Ground level measurements of aerosol physical and chemical properties were conducted at Ny ¿lesund on Spitsbergen from late March to mid-May 1996 as part of the European Arctic Tropospheric Ozone Chemistry study (ARCTOC'96) using laser-scattering aerosol-spectrometer probes and a denuder-filterpack system. When the air comes from Eurasia, the number, area, and volume distributions of aerosols at Ny ¿lesund are similar to those observed in the high Arctic at Alert, Canada, while when air comes off open water areas of the North Atlantic or Greenland, they are most different. The marine air has relatively more supermicrometer particles than the polluted arctic air. In Eurasian air the inorganic ionic component is dominated by H+ and SO4= and to a lesser extent NH4+. The average molar ratio of NH4+/H+ is 1/3. The average dry aerosol mass concentration and composition at Ny ¿lesund varied depending on origin of air. The period mean mass concentration was 4.1 μg m-3. It was lowest (2.2 μg m-3) in air coming from the open North Atlantic and Greenland and was highest (6.2 μg m-3) in air coming from the east. On average, aerosols in the North Atlantic and Greenland air were composed of 19% non--sea-salt SO4=, 22% sea salt, 25% other inorganic ions, and 34% of an uncharacterized residual fraction that is likely mostly carbonaceous compounds. In contrast, for the most polluted eastern air these fractions were 48% non--sea-salt SO4=, 7% sea salt, 8% other inorganic ions, and 37% residual. For the period the median diameters of the mean accumulation mode number, surface area, and volume distributions at ambient relative humidity (78--90%) were 0.23 μm, 0.37 μm, and 0.50 μm effective scattering diameter (ESD), respectively. Number concentrations (for dry particle diameters >0.065 μm) ranged from 10 to 922 cm-3 with an average of 270 cm-3. Mixing ratios of particulate Br- compounds were observed to be anticorrelated with that of ozone, increasing markedly when the ozone mixing ratio was <10 nmol/mol. There was no obvious dependence of the ratio of gas to particle Br- on ozone mixing ratio. Observations suggested that during ozone depletion events the number concentration of particles increased in the 1.0 to 1.4 μm ESD diameter range (ambient relative humidity). ¿ 1999 American Geophysical Union