Fast response measurements of particle size distributions, bulk submicron particle composition, and single particle composition were made aboard the NOAA WP-3D research aircraft in the free troposphere over the eastern Pacific Ocean and the western coast of North America. Measurements of gas-phase compounds and meteorological analysis show evidence of long-range transport of layers of aerosol particles from anthropogenic and biomass-burning sources in eastern Asia. Layers of crustal particles with no evident sources were also encountered. Measurements of substantially enhanced particulate sulfate mass and gas-phase H2SO4 encountered during one transport event were interpreted with the aid of a numerical model of particle formation and growth and trajectory analysis. The observed particle size distributions and sulfate mass concentration were consistent with the simulation, indicating that the particles were formed over the mid-Pacific from gas-to-particle conversion following long-range transport of SO2 through a midlatitude cyclonic system. Such cloud systems appear to effectively scavenge most pre-existing particle mass, but can allow transport of gas-phase precursors which substantially alter downstream particle microphysical and chemical properties.