Indirect Effect
Indirect effect is influence of aerosol on cloud droplet or crystal number and hence cloud optical depth.
There are two kind of indirect effect; 'First indirect effect', 'Second indirect effect'
First Indirect Effect (Cloud Albedo Effect)
influence through impact on effective radius, with no change in water content of
cloud
The cloud-albedo effect(=First indirect effect)that is, the distribution of the same cloud liquid water content over more, hence smaller, cloud droplets leading to higher cloud reflectivity, is a purely radiative forcing |  |
The changes arising from the aerosol scattering and absorption of radiation are referred to as the direct radiative forcing. Aerosols influence climate through their role in cloud condensation nuclei.
Increase in aerosols causes an increase in the droplet number concentration. The increase in the number of drops leads to an increase in the reflection to space of solar radiation from clouds, leading to a climate cooling. This is called the first indirect radiative forcing
 [Fig.] Aircraft data illustrating the increase in cloud drops with aerosol number concentration (reference : 7 December 2001 Vol. 294 SCIENCE) | Aerosols, larger than about 0.05 mm in diameter provide the nuclei for most cloud drops. Organic aerosols also serve as cloud condensation nuclei (CCN) on a par with the role of sulfate particles. In situ aircraft observations of continental and marine clouds show that anthropogenic aerosols enhance CCN and cloud drop number density. |
Second Indirect Effect (Cloud Lifetime Effect)
influence on cloud optical depth through influence of droplet number on mean droplet size and hence initiation of precipitation
 | The other effects involve feedbacks in the climate system and are discussed here. The albedo effect cannot be easily separated from the other effects; in fact, the processes that decrease the cloud droplet size per given liquid water content also decrease precipitation formation, presumably prolonging cloud lifetime. In turn, an increase in cloud lifetime also contributes to a change in the time-averaged cloud albedo. |
If the condensed moisture inside the cloud is not altered by the increase in aerosols, the droplet radius will decrease because of the increase in its number concentration resulting in a decrease in the precipitation efficiency.
This effect leads to suppression of precipitation in polluted clouds. In addition, it can also lead to an increase in cloud lifetime and the amount of clouds. The increase in cloudiness will lead to a further increase in the reflection of solar radiation, giving rise to the second indirect radiative forcing.
 [Fig.]Effective radius particle within the polluted area (reference : 7 December 2001 Vol. 294 SCIENCE) | Aerosols containing large concentrations of small CCN nucleate many small cloud droplets, which coalesce very inefficiently into raindrops. One consequence of this is suppression of rain over polluted regions In satellite studies, the clouds within and outside of pollution tracks had similar dimensions and contained similar amounts of water. The only difference was in the reduction of the cloud particle effective radius within the pollution tracks to less than 14 mm, the precipitation threshold radius below which precipitation particles do not normally form. |
[Table.] Overview of the Aerosol effects
(reference : IPCC Four Assement Report, Climate Change 2007)
