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    A global study of hygroscopicity-driven light-scattering enhancement in the context of other in situ aerosol optical properties
    (Katlenburg-Lindau : European Geosciences Union, 2021) 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
    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, 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 D 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., w0 and scattering Ångström exponent, asp) to parameterize f (RH). The majority of sites (75 %) showed a consistent trend with w0 (higher f (RH D 85 %) for higher w0), while no clear pattern was observed between f (RH D 85 %) and asp. This suggests that aerosol w0 is more promising than asp as a surrogate for the scattering enhancement factor, although neither parameter is ideal. Nonetheless, the qualitative relationship observed between w0 and f (RH) could serve as a constraint on global model simulations. © 2021 The Author(s).
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    Optical properties of coated black carbon aggregates: numerical simulations, radiative forcing estimates, and size-resolved parameterization scheme
    (Katlenburg-Lindau : European Geosciences Union, 2021) Romshoo, Baseerat; Müller, Thomas; Pfeifer, Sascha; Saturno, Jorge; Nowak, Andreas; Ciupek, Krzysztof; Quincey, Paul; Wiedensohler, Alfred
    The formation of black carbon fractal aggregates (BCFAs) from combustion and subsequent ageing involves several stages resulting in modifications of particle size, morphology, and composition over time. To understand and quantify how each of these modifications influences the BC radiative forcing, the optical properties of BCFAs are modelled. Owing to the high computational time involved in numerical modelling, there are some gaps in terms of data coverage and knowledge regarding how optical properties of coated BCFAs vary over the range of different factors (size, shape, and composition). This investigation bridged those gaps by following a state-of-the-art description scheme of BCFAs based on morphology, composition, and wavelength. The BCFA optical properties were investigated as a function of the radius of the primary particle (ao), fractal dimension (Df), fraction of organics (forganics), wavelength (λ), and mobility diameter (Dmob). The optical properties are calculated using the multiple-sphere T-matrix (MSTM) method. For the first time, the modelled optical properties of BC are expressed in terms of mobility diameter (Dmob), making the results more relevant and relatable for ambient and laboratory BC studies. Amongst size, morphology, and composition, all the optical properties showed the highest variability with changing size. The cross sections varied from 0.0001 to 0.1 μm2 for BCFA Dmob ranging from 24 to 810nm. It has been shown that MACBC and single-scattering albedo (SSA) are sensitive to morphology, especially for larger particles with Dmobg > 100 nm. Therefore, while using the simplified core-shell representation of BC in global models, the influence of morphology on radiative forcing estimations might not be adequately considered. The Ångström absorption exponent (AAE) varied from 1.06 up to 3.6 and increased with the fraction of organics (forganics). Measurement results of AAE ≫1 are often misinterpreted as biomass burning aerosol, it was observed that the AAE of purely black carbon particles can be ≫1 in the case of larger BC particles. The values of the absorption enhancement factor (Eλ) via coating were found to be between 1.01 and 3.28 in the visible spectrum. The Eλ was derived from Mie calculations for coated volume equivalent spheres and from MSTM for coated BCFAs. Mie-calculated enhancement factors were found to be larger by a factor of 1.1 to 1.5 than their corresponding values calculated from the MSTM method. It is shown that radiative forcings are highly sensitive to modifications in morphology and composition. The black carbon radiative forcing FTOA (Wgm-2) decreases up to 61% as the BCFA becomes more compact, indicating that global model calculations should account for changes in morphology. A decrease of more than 50% in FTOA was observed as the organic content of the particle increased up to 90%. The changes in the ageing factors (composition and morphology) in tandem result in an overall decrease in the FTOA. A parameterization scheme for optical properties of BC fractal aggregates was developed, which is applicable for modelling, ambient, and laboratory-based BC studies. The parameterization scheme for the cross sections (extinction, absorption, and scattering), single-scattering albedo (SSA), and asymmetry parameter (g) of pure and coated BCFAs as a function of Dmob were derived from tabulated results of the MSTM method. Spanning an extensive parameter space, the developed parameterization scheme showed promisingly high accuracy up to 98% for the cross sections, 97% for single-scattering albedos (SSAs), and 82% for the asymmetry parameter (g). © 2021 The Author(s).