I

ntroductiontoVisibility William C. Malm 

Introduction Introduction


to Visibility to Visibility


William C. Malm 

CIRA Colorado State 

University 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 
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

iii 
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 
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

v 
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 
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

vii 
viii
INTRODUCTION NTRODUCTION
 ....................................................


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 1 
(a) The farthest scenic feature is the 130 km distant 

Navajo Mountain, as seen from Bryce Canyon 

National Park. 

(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. 

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

River, are a dominant form on the distant horizon. 

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

the highly textured and brightly colored foreground 

features. 

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. 

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


ON THE NA ON THE NATURE OF LIGHT TURE OF LIGHT


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. 

lengths. A wavelength is defined as the distance from 

one crest to the next. 

Fig. 1.1 Water waves illustrate the concept of waveFigure 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 

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. 

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. 

Section 1: On the Nature of Light 3 
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. 

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. 1.4 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. 

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. 

4 Introduction to Visibility 
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. 

- Target characteristics that include color, tex- It is important to understand the significance of 

ture, form, and brightness. the light that is scattered in the sight path toward 

- Optical characteristics of intervening atmos­ the observer. The amount of light scattered by the 

phere: atmosphere and particles between the object and 

i. image-forming information (radiation) observer can be so bright and dominant that the 

originating from landscape features is light reflected by the landscape features becomes 

scattered and absorbed (attenuated) as it insignificant. This is somewhat analogous to 

passes through the atmosphere toward viewing a candle in a brightly lit room and in a 

the observer, and room that would otherwise be in total darkness. In 

ii. sunlight, ground reflected light, and light the first case, the candle can hardly be seen, while 

reflected by other objects are scattered in the other it becomes the dominant feature in the 

by the intervening atmosphere into the room. 

sight path. 

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

Section 1: On the Nature of Light 5 
6 Introduction to Visibility 
SECTION SECTION 2
 ...................................................................................................


INTERACTION OF LIGHT AND P INTERACTION OF LIGHT AND PARTICLES


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 

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. 

Section 2: Interaction of Light and Particles 7 
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). 

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

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

forward direction. 

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. 

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. 

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. 2.5 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. 

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. 

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. 2.7 When particles are near or larger than the 

wavelength of the incident light, photons of all colors 

are scattered out of the beam path. 

Section 2: Interaction of Light and Particles 9 
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. 

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. 

(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. 

10 Introduction to Visibility 
SECTION SECTION 3
 .........................................................................................


VISION THROUGH THE A VISION THROUGH THE ATMOSPHERE TMOSPHERE


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 Fig. 3.1 The human eye operates much like a photoeye measures contrast between adjacent objects or graphic camera. It has a lens to focus an image on a very 

between an object and its background. Contrast of sensitive detector called the retina. Also, the amount of 

an object is simply the percent difference between light entering the eye is controlled by an aperture called 

object luminance and its background luminance. 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. 

Section 3: Vision Through the Atmosphere 11 
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. 

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. 

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. 

12 Introduction to Visibility 
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. 

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 
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 

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

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

States. Figure 3.16 demonstrates how the effect of 

nitrogen dioxide gas 

scenic features. Remember that carbon absorbs (NO2), in combination with varied background 

all wavelengths of light and scatters very little. illumination, can combine to yield a very brown 

Thus the scene will always tend to be darkened. atmospheric discoloration. If a volume of atmosphere containing NO2 is shaded and if light passes 

Figure 3.15 shows the effects of illumination through this shaded portion of the atmosphere, the 

on the appearance of power plant plumes. The light reaching the eye will be deficient in photons 

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

14 Introduction to Visibility 
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. 

Section 3: Vision Through the Atmosphere 15 
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 

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

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

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 Figure 3.17 is an easterly 

first plume appears white and the second appears almost black. The two view of the La Sal 

plumes on the left are fly-ash plumes. Mountains in southeastern 

Utah as seen from an elevated point that is some 100 

in the blue part of the spectrum. As a conse- kilometers distant. The photograph shown in 

quence, the light will appear brown or reddish in Figure 3.17a was taken at 9:00 a.m., while the 

color. However, if light is allowed to shine on, but photograph shown in 3.17b was taken later in the 

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). 

16 Introduction to Visibility 
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 

(a)	 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, 

(b)	 Canyonlands National Park, 

looking out over 

Fig. 3.17 Photographs showing how the vistas appeared on a day when pol- Canyonlands with its many 

lutants were trapped under an inversion layer. In (a) the haze appears white; colorful features toward the 

in (b) the identical haze is dark or gray. Because most of the light energy is 50 km distant La Sal 

scattered in the forward direction (white haze), it can be concluded that the Mountains. Figure 3.18a particles must be quite large in comparison to the wavelength of light. shows how the canyon 

appears when it is in total 

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

day. These photographs show how these views, or 3.18a, b, and c show a progressively higher sun 

vistas, appear when obscured by a layer of haze. angle until in Figure 3.18d the scene is entirely 

In the first view the haze layer appears white, but illuminated. In each case, the air quality is the 

the same air mass viewed later in the day has a same. The only change is in the angle at which 

dark gray appearance. This effect is entirely due the sun illuminated the vista. There are primarily 

to the geometry involved with the observer and two reasons for the apparent change in visual air 

the sun. In the first view the sun is low in the east- quality. First, at higher sun angles, there is less 

ern sky. Consequently, the photons reaching the scattering of light by the intervening atmosphere 

Section 3: Vision Through the Atmosphere 17 
(a) (b) 

(c) (d) 

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. 

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 
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

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. 

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. 

Section 4: Transport and Transformation of Particulates and Gases 19 
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 
(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). 

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

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. 

Section 4: Transport and Transformation of Particulates and Gases 21 
Table 4.2 Typical size ranges of a number of aerosols commonly found in the atmosphere. 

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 
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. 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, 

The size of most sec- and sulfur. Through complex sets of chemical reactions, gases are formed that, 

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

between 0.1 and 1.0 to form ammonium sulfate, nitrogen oxide forms ammonium nitrate, oxygen is 

converted to ozone, and carbon, hydrogen, and oxygen form hydrocarbon par- microns. For reference, the ticles. 

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 Fig. 4.4 Sulfur dioxide gas converts in the atmosphere to ammonium sul- effects. The formation mecha­ fate particles. These particles are hygroscopic, meaning they grow 

rapidly in the presence of water to reach a size that is disproportionately nisms are also schematically 

responsible for visibility impairment. shown in Figure 4.6. 

Section 4: Transport and Transformation of Particulates and Gases 23 
ondary 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 

Fig. 4.5 Relative size of beach sand, a grain of flour, These concepts are summarized in Figures 4.7 and a secondary fine particle. and 4.8. Emissions are transported (or accumulated depending on inversion characteristics), 

transformed into other gaseous or particle species, 

Near a source (within 0-100 km), such as an and deposited to the terrestrial ecosystem. In 

urban center, power plant, or other industrial facil- Figure 4.7, SO2 emissions and (NH4)2SO4 are 

ities, haze is usually a mixture of gases and sec- 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 leavFig. 4.6 The particles are arranged by their typical mass/size distribution ing behind SO4 particles that 

in the atmosphere. Coarse particles tend to have natural origins and affect visibility or the SO4 can 

deposit out close to the source. Fine particles are usually man-made, can deposit out of the cloud as acid transport great distances, and cause the greatest visibility impairment. 

24 Introduction to Visibility 
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 

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. 

Section 4: Transport and Transformation of Particulates and Gases 25 
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. 

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

particle mass: sulfates, organics, elemental carbon, 

soil, and nitrates. 

26 Introduction to Visibility 
SECTION SECTION 5
 .....................................................................


VISIBILITY MEASUREMENTS VISIBILITY MEASUREMENTS


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. 

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. 

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 5: Visibility Measurements 27 
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. 

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 
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. 

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. 

Section 5: Visibility Measurements 29 
30 Introduction to Visibility 
SECTION SECTION 6
 ...............................................................................


PARTICLE CONCENTRA TICLE CONCENTRATION AND TION AND


VISIBILITY TRENDS VISIBILITY TRENDS

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 possiFig. 6.1 Location of the monitoring sites used in IMPROVE. (1999) 

Section 6: Particle Concentration and Visibility Trends 31 
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. 

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 

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. 

Dry Extinction % Contribution to 

Concentrations Efficiencies Visibility Reduction

 EAST WEST WEST 

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

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 2/gm 2-3% 4-4% 

3.0 μg/m3 3.0 μg/m3 0.6 m2/gm 6-8% 

NA NA NA 33-43% 61-64% 

NA NA NA 100-130 km 182-193 km 

Average Mass 

EAST 

FINE PARTICLES 

0.115 

Organics 

1.25 m

COARSE PARTICLES 11-12% 

CLEAN AIR 

VISUAL RANGE 

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 

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 
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. 

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 

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 

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. 6.3 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. 

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. 6.4 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 

portional to the contribution of various particle species. 

and the shaded patterns are pro6.2.2 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. 

Fig. 6.5 Comparison of extinction (Mm-1), deciview (dv), and visual range (km). 

Section 6: Particle Concentration and Visibility Trends 35 
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, 

Fig. 6.6 

cient (Mm-1

tions measured during IMPROVE. 

Average reconstructed light extinction coeffi­

) calculated from the aerosol concentraFig. 6.7 Average visibility, expressed as deciviews, 

calculated from aerosol concentrations measured in 

the IMPROVE monitoring program. 

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. 

(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. 

36 Introduction to Visibility 
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 

(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. 

Section 6: Particle Concentration and Visibility Trends 37 
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. 

(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. 

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 

(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. 

38 Introduction to Visibility 
(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. 

ern 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 

Fig. 6.13 Trends in median visual range over the eastern United States from have also been examined for 

trends in visual air quality in 1948 through 1982. 

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 eastthe 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 
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 

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. 

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 
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. 

Fig. 6.15 Scatter plot of de-seasonalized sulfur con- These data show that trends in sulfur dioxide 

centrations (μg/m3) vs. smelter emissions (tons/day) emissions provide a plausible explanation for 

for monthly data at Hopi Point in Grand Canyon variability observed in regional visibility and sulNational Park and Chiricahua National Monument. fate concentration variations. 

(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. 

Section 6: Particle Concentration and Visibility Trends 41 
42 Introduction to Visibility 
SECTION SECTION 7
 ..........................................................................................................


IDENTIFICA IDENTIFICATION OF SOURCES TION OF SOURCES

CONTRIBUTING T CONTRIBUTING TO VISIBILITY IMP O VISIBILITY IMPAIRMENT AIRMENT

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 
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 sul-

(a) 

fur 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 Texas-

(b) 

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. 

46 Introduction to Visibility 
(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. 7.5 (a) Extreme fine sulfur concentration source contribution, Grand Canyon National Park. (b) Extreme 

fine sulfur concentration probability, Grand Canyon National Park. 

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

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. 7.7 Extreme fine sulfur concentration source 

contribution, Chiricahua National Monument. 

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. 

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

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 

Fig. 7.9 

tribution, Shenandoah National Park. 

Extreme fine sulfur concentration source conRocky 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 
(a) (b) 

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. 

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 
(a) (b) 

(c) (d) 

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. 

(e) 

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 
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 
(a) (b) 

(c) (d) 

(e) (f) 

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. 

52 Introduction to Visibility 
SECTION SECTION 8
 ..............................................................................


HUMAN PERCEPTION OF VISUAL HUMAN PERCEPTION OF VISUAL


AIR QUALITY AIR QUALITY

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 
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 

Fig. 8.1 Configuration of the laboratory setup used to conduct the visual sensitivity experiment. 

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. 8.2 Predicted probability of detection curves for 

one subject used in the full length plume study. 

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 

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. 

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 
(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. 

to orient the observers to the full range of visual they concurred, the visitors were seated in a trailer 

air quality conditions. and verbally instructed on how to rate the slides. 

Park visitors were approached and asked if To determine the accuracy with which the 

they would like to participate in a study designed observers used the rating scale, 15 identical conto evaluate visual air quality in national parks. If trol slides were mixed with evaluation slides. 

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

(f) 

(g) 

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 

(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. 

Section 8: Human Perception of Visual Air Quality 57 
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 

(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. 

58 Introduction to Visibility 
(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.

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 

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. 

Section 8: Human Perception of Visual Air Quality 59 
Fig. 8.8 Mt. Trumbull as 

viewed from Hopi Point under 

two different lighting conditions. Foreground features (a) change dramatically, but 

studies show that the sensitivity of the vista to air pollution 

impact remains unchanged. 

(b) 

Fig. 8.9 The effect of changes 

in sun angle as well as the (a)


visual effect of cumulus clouds


in the La Sal Mountains.


(b) 

60 Introduction to Visibility 
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 

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. 

Section 8: Human Perception of Visual Air Quality 61 
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. 

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 

The yellow 

Most 

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. 

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

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. 

62 Introduction to Visibility 
(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. 

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. 

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. 

Section 8: Human Perception of Visual Air Quality 63 
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 
GLOSSARY OF TERMS Y OF TERMS
 ..............................................................


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 
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