Sulphates in particles of different sizes in the marine boundary layer over the Southern Baltic Sea

Concentrations of sulphate ions and particle size distributions were measured in the marine boundary layer using a ten-cascade Berner impactor at Hel between December 1997 and March 1998, and with a filter packin Gdynia from February to May 2001. In the marine boundary layer over the Hel Peninsula the presence of natural sea-salt and non-sea-salt sulphate ions (nssSO4 2−) was confirmed in the aerosols. The nssSO4 were present in all size ranges only in December 1997 and in the first half of January 1998, making up an average of 83.7±2.4% of the total sulphate content. In the other winter months, nssSO4 2− were recorded only in aerosols of diameters from 0.4 to 1.5 μm, and their average contribution dropped by over 20%. The extended growing season in the Southern Baltic resulted in the formation of fine particles that appeared in the marine boundary layer in early winter. At that time the proportion of nssSO4 2− in ultra-fine particles ranged from 75.7% to 100%. The concentration of nssSO4 2−, which varied from 0.8 to 2.3 nmol m−3 in particles < 0.4 μm in size probably constituted the natural background aerosol population in the relatively clean coastal environment of the Southern Baltic Sea. In the coastal urbanised area of Gdynia, the sulphate concentration in aerosols was higher than at Hel (av. 75.3 nmol m−3) but the proportion of nssSO4 was low, of the order of 24%. Sea salt was then the dominant source of sulphate.