Geoscience Reference
In-Depth Information
Cloud condensation nuclei
There are always numerous small particles, called
aerosols
, suspended in the atmos-
phere. Some are important because they act as the condensation nuclei required for
cloud droplet formation. Aerosols are classified according to their size as:
Aitken Nuclei, if they have diameter in the range 0 to 0.2
μ
m;
●
Large Nuclei, if they have diameter in the range 0.2 to 2
μ
m;
●
Giant Aerosols, if they have diameter greater than 2
μ
m.
●
Typically the concentration of aerosols in the troposphere is on the order of 10
12
per cubic meter but it can be as high as 10
14
per cubic meter downwind of pollution
sources. Higher in the atmosphere, aerosol concentration falls off to about 10
10
per
cubic meter at 15 km. The concentration of aerosols falls off rapidly with the
size of the aerosols so the concentration of Aitken nuclei greatly exceeds that for
giant nuclei.
Aerosols originate in many different ways but their origin can be broadly classi-
fied according to whether they result from natural phenomena or human activity.
The main source of aerosols and the amount of aerosols these sources produce
annually are listed in Table 10.1. On average natural sources are dominant,
Table 10.1
Sources of atmospheric aerosols and estimates of
the amount of aerosols produced in tonnes per year.
Natural phenomena
(tonnes per year)
Airborne Sea salt
1000
Gas to particle conversion
570
Windblown dust particles
500
Natural forest fires
35
Meteoric debris
20
Volcanoes (a highly variable source)
25
Total from natural sources
>2150
Human activities
(tonnes per year)
Gas to particle conversion
275
Industrial processes
56
Fuel combustion (stationary sources)
44
Solid waste disposal
3
Transportation
3
Miscellaneous
28
Total from human activities
∼
410
