This document describes a standard method for determining particle number size distributions in ambient air in the size range from 10 nm to 800 nm at total concentrations up to approximately 105 cm 3 with a time resolution of a few minutes. The standard method is based on a Mobility Particle Size Spectrometer (MPSS) used with a bipolar diffusion charger and a Condensation Particle Counter (CPC) as the detector. The document describes the performance characteristics and minimum requirements of the instruments and equipment to be used, and describes sampling, operation, data processing and QA/QC procedures, including calibration.