Particulate Constituents Of The Atmosphere

Table 2-1 does not include the "particulate matter,'' dust, or, in general, the nongaseous matter that is usually present in the atmosphere. These are particles, which are 10—7 —10—2 cm in radius, that is, from approximately molecular dimensions to sizes that settle fairly rapidly. Particulate matter in the atmosphere can be natural or man-made. Table 2-2 gives some estimates of the tonnage of particles with radii less than 2 x 10—3 cm (20 ^m) emitted to the atmosphere each year.

Table 2-2 indicates that particles of human origin constitute between 5 and 45% of the total appearing in the atmosphere annually at the present time. Other estimates are as high as 60% These estimates cover a large range because some occurrences, like volcanic action, are inherently variable and because some of the other sources of particulate matter are very difficult to estimate.

Large particles can settle out of the atmosphere by sedimentation in the earth's gravitational field. Furthermore, it is estimated that approximately 70% of the particulate matter that enters the atmosphere is eventually rained back out, probably because the particles act as condensation nuclei for the water droplets that form rain clouds. The particles are accelerated by the gravitational force but retarded by friction with the gas molecules of the air, thus reaching a terminal sedimentation velocity which is related to the radius of each particle as shown in Table 2-3, which, by the way, refers to water droplets as well as particles. Water droplets with radii from 1 to 50 ^m may be present in clouds, while droplets with radii of 100 ^m generally fall as a drizzle. Larger droplets fall as rain.

TABLE 2-2

Estimate of Particles Emitted into or Formed in the Atmosphere Each Year

Particle sources Quantity (megatons/year)

Natural

Soil and rock debris

100-500

Forest fires and slash burning

3-150

Sea salt

300

Volcanic action

25-150

Gaseous emissions

Sulfate from H2S

130-200

NH4 salts from NH3

80-270

Nitrate from NO*

60-430

Hydrocarbons from vegetation

75-200

Natural particle subtotal

773-2200

Human origin

Direct emissions, smoke, etc.

10-90

Gaseous emissions:

Sulfate from SO2

130-200

Nitrate from NO*

30-35

Hydrocarbons

15-90

Human origin particle subtotal

185-415

Grand total

958-2615

Source: Reprinted, with some modification, from Inadvertent Climate Modification, Report on the Study of Man's Impact on Climate (SMIC), MIT Press, Cambridge, MA, 1971.

The smallest solid particles, many with radii of only 0.1-1.0 ^m, have a maximum concentration about 18 km above the earth's surface. Some of these arise from volcanic eruptions, while others are particles of ammonium sulfate formed when H2S and SO2 in the atmosphere are oxidized and the resulting

TABLE 2-3

Sedimentation Velocity of Particles with Density 1 g/cm3 in Still Air at 0°C and 1 atm Pressure

TABLE 2-3

Sedimentation Velocity of Particles with Density 1 g/cm3 in Still Air at 0°C and 1 atm Pressure

Radius of particle (pm)

Sedimentation velocity (cm/s)

< 0.1

Negligible

0.1 (IO-5 cm)

8 x 10-5

1.0

4 x 10-3

10.0

0.3

100.0 (10-2 cm)

25

H2S04 neutralized another atmospheric constituent, ammonia. The mean residence time of particulate matter in the lower atmosphere is between 3 and 22 days. In the stratosphere it is 6 months to 5 years, in the mesosphere from 5 to 10 years. (The meaning of "stratosphere'' and "mesosphere'' will be explained in the next section.) At any rate, the higher in the atmosphere—that is, the farther above the surface of the earth the particles have reached—the longer it takes them to come down.

Particles with radii from 0.02 to 10 ^m may contribute to the turbidity of the atmosphere. We shall see later that the turbidity of the atmosphere is an important determinant of the earth's climate. The atmosphere has become more turbid over the past half-century, as shown by monitoring both in urban locations such as Washington, D.C., and in the mountains, such as Davos, Switzerland.

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