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

Sulfur isotope measurements were performed on size-segregated aerosols collected during the Albatross oceanographic campaign from 61 degrees N to 35 degrees 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 S-34 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. Tn the fine mode a different picture was obtained. In the Not-them 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.