<p>The scattering and backscattering enhancement factors (<i>f</i>(RH) and <i>f</i><sub><i>b</i></sub>(RH)) describe how aerosol particle light scattering and backscattering, respectively, change with relative humidity (RH). They are important parameters in estimating direct aerosol radiative forcing (DARF). In this study we use a dataset presented in Burgos et al. (2019) that compiles <i>f</i>(RH) and <i>f</i><sub><i>b</i></sub>(RH) measurements at three wavelengths (i.e. 450, 550 and 700 nm) performed with tandem nephelometer systems at multiple sites around the world. We present an overview of <i>f</i>(RH) and <i>f</i><sub><i>b</i></sub>(RH) based on both long-term and campaign observations from 23 sites representing a range of aerosol types. The scattering enhancement shows a strong variability from site to site, with no clear pattern with respect to total scattering coefficient. In general, higher <i>f</i>(RH) is observed at Arctic and marine sites while lower values are found at urban and desert sites, although a consistent pattern as a function of site type is not observed. The backscattering enhancement <i>f</i><sub><i>b</i></sub>(RH) is consistently lower tan <i>f</i>(RH) at all sites, with the difference between <i>f</i>(RH) and <i>f</i><sub><i>b</i></sub>(RH) increasing for aerosol with higher <i>f</i>(RH). This is consistent with Mie theory which predicts higher enhancement of the light-scattering in the forward than in the backward direction as the particle takes up water. Our results show that the scattering enhancement is higher for PM<sub>1</sub> than PM<sub>10</sub> at most sites, which is also supported by theory due to the change in scattering efficiency with the size parameter that relates particle size and wavelength of incident light. At marine-influenced sites this difference is enhanced when coarse particles (likely sea salt) predominate. For most sites, <i>f</i>(RH) is observed to increase with increasing wavelength, except at sites with a known dust influence where the spectral dependence of <i>f</i>(RH) is found to be low or even exhibit the opposite pattern. The impact of RH on aerosol properties used to calculate radiative forcing (e.g., single scattering albedo, ω<sub>0</sub>, and backscattered fraction, <i>b</i>) is evaluated. The single scattering albedo generally increases with RH while <i>b</i> decreases. The net effect of aerosol hygroscopicity on radiative forcing efficiency (RFE) is an increase in the absolute forcing effect (negative sign) by a factor of up to 4 at RH = 90 % compared to dry conditions (RH < 40 %). Because of the scarcity of scattering enhancement measurements, an attempt was made to use other, more commonly available aerosol parameters (i.e., ω<sub>0</sub> and scattering Angström exponent, α<sub>sp</sub>) to parameterize <i>f</i>(RH). The majority of sites (75 %) showed a consistent trend with ω<sub>0</sub> (higher <i>f</i> (RH = 85 %) for higher ω<sub>0</sub>), while no clear pattern was observed between <i>f</i> (RH = 85 %) and α<sub>sp</sub>. This suggests that aerosol ω<sub>0</sub> is more promising tan α<sub>sp</sub> as a surrogate for the scattering enhancement factor, although neither parameter is ideal. Nonetheless, the qualitative relationship observed between ω<sub>0</sub> and <i>f</i>(RH) could serve as a constraint on global model simulations.</p>