Aerosol Chemical Composition

Atmospheric aerosol particles contain sulfates, nitrates, ammonium, organic material, crustal species, seasalt, metal oxides, hydrogen ions, and water. From these species sulfate, ammonium, organic and elemental carbon, and certain transition metals are found predominantly in the fine particles. Crustal materials, including silicon, calcium, magnesium, aluminum, and iron, and biogenic organic particles (pollen, spores, plant fragments) are usually in the coarse aerosol fraction. Nitrate can be found in both the fine and coarse modes. Fine nitrate is usually the result of the nitric acid/ammonia reaction for the formation of ammonium nitrate, while coarse nitrate is the product of coarse particle/ nitric acid reactions.

A typical urban aerosol size/composition distribution is shown in Figure 8.23 (Wall et al. 1988). These results indicate that sulfate, nitrate, and ammonium have two modes in the 0.1-1.0 pm size range (the condensation and droplet modes), and a third one over 1 pm (coarse mode). The condensation mode has a peak around 0.2 pm and is the result of condensation of secondary aerosol components from the gas phase. The droplet mode peaks around 0.7 pm in diameter and its existence is attributed to heterogeneous, aqueous-phase reactions discussed in Chapter 7 (Meng and Seinfeld 1994). More than half of the nitrate is found in the coarse mode together with most of the sodium and chloride. This coarse nitrate is the result of reactions of nitric acid with sodium chloride or aerosol crustal material (see Chapter 10). This is an interesting case where secondary aerosol matter (nitrate) is formed through the reaction of a naturally produced material (seasalt or dust) and an anthropogenic pollutant (nitric acid).

More than 40 trace elements are routinely found in atmospheric particulate matter samples. These elements arise from dozens of different sources including combustion of coal, oil, woodburning, steel furnaces, boilers, smelters, dust, waste incineration, and brake wear. Depending on their sources, these elements can be found in either the fine or the coarse mode. Concentrations of selected elements together with the size mode where these elements are usually found are shown in Table 8.6. The concentrations of these elements even for similar pollution levels vary over almost three orders of magnitude, indicating the strong effect of local sources. In general, elements such as lead, iron, and copper have the highest concentrations, while elements such as cobalt, mercury, and

Effect Hydrogen Iron Particle

FIGURE 8.23 Measured size distributions of aerosol sulfate, nitrate, ammonium, chloride, sodium, and hydrogen ion in Claremont, CA (Wall et al. 1988).

Aerodynamic Diameter, Dae, |im

FIGURE 8.23 Measured size distributions of aerosol sulfate, nitrate, ammonium, chloride, sodium, and hydrogen ion in Claremont, CA (Wall et al. 1988).

TABLE 8.6 Concentrations (ng m 3) and Size Distribution of Various Elements Found in Atmospheric Particles

Concentration (ng m-3)

TABLE 8.6 Concentrations (ng m 3) and Size Distribution of Various Elements Found in Atmospheric Particles

Element

Mode"

Remote

Rural

Urban

Fe

F and C

0.6-4,200

55-14,500

130-13,800

Pb

F

0.01-65

2-1,700

30-90,000

Zn

F

0.03^150

10-400

15-8,000

Cd

F

0.01-1

0.4-1,000

0.2-7,000

As

F

0.01-2

1-28

2-2,500

V

F and C

0.01-15

3-100

1-1,500

Cu

F and C

0.03-15

3-300

3-5,000

Mn

F and C

0.01-15

4-100

4-500

Hg

0.01-1

0.05-160

1-500

Ni

F and C

0.01-60

1-80

1-300

Sb

F

0-1

0.5-7

0.5-150

Cr

F and C

0.01-10

1-50

2-150

Co

F and C

0-1

0.1-10

0.2-100

Se

F and C

0.01-0.2

0.01-30

0.2-30

"F = fine mode; C = coarse mode. Source: Schroeder et al. (1987).

"F = fine mode; C = coarse mode. Source: Schroeder et al. (1987).

TABLE 8.7 Comparison of Ambient Fine and Coarse Particles

Fine Particles

Coarse Particles

Formation pathways

Chemical reactions

Mechanical disruption

Nucleation

Suspension of dusts

Condensation

Coagulation

Cloud/fog processing

Composition

Sulfate

Resuspended dust

Nitrate

Coal and oil fly ash

Ammonium

Crustal element (Si, Al, Ti,

Hydrogen ion

Fe) oxides

Elemental carbon (EC)

CaC03, NaCl

Organic compounds

Pollen, mold, spores

Water

Plant, animal debris

Metals (Pb, Cd, V, Ni,

Tire wear debris

Cu, Zn, Mn, Fe, etc.)

Solubility

Largely soluble, hygroscopic

Largely insoluble and non-hygroscopic

Sources

Combustion (coal, oil,

Resuspension of industrial

gasoline, diesel, wood)

dust and soil

Gas-to-particle conversion

Suspension of soil (farming,

of NO*, S03, and VOCs

mining, unpaved roads)

Smelters, mills, etc.

Biological sources Construction/demolition Ocean spray

Atmospheric lifetime

Days to weeks

Minutes to days

Travel distance

100s to 1000s of km

< to 10s of km

antimony are characterized by low concentrations. Elements produced during combustion usually exist in the form of oxides (e.g., Fe203, Fe304, A1203), but their chemical form is in general uncertain.

A summary of chemical information regarding the coarse and fine modes is presented in Table 8.7.

The composition of seasalt reflects the composition of seawater enriched in organic material (marine-derived sterols, fatty alcohols, and fatty acids) that exists in the surface layer of the oceans (Schneider and Gagosian 1985). Seawater contains 3.5% by weight seasalt, and when first emitted, the seasalt composition is the same as that of seawater (Table 8.8). Reactions on seasalt particles modify its chemical composition; for example, sodium chloride reacts with sulfuric acid vapor to produce sodium sulfate and hydrochloric acid vapor

H2S04(g) -I- 2NaCl — Na2S04 + 2HCl(g) leading to an apparent "chloride deficit" in the marine aerosol.

TABLE 8.8 Composition of Seasalt"

Species

Percent by Weight

CI

55.04

Na

30.61

SQJ-

7.68

Mg

3.69

Ca

1.16

K

1.1

Br

0.19

C (noncarbonate)

3.5 x 10~3 - 8.7 x 10

-3

A1

4.6 x 10-4 - 5.5 x 10

-3

Ba

1.4 x 10~4

I

1.4 x 10~4

Si

1.4 x 10~4 - 9.4 x 10"

-3

NOj

3 x 10"6 - 2 x 10~3

Fe "

5 x 10~5 - 5 x 10~4

Zn

1.4 x 10"5 - 4 x 10~5

Pb

1.2 x 10~5 - 1.4 x 10"

-5

NH+

1.4 x 10~6 - 1.4 x 10"

-5

Mn

2.5 x 10~6 - 2.5 x 10"

-5

V

9 x 10"7

"Based on the composition of seawater and ignoring atmospheric transformations.

"Based on the composition of seawater and ignoring atmospheric transformations.

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