Vertical profiles of aerosol and gas phase species were measured on flights near Hawaii on April 9 and 10, 1999, during NASA's Pacific Exploratory Mission (PEM) Tropics B program. These measurements characterized aerosol microphysics, inferred chemistry, optical properties, and gases in several extensive dust and pollution plumes, also detected by satellites, which had 10,000-km trajectories back to sources in Asia. Size-resolved measurements indicative of aerosol sulfate, black carbon, dust, light scattering, and absorption allowed determination of their concentrations and contributions to column aerosol optical depth. A new Chemical Transport Model (CTM) that includes aerosol, meteorological fields, dynamics, gas and particle source emissions, a chemistry component (MATCH), and assimilated satellite data was used to predict aerosol and gas concentrations and the aerosol optical effects along our flight path. Flight measurements confirmed the “river-like” plume structures predicted by the CTM and showed close agreement with the predicted contributions of dust and sulfate to aerosol concentrations and optical properties for this global-scale transport path. Consistency between satellite, model and in situ assessment of aerosol optical depth was found, with noted exceptions, within ∼25%. Both observations and model results confirmed that this aerosol was being entrained into the marine boundary layer between Hawaii and California where it can be expected to modify the type and concentration of cloud condensation nuclei in ways that may alter properties of low-level clouds. These observations document the significance and complexity of long-range aerosol transport and highlight the potential of emerging CTM models to extend observational data and address related issues on global scales.