The Aerosols99 cruise crossed the Atlantic Ocean from Norfolk, Virginia, to Cape Town, South Africa, during January and February of 1999. On the basis of back trajectories, aerosol number concentrations and size distributions, and trace gas concentrations, seven air mass regions were encountered. These included North America, Northern Hemisphere (NH) marine, African dust, a mixture of dust and biomass burning from Africa, biomass burning from Africa, Southern Hemisphere (SH) marine tropics, and SH marine temperate. Simultaneous measurements of aerosol chemical composition, number size distribution, scattering and absorption coefficients, vertical profiles, and optical depth allowed for a thorough characterization of the aerosol. Presented here are the concentrations and mass fractions of the aerosol chemical components that were dominant in each region and the aerosol scattering and absorption coefficients, single scattering albedos, ¿ngstr¿m exponents, and optical depths measured in each region. Also presented is the percent of the extinction measured at the surface due to each chemical component and mass extinction efficiencies of the individual aerosol components estimated from Mie calculations and a multiple linear regression. All results are reported at the measurement relative humidity of 55¿5%. Non-sea-salt (nss) SO4= aerosol was a significant contributor to the submicron mass concentration in all air mass regions (mean mass fractions ranged from 20 to 67%). It made the largest contribution to submicron extinction in the North America region (45¿30%, mean and 1&sgr; standard deviation). Sea-salt mean submicron mass fractions ranged from 9 to 49% with the lowest value in the biomass burning region and highest values in the NH marine and dust regions. Its contribution to submicron extinction ranged from a mean of 29 to 66%. Sea-salt mean supermicron mass fractions ranged from 52 to 98% with the highest values in the marine regions. Its contribution to supermicron extinction ranged from 60 to 98%. Mean submicron and supermicron mass fractions of dust in the dust region were 22¿3.3% (mean and 95% uncertainty) and 26¿3.9%, respectively. Corresponding submicron and supermicron extinction contributions were 24¿7.5 and 18¿4.0%, respectively. Submicron mass fractions of particulate organic matter (POM) ranged from below detection limits in the dust region to 18¿11% in the biomass burning region. Contributions to submicron extinction ranged from below detection limits to 24% in the North America region. In the biomass burning region the black carbon mean submicron mass fraction was 6.7¿1.3% with a contribution of 6.4¿2.7% to the submicron extinction. Extinction fractions of each component for particles with aerodynamic diameters less than 10 μm also are reported in the paper. Sea salt dominated the extinction measured at the surface due to sub-10 μm aerosol for all air mass regions, even those influenced by continental sources. The fraction of the measured column aerosol optical depth due to aerosol within the boundary layer was estimated for the NH marine, dust, biomass burning, and SH marine tropics regions. Mean values ranged from 35¿15% for the biomass burning region to 95¿46% for the NH marine region. ¿ 2001 American Geophysical Union