A new instrument capable of measuring aerosol size distribution with high time and size resolution, and high signal-to-noise ratio is described. The instrument, referred to as Fast Integrated Mobility Spectrometer (FIMS), separates charged particles based on their electrical mobility into different trajectories in a uniform electric field. The particles are then grown into super-micrometer droplets, and their locations on the trajectories are recorded by a fast charge-coupled device (CCD) imaging system. Images captured by the CCD reveal mobility-dependent particle positions and their numbers, which are then used to derive a particle size distribution spectrum. By eliminating the need to scan over a range of voltages, FIMS significantly improves the measurement speed and counting statistics. A theoretical framework has been developed to quantify the measurement range, mobility resolution, and transfer function of FIMS. It is shown that FIMS is capable of measuring aerosol size distributions with high-time and size resolution. (c) 2006 Elsevier Ltd. All fights reserved.