Aircraft-based measurements of sulfur dioxide, acetone, carbon dioxide, and condensation nuclei (CN) were made over the north-eastern Atlantic at upper tropospheric altitudes, around 9000 m. On October 14, 1993, strong SO2- and acetone-pollution (both up to 3 ppbv) were observed, which were accompanied by a CO2-enhancement of up to 6 ppmv, and large CN-concentrations of up to about 1500 cm-3 (for radii≥6 nm). CN, excess CO2, and to a lesser degree also acetone, were positively correlated with SO2. Air mass trajectory analyses indicate, that most of the air masses encountered by our aircraft originated from the polluted planetary boundary layer of the North-Eastern U.S. approximately 4--5 days prior to our measurements, and that polluted boundary layer air experienced fast vertical transport to the upper troposphere as well as horizontal transport across the Atlantic. From our data we conclude, that in the polluted air mass around 9000 m altitude HOx-formation, photochemical SO2-conversion to gaseous H2SO4, and eventually also CN-formation by homogeneous bimolecular (H2SO4-H2O) nucleation may have taken place with enhanced efficiency.¿ 1997 American Geophysical Union