Many state-of-the-art aerosol measurement instruments of today are large, heavy and expensive; and therefore not well suited for field measurements. We have built a diffusion size classifier (DiSC), which trades the accuracy and size distribution information of the larger instruments against simplicity and portability. It measures number concentration and an average diameter with a high time resolution (approximately 2s). After a correction for polydisperse aerosol, the measured numbers agree reasonably well with SMPS measurements. The diffusion size classifier is comparatively small and lightweight. It is battery-powered and can be used with a PDA as a portable measurement unit. The measurable aerosol size range of the instrument is between 20 and 200 nm. Detectable aerosol concentration ranges are typically between 3E3…1E6 pt/ccm (with an additional dependence on particle size and sampppppppling time constants).