 

A similar analysis can be carried out to show

the origins of air masses that are associated with

sulfur concentrations lower than one standard

deviation below the mean. Figure 7.10a shows

the conditional probability surface map for low

sulfur concentrations for Grand Canyon National

Park. Notice that only air masses arriving from

the north are associated with low sulfur concentrations. In general, similar maps for other

national parks show the Great Basin region associated with low sulfur concentrations. Figure

7.10b, a low sulfur conditional probability surface

map for Mount Rainier, shows that air mass arriving from east of the Cascades but west of the

Rocky Mountains, and from northern California

and southern Oregon, are associated with low sulfur concentrations.

Source contribution function and conditional

probability surface analysis has been carried out

for all National Park Service monitoring sites, and

similar source areas show up for many areas, but

with varying probabilities of impacting specific

receptor sites. By comparing maps like those in

Figures 7.3 through 7.10, it is possible to begin to

understand the geographic extent to which

sources affect various parts of the country.

48 Introduction to Visibility

Fig. 7.8 Extreme fine sulfur concentration source contribution, Big Bend National Park.

Fig. 7.9 Extreme fine sulfur concentration source contribution, Shenandoah National Park.

Fig. 7.7 Extreme fine sulfur concentration source

contribution, Chiricahua National Monument.
However, comparison of a number of source contribution function surface figures is difficult and

sometimes confusing. The source contribution

function analysis can be modified to show the

geographic extent of a single source area, rather

than various sources impacting a single receptor

site. To do this, the relative probability of a single

source area at each of the receptor sites is contoured.

Figures 7.11a through 7.11e show the geographic extent to which emissions from four

source areas contribute to high sulfate concentrations. The first area (Figure 7.11a), southern

California, has its highest probability of impact in

the Mohave Desert and Colorado Plateau, an area

containing a number of national parks, such as

Grand Canyon, Canyonlands, Arches, and Bryce

Canyon. The graph also shows that southwestern

California has a reasonably high probability of

impacting areas as far north as Wind Cave

National Park, South Dakota, Grand Teton

National Park, Wyoming, and Craters of the Moon

National Monument, Idaho. Figure 7.11b shows

the extent to which copper smelter emissions contribute to elevated sulfate. The geographic area

associated with the highest probability source

contribution function extends from southern

Arizona along the Arizona-New Mexico border,

central Colorado, western Nebraska and southwestern South Dakota. Figure 7.11c shows that

emissions from the industrial area near Monterrey,

Mexico affect Big Bend and Guadalupe

Mountains National Parks and as far north as

Grand Canyon National Park. The source area

associated with the uncontrolled Navajo

Generating Station, a large, coal-fired power

plant, contributes to extreme sulfate concentration

throughout the central Rocky Mountains, southern

Idaho, and Arizona (see Figure 7.11d). Finally,

Figure 7.11e shows that sources around the Salt

Lake City area have their largest impact on southern Idaho, but can be detected as far south as

Grand Canyon. 

7.2 Trajectory Apportionment Model

The previously described techniques identify

source areas and the geographic extent to which

these source areas contribute to extreme sulfur.

These techniques do not allow for estimation of

the fraction of sulfur contributed by each of the

sources. However, a source receptor model utilizing trajectory endpoints in conjunction with

known source areas approximates the relative

contribution of different source areas to aerosol

concentrations at receptor sites. The technique

uses statistical models to relate the number of

Section 7: Identification of Sources Contributing to Visibility Impairment 49

Fig. 7.10 (a) Low fine sulfur concentration conditional probability, Grand Canyon National Park. (b) Low fine

sulfur concentration conditional probability, Mount Rainier National Park.

(a) (b)
back trajectory endpoints

over source areas to sulfur

concentrations at the

receptor site as a function

of time. The following

discussion pertains to an

analysis using data and

emission inventories

developed previous to

1991.

Figures 7.12a-f show

relative contribution of

various source areas to

measured sulfur concentrations at a number of

national parks. Relative

source contributions were

calculated for Mount

Rainier, Grand Canyon,

and Big Bend National

Parks in the West, and at

Acadia, Shenandoah, and

Great Smoky Mountains

National Parks in the East. 

At Mount Rainier (see

Figure 7.12a), sources on

Vancouver Island, in western British Columbia, and

in the Puget Sound area

are estimated to contribute

approximately 35% of the

ambient sulfur, while

sources in northern

Washington are estimated

to contribute 30% of

ambient sulfur. The

source regions north of

Mount Rainier do not contain a single large source

of SO2, but a rather large

number of smaller

sources. These sources

include industrial boilers,

some mining and smelting

activity, and pulp and

50 Introduction to Visibility

Fig. 7.11 Plots of constant source

contribution function lines for (a)

southern California, (b) southern

Arizona copper smelter region, (c)

Monterrey, Mexico region, (d) Navajo

Generating Station, (e) northern Utah,

Salt Lake City, and surrounding area.

(a) (b)

(d)

(e)

(c)
paper mill operations. The Pacific Power source

region, which is estimated to contribute 34% of

measured sulfur, includes the Centralia coal-fired

power plant, which has SO2 emissions in excess

of 65,000 tons per year. Sources to the south of

Mount Rainier are estimated to contribute less

than 2% of ambient sulfur. 

At Grand Canyon (see Figure 7.12b), sources

in California contribute about 33% of measured

sulfur at Grand Canyon, while southern Arizona

accounts for 14%. California SO2 sources consist

of oil refining activities, electric generating facilities, and in Los Angeles, automobile and diesel

emissions. The southern Arizona SO2 sources are

primarily associated with copper smelter activities. Other large sources of SO2 contributing to

sulfur at Grand Canyon (23%) are the Mohave,

Reid Gardner, and Navajo Power Plants. Sources

in Utah are estimated to contribute approximately

12% of measured sulfur, while other sources,

including coal-fired power plants, some smelting

activity in southeastern New Mexico, and the El

Paso, Texas area, are estimated to contribute about

18%. 

At Big Bend, (see Figure 7.12c), it is estimated that about 41% of the ambient sulfur is

associated with emissions in the Monterrey,

Mexico area, and about 29% from central Mexico.

Thus, about 70% of the sulfur found at Big Bend

has its origin in Mexico. It is not known what

types of sources are responsible for the SO2 emissions.

The source area contributing the most sulfur,

29%, to Acadia is the area around Sudbury,

Canada (see Figure 7.12d). The nickel smelter

operations at Sudbury emit approximately

700,000 tons of SO2 per year during the time

period this analysis was conducted. Coal-fired

power plants in the New York-Philadelphia area

are estimated to contribute approximately 15%,

while power plants in northern New York contribute 24%. SO2 sources in the Midwest, primarily Michigan, contribute approximately another

20% to the sulfur measured at Acadia.

About 30% of the sulfur measured at

Shenandoah is from power plants in the

Pittsburgh-Cleveland area (see Figure 7.12e).

This source area contains the Environmental

Protection Agency air quality control regions with

the highest SO2 annual average emission rates in

the United States. Coal-fired power plants in that

region emit over 3 million tons of SO2 per year.

The coal-fired power plants in the ColumbusDayton-Cincinnati region contribute another 12%

of sulfur at Shenandoah. Together, these two geographic areas, usually referred to as the Ohio

River Valley, contribute 42% of the ambient sulfur

at Shenandoah. Another dominant source area for

Shenandoah, labeled as Piedmont-northern

Tennessee, contributes 16% of ambient sulfur.

These emissions are again associated with large

coal-fired power plants. Coal-fired power plants

in the Southeast contribute another 23% to the

measured sulfur.

For Great Smoky Mountains (see Figure

7.12f), more than 35% of the sulfur is estimated to

arrive from the Ohio River Valley region. Other

dominant source regions are both areas containing

large coal-fired power plants: Tennessee Valley

Authority (western TVA and Piedmont) and

Memphis regions contribute about 17%, while

sources along the Gulf Coast’s contribution to sulfur contribute more than 10%.

Section 7: Identification of Sources Contributing to Visibility Impairment 51
52 Introduction to Visibility

Fig. 7.12 Fraction of sulfate arriving at (a) Mount Rainier, (b) Grand Canyon, (c) Big Bend, (d) Acadia, (e)

Shenandoah, and (f) Great Smoky Mountains National Parks, from various source regions.

(a)

(c)

(e)

(b)

(d)

(f)
Amajor challenge in establishing visibility

values is to develop ways of quantitatively measuring visibility impairment as perceived by the human eye. Quantification of visual

impairment of a scenic resource requires two crucial components: 1) the establishment of the level

of air pollution that is just noticeable, and 2) a

determination of the functional relationship

between air pollution and perceived visual air

quality.

The first goal is important when it is necessary

to quantitatively specify visible pollution under a

given atmospheric condition. The second object

is important when trying to assess the societal

value of clear air, whether it be social, psychological, or economical. The first step in assessing

value is to understand the relationship between

perceived changes in visual air quality and an

appropriate physical parameter, such as vista contrast or atmospheric extinction. For example, if a

visitor is willing to pay $5.00 for a given decrease

in atmospheric extinction (air pollution) at the

Grand Canyon, but is unwilling to pay that same

amount for a similar decrease at some other park,

is it because a) that person values the scenic

resource differently at the two parks, or b) the perceived change in visual air quality is different at

the two parks? That is, at one park a given

decrease in extinction can readily be seen, while

at another that same decrease may go unnoticed.

Developing relationships between air pollution and visitor perception falls into two uniquely

different categories. Air pollution can manifest

itself either as layer or as uniform haze. Layered

haze can be thought of as any confined layer of

pollutants that results in a visible spectral discontinuity between that layer and its background (sky

or landscape). Uniform haze exhibits itself as an

overall reduction in air clarity. As discussed in

Section 3, the classic example of a layered haze is

a tight, vertically constrained, coherent plume

(plume blight). However, as an atmosphere

moves from a stable to unstable condition and a

plume mixes with the surrounding atmosphere,

the plume impact on visual air quality may manifest itself in an overall reduction in air clarity (uniform haze) rather than as a layer of haze.

The eye is much more sensitive to a sharp

demarcation in brightness or color than it is to a

gradual change in brightness or color, whether

that change takes place in space or time. Layered

haze falls into the first category, in that the layer

of haze is observed at some specific time and

against some background (sky or landscape element), while uniform haze falls into the second.

Because changes in uniform haze usually take

place over the course of hours or days, an evaluation of visual air quality change resulting from a

uniform haze requires a person to “remember”

what the scene looked like before a given change

in air pollution took place. An evaluation of the

impact a uniform haze has on visual air quality

requires identification of those elements of the

total vista that are deemed important to visitor

experience. On the other hand, a layered haze, if

visible, could constitute impairment regardless of

background features.

Section 8: Human Perception of Visual Air Quality 53

SECTION SECTION 8 ..............................................................................

HUMAN PERCEPTION OF VISUAL HUMAN PERCEPTION OF VISUAL 

AIR QUALITY AIR QUALITY
It is also important to point out that judgments

of visual air quality as a function of air pollution,

whether manifest as layered or uniform haze,

might be altered by variations in sun angle, cloud

cover, or landscape features.

8.1 Perception Thresholds of

Layered Haze (Plume Blight)

A first step to determining whether a plume

with given size, shape, and contrast characteristics

constitutes visibility impairment is to determine

whether the plume can be detected. A Yes/No

high threshold psychophysical procedure was

used as the methodology to conduct laboratory

experiments of plume perceptibility. The Yes/No

method consists of randomly presenting subjects

with stimuli, via computer generated photographic slides that show only the blue sky background, or show the background with the plume.

The subjects’ task is to indicate whether a plume

could be seen in each of the presentations. Figure

8.1 depicts the physical layout of the testing room. 

The projector was mounted on a platform

1.80 m high in a room adjacent to the back of the

testing laboratory. It projected an image 0.87 m

high x 1.31 m wide through an opening in the wall

and onto a large screen mounted on the opposite

wall 6.71 m away. Viewing chairs were located

4.80 m from the projected image and elevated on

a platform 0.43 m high to allow subjects to view

the images with a vertical offset and azimuth

angle of approximately 0. This configuration

resulted in a projected image that subtended viewing angles of 10 vertically by 16 horizontally.

The stimuli consisted of varying sizes of full

length, oval, and circular plumes with Gaussian

luminance distributions. In each case, the protocol for observer detection was the same for all

experiments, the surround was kept at the same

brightness, edge effects were dealt with uniformly, and stimuli representative of Gaussian

plume brightness profiles were used. 

Sixteen subjects participated in the full length

plume experiment. The stimuli consisted of

plumes with vertical angular sizes of 0.09, 0.18,

0.36, 0.72, 1.44, and 2.88 and a horizontal angular extent of 16.0. Contrast values of 0.050,

0.040, 0.030, 0.020, 0.017, 0.015, 0.013, 0.011,

and 0.005 were used for all sizes. Figure 8.2

shows the predicted probability of detection

curves. As plume contrast increases the probability of detecting the plume increases. If a plume

has a modulation contrast of greater than

about 0.01, it will be

detected nearly 100% of

the time for all sizes.

Furthermore, these

curves show that the

size of the plume is

quite important! Plumes

that subtend an angle of

about 0.3 can be

detected more easily

than plumes that are

larger or smaller.

Results for the oval and

circular plumes were

similar.

54 Introduction to Visibility

Fig. 8.1 Configuration of the laboratory setup used to conduct the visual sensitivity experiment.
To more clearly see how the three shapes compared, the modulation contrast corresponding to

50% probability of detection for each shape is

plotted against plume size in Figure 8.3. Notice

that the general trend for all stimuli is the same,

with plumes subtending about a 0.3 width being

the easiest to detect. However, observers are most

sensitive to full length plumes and least sensitive

to circular stimuli, with the oval plumes being

intermediate. The full length, oval, and circular

plume contrast threshold data have been incorporated into a linear interpolation algorithm that

allows plumes of any size to be estimated. 

8.2 Perceived Visual Air Quality

(PVAQ)

A good deal has been learned about the relationship between Perceived Visual Air Quality

(PVAQ) and various visibility parameters for both

layered and uniform haze. A number of studies

have established relationships between PVAQ and

various physical variables for vistas similar to

those shown in the eight photographs of Figure

8.4. It would be ideal to find one variable that

represents the same perceived change in air quality, whether the background atmosphere was clean

or dirty, whether the vista was near or far, or

whether the haze was layered or uniform.

To address these questions, a study was formulated that involved a visitor survey. Visitors to

a number of national parks were asked to rate

slides, on a scale of one (poor) to ten (good), that

represented various levels of air quality. It was

expected that sun angle, amount of snow cover,

meteorological conditions, and other factors

might affect ratings of visual air quality. Thus, a

special effort was made to select slides that

showed various air pollution levels under a number of atmospheric conditions. Specifically, a

number of slides representing the best, worst, and

intermediate levels of air quality were chosen to

correspond to various cloud, snow cover, and sun

angle conditions. These randomly ordered evaluation slides were preceded by ten preview slides

Section 8: Human Perception of Visual Air Quality 55

Fig. 8.2 Predicted probability of detection curves for

one subject used in the full length plume study.

Fig. 8.3 Threshold modulation contrast plotted as a

function of plume width in degrees for full length,

oval, and circular plumes. The human observer is

most sensitive to all plumes if they have a width of

about 0.3. Plumes larger or smaller than about 0.3

require increased contrast to be seen.
to orient the observers to the full range of visual

air quality conditions.

Park visitors were approached and asked if

they would like to participate in a study designed

to evaluate visual air quality in national parks. If

they concurred, the visitors were seated in a trailer

and verbally instructed on how to rate the slides. 

To determine the accuracy with which the

observers used the rating scale, 15 identical control slides were mixed with evaluation slides.

Calculations showed that if 50 ratings of the con56 Introduction to Visibility

(a) (b)

(c)

(d)

Fig. 8.4 Sample slides used to gauge public perception of visual air quality. Photographs (a) and (b) depict the

50 km distant La Sal Mountains as seen from Canyonlands National Park. Photograph (c) is of the 96 km distant

Chuska Mountains as seen from Mesa Verde National Park. Photograph (d) is of a forest fire plume as seen from

Grand Canyon National Park.
trol slides were chosen at random

from the total data set, the mean

rating of different groups of 50

changed by less than 0.1 points,

while the standard deviations

varied by only 0.4. However,

there is the possibility that, even

though there may be some variability due to the observers’

demographic backgrounds, a

random selection would tend to

average out the differences.

Thus, an additional analysis

involving the calculations of

mean and standard deviations of

the control slides as a function of

demographic background determined that regardless of educational level, age, sex, or location

of residence, individuals judged

visual air quality essentially the

same; means and standard deviations varied by as little as 0.3.

A most important result of

these studies was the close agreement between on-site and slide

ratings. Correlation between onsite and slide ratings was 0.94.

In almost all cases, statistical

tests indicated little significant

Section 8: Human Perception of Visual Air Quality 57

(e)

(f)

(h)

Fig. 8.4 (Continued). Photographs (e) and (f) are of Desert View (Grand Canyon National Park) as seen from

Hopi Point. Photograph (g) is also taken from Hopi Point but in the opposite direction of Desert View. The distant mountain (96 km) is Mt. Trumbull. Photograph (h) is of the 50 km distant San Francisco Peaks as seen from

Grand Canyon National Park.

(g)
difference between the window and slide ratings.

These studies hence suggest that slides can serve

as reasonable representations of actual scenes.

Notably, one simple relationship between

vista contrast and PVAQ became apparent time

and again. For example, one portion of the study

used the photographs shown in Figures 8.5 and

8.6, i.e., two vistas under clear sky conditions but

with various levels of air pollution. In Figure 8.5,

a 50 km distant mountain range dominates the

scenic vista, while in Figure 8.6, the distant mountain feature, which is the only scenic element that

shows a visual effect from an increase in air pollution, takes up only 4% of the total scene. Yet

when PVAQ, which is the average of the one-toten ratings assigned to each slide by the park visitors, is plotted against the contrast of the most

distant scenic element, a straight line relationship

results for either scene. This relationship is surprising for the second scene because the distant

feature is quite small. A representative relationship of the change in PVAQ evoked by a given

change in contrast is shown by the orange line in

Figure 8.7.

Because visitors see a visual resource under a

variety of atmospheric conditions it is important

to determine the effect that cloud cover or changes

in sun angle will have on the sensitivity of a vista

to changes in contrast. Surprisingly, the answer is

“None!” Figure 8.7 also shows how a PVAQ versus contrast curve changes when sun angle is

changed or cloud cover is added. In both cases the

overall ratings are higher but the slopes of the

curves, representing the sensitivities, remain the

same. The photographs of Figure 8.8 show the

same two vistas under different sun angle conditions; the photographs of Figure 8.9 show the La

Sal Mountains in the presence of cumulus clouds.

When sun angle is changed, the foreground features are illuminated and their color enhanced.

Changing either sun angle or cloud cover results

in an increase in scenic beauty and thus an

increase in the average PVAQ ratings. However,

the sensitivity of the vista-to-contrast-change is

58 Introduction to Visibility

(a)

(b)

(c)

Fig. 8.5 The appearance of one Canyonlands

National Park vista under various air quality levels.

The distant (50 km) mountain range is the La Sal

Mountains. Notice that the foreground features,

because of the proximity to the observer and the bright

color, show little change as air quality changes.

Hatchett Peak goes from being very clear to almost

disappearing. The sky-mountain contrast in (a) is

-0.39, (b) is -0.26, and (c) is -0.23.
not altered. This means that visitors to this type of

vista would be equally sensitive to air pollution

changes in the morning or afternoon, with or without clouds in the sky. 

This linear relationship appeared consistent

for all scenes as long as each scene had one specific scenic element that changed as a function of

air pollution. Notice in the photographs of Figure

8.6 that the foreground features remain unchanged

even though the visual air quality or contrast of

the distant scene changes dramatically. The

observers judging visual air quality appear to “key

in” on the scenic element that is most sensitive to

changes in air pollution. The visual sensitivity of

Section 8: Human Perception of Visual Air Quality 59

(a)

(b)

(c)

Fig. 8.6 Mt. Trumbull, 96 km distant, as seen from

Grand Canyon National Park. Impairment is from a

uniform haze. The sky-mountain contrasts for the

three photographs, from best to worst, are (a) -0.32,

(b) -0.29, and (c) -0.15.

Fig. 8.7 Plot of judgments of perceived air quality

(PVAQ) as a function of apparent vista contrast of most

distant landscape feature for a typical scene. The

orange line shows the linear relationship between these

two variables where the scene is shaded from sunlight.

The yellow line shows a similar relationship of the

same vista but in direct sunlight. The blue line shows

the relationship when cumulus clouds are present.

Notice that the slopes of the PVAQ contrast lines do not

change. The sensitivity of an observer to visibility haze

is independent of sun angle and meteorological conditions.
60 Introduction to Visibility

(a)

(b)

Fig. 8.9 The effect of changes

in sun angle as well as the

visual effect of cumulus clouds

in the La Sal Mountains.

(a)

(b)

Fig. 8.8 Mt. Trumbull as

viewed from Hopi Point under

two different lighting conditions. Foreground features

change dramatically, but

studies show that the sensitivity of the vista to air pollution

impact remains unchanged.
an observer to increases in air pollution is represented by the slope of the PVAQ versus contrast

curve in Figure 8.7.

An increase in vista color exists as the one

common denominator between the effects that sun

angle, meteorological conditions, and air pollution have on PVAQ. If vista color increases as a

result of decreased air pollution, changing sun

angle, or meteorological conditions, then the

PVAQ increases. This does not mean that other

perceptual clues, such as change in contrast detail

or texture, should be ruled out as being important

to PVAQ. However, there does not appear to be a

systematic relationship between texture and

PVAQ or contrast detail and PVAQ.

The effect of changing sun angle on color was

further investigated using scenes similar to those

shown in Figure 3.18. When the scene was in full

shadow, the lowest PVAQ was evoked. As the

sun rose in the sky, illuminating the vista and thus

increasing its color, PVAQ also increased. This

increase in PVAQ continued until the scene was

fully illuminated (approximately noon) and then

remained constant through the rest of the day (see

Figure 8.10). These results support the idea of

color change and PVAQ.

There does appear to be a simple linear relationship between PVAQ and other visibility parameters. One variable of particular interest is the

particulate mass concentration, a direct measure

of the amount of pollution in the atmosphere. The

relationship between particulate mass concentration and PVAQ is shown in Figure 8.11. Notice

the nonlinear nature of the relationship. A given

change in mass concentration results in a much

larger change in PVAQ when the air is clean than

when it is dirty.

The effect of mass concentration on PVAQ is

further illustrated by Figure 8.12. This graph

shows the change in PVAQ resulting from a given

increase in air pollution as a function of distance.

First, a specified amount of air pollution increase

has a much greater effect on PVAQ when the vista

is seen in a clean atmosphere; second, there is an

observer-target distance that is perceptually most

sensitive to air pollution increases. In relatively

clean areas, like the Grand Canyon, this distance

is 60 to 100 km. In the East, where the atmosphere is already quite polluted, the most sensitive

distance is closer to 10 km.

The above comments apply to vistas containing certain scenic elements that are substantially

more sensitive to air pollution change than the rest

of the scene. For more complicated scenes, it

appears that the PVAQ varies as a function of the

contrast change of each scenic element weighted

in proportion to the area subtended by that element and to the inherent scenic beauty of each

Section 8: Human Perception of Visual Air Quality 61

Fig. 8.10 Perceived visual air quality plotted as a

function of sun angle. During early morning hours the

vista is shaded from sunlight, and color saturation of

the scene is low. As the sun rises in the sky the scene

moves from shade into direct illumination and

becomes saturated with color. Judgments of visual air

quality increase over this time period until the scene is

fully illuminated. It then remains constant during

afternoon hours.
62 Introduction to Visibility

scenic feature. However, in general these conclusions hold for these scenes as well.

Layered haze, whether it appears as a coherent plume or as a layer of pollutants trapped near

the ground, is recognized by an abrupt change in

color between itself and some background. Haze

layers can be lighter or darker than the background sky, and under the right lighting conditions can also have a brown coloration. Figure

8.13 shows examples of three different haze layers. Figure 8.13a shows a white coherent plume

over Navajo Mountain; Figure 8.13b shows a

dark plume that just obscures the mountain top;

and Figure 8.13c shows a trapped haze layer that

obscures approximately half of Navajo Mountain.

Slides similar to those shown in Figure 8.13

were used in studies to determine individuals’

perceptions of layered haze. Results of these

studies are summarized in Figure 8.14; it should

be kept in mind that a plume with a contrast of

Fig. 8.12 Relationships between perceived visual air quality (PVAQ) and vista distance for different levels of air

quality. The blue line shows the decrease in perceived visual air quality of hypothetical vista as a result of adding

a small amount of particulate matter to a “clean” atmosphere as a function of distance to the vista. The yellow

curve is similar in nature but shows perceived changes in air quality when the atmosphere is already “dirty.” Most

importantly, the graph shows that vistas viewed in a clean atmosphere are many more times sensitive to an incremental change in air pollution than when viewed under more impaired conditions, and secondly, there is a vistaobserver distance that will result in a perceptual sensitivity that is greater than for any other distance.

Fig. 8.11 In contrast to the very simple relationship

between perceptions of visual air quality and vista

contrast, perceived visual air quality plotted against

particulate concentration shows a very nonlinear relationship.
0.02 to 0.05 is visible. The results indicate that

plumes, when positioned in the sky in such a way

as to not obscure the vista, have a minimal impact

on PVAQ. However, dark plumes were rated

lower or perceived to have a greater impact on

visual air quality than light-colored plumes; as the

plume, whether dark or light, obscured more and

more of the vista, the ratings went down. A plume

or haze layer with a sky-haze contrast of 0.3 is

perceived to be worse if it obscures two thirds of

the mountain than if it obscures only one third of

the scenic element.

Section 8: Human Perception of Visual Air Quality 63

(a)

(b)

(c)

Fig. 8.13 Three ways in which air pollutants can manifest themselves as layered haze. (a) Shows a white

plume positioned over Navajo Mountain as seen from

Bryce Canyon National Park, (b) shows a dark plume

that just obscures the mountain top, and (c) is a dark

haze layer that results from pollutants being trapped

in a ground inversion layer.

Fig. 8.14 Summarization of the results of the layered

haze perception studies. The orange line shows the

relationship between PVAQ and plume contrast for a

white plume placed in the sky in such a way as to not

touch any portion of the scenic vista. There is little

change in PVAQ as plume contrast increases. The yellow line and blue lines suggest the relationship

between PVAQ and layered haze contrast for a plume

just obscuring the mountain top and for a haze layer

obscuring the lower one half of the mountain, respectively. As haze obscures more of the mountain, the

effect that a specific pollution level has on PVAQ

increases. This is indicated by the increased slopes of

the blue and yellow lines over that of the orange line.
The data presented in this document is a summation of the work of

many scientists in the field. The current trends and particle contributions to visibility impairment were derived from the Interagency

Monitoring of Protected Visual Environments (IMPROVE) program.

IMPROVE is an interagency monitoring program funded and administered by the USDI National Park Service (NPS), USDA Forest

Service (USFS), USDI Bureau of Land Management (BLM), USDI

Fish and Wildlife Service (FWS), and the US Environmental

Protection Agency (EPA). Other information presented in this document can be found in the National Acid and Precipitation

Assessment Program (NAPAP) State of Science and Technology

Report Number 24 (1990).

The assumptions, findings, conclusions, judgements, and views

presented herein are those of the author and should not be interpreted as necessarily representing official National Park Service

policies.

Acknowledgement

The author wishes to thank:

 Kristi Gebhart and Jim Sisler in helping with data analysis.

 The University of California at Davis and the Air Resource 

Specialists staffs for their diligent efforts in operating the 

IMPROVE monitoring network.

 Site operators for their efforts in site maintenance and 

operation.

 Rebecca Burke for editing and project coordination.

 Jeff Lemke for graphics, layout, and printing coordination.

 The many reviewers and their helpful comments.

64 Introduction to Visibility
absorption: a class of processes by which one

material is taken up by another.

absorption coefficient: a measure of the ability of

particles or gases to absorb photons; a number that

is proportional to the number of photons removed

from the sight path by absorption per unit length.

absorption cross section: the amount of light

absorbed by a particle divided by its physical cross

section.

aerosol: a dispersion of microscopic solid or liquid

particles in a gaseous medium, such as smoke and

fog.

air parcel: a volume of air that tends to be transported as a single entity.

anthropogenic: produced by human activities.

apportionment: to distribute or divide and assign

proportionately.

attenuation: the diminuation of quantity. In the

case of visibility, attenuation or extinction refers to

the loss of image-forming light as it passes from an

object to the observer.

back trajectory: a trace backwards in time showing where an air mass has been.

bimodal distribution: a plot of the frequency of

occurrence of a variable versus the variable. A

bimodal distribution exists if there are two maxima

of the frequency of occurrence separated by a minimum. See mode.

budget: See light extinction budget.

coagulation: the process by which small particles

collide with and adhere to one another to form larger

particles.

condensation: the process by which molecules in

the atmosphere collide and adhere to small particles.

condensation nuclei: the small nuclei or particles

with which gaseous constituents in the atmosphere

(e.g., water vapor) collide and adhere.

deciview: a unit of visibility proportional to the

logarithm of the atmospheric extinction. Under

many circumstances a change in one deciview will

be perceived to be the same on clear and hazy days.

diffraction: modification of the behavior of a light

wave resulting from limitations of its lateral extent

by an obstacle. For example, the bending of light

into the “shadow area” behind a particle.

diffusion: a process by which substances, heat, or

other properties of a medium are transferred from

regions of higher concentration to regions of lower

concentration.

extinction: the attenuation of light due to scattering

and absorption as it passes through a medium.

extinction coefficient: a measure of the ability of

particles or gases to absorb and scatter photons from

a beam of light; a number that is proportional to the

number of photons removed from the sight path per

unit length. See absorption.

Glossary of Terms 65

GLOSSARY OF TERMS Y OF TERMS ..............................................................
extinction cross section: the amount of light scattered and absorbed by a particle divided by its physical cross section.

haze: an atmospheric aerosol of sufficient concentration to be visible. The particles are so small that

they cannot be seen individually, but are still effective in visual range restriction. See visual range.

homogenous nucleation: process by which gases

interact and combine with droplets made up of their

own kind. For instance, the collision and subsequent adherence of water vapor to a water droplet is

homogenous nucleation. See nucleation.

hydrocarbons: compounds containing only hydrogen and carbon. Examples: methane, benzene,

decane, etc.

hygroscopic: readily absorbing moisture, as from

the atmosphere.

IMPROVE: Interagency Monitoring of PROtected

Visual Environments.

integrating nephelometer: an instrument that

measures the amount of light scattered (scattering

coefficient).

inversion: See temperature inversion.

isopleth: a line drawn on a map through all points

having the same numerical value.

isotropic: a situation where a quantity (or its spatial derivatives) are independent of position or

direction.

isotropic scattering: the process of scattering light

equally in all directions.

LAC: See Light-Absorbing Carbon.

light-absorbing carbon: carbon particles in the

atmosphere that absorb light. Black carbon.

light extinction budget: the percent of total atmospheric extinction attributed to each aerosol and

gaseous component of the atmosphere.

long path measurement: an atmospheric measurement process that is made over distances in excess

of a few hundred meters.

micron: a unit of length equal to one millionth of a

meter; the unit of measure for wavelength.

mode: the maximum point in a plot of the frequency of occurrence of a variable versus the variable.

nitrogen dioxide: a gas (NO2) consisting of one

nitrogen and two oxygen atoms. It absorbs blue

light and therefore has a reddish-brown color associated with it.

NO2: See nitrogen dioxide.

nucleation: process by which a gas interacts and

combines with droplets. See homogenous nucleation.

Perceived Visual Air Quality (PVAQ): an index

that relates directly to how human observers perceive changes in visual air quality.

phase shift: a change in the periodicity of a waveform such as light.

photometry: instrumental methods, including analytical methods, employing measurement of light

intensity. See telephotometer.

photon: a bundle of electromagnetic energy that

exhibits both wave-like and particle-like characteristics.

plume blight: visual impairment of air quality that

manifests itself as a coherent plume.

66 Introduction to Visibility
point source: a source of pollution that is point-like

in nature. An example is the smoke stack of a coalfired power plant or smelter. See source.

polar nephelometer: an instrument that measures

the amount of light scattered in a specific direction.

See integrating nephelometer.

precursor emissions: emissions from point or

regional sources that transform into pollutants with

varied chemical properties.

psychophysical: the branch of psychology that

deals with the relationships between physical stimuli and resulting sensations and mental states.

PVAQ: See Perceived Visual Air Quality.

Rayleigh scattering: the scattering of light by particles much smaller than the wavelength of the light.

In the ideal case, the process is one of a pure dipole

interaction with the electric field of the light wave.

refraction: the change of direction of a ray of light

in passing obliquely from one medium into another

in which the speed of propagation differs.

relative humidity: the ratio of the partial pressure

of water to the saturation vapor pressure, also called

saturation ratio; often expressed as a percentage.

scattering (light): an interaction of a light wave

with an object that causes the light to be redirected

in its path. In elastic scattering, no energy is lost to

the object.

scattering angle: the angle between the direction

of propagation of the scattered and incident light (or

transmitted light):

incident transmitted light

light

scattered light

scattering coefficient: a measure of the ability of

particles or gases to scatter photons out of a beam of

light; a number that is proportional to the amount of

photons scattered per unit length.

scattering cross section: the amount of light scattered by a particle divided by its physical cross section.

secondary aerosols: aerosol formed by the interaction of two or more gas molecules and/or primary

aerosols.

SO2: See sulfur dioxide.

source: in atmospheric chemistry, the place, places,

group of sites, or areas where a substance is injected

into the atmosphere. Can include point sources, elevated sources, area sources, regional sources, multiple sources, etc.

spectral: an adjective implying a separation of

wavelengths of light or other waves into a spectrum

or separated series of wavelengths.

stable air mass: an air mass which has little vertical mixing. See temperature inversion.

stagnant: referring to meteorological conditions

that are not conducive to atmospheric mixing.

stagnation episodes: See stagnation periods.

stagnation periods: lengths of time during which

little atmospheric mixing occurs over a geographical area, making the presence of layered hazes more

likely. See temperature inversion.

sulfates: those aerosols which have origins in the

gas-to-aerosol conversion of sulfur dioxide; of primary interest are sulfuric acid and ammonium sulfates.

sulfur dioxide: a gas (SO2) consisting of one sulfur

and two oxygen atoms. Of interest because sulfur

dioxide converts to an aerosol that is a very efficient

light scatterer. Also, it can convert into acid

droplets consisting primarily of sulfuric acid.

Glossary of Terms 67
sun angle: refers to the angle of the sun above the

horizon of the earth.

telephotometer: an instrument that measures the

brightness of a specific point in either the sky or

vista.

temperature inversion: in meteorology, a departure from the normal decrease of temperature with

increasing altitude such that the temperature is

higher at a given height in the inversion layer than

would be expected from the temperature below the

layer. This warmer layer leads to increased stability

and limited vertical mixing of air.

transmissometer: an instrument that measures the

amount of light attenuation over a specified path

length.

unstable air mass: an air mass that is vertically

well mixed. See also stable air mass, temperature

inversion.

visual range: the distance at which a large black

object just disappears from view.

VOC: Volatile Organic Carbon - gaseous hydrocarbon.

wavelength: the distance, measured in the direction

of propagation of a wave, between two successive

points in the wave that are characterized by the same

phase of oscillation.

68 Introduction to Visibility
Section 1: On the Nature of Light 69
70 Introduction to Visibility