A particle size sampler includes a cylindrical housing with an inlet end and an outlet end. A pump, connected to an outlet pipe at the outlet end, draws air through the particle size sampler. Aerosols or other particles are removed from the air during a series of filtering steps. The filtering steps include an impactor, a plurality of mesh screens, and a backup filter to remove any particles remaining after the previous filtering steps. The particle size sampler may operate for long periods of time providing a size distribution and concentration of small particles in the air.