What are the probable sources of dust in the atmosphere?

Dust is often airborne because of horizontal winds combined with turbulence and convection due to rising currents in heated air. Such dust can travel for thousands of miles. This is commonly seen on a large scale in Spring storms that develop in the Great Plains, or Sahara desert dust carried into the atmosphere from a strong easterly jet stream and deep dry convection. On a smaller scale dust becomes airborne at construction sites in cities where preparatory work for road building or foundation laying is underway.

Another important source may not be so obvious, but is and has been very important in the history of our planet. This source is from "outer space" in what is commonly referred to as cosmic dust. The mass of the earth easily attracts such small particles by the laws of Physics. This cosmic dust has been accumulating on the earth since the planet's existence and will continue. The process is actually increasing the mass of the earth.