We have investigated the potential of upper tropospheric cloud condensation nuclei (CCN) formation from sulfur dioxide. This process involves three steps including SO2 conversion to gaseous sulfuric acid (GSA), homogeneous bimolecular nucleation of GSA and water vapor leading to new aerosol particles, and new aerosol particle growth via mutual coagulation and GSA-H2O condensation. Our investigation includes both measurements of SO2 and GSA as well as model calculations of SO2 conversion to new aerosol particles in the presence of preexisting aerosols which act as a competing sink for GSA. The model reproduces the measured abundance ratio GSA-SO2 reasonably well. Furthermore, the model predicts that efficient upper tropospheric CCN formation from SO2 may, in fact, occur under certain plausible conditions. These conditions are explored utilizing the factor FCCN, defined as a ratio of the source and sink terms of gaseous sulfuric acid. In particular, a threshold FCCN value for efficient CCN formation is calculated with the model. ¿ 2000 American Geophysical Union