A new optical particle counter, the Particle Metrics, Inc., ultra high sensitive aerosol spectrometer (UHSAS), has been used to size aerosol particles both in our laboratory, and at a remote mountain-top site in Wyoming. The laboratory studies show that the UHSAS measured diameters of polystyrene latex (PSL), ammonium sulfate, and ammonium nitrate particles are within 10% of those determined by a scanning mobility particle sizer (SMPS), as are those for non-spherical sodium chloride particles and PSL doublets. The UHSAS detection efficiency is close to 100% for particles larger than 100 nm and concentrations below 3000 cm(-3), but decreases both for smaller particles and for higher concentrations considerably. The field measurements confirm these observations and also demonstrate good agreement of particle concentration measurements between the UHSAS, SNIPS, and a passive cavity aerosol spectrometer probe (PCASP) in the same size ranges. (C) 2008 Elsevier Ltd. All rights reserved.