Using an innovative arrangement of instruments to obtain a comprehensive set of properties, we present a description of the submicron aerosol properties for two distinct regions. During the 2009 SHARP/SOOT campaign in Houston, TX, the average effective density was 1.54 +- 0.07 g cm^-3, consistent with a population comprised largely of sulfates and organics Even in low concentrations (0.31 +- 0.22 ug m^-3), black carbon concentration has a significant impact on the overall density and optical properties. Under prevailing northerly winds, the average black carbon concentration increases from 0.26 +- 0.18 ug m^-3 to 0.60 +- 0.21 ug m^-3. Throughout the campaign, aerosols are often internally mixed, with one peak in the effective density distribution located at 1.55 +- 0.07 g cm^-3. In addition, we conclude that in this region the meteorology has a discernible impact on the concentration and properties of aerosols. After a frontal passage, there is a significant shift in the size distribution as the concentration of <100 nm particles increase and the average effective density decreases to 1.43 +- 0.08 g cm^-3. In Tijuana, Mexico, the submicron aerosols are heavily influenced by vehicle emissions. We observe an average single scattering albedo of 0.75. This average SSA is lower than observed in many US urban environments, and indicates a high concentration of black carbon. The average black carbon concentration is 2.71 +- 2.65 g cm^-3. The aerosol size distributions reveal a high concentration of small particles (< 100 nm) during the day, which are frequently associated with vehicle emissions. Overall, 46 and 81 nm particles are hydrophobic, have an average effective near 1.30 g cm^-3, a higher volatile growth factors than the larger particles, and exhibit a distinct diurnal cycle, which, on average, ranges between 0.80 during the afternoon and 1.70 g cm^-3 overnight. 46 and 81 nm distributions indicate a uniform aerosol composition. 151 and 240 nm aerosols are less cyclical, and the hygroscopicity, volatility, and effect density distributions all exhibit a bimodal distribution, which indicates an external mixture of aerosols. Black carbon and vehicle and industrial organic emissions appear to be the main components of the external mixture.
