In a numerical study, influences of relative humidity (RH) on aerosol optical properties and direct radiative forcing in the surface boundary layer at Xinken (XK) in Pearl River Delta of China are investigated based on the observed particle hygroscopic growth (fg(Dp,RH)) and the relevant aerosol physical and chemical properties. The model is validated by comparing the simulated ambient extinction coefficients with those measured by a Raman LIDAR. At 550 nm, the scattering coefficient increases with a factor of 1.54 and 2.31 at a RH increase from 30% to 80% and 90%, respectively. This ratio is mainly controlled by fg, and generally increases with increasing wavelength and decreases with increasing particle effective diameter. From RH 30% to 90%, the average single scattering albedo (ω0) changes from 0.77 to 0.88, 0.91 to 0.97, and 0.86 to 0.94 for internal, respectively, external mixtures, and the retrieved mixing state of the elemental carbon (EC) at XK. The assumption that the absorption coefficient does not change upon humidification alone can introduce an overestimation of ω0 of 0.02 (∼5%) at RH 90% for an internal mixture. However, accounting for the EC mixing state at XK, this error is only about 0.3%. The mean upscatter fraction () is evaluated as 0.233 at RH 30% with a 30% decrease to 0.191 at RH 90%. The estimation of aerosol direct radiative forcing (ΔFR) in the surface boundary layer at XK strongly depends on RH and the EC mixing state. The critical ω0 at XK is estimated to be about 0.77–0.80 at 550 nm. Assuming an internal or coated mixture of EC, no pronounced ΔFR is observed when RH<60%, whereas a cooling effect arises while RH>60%. Under the actual EC mixing conditions at XK, the effect of ΔFR is cooling. Over 40% of this cooling effect is contributed by water at RH 80% and ΔFR at RH 90% exceeds that at RH 30% by about a factor of 2.7.