The production of sulfuric acid vapor and new particle formation are studied over the northwest Atlantic for two synoptic cases: a wet case in April with a large amount of cloud and precipitation in the simulation domain and a relatively dry one in March. The Fifth-Generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5) is utilized coupled on-line with a simple sulfur chemistry mechanism. Our numerical simulations show that there are three regions where significant binary homogeneous nucleation occurs. One region is in convective outflow where high anthropogenic SO2 concentration transported upward and low air temperature in the free troposphere (FT) favor the binary homogeneous nucleation of sulfuric acid gas and water vapor. The second region is near the top of marine boundary layer (MBL) clouds. The third region is in the clear FT at 500--600 hPa. In the three regions, significant nucleation events are all connected with relative humidity (RH) higher than 60%. For the northwest Atlantic we found that homogeneous nucleation generally occurs from the late morning through the afternoon on the northwest side of our simulation domain where anthropogenic SO2 concentration is high enough to produce sufficient sulfuric acid gas through the oxidation by OH radical. Significant nucleation does not necessarily happen in regions with very low preexisting particle surface area in our simulated cases. Our simulated total aerosol number concentrations in regions with significant new particle formation are generally consistent with field measurements. Nucleation rates are enhanced by NH3 especially in the MBL. Sensitivity tests show that a grid of 10 km (and time step of 30 s) yielded significantly higher nucleation as compared with a grid size of 30 km (and time step of 90 s). The latter grid may be too coarse to resolve the concentration gradients, which likely strongly impact the highly nonlinear homogeneous nucleation system. ¿ 2001 American Geophysical Union