More than 400 firn samples were collected at an altitude of 1660 m on the small ice sheet covering James Ross Island (northeastern coast of the Antarctic Peninsula). Chemical analysis of these samples provides information about the marine background aerosol composition in the area. The samples cover the years 1975--1979 (pit samples) and the last 15 years (firn core samples). Major impurities (Na, K. Ca. Al, H, SO4, and NO3) were determined by using flameless atomic absorption, neutron activation, acid titration, or ion chromatography. Aerosols are contributed mainly by the surrounding ocean; however, secondary aerosol deposition (H2SO4 and HNO3) is also important. All samples were found to be slightly acidic (pH generally in the range 5--6). Strong seasonal variations (maximum values in summer) are found in the H2SO4 deposition. A photochemical mechanism is proposed to explain the local production of H2SO4 from gaseous marine sulfur compounds. From these results, a yearly deposition of ~0.13 Tg of H2SO4 is calculated for the entire Antarctic continent.