Large uncertainties remain in understanding the effects of ambient aerosols on climate. As part of the First Aerosol Characterization Experiment (ACE 1, November 15 to December 15, 1995), the total light scattering and backscattering coefficients by particles (&sgr;sp and &sgr;bsp) were measured at Cape Grim, Tasmania. Such measurements were made to characterize aerosol radiative forcing relevant to climate change at a southern hemisphere site. A controlled relative humidity (RH) nephelometry system (humidograph) was used to measure the dependence of &sgr;sp and &sgr;bsp upon controlled RH, upper particle diameter (Dp), and wavelength of light (λ). Parameters derived from the measurements necessary to incorporate aerosols into climate models include the dependence of light scattering on aerosol hygroscopic growth (f(RH)) and the hemispheric backscatter fraction (b), both as functions of wavelength of light. Differences in parameter values during clean marine and non-clean-marine time periods were greatest for &sgr;sp and &sgr;bsp for Dp≤1 μm and were least for f(RH) and b. During clean marine conditions, for Dp≤10 μm, and at λ=550 nm, means and standard deviations of key parameters measured included f(RH equal to 82%)=1.98¿0.08 for &sgr;sp, f(RH equal to 82%)=1.48¿0.09 for &sgr;bsp, b=0.16¿0.01 at RH equal to 22%, and b=0.12¿0.03 at RH equal to 82%. These measurements contribute to characterizing aerosol optical properties at a remote marine site, serve as a basis of comparison to polluted sites, and may provide input to climate models. ¿ 1998 American Geophysical Union<