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Visibility and Light Scattering in the Atmosphere


Origin of Particulate Matter Particulates and gases in the atmosphere can originate from natural or manmade sources. Table 35.1 includes the terms that are usually used to describe airborne particles. 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 natural aerosols that limit our ability to see are condensed water vapor (water droplets), windblown dust, and secondary aerosols. Secondary aerosols are airborne dispersions of particles formed by atmospheric reaction of gaseous "precursor" emissions.

Table 35.1 Terms That Describe Airborne Particulate Matter Particulate

Particulate matter

Any material, except uncombined water, that exists in the solid or liquid state in the atmosphere or gas stream at standard conditions

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

Fume

Particles formed by condensation, sublimation, or chemical reaction, predominantly smaller than 1g (tobacco smoke)

Mist

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

Smoke

Small gasborne particles resulting from combustion

Particle Discrete mass of solid or liquid matter
Fog Visible aerosol
Soot An agglomeration of carbon particles

 

Figure 35.1 is a schematic summary of how gases are converted into aerosols of various sizes. The reactions are very complex and only in recent years have the physical processes been understood. The gas to aerosol conversion process takes place by essentially three processes: condensation, nucleation, and coagulation. Condensation involves gaseous vapors condensing or combining with existing small nuclei, usually referred to as condensation nuclei. The small condensation nuclei may have their origin in sea salts or from combustion processes. Gases may also interact and combine with droplets of their own kind and form larger aerosols. This process is referred to as homogenous nucleation. Once aerosols are formed they can grow in size by a process called coagulation. In coagulation, particles essentially bump into each other and "stick" together. In each of these processes the interaction takes place via the electronic structure of the molecule or aerosol, and the subsequent formation of new particles results in a lower overall energy state.

Figure 35.1 How atmospheric aerosols are formed.

Table 35.2 summarizes the sources of manmade, or anthropogenic, emissions. The pollutants of primary concern to visibility reduction, because they are or become efficient light scatterers, are fine particulates, nitrogen dioxide, and sulfur dioxide gas. Sulfur dioxide gas is of interest because it can, under the right conditions, convert into a sulfate aerosol through the gas-to-particle mechanisms already discussed. Nitrogen dioxide, on the other hand, absorbs blue light as a gas; consequently it is, by itself, an important pollutant. Organic compounds are also of interest because they can contribute to the formation of sulfates and nitrogen dioxide and can form organic aerosols.

 

Table 35.2 1997 Anthropogenic emissions in the United States.

Source Category
CO
NOx
VOC
SO2
PM-10
FUEL COMBUSTION
4,817
10,724
861
17,260
1,101

Electric Utilities

406
6,178
51
13,082
290

Coal

254
5,588
29
12,529
265

Oil

12
131
3
486
6

Gas

79
286
8
4
0

Other

62
159
10
61
19
Industrial
1,110
3,270
217
3,365
314

Coal

100
614
6
1769
72

Oil

73
240
12
847
48

Gas

362
1,385
77
572
47

Other

775
1,032
122
177
146
Other
3,301
1,276
593
813
497

Misc. Fuel Comb. (Except Residential)

1
417
57
637
129
INDUSTRIAL PROCESSES
6,052
917
9,836
1,718
1,277
Chemical & Allied Processing
1,287
167
458
301
70
Metals Processing
2,465
102
73
552
220
Petroleum & Related Industries
364
115
538
385
41
Waste Disposal & Recycling
1,242
103
449
50
296
Solvent Utilization
0
0
6,483
1
6
Other Industrial Processes
694
430
1,835
429
644
TRANSPORTATION
67,014
11,595
7,660
1,380
734
On-Road Vehicles
50,257
7,035
5,230
320
268
Non-Road Sources
16,775
4,560
2,430
1,060
466
MISCELLANEOUS
9,568
346
858
13
0
TOTAL ALL SOURCES
87,451
23,582
19,214
20,371
3,112
Note(s): Values expressed in thousand short tons. Multiply these values by 0.9072 to convert to metric ton. Source of Data: U.S. EPA, Office of Air and Radiation

The size distribution of atmospheric aerosols is bimodal, with maximums at 10 microns and 0.1 microns. Particles larger than 2.5 microns are called coarse particles and particles with diameters less than 2.5 microns are called fine particles. Coarse particles originate from entrainment of wind-blown particles, while fine particles result from either high temperature combustion or secondary conversion of atmospheric gases. Figure 35.2 illustrates the typical bimodal distribution of atmospheric particulate matter, and shows the usual chemical composition found for each size distribution.

Figure 35.2 Bimodal size distribution observed for atmospheric particulate matter.

 

Once the gases and particulate matter are emitted from a source into the atmosphere, they are entirely responsive to meteorological conditions. Pollutant transport and transformation (gas-to-aerosol conversion) depend on wind speed and atmospheric stability, as well as solar radiation. Atmospheric stability determines the movement of air parcels within the atmosphere as a whole. If air parcels are moving vertically up and down, the atmosphere is said to be mixed and unstable. On the other hand, if an air parcel does not have vertical motion, it is said to be stable and usually is referred to as a stagnant air mass.

The amount of solar radiation that is allowed to heat the ground essentially determines whether the atmosphere is stable or unstable. The sun's radiation passes through the atmosphere, transferring 'little of its energy to air molecules or atmospheric aerosols. After passing through the atmosphere it strikes the earth where it is absorbed and converted into heat energy. The earth, in turn, heats the air near the ground. These warm air parcels then rise, allowing colder air parcels to move down near the earth. The cold air parcels are heated and the process continues. Under conditions where sufficient solar energy exists to heat the earth and in turn the air next to the earth, the result will be a well mixed, unstable atmosphere. Pollutants emitted into this type of atmosphere appear as a uniform haze.

On the other hand, if there is insufficient radiation to heat the earth, the air next to the earth becomes cooler than the air farther up in the atmosphere and a stable, non-mixing system results (inversion layer). Stable conditions usually occur at night and during winter months. During winter months, not does less heating of the earth occur, but in some instances the white snow covering parts of the earth reflects radiation back into the atmosphere. Because of these compound effects it is possible to have stable or stagnant air for a week or more. These conditions are usually referred to as stagnation periods or episodes.

When pollutants are emitted into a stable atmosphere, 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. If there are no surface winds or if pollutants are emitted into a stagnant air mass over periods of days, a layer of haze forms near the ground and continues to build as long as the stagnation condition persists. Whether the pollutants are transported from one location to another or trapped in a stagnant air mass, they will transform from gases to aerosols and from small to larger particles. Their ability to scatter and absorb light changes during this transformation. The typical composition of atmospheric particulate matter, collected from air over the Grand Canyon, is given in Table 35.3. Ammonium sulfate, (NH4)2SO4, results from sulfur dioxide gas emissions from distant copper smelters and coal-fired power plants. As the sulfur dioxide is transported through the atmosphere it converts to an aerosol responsible for much of the atmospheric extinction. Organic carbon can have its origins from automobile hydrocarbon emissions and/or vegetation. Soot (elemental carbon) can come from diesel engines or from forest fires. Fine crustal material is from wind-blown dust, and because it is fine, it can be transported hundreds of kilometers. Coarse soil, on the other hand, is local in nature and is usually transported only a few kilometers.

Table 35.3 Typical composition of particulate matter over the Grand Canyon.

Substance
Concentration in µg/m3
Sulfate
1.01
Nitrate
0.21
Organics
0.94
Soot
0.16
Soil
0.64
Coarse Mass
4.86

The composition of particulate matter in air reflects the origin of the air mass. At the Grand Canyon, for instance, the air tends to contain lower concentrations of man-made pollutants if the air originates from the north. It is very high in sulfate and organics if the air mass arrives at the Grand Canyon after passing over southern California, Arizona, and New Mexico.

Nature of Light

Light can be thought of as waves, and to a certain extent they are analogous to water and sound waves. Electric and magnetic fields transmit energy in waves that are called electromagnetic radiation. Ordinary light is a form of electromagnetic radiation, as are x-rays, ultraviolet, infrared, radar, and radio waves. All electromagnetic radiation travels at approximately 300,000 km./sec. (186,000 mi./sec.). The various forms of electromagnetic radiation differ from one only in wavelength, and therefore in the energy they can transmit. Figure 35.3 is a 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), extends from about 0.4 to 0.7 micron.

Figure 35.3 The electromagnetic spectrum, showing the visible portion of the spectrum in color.

Waves of all kinds, including light waves, carry energy. Electromagnetic energy is unique in that energy is carried in small, discrete parcels called photons. Representations of a blue, green, and red photon are shown in Figure 35.4. Blue, green, and red photons have wavelengths of around 0.45, 0.55 and 0.65 micron 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 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 capture photons of red and blue light and allow green photons to bounce back, thus providing the green

Figure 35.4 Representation of blue, green and red photons, demonstrating their relative wavelengths.

 

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 35.5 is an example of an eggshell reflecting all wavelengths of light. The eye perceives the eggshell as white. An apple, on the other hand, reflects mostly red light while absorbing all others, so the apple appears red. For all practical purposes, in visibility, it is most convenient to think of light as being made of small colored particles.

Figure 35.5 Effects of white light reflected from a white egg and a red apple.

Light scattering is caused by all atmospheric particles, but particles very near the same diameter as the wavelength of light scatter photons most effectively. Figure 35.6 illustrates the size relationship between light and different types of particles. Aerosols grow in size as they remain suspended in the atmosphere, and as the relative humidity increases. Combustion nuclei range in size from 0.01 to 0.1 microns when first released into the atmosphere, but they quickly grow to diameters of 0.3 to 0.7 microns-the ideal size to scatter visible light very effectively!

Figure 35.6. Size relationship between atmospheric particles and electromagnetic radiation wavelength

 

Interaction of Light with Particulate Matter

A photon of light is scattered when it is received by a particle and re-radiated at the same wavelength. Visibility degradation results from light scattering and absorption by atmospheric particles and gases which are nearly the same size as the wavelength of the light. Particles larger than the wavelength of light can scatter light through three processes: (1) diffraction, (2) refraction, and (3) phase shift. Photons of light can also be absorbed by particles and converted to internal molecular energy. 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. Figure 35.7 illustrates these four processes.

Figure 35.7 Physical processes between light and particles.

 

Photons can be scattered equally in all directions (isotropic scattering), but in most instances photons are scattered in a forward direction. If the particles are small (such as the air molecules themselves) the amounts of light 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. Figure 35.8 showes the distribution of scattered light for particles which are respectively much smaller and much larger than the wavelength of light.

Figures 35.8 Scattering of light as a function of particle size illustrating isotropic scattering by small particles (Rayleigh Scattering) and forward scattering by large particles.

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 particles of different sizes are able to scatter light with varying degrees of efficiency. The efficiency with which an individual particle can scatter light is expressed as a ratio of a particle's effective cross section to its actual cross section. Figure 35.9 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 very large particles scatter light, as if they were twice as big as they actually measure. Each of these particles removes twice the amount of light intercepted by its geometric cross sectional area.

Figure 35.9 Efficiency of light scattering as a function of particle size

Figure 35.10 shows the relative amounts of small and large particles found in the atmosphere. The green line is a typical mass size distribution of particles. The y-axis is the amount of mass in a given size range; the x-axis is particle size measured in microns. Notice the two-humped, or modal curve. Those particles less than 2.5 microns are referred to as "fine particles" and particles larger than 2.5 microns are called "coarse particles". The red line represents the amount of light scattering 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 hazes 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, orange, 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 35.10 Mass to particle size distribution (green line) compared to the relative amount of light scattered by size (red line).

Figure 35.111 shows what happens when the red, blue, and green photons of white light strike small particles (top illustration) and large particles (bottom illustration). Only the blue photons are scattered because scattering efficiency is greatest when the size relationship of photon 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. The bottom drawing in Figure 35.10 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.

Figure 35.11

 

Suggested Reading: Malm, William C., 1983, Introduction to Visibility, Air and Water Quality Division, National Park Service, Fort Collins, Colorado


Original page created by Dr.Richard Foust, Professor of Chemistry and Environmental Science, Northern Arizona University