In situ measurements of aerosol optical and hygroscopic properties were made on board the National Oceanic and Atmospheric Administration R/V Ronald H. Brown during the Texas Air Quality Study–Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS-GoMACCS). The aerosol light extinction coefficient (σep) was measured at 355, 532, and 1064 nm at 25%, 60%, and 85% relative humidity (RH) for both sub-1- and sub-10-μm-diameter particles with a cavity ring–down aerosol extinction spectrometer. The 532-nm σep was coupled with the 532-nm light absorption coefficient (σap) measured with a photoacoustic absorption spectrometer to calculate the aerosol single scattering albedo (ω) with absolute uncertainty <0.01. The σep dependence on RH was expressed in terms of gamma (γ). The sampled aerosols covered a broad spectrum of γ and ω values; aerosols from traffic emissions were hydrophobic and highly light-absorbing with γ ∼ 0.4 and ω ∼ 0.6, whereas the regional aerosols exhibited variable values of both γ and ω. Aerosols with the highest sulfate content also had the highest γ and ω values (>0.65 and >0.9, respectively). The optical data were used to estimate local, top of atmosphere aerosol-induced climate forcing (ΔFR). The ΔFR calculations were performed using both ω values measured at 25% RH and ω values converted to ambient RH. The calculated ambient ΔFR ranged from −7 to −40 W/m2 with absolute uncertainty between 0.7 and 2.5 W/m2. The results show that including aerosol hygroscopic properties in climate calculations is critical for improving estimates of aerosol forcing on climate.