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

• Main Authors: Titos, Gloria, Burgos, María A., Zieger, Paul, Alados-Arboledas, Lucas, Baltensperger, Urs, Jefferson, Anne, Sherman, James, Weingartner, Ernest, Henzing, Bas, Luoma, Krista, O'Dowd, Colin, Wiedensohler, Alfred, Andrews, Elisabeth
• Format: Article in Journal/Newspaper
• Language: English
• Published: Katlenburg-Lindau : European Geosciences Union 2021
• Subjects: ddc:550; relative humidity (RH); direct aerosol radiative forcing (DARF); aerosol optical properties; hygroscopicity-driven light-scattering enhancement; scattering efficiency; albedo
• Online Access: https://oa.tib.eu/renate/handle/123456789/8159; https://doi.org/10.34657/7200

The scattering and backscattering enhancement factors (f (RH) and fb(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 the dataset presented in Burgos et al. (2019) that compiles f (RH) and fb(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 fb(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 the 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 fb(RH) is consistently lower than f (RH) at all sites, with the difference between f (RH) and fb(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 the 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, w0, ...