Numerical simulation of new particle formation over the northwest Atlanticusing the MM5 mesoscale model coupled with sulfur chemistry

The production of sulfuric acid vapor and new particle formation are studie d 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 an d a relatively "dry" one in March. The Fifth-Generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5) i s utilized coupled on-line with a simple sulfur chemistry mechanism. Our nu merical simulations show that there are three regions where significant bin ary homogeneous nucleation occurs. One region is in convective outflow wher e high anthropogenic SO2 concentration transported upward and low air tempe rature 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 c onnected with relative humidity (RH) higher than 60%. For the northwest Atl antic we found that homogeneous nucleation generally occurs from the late m orning 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 nucleat ion does not necessarily happen in regions with very low preexisting partic le surface area in our simulated cases. Our simulated total aerosol number concentrations in regions with significant new particle formation are gener ally consistent with field measurements. Nucleation rates are enhanced by N H3 especially in the MEL. 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 c oarse to resolve the concentration gradients, which likely strongly impact the highly nonlinear homogeneous nucleation system.