A portable photoelectric nucleus counter, with similar sensitivity to the Pollak photoelectric nucleus counter with convergent light beam, has been developed and calibrated. This instrument has been incorporated into a packaged measurement system which allows the experimenter to determine the effective diffusion coefficient and fraction charged, of the natural aerosol, in uncontaminated areas. The photoelectric counter has comparable accuracy to the absolute (Aitken, Scholz) counters in the concentration range of interest, and is capable of determining the concentration once per minute.Field tests of the prototype instrument were conducted near sea level in Greenland. The concentration of natural aerosol in this area ranged from 150 to 200 particles cm−3. The instrumentation had sufficient sensitivity to detect a gradual increase in particle size at this low concentration.