Aerosol optical properties in the southeastern United States were measured at two research sites in close horizontal proximity but at different altitudes at Black Mountain (35.66 ¿N, 82.38 ¿W, 951 m msl) and Mount Gibbes (35.78 ¿N, 82.29 ¿W, 2006 m msl) to estimate the direct radiative forcing in the lowest 1 km layer of the troposphere during the summer of 1998. Measurements of light scattering and light absorption at ambient relative humidity (RH) are categorized by air mass type (polluted continental, marine with some continental influence, continental) according to 48-hour back-trajectory analysis. At a wavelength of 530 nm the average total scattering coefficient (&sgr;sp) measured at the valley site was 1.46¿10-4 m-1 for polluted continental air masses, 7.25¿10-5 m-1 for marine air masses, and 8.36¿10-5 m-1 for continental air masses. The ratio of &sgr;sp at the mountain site to &sgr;sp at the valley site was 0.64, 0.58, and 0.45 for polluted continental, marine, and continental air masses, respectively. The hygroscopic growth factor (&sgr;sp(RH=80%)/&sgr;sp(RH=30%)) was calculated to be almost a constant value of 1.60¿0.01 for polluted continental, marine, and continental air masses. As the RH increased from 30% to 80%, the backscatter fraction decreased by 23%. On the basis of these measurements, direct radiative climate forcing (ΔFR) by aerosols in the lowest 1 km layer of the troposphere was estimated. The patterns of ΔFR for various values of RH were similar for the three air masses, but the magnitudes of ΔFR(RH) were larger for polluted continental air masses than for marine and continental air masses by a factor of about 2 due to higher sulfate concentration in polluted continental air masses. The average value of ΔFR(RH=80%)/ΔFR(RH=30%) was calculated to be almost a constant value of 1.45¿0.01 for all three types of air masses. This implies little dependence of the forcing ratio on the airmass type. The averaged ΔFR for all the observed ambient RHs, in the lowest 1 km layer during the 3-month summer period, was -2.95 W m-2 (the negative forcing of -3.24 W m-2 by aerosol scattering plus the positive forcing of +0.30 W m-2 by aerosol absorption) for polluted continental air masses, -1.43 W m-2 (-1.55 plus +0.12) for marine air masses, and -1.50 W m-2 (-1.63 plus +0.14) for continental air masses. The ΔFR for polluted continental air masses was approximately twice that of marine and continental air masses. These forcing estimates are calculated from continuous in situ measurements of scattering and absorption by aerosols without assumptions for Mie calculations and global mean column burden of sulfates and black carbon (in g m-2) used in most of the model computations. ¿ 2001 American Geophysical Union