Isotopic composition of sulfur in size‐resolved marine aerosols above the Atlantic Ocean

Abstract

Sulfur isotope measurements were performed on size‐segregated aerosols collected during the Albatross oceanographic campaign from 61°N to 35°S above the Atlantic Ocean in October and November 1996. Results obviously showed the dependence of the sulfur isotope ratio upon particle size, the finest particles being depleted in 34S compared to coarse particles, suggesting a heavier continental influence in the fine mode. In the coarse mode, 50–90% of the excess sulfate in both hemispheres was found to be of biogenic origin. In the fine mode a different picture was obtained. In the Northern Hemisphere the contribution of biogenic sulfur was found to be less than 35% of the excess sulfur even in relatively clean air masses. On the other hand, in the Southern Hemisphere the participation of biogenic sulfur was about 60% of the excess sulfur in purely marine air. The contribution of continental sulfur to the fine fraction in the Southern Hemisphere was up to 40±25% even under pure oceanic conditions and far more in the Northern Hemisphere. These results attest to the possible importance of long‐range transport of fine sulfate particles or SO2, possibly through the free troposphere, or the importance of anthropogenic emissions due to shipping.