The Pacific Exploratory Mission Tropics (PEMT) A (1996) and B (1999) field campaigns occurred over a large area of the Pacific Basin and revealed the presence of “rivers” of continental outflow propagating into the remote marine atmosphere that were also supported by remote sensing and modeling efforts. Airborne measurements of both the coarse and fine mode aerosol during these campaigns provided assessment of the spatial variability in aerosol parameters (including optical properties and degree of internal versus external mixing) in continental plumes encountered over the Pacific Ocean. Large perturbations to the “pristine” marine atmosphere were observed. Most plumes were encountered in the Southern Hemisphere during PEMT A, while the opposite was observed during PEMT B. A variety of anthropogenic and natural sources for these continental plumes are suggested by the data, including biomass burning, urban/industrial emissions, and in the case of Asian outflow, dust storms. Aerosol size distributions (particularly for the refractory component) varied from one plume to another and most combustion-derived aerosol appeared to be an internal mix of a refractory soot-like constituent in a volatile matrix. Within the sampled plumes, size-resolved volatility suggested that this volatile matrix was relatively well neutralized, implying the presence of ammonia in the particle phase. The radiatively important single scatter albedo (ω) obtained from measured “dry” scattering and absorption coefficients ranged from approximately 0.88 (pollution with no coarse particles) to 0.94 (pollution and dust) in the free troposphere (FT) to 0.98 (pollution and sea salt) within the marine boundary layer (MBL). Vertical profiles often revealed more concentrated plumes aloft, typically situated in dry air with ambient relative humidity (RH) <40%, and much lower values of ω than in the underlying MBL.