Chemical size distributions of aerosols were measured using a 12-stage low-pressure impactor over the Atlantic Ocean in November--December 1999 and at an Antarctic site, Aboa, in January 2000. In the polluted latitudes north of the equator, particles were neutral in all sizes but accumulation mode was acidic in the clean areas. Chloride depletion was analyzed in detail. In most areas, chloride depletion could be explained by replacement by sulfate, nitrate, and methane sulfonate. In Antarctica the chloride losses increased in all size ranges and the contribution of each anion to the chloride depletion varied with increasing air mass residence time over continental Antarctica. The modal structure of the size distributions of ionic compounds was analyzed. The modes derived from chemical mass size distributions were compared with modes obtained from number size distributions. The modes of non-sea-salt sulfate and MSA were very similar suggesting that these particles were internally mixed. The nitrate mass modes were closer to the sea salt surface modes than the sea salt mass modes, suggesting that nitrate was on the surface of sea salt particles.