Atmospheric particles were collected in the high Arctic at Alert during winter (February) and spring (April-May) and were subjected to stable carbon isotopic (delta C-13) measurements to better understand the source of carbonaceous aerosols. The mean delta C-13 values of aerosol total carbon (TC) were observed to increase from winter (-25.7 +/- 0.7%) to spring (-23.7 +/- 0.8%). A strong correlation (r(2) = 0.92, p < 0.001) was found between the delta C-13 values and Na+/TC ratios. The increased delta C-13 values were most likely explained by an enhanced sea-to-air emission of marine organic matter to the high Arctic and also by a decreased atmospheric transport of anthropogenic carbon from the midlatitudes. The backward trajectory analysis together with inorganic ion analysis indicated that spring aerosols were more affected by the Arctic Ocean than winter aerosols that were mainly derived from the primary pollutants emitted in the midlatitudes. On the basis of the delta C-13 values and Na+/TC ratios, contribution of marine organic matter to aerosol TC was estimated to be 45% in late spring. The enhanced sea-to-air emission of marine organic carbon is probably linked with a melting of sea ice, expansion of leads, and increased biological activity in the Arctic Ocean after the polar sunrise in spring.