This invention is an apparatus and method of real time determination of particle size and optionally chemical composition or both. An aerosol beam generator focuses a beam containing sample particles that passes through a sizing laser beam of approximately constant width to produce light scattering that is detected by a light detection means, allowing generation of electrical pulses that may be used to compute particle velocity. In being formed into a beam, the particles are accelerated to terminal velocities that are functions of their sizes. The duration of time elapsed while a particle passes through the width of the sizing laser beam is a function of its velocity which, in turn, is a function of its size. Chemical composition of the particle is determined by suitable analytical means included in the apparatus, such as mass spectrometry.