Atmospheric aerosols are any solid or liquid particles suspended in the air. There are many sources of natural and man-made aerosols. Strong winds blow desert dust and sea salt up to higher altitudes. Factories and cars emit black carbon while forest fires are sources of organic carbon.
In remote, clean environments or at high altitudes, the primary source of aerosols is nucleation, when new particles form from the gas phase. These particles are created at very small sizes, no more than a few nanometres, and must grow through condensation and coagulation to several tens of nanometres before they can have an impact on climate.