Abstract
Abstract. 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, 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 Ångström exponent, αsp) 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 αsp. This suggests that aerosol ω0 is more promising than αsp 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.
Highlights
Aerosol particles from both natural and anthropogenic sources interact with solar radiation through scattering and absorption
In this paper we have presented an extended overview and analysis of the range and variability of the scattering enhancement factor, f (RH = 85 %), at 23 diverse sites across the globe based on the dataset developed by Burgos et al (2019)
The variability in f (RH = 85 %) observed at each site suggests that simple assumptions about f (RH) based on dominant aerosol type will not capture the actual range observed in this parameter for a given location
Summary
Aerosol particles from both natural and anthropogenic sources interact with solar radiation through scattering and absorption. An early example of this was provided by Sheridan et al (2001), who showed different probability distribution functions of f (RH) for different single-scattering albedo (ω0) and submicron scattering fraction constraints at the Southern Great Plains (SGP), a rural site in the continental US They linked the decrease in ω0 to the presence of smoke aerosol and the increase in particle size to the presence of dust aerosol. We first describe the variability of scattering enhancement at 23 of these sites in order to present a climatological overview of hygroscopicity observations (three of the sites in the Burgos et al, 2019, dataset are not included in this analysis because of their lower time resolution or because colocated measurements of the aerosol absorption coefficient were not available). We combine the hygroscopicity dataset with simultaneous and colocated measurements of dry aerosol optical properties (i.e., dry spectral aerosol light-scattering and absorption coefficients) to investigate the impact of relative humidity on radiative forcing calculations. We extend previous investigations of the viability of using aerosol optical properties (i.e., scattering Ångström exponent and singlescattering albedo) as constraints for f (RH) across sites and aerosol types
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