23 Dec 2020

23 Dec 2020

Review status: this preprint is currently under review for the journal ACP.

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

Gloria Titos1,2, María A. Burgos3, Paul Zieger3, Lucas Alados-Arboledas1,2, Urs Baltensperger4, Anne Jefferson5, James Sherman6, Ernest Weingartner4,a, Bas Henzing7, Krista Luoma8, Colin O'Dowd9, Alfred Wiedensohler10, and Elisabeth Andrews5,11 Gloria Titos et al.
  • 1Andalusian Institute for Earth System Research, University of Granada, 18006, Granada, Spain
  • 2Department of Applied Physics, University of Granada, 18071, Granada, Spain
  • 3Department of Environmental Science & Bolin Centre for Climate Research, Stockholm University, 11418, Stockholm, Sweden
  • 4Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen, Switzerland
  • 5Cooperative Institute for Research in Environmental Studies, University of Colorado, 80309, Boulder, USA
  • 6Department of Physics and Astronomy, Appalachian State University, Boone, USA
  • 7Netherlands Organisation for Applied Scientific Research (TNO), Princetonlaan 6, 3584 Utrecht, the Netherlands
  • 8Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, P.O. Box 68, 00014 Helsinki, Finland
  • 9School of Physics, Ryan Institute's Centre for Climate and Air Pollution Studies, National University of Ireland Galway, University Road, H91 CF50 Galway, Ireland
  • 10Leibniz Institute for Tropospheric Research, DE-04318 Leipzig, Germany
  • 11Earth System Research Laboratory, National Oceanic and Atmospheric Administration, 80305, Boulder, USA
  • anow at: Institute for Sensing and Electronics, University of Applied Sciences, Windisch, Switzerland

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 a 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 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 tan 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 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 Angström 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 tan 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.

Gloria Titos et al.

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Gloria Titos et al.

Data sets

ime series of aerosol light scattering coefficients and enhancement factors from humidified tandem nephelometers at twenty-six stations between 1998 and 2017 Maria A. Burgos, Elisabeth Andrews, Gloria Titos, Lucas Alados-Arboledas, Urs Baltensperger, Derek Day, Anne Jefferson, Nikos Kalivitis, Nikos Mihalopoulos, James Sherman, Junying Sun, Ernest Weingartner, and Paul Zieger

Gloria Titos et al.


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Short summary
This manuscript investigates the impact of water uptake on the aerosol optical properties, in particular the aerosol light-scattering coefficient. Although in-situ measurements are performed at low relative humidity (typically at RH < 40 %), to address the climatic impact of aerosol particles it is necessary to take into account the effect that water uptake may have on the aerosol optical properties.