This depends critically on how much aerosol is present, how absorptive the aerosol is, how large the aerosol particles are, and how high the aerosol is in the atmosphere. For example, the aerosols emitted by the Mt. Pinatubo eruption in 1991 were very small particles that absorbed very little sunlight and primarily reflected it. As predicted, the aerosols from Mt. Pinatubo acted to cool the planet for a couple of years before settling out of the atmosphere. Man-made aerosols tend to be processed out of the atmosphere by clouds within a few weeks. As the number of aerosols increases, the water in the cloud gets spread over many more particles, each of which is correspondingly smaller.
Smaller particles fall more slowly in the atmosphere, and decrease the amount of rainfall.
In this way, changing aerosols in the atmosphere can change the frequency of cloud occurrence, cloud thickness, and rainfall amounts. It is thought that the indirect effect of aerosols can be even larger than their direct effect, but at present it is not known whether such an effect is a net cooling or warming of the planet. From Measurements to Climate Models With clouds, aerosols, and surface properties varying greatly over the Earth, and with our inability to model Earth's complex climate system, we need a hierarchy of measurements varying from laboratory studies of cloud particles and aerosol properties, through aircraft and surface-based field experiment measurements, to observations of the entire Earth from space. Only from space, however, can we observe the surface, aerosol, and cloud changes in anything approaching a complete set of conditions occurring in the climate system. Ultimately, we are searching for a set of mathematical models that allow us to span the incredibly large range of space and time scales important to aerosols, water vapor, clouds, the land surface, and the oceans.