Jacobson M.Z. Atmospheric Pollution

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138 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Table 5.6. Dominant Sources and Components of Nucleation,

Accumulation, and Coarse Mode Particles

Nucleation Fossil-fuel emissions Sea-spray emissions

O(aq), SO

2

, NH



BC, OM, SO

2

, Fe, Zn H

O, Na



, Ca

2

, Mg

2

, K



, Cl



2

, Br



, OM

Fossil-fuel emissions Biomass-burning emissions Soil-dust emissions

BC, OM, SO

2

, Fe, Zn BC, OM, K



, Na



, Ca

2

, Mg

2

, Si, Al, Fe, Ti, P, Mn, Co, Ni, Cr,

2

, NO



, Cl



, Fe, Mn, Zn, Na



, Ca

2

, Mg

2

, K



, SO

2

Pb, V, Cd, Cu, Co, Sb, As, Ni, Cl



, CO

2

, OM

Biomass-burning emissions Industrial emission Biomass burning ash, industrial

BC, OM, K



, Na



, Ca

2

, Mg

2

, BC, OM, Fe, Al, S, P, Mn, Zn, Pb, ﬂy-ash, tire-particle emissions

2

, NO



, Cl



, Fe, Mn, Ba, Sr, V, Cd, Cu, Co, Hg, Sb,

Zn, Pb, V, Cd, Cu, Co, Sb, As, Sn, Ni, Cr, H

O, NH



, Na



As, Ni, Cr Ca

2

, K



, SO

2

, NO



, Cl



2

Condensation/dissolution Condensation/dissolution Condensation/dissolution

O(aq), SO

2

, NH



, OM H

O(aq), SO

2

, NH



, OM H

O(aq), NO



Coagulation of all components from Coagulation of all components

nucleation mode from smaller modes

Nucleation Mode Accumulation Mode Coarse Mode

efﬁcient for particles than for gases because particles are heavier than are gases. As

such, particles fall and tend to stay on a surface more readily than do gases, unless

wind speeds are high. Gases, especially if they are chemically unreactive, are more

readily resuspended into the air.

Aerosol particle rainout is a process by which aerosol particles coagulate with

raindrops, which subsequently fall to the ground. Rainout is an important removal

process for aerosol particles. Because rain clouds occur only in the troposphere, rain-

out is not a process by which stratospheric particles are removed. Rainout is an

effective removal process for volcanic particles.

5.4. SUMMARY OF THE COMPOSITION OF AEROSOL PARTICLES

The composition of aerosol particles varies with particle size and location. Some gen-

eralities about composition include the following:

• Newly nucleated particles usually contain sulfate and water, although they may

also contain ammonium.

• Biomass burning and fossil-fuel combustion produce primarily small accumulation

mode particles, but coagulation and gas-to-particle conversion move these particles

to the middle and high accumulation modes. Coagulation also moves some parti-

cles to the coarse mode.

• Metals that evaporate during industrial emissions recondense, primarily onto accu-

mulation mode soot particles and coarse mode ﬂy-ash particles. The metal emitted

in the greatest abundance is usually iron.

• Sea spray and soil particles are primarily in the coarse mode.

• When sulfuric acid condenses, it usually condenses onto accumulation-mode parti-

cles because these particles have more surface area, when averaged over all

particles in the mode, than do nucleation-or coarse-mode particles.

• Once in accumulation-mode particles, sulfuric acid dissociates primarily to sulfate

[SO

2

]. To maintain charge balance, ammonia gas [NH

(g)], dissolves and disso-

ciates, producing the ammonium ion [NH



]; the major-cation in particles. Thus,

ammonium and sulfate often coexist in accumulation-mode particles.

• Because sulfuric acid has a lower SVP and a greater solubility than does nitric

acid, nitric acid is inhibited from entering accumulation-mode particles that

already contain sulfuric acid.

• Nitric acid tends to dissolve in coarse-mode particles and displace chloride in sea-

spray drops and carbonate in soil-dust particles during acidiﬁcation. Sulfuric acid

also displaces chloride and carbonate during acidiﬁcation.

Table 5.6 summarizes the predominant components and their sources in each the

nucleation, accumulation, and coarse particle modes.

5.5. AEROSOL PARTICLE MORPHOLOGY AND SHAPE

The morphologies (structures) and shapes of aerosol particles v

ary with composition.

The older an aerosol particle, the greater the number of layers and attachments the par-

ticle is likely to have. If the aerosol particle is hygroscopic, it absorbs liquid water at

high relative humidities and becomes spherical. If

ions are present and the relative humidity decreases,

solid crystals may form within the particle. Some

observed aerosol particles are ﬂat, others are globu-

lar, others contain layers, and still others are ﬁbrous.

Of particular interest is the morphology and

shape of soot particles, which contain BC, OM, O, N,

and H. Soot particles have important optical effects.

The only source of soot is emissions. Globally, about

55 percent of soot originates from fossil-fuel com-

bustion, and the rest originates from biomass burning

(Cooke and Wilson, 1996; Liousse et al., 1996). An

emitted soot particle is irregularly shaped and mostly

solid, containing from 30 to 2000 graphitic spherules

aggregated with random orientation by collision dur-

ing combustion (Katrlnak et al., 1993). An example

of a soot aggregate is shown in Fig. 5.15(b).

Once emitted, soot particles can coagulate or

grow. Because soot particles are porous and have a

large surface area, they serve as sites on which con-

densation occurs. Although BC in soot is

hydrophobic, some organics in soot attract water, in

which inorganics gases dissolve (Andrews and Larson, 1993). Evidence of condensa-

tion and coagulation is abundant because trafﬁc tunnel studies (Venkataraman et al.,

1994) and test vehicle studies (Maricq et al., 1999; ACEA, 1999) indicate that most

fossil-fuel BC is emitted in particles smaller than 0.2 m in diameter, but ambient

AEROSOL PARTICLES IN SMOG AND THE GLOBAL ENVIRONMENT 139

Figure 5.15. Transmission electron microscopy

(TEM) images of (a) ammonium sulfate parti-

cles containing soot (arrows point to soot

inclusions), (b) a chainlike soot aggregate,

and (c) ﬂy-ash spheres consisting of amor-

phous silica collected from a polluted marine

boundary layer in the North Atlantic Ocean by

Pósfai et al. (1999).

measurements in Los Angeles, the Grand Canyon, Glen Canyon, Chicago, Lake

Michigan, Vienna, and the North Sea show that accumulation mode BC often exceeds

emissions mode BC (McMurry and Zhang, 1989; Hitzenberger and Puxbaum, 1993;

Venkataraman and Friedlander, 1994; Berner et al., 1996; Offenberg and Baker, 2000).

The most likely way that ambient BC redistributes so dramatically is by coagulation

and growth. Similarly, the measured mean number diameter of biomass-burning

smoke less than 4 minutes old is 0.10 to 0.13 m (Reid and Hobbs, 1998), yet the

mass of such aerosol particles increases by 20 to 40 percent during aging, with one-

third to one-half the growth occurring within hours after emissions (Reid et al., 1998).

Transmission electron microscopy (TEM) images

support the theory that soot particles can become coat-

ed once emitted. Katrlnak et al. (1992, 1993) show

TEM images of soot from fossil-fuel sources coated

with sulfate or nitrate. Martins et al. (1998) show a

TEM image of a coated biomass-burning soot particle.

Pósfai et al. (1999) show TEM images of North

Atlantic soot particles containing ammonium sulfate.

This image is reproduced in Fig. 5.15(a). They found

that internally mixed soot and sulfate appear to com-

prise a large fraction of aerosol particles in the

troposphere. Almost all soot particles found in the

North Atlantic contained sulfate. Strawa et al. (1999)

took scanning electron microscopy (SEM) images of

black carbon particles in the Arctic stratosphere. One

such image is reproduced in Fig. 5.16. The rounded

edges of the particle seems to indicate that the particle

is coated. Katrlnak et al. (1993) report that rounded

grains on black carbon aggregates indicate a coating.

In sum, whereas emitted soot particles are relatively distinct, or externally mixed

from other aerosol particles, soot particles typically coagulate or grow to become

internally mixed with other particle components. Although soot becomes internally

mixed, it does not become “well mixed” (diluted) in an internal mixture because soot

consists of a solid aggregate of many graphite spherules. Thus, soot is a distinct com-

ponent in a mixed particle. Katrlnak et al. (1992) found that coated soot aggregates

have carbon structures similar to uncoated aggregates; thus, individual spherules in BC

structures do not readily break off or compress during coating.

5.6. HEALTH EFFECTS OF AEROSOL PARTICLES

Aerosol particles contain a variety of hazardous inorganic and organic substances.

Some hazardous organic substances include benzene, polychlorinated biphenyls, and

polycyclic aromatic hydrocarbons (PAHs). Hazardous inorganic substances include

metals and sulfur compounds. Metals cause lung injury, bronchioconstriction, and

increased incidence of infection (Ghio and Samet, 1999). Particles smaller than 10 m

in diameter (PM

) have been correlated with asthma and chronic obstructive pul-

monary disease (MacNee and Donaldson, 1999).

A major worldwide health problem directly linked to aerosol particles is Coal

Workers’ Pneumoconiosis (CWP), more commonly known as black-lung disease.

140 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Figure 5.16. Scanning electron microscopy

(SEM) image of a coated soot particle from the

Arctic stratosphere by Strawa et al. (1999).

Coal workers develop black-lung disease over many years of exposure to coal dust.

The dust first builds up in air sacs in the lungs, then scars the sacs, making breath-

ing difficult. During the last 30 years alone in the United States, black lung disease,

first identified in 1831, has killed an average of 2000 coal workers per year

(NIOSH, 2001).

A more deadly health hazard linked to aerosol particles is indoor-burning of bio-

mass and coal. Such burning is carried out daily for cooking and home heating by

large segments of the population in many developing countries. The World Health

Organization estimates that, of 2.7 million people who die each year from air pollu-

tion, 1.8 million die in rural areas, where the largest source of mortality is indoor-

burning of biomass and coal (WHO, 2000).

With respect to outdoor air, some studies have found that there may be no low

threshold for PM

-related health problems (Pope et al., 1995). Because most mass of

is not hazardous, damage from PM

may be due primarily to small particles,

particularly ultraﬁne particles (smaller than 0.1 m in diameter). Such particles may

be toxic to the lungs, ev

en when they contain components that are not toxic when pres-

ent in larger particles (MacNee and Donaldson, 1999).

Studies in the 1970s found a link between cardiopulmonary disease and high

concentrations of aerosol particles and sulfur oxides. Other studies in the 1970s and

1980s found a link between low concentrations of aerosol particles and health. A sum-

mary of several studies by Pope (2000) concluded that short-term (acute) increases of

10 g m

3

were associated with a 0.5 to 1.5 percent increase in daily mortality,

higher hospitalization and health-care visits for respiratory and cardiovascular disease,

and enhanced outbreaks of asthma and coughing. Increased death rates usually

occurred within 1 to 5 days following an air pollution episode. Long-term exposures to

5 g m

3

of particles smaller than 2.5 m in diameter (PM

2.5

) above background lev-

els resulted in a variety of cardiopulmonary problems, including increased mortality,

increased disease, and decreased lung function in adults and children (Pope and

Dockery, 1999).

Additional studies have shown that PM

2.5

results in more respiratory illness

and premature death than do larger aerosol particles (Özkatnak and Thurston, 1987;

U.S. EPA, 1996). One six-city, 16-year study concluded that people living in

areas where aerosol particle concentrations were lower than even the federal PM

standard had a lifespan two years shorter than people living in cleaner air (Dockery -

et al., 1993). Air pollution was correlated with death from lung cancer and

cardiopulmonary disease. Mortality was correlated with ﬁne particulates, including

sulfates.

Finally, several studies have examined the effect of both gas and particle pollution

on health. A study by Hall et al. (1992) found that 98 percent of the 12 million people

living in the Los Angeles Basin experienced ozone-related symptoms for 17 days each

year, and air pollution in the basin caused 1600 premature deaths per year. Children

and people working outdoors were most likely to experience respiratory problems. A

study by Dr. Kay Kilburn of the University of Southern California found that the lung

function of children raised in Los Angeles was 10 to 15 percent lower than that of

children raised in cleaner environments. A study by Dr. David Abbey of Loma Linda

University found that people living in areas of Los Angeles that violated federal partic-

ulate standards at least 42 days per year had a 33 percent greater risk of bronchitis and

74 percent greater risk of asthma than a control group. Women living in high-particulate

areas had a 37 percent higher risk of developing cancer. A 1987 study by Drs. Russell

AEROSOL PARTICLES IN SMOG AND THE GLOBAL ENVIRONMENT 141

Sherwin and Valda Richters of the University of Southern California found that 75

percent of a group of young people who died accidentally had airspace inﬂammation,

27 percent of the group had severe damage to their lungs, 39 percent had severe illness

to the bronchial glands, and 29 percent had severe illness in their bronchial linings

(SCAQMD, 2000).

Although epidemiological studies have found an association between short-term

exposure to outdoor particulate air pollution and health problems, people spend most

of their time indoors, and concentrations of aerosol particles are often greater indoors

than they are outdoors. Concentrations of gases and aerosol particles measured in the

vicinity of an individual indoors are even greater than are concentrations measured

from a stationary indoor monitor away from the individual. The relatively high con-

centration of pollution measured near an individual is called the personal cloud

(Rodes et al., 1991; McBride et al., 1999). A personal cloud may arise when a person’s

movement stirs up gases and particles on clothes and nearby surfaces, increasing pol-

lutant concentrations. People also release thermal-IR radiation, which rises, stirring

and lifting pollutants. Personal cloud concentrations can range from 3 to 67 g m

3

for PM

and from 6 to 27 g m

3

for PM

2.5

(Wallace, 2000). Personal cloud concen-

trations must be separated from background outdoor or indoor air concentrations when

determining the effects of particles on health.

5.7. SUMMARY

Aerosol particles appear in a variety of shapes and compositions and vary in size from

a few gas molecules to the size of a raindrop. Natural sources of aerosol particles

include sea-spray uplift, soil-dust uplift,

volcanic eruptions, natural biomass burning,

plant material emissions, and meteoric debris. Anthropogenic sources include fugitive

dust emissions, biomass burning, fossil-fuel combustion, and industrial sources.

Aerosol particle size distributions are usually trimodal or quadrimodal, consisting of a

nucleation, one or two subaccumulation, and a coarse particle mode. Homogeneous

nucleation and emissions from combustion and biomass burning dominate the nucle-

ation mode. Emissions of sea-spray, natural soil dust, and fugitive soil dust dominate

the coarse mode. Aerosol particles coagulate and grow by condensation or dissolution

from the nucleation mode to the accumulation mode. Chemistry within aerosol parti-

cles and between gases and aerosol particles affects gro

wth. Growth does not move

accumulation mode particles to the coarse mode, except when water vapor grows onto

aerosol particles to form cloud drops. The main removal processes of aerosol particles

from the atmosphere are rainout, sedimentation, and dry deposition. Aerosol particles

are responsible for a variety of health problems.

5.8. PROBLEMS

5.1. Why do accumulation mode aerosol particles not grow readily into the coarse

mode?

5.2. Why do accumulation mode particles generally contain more sulfate than do

coarse mode particles?

142 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

5.3. On a global scale, why is most chloride observed in the coarse mode?

5.4. Why is most ammonium found in accumulation mode particles?

5.5. Write an equilibrium reaction showing nitric acid gas reacting with magnesite,

a solid. What particle size mode should this reaction most likely occur in?

5.6. Why is the carbonate ion not abundant in aerosol particles?

5.7. Why might a sea-spray drop over midocean lose all its chloride when it reach-

es the coast?

5.8. Why is more nitrate than sulfate generally observed in particles containing soil

minerals?

5.9. Why is coagulation not an important process for moving particle mass from the

lower to the upper accumulation mode?

5.10. Visibility is affected primarily by particles with diameter close to the wave-

length of visible light, 0.5 m. Which particle mode does this correspond to,

and which three particle components in this mode do you think affect visibility

in the background troposphere the most?

5.11. Particles smaller than 2.5 m in diameter affect human health more than do

larger particles. Identify ﬁve chemicals that you might e

xpect to see in high

concentrations in these particles in polluted air. Why did you pick these

chemicals?

AEROSOL PARTICLES IN SMOG AND THE GLOBAL ENVIRONMENT 143

EFFECTS OF

METEOROLOGY ON

AIR POLLUTION

n this chapter, the effects of meteorology on air pollution are discussed. The

concentrations of gases and aerosol particles are affected by winds, tempera-

tures, vertical temperature proﬁles, clouds, and the relative humidity. These

meteorological parameters are influenced by large-and small-scale weather systems.

Large-scale weather systems are controlled by large-scale regions of high and low

pressure. Small-scale weather systems are controlled by ground temperatures and

small-scale variations in pressure. The first section of the chapter examines the

forces acting on air. The second section examines how forces combine to form

winds. The third section discusses how radiation, coupled with forces and the rota-

tion of the Earth, generates the global circulation of the atmosphere. Sections 6.4

and 6.5 discuss the two major types of large-scale pressure systems. Section 6.6

discusses the effects of such pressure systems on air pollution. The last section

focuses on the effects of local meteorology on air pollution.

6.1. FORCES

Winds arise due to forces acting on the air. Next, the major forces are described.

6.1.1. Pressure Gradient Force

When high air pressure exists in one location and low pressure exists nearby, air moves

from high to low pressure. The force causing this motion is the pressure gradient

force (PGF). The force is proportional to the difference in pressure di

vided by the

distance between the two locations and always acts from high to low pressure.

6.1.2. Apparent Coriolis Force

When air is in motion over a rotating Earth, it appears to an observer ﬁxed in space to

be deﬂected to the right in the Northern Hemisphere and to the left in the Southern

Hemisphere by the apparent Coriolis force (ACoF). The ACoF is not a real force; it

is an apparent or ﬁctitious force that, to an observer ﬁxed in space, is really an acceler-

ation of moving air to the right in the Northern Hemisphere and left in the Southern

Hemisphere and arises when the Earth rotates under a body (air in this case) in motion.

The ACoF is zero at the equator, maximum at the poles, zero for bodies at rest, propor-

tional to the speed of the air, and always acts 90 to the right (left) of the moving body

in the Northern (Southern) Hemisphere.

Figure 6.1 illustrates the ACoF. If the Earth did not rotate, an object thrown from

point A directly north would be received at point B, along the same longitude as point A.

Because the Earth rotates, objects thrown to the north have a west-to-east velocity equal

to that of the Earth’s rotation rate at the latitude they originate from. The Earth’s rotation

rate near the equator (low latitude) is greater than that near the poles (high latitudes);

thus, objects thrown from low latitudes have a greater west-to-east velocity than does the

Earth below them when they reach a high latitude. For example, an object thrown from

point A toward point B in the north will end up at point C, instead of at point B by the

time the person at point A reaches point A, and the object will appear as if it has been

deﬂected to the right (from point B to point C). Similarly, an object thrown from point

B toward point A in the south will end up at point D, instead of at point A by the time

the person at point B reaches point B. The Coriolis effect, therefore, appears to deﬂect

146

moving bodies to the right in the Northern Hemisphere. Moving bodies in the Southern

Hemisphere appear to be deﬂected to the left.

6.1.3. Friction Force

A third force that acts on moving air is the friction force (FF). This force is important

near the surface only. The FF slows the wind. Its magnitude is proportional to the wind

speed, and it acts in exactly the opposite direction from the wind. The rougher the sur-

face, the greater the FF. The FF over oceans and deserts is small, whereas the FF over

forests and buildings is large.

6.1.4. Apparent Centrifugal Force

A fourth force, which also acts on moving air, is the apparent centrifugal force

(ACfF). This force is also a ﬁctitious force. The force arises when an object rotates

around an axis. The apparent force is directed outward, away from the axis of rotation.

When a passenger in a car rounds a curve, for example, a viewer travelling with the

passenger sees the passenger being pulled outward, away from the axis of rotation, by

this force. By contrast, a viewer ﬁxed in space sees the passenger accelerating inward

due to a centripetal acceleration, which is equal in magnitude to but opposite in

direction from the apparent centrifugal force.

6.2. WINDS

The major forces acting on the air in the horizontal are the PGF, ACoF, FF, and ACfF.

In the vertical, the major forces are the upward-directed vertical pressure-gradient

force and the downward-directed force of gravity. These forces drive winds. Examples

of horizontal winds arising from force balances are given next.

EFFECTS OF METEOROLOGY ON AIR POLLUTION 147

Equator

North Pole

South Pole

A′

B′

E E′

F′

Direction

of the Earth’s

rotation

West East

Figure 6.1. Example of the apparent Coriolis force (ACoF), described in the text. Thin arrows

in the ﬁgure are intended paths, thick arrows are actual paths.