Size resolved aerosol components (major ions, selected organic acids, and crustal elements) were investigated with 12-stage impactors during the summers 1997 and 1998 and winter 1997--1998 at Sevettij¿rvi in Finnish Lapland. Concentrations are compared with those obtained with simultaneous measurements with bulk sampling and show agreement within 15%. The samples were classified depending on their degree of marine and continental origin. Modal parameters (aerodynamic diameters and dispersion) were identified from the size distribution of each species. In general, the aerosol comprises two submicron modes (condensation mode at 0.30 ¿ 0.05 ¿m and droplet mode at 0.65 ¿ 0.15 ¿m) and one or two supermicron modes (the first one between 3.5 and 4.5 ¿m associated with marine species and the second one between 5 and 6 ¿m associated with crustal species). The results suggest internal mixing of nss sulfate, ammonium, and organic acids in the submicron modes, and differences between the classes of air masses allow to draw hypotheses on the formation processes involved for each species. The results for the supermicron modes indicate interactions of nss sulfate, ammonium, nitrate, and organic acids with the marine and crustal particles, which increase from the marine to the continental classes. During summer and winter the supermicron mass fraction largely decreases from the marine air masses to continental ones.