Aerosol optical properties were measured in situ from a research aircraft during three recent field experiments in the central and eastern parts of North America as well as over areas of the western Atlantic Ocean. Regional and vertical variability of the aerosol properties for boundary layer and free tropospheric air were determined. In general, the differences between distributions of aerosol properties measured at low (planetary boundary layer) and high (free troposphere) altitudes were small but statistically significant at the 3% level or better. However, most of the aerosol optical thickness of the layers studied (~5 km down to ~100 m) was encountered in the lowest 1 km of each layer. As a result, the near-surface measurements of aerosol optical properties adequately represented the portion of the lower column that dominates the radiative effects. The estimated error encountered by using near-surface aerosol measurements to calculate the layer forcing was typically <10%. ¿ 1999 American Geophysical Union