Ammonia concentrations in aerosols and ammonium concentrations in the air were measured in the coastal zone of the southern Baltic Sea. The main study area was the Hel Peninsula, where measurements were carried out from December 1997 to March 1998. There was a second such area in Gdynia, where the first measurement period extended from May to December 2000, and the second one lasted from February to May 2001. At the same time, chloride, sulphate and sodium concentrations in the aerosols were determined; meteorological parameters were also measured. Aerosol samples were collected with a filter pack, and the gaseous phase species were collected on denuders. At both stations, the ammonium concentration in aerosols was at least one order of magnitude higher than that of the ammonia in the air samples. It was found that the marine boundary layer always contained ammonium chloride and ammonium sulphate. The presence of ammonium nitrate was detected only when winds were slight and was attributed to local anthropogenic sources. The ammonia concentration related to nitrate increased with rising air humidity and a falling temperature. The contribution of marigenic ions in aerosols was noted at onshore wind of speeds >3.5 m s^-1. In winter as in summer, conditions occurred facilitating light backscattering by a "wet" aerosol, which contained ammonium nitrate and ammonium sulphate in proportions of 2.5:1 and 1:1.