The total number concentration of atmospheric particles, as measured with a variety of Nolan--Pollak derived photo electric nucleus counters, is often indistinguishable from the concentration measured in filtered air. These periods of near zero aerosol concentration persist for periods of a few minutes to tens of hours at many places.

Near zero concentrations are frequently measured over calm seas following dissipation of fog, but aerosol generation by bubble mechanisms increase the aerosol concentration in a few minutes. Interesting cases of near zero surface aerosol concentration are found over the Arctic in summer when such concentrations persist at the surface for periods of time dependent on sky conditions, surface vegatation and near surface meteorological parameters. Size measurements of the reappearing aerosol indicate that it is formed in situ.