Measurements of particle number and mass size distributions of the atmospheric aerosol in the lower troposphere were made during the Polar Sunrise Experiment from January 20 to April 20, 1992, at Alert, Canada (82.5 ¿N, 62.3 ¿W). Particle number concentrations were determined by means of laser scattering particle counters and ranged from 50 to 600 cm-3 in the accumulation mode. Particle mass size distributions were measured using a Berner low-pressure impactor; the total suspended particle mass ranged from 2.4 to 8.6 μg m-3. Average densities of (1.52¿0.07) and (2.58¿0.48) g cm-3 were calculated for accumulation and coarse mode particles, respectively, by combining impactor and optical size distributions. Boundary layer ozone concentrations were positively correlated with particles of diameters less than 0.35 μm and negatively with particles larger than 0.35 μm. Particles in the size range 6--12 μm were detected exclusively during low ozone episodes. ¿ American Geophysical Union 1994