On the same atmospheric particles, we have measured the aerodynamic diameter, the height of the scattered light pulse to estimate the optical diameter, and the composition with a laser ionization mass spectrometer. Different types of particles, as determined by the mass spectra, have distinct relationships between optical and aerodynamic diameters. The relative densities of organic and sulfate particles inferred from the aerodynamic and optical diameters are consistent with the composition inferred from the mass spectra. (C) 2003 Elsevier Ltd. All rights reserved.