A global study of hygroscopicity-driven light scattering enhancement in the context of other in-situ aerosol optical properties

Abstract. The scattering and backscattering enhancement factors (f(RH) and f_b(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 f(RH) and f_b(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 f(RH) and f_b(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 f(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 f_b(RH) is consistently lower tan f(RH) at all sites, with the difference between f(RH) and f_b(RH) increasing for aerosol with higher f(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 PM₁ than PM₁₀ 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, f(RH) is observed to increase with increasing wavelength, except at sites with a known dust influence where the spectral dependence of f(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, ω₀, and backscattered fraction, b) is evaluated. The single scattering albedo generally increases with RH while b 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., ω₀ and scattering Angström exponent, α_sp) to parameterize f(RH). The majority of sites (75 %) showed a consistent trend with ω₀ (higher f (RH = 85 %) for higher ω₀), while no clear pattern was observed between f (RH = 85 %) and α_sp. This suggests that aerosol ω₀ is more promising tan α_sp as a surrogate for the scattering enhancement factor, although neither parameter is ideal. Nonetheless, the qualitative relationship observed between ω₀ and f(RH) could serve as a constraint on global model simulations.