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

Journal Article · · Atmospheric Chemistry and Physics (Online)
ORCiD logo [1];  [2]; ORCiD logo [2]; ORCiD logo [1];  [3];  [4];  [5]; ORCiD logo [6]; ORCiD logo [7]; ORCiD logo [8];  [9];  [10]; ORCiD logo [11]
  1. Univ. of Granada (Spain)
  2. Stockholm Univ. (Sweden)
  3. Paul Scherrer Inst. (PSI), Villigen (Switzerland)
  4. Univ. of Colorado, Boulder, CO (United States)
  5. Appalachian State Univ., Boone, NC (United States)
  6. Paul Scherrer Inst. (PSI), Villigen (Switzerland); Univ. of Applied Sciences, Windisch (Switzerland)
  7. Netherlands Organisation for Applied Scientific Research (TNO), Utrecht (Netherlands)
  8. Univ. of Helsinki (Finland)
  9. National Univ. of Ireland, Galway (Ireland)
  10. Leibniz Inst. for Tropospheric Research (ITR), Leipzig (Germany)
  11. Univ. of Colorado, Boulder, CO (United States); National Oceanic and Atmospheric Administration (NOAA), Boulder, CO (United States)
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The scattering and backscattering enhancement factors (ƒ(RH) and ƒb(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 ƒ(RH) and ƒb(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 ƒ(RH) and ƒb(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 ƒ(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 ƒb(RH) is consistently lower than ƒ(RH) at all sites, with the difference between ƒ(RH) and ƒb(RH) increasing for aerosol with higher ƒ(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, ƒ(RH) is observed to increase with increasing wavelength, except at sites with a known dust influence where the spectral dependence of ƒ(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 ƒ(RH). The majority of sites (75 %) showed a consistent trend with ω0 (higher ƒ(RH = 85 %) for higher ω0), while no clear pattern was observed between ƒ(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 ƒ(RH) could serve as a constraint on global model simulations.

Research Organization:
Univ. of Colorado, Boulder, CO (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Biological and Environmental Research (BER). Earth and Environmental Systems Science Division; European Research Council (ERC); Spanish Ministry of Economy and Competitiveness
Grant/Contract Number:
SC0016541; 654109; 739530; RTI2018-101154-A-I00
OSTI ID:
1818907
Journal Information:
Atmospheric Chemistry and Physics (Online), Vol. 21, Issue 17; ISSN 1680-7324
Publisher:
Copernicus Publications, EGUCopyright Statement
Country of Publication:
United States
Language:
English

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54 ENVIRONMENTAL SCIENCES
aerosol hygroscopicity
aerosol light scattering coefficient
model evaluation