A global study of hygroscopicity-driven light-scattering enhancement in the context of other in situ aerosol optical properties
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Authors
Titos, Gloria
Burgos, María A.
Zieger, Paul
Alados-Arboledas, Lucas
Baltensperger, Urs
Jefferson, Anne
Sherman, James
Henzing, Bas
Luoma, Krista
Author (Corporation)
Publication date
2021
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Type
01A - Journal article
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Parent work
Atmospheric Chemistry and Physics
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DOI of the original publication
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Series number
Volume
21
Issue / Number
17
Pages / Duration
13031-13050
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Publisher / Publishing institution
Copernicus
Place of publication / Event location
Göttingen
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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.
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620 - Ingenieurwissenschaften und Maschinenbau
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1680-7324
1680-7316
1680-7316
Language
English
Created during FHNW affiliation
Yes
Strategic action fields FHNW
Publication status
Published
Review
Peer review of the complete publication
Open access category
Gold
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Citation
TITOS, Gloria, María A. BURGOS, Paul ZIEGER, Lucas ALADOS-ARBOLEDAS, Urs BALTENSPERGER, Anne JEFFERSON, James SHERMAN, Ernest WEINGARTNER, Bas HENZING, Krista LUOMA, Colin O’DOWD, Alfred WIEDENSOHLER und Elisabeth ANDREWS, 2021. A global study of hygroscopicity-driven light-scattering enhancement in the context of other in situ aerosol optical properties. Atmospheric Chemistry and Physics. 2021. Bd. 21, Nr. 17, S. 13031–13050. DOI 10.5194/acp-21-13031-2021. Verfügbar unter: https://doi.org/10.26041/fhnw-7634