Atmospheric particles originate from many sources. Globally, natural particle-production mechanisms predominate, but over large urban areas man-made sources produce a major fraction of total airborne particles. Within the planetary boundary layer, the atmospheric aerosol contains large numbers of preformed primary particles that have been discharged to the atmosphere from ground sources. At higher elevations, secondary particles, formed by gas phase reactions between gases and vapors, often with the aid of activation energy from sunlight, predominate. All viable particles in the atmosphere are discharged from the earth. Particles have a relatively brief existence in the lower atmosphere and a longer, though finite, residence time at higher altitudes. None remain airborne permanently and measured concentrations represent a temporary equilibrium between rates of formation and destruction during the sampling period.