A gas laser cavity beam is used as monochromatic radiation source for detecting and sizing aerosol particles. The aerosol is forced through the cavity of a (1/2) mW open cavity He&sngbnd;Ne laser having one plane mirror and one spherical mirror. Light scattered from individual particles is collected by a hemispherical mirror and is focused onto a detector. The system has good resolution and sensitivity and has the capability of measuring high aerosol concentrations without dilution.