A method and apparatus for detecting, sizing, and counting gas borne particulate matter are described. The particles are impacted onto a heated surface capable of pyrolyzing them. Some of the products of the pyrolysis are then surfaceionized, producing a burst of ions which is detected electronically producing a voltage pulse proportional to the amount of surfaceionizable matter originally present in the particle. Methods are described for adding to each particle an amount of surfaceionizable material proportional to the particle size, enabling the instrument to measure the particle size distribution of a collection of gasborne particles. The basic apparatus may detect particles by means of either positive or negative surfaceionization, and is capable of operation either when situated in air or when located in a vacuum system.