Aerosol Particles in the Polluted and Global Atmosphere - Air Pollution and Global Warming: History, Science, and Solutions - page 102

Table 5.1. Characteristics of gases, aerosol particles, and hydrometeor particles

Number
concentration

Mass

Diameter (molecules or concentration

particles cm −3 ) gm −3 ) range ( m) (

2.45 × 10 19 1.2 × 10 9 Gas molecules 0.0005

Aerosol particles
Small

<

10 3 -10 6

<

0.1 1

Medium 0.1-2.5 1-104 < 250

Large 2.5-8,000 < 1-10 < 500

Hydrometeor particles
Fogdrops 5-20 1-500 10 4 -10 6

Cloud drops 10-200 1-1,000 10 4 -10 7

Drizzle 200-1,000 0.01-1 10 5 -10 7

Raindrops 1,000-8,000 0.001-0.01 10 5 -10 7

Hail 5,000-115,000 0.0001-0.001 10 5 -10 7

Data are for typical lower tropospheric conditions.

lognormal function ,which is a bell curve distribu-
tion on a log-log scale, as shown in Figure 5.1a.
Figure 5.1b shows the same distribution on a log-linear
scale.
Aerosol particle size distributions with one, two,
three, four, and even five modes are called uni-
modal, bimodal, trimodal, quadramodal, or pentamodal,
respectively. Distributions may include one or two
nucleation modes, one or two accumulation modes, and
a coarse mode. A nucleation mode is a mode with

mean diameter smaller than 0.1 m(100 nm). Thus,

nucleation mode particles are nanoparticles or ultrafine
particles. The nucleation mode contains newly nucle-
ated particles (those formed directly from the gas
phase) or small emitted particles. Nucleated particles
are between 1 and 3 nm in diameter because they con-
sist of a small cluster of gas molecules that have changed
phase to a liquid.
Two nucleation modes often exist in the case of vehi-
cle emissions. One mode has a peak smaller than 10 nm

10 2 10 2

10 1

10 1

10 0 10 0

10 -1

10 -1

10 -2 10 -2

D 1 D 2

10 -3 10 -3

0.001 0.01 0.1 1 0 0.05 0.1 0.15

Particle diameter (D, µm) Particle diameter (D, µm)

(a) (b)

Figure 5.1. (a) A lognormal particle volume distribution on a log-log scale. The incremental volume
concentration (dv, m 3 cm −3 -air) of material between any two diameters (D 1 and D 2 , m) is estimated by

multiplying the average value from the curve between the two diameters by d log 10 D = log 10 D 2 − log 10 D 1 .

m 3 cm −3

Thus, for example, the volume concentration between diameters D 1 and D 2 is approximately 6

m −1 × 0.15 m = 0.9 m 3 cm −3 .(b)The lognormal curve shown in (a), drawn on a log-linear scale.