Preprints
https://doi.org/10.5194/amt-2021-369
https://doi.org/10.5194/amt-2021-369

  21 Dec 2021

21 Dec 2021

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

Impact of particle size, refractive index, and shape on the determination of the particle scattering coefficient – an optical closure study evaluating different nephelometer angular truncation and illumination corrections

Marilena Teri1, Thomas Müller2, Josef Gasteiger1, Sara Valentini3,1, Helmuth Horvath1, Roberta Vecchi3, Paulus Bauer1, Adrian Walser1, and Bernadett Weinzierl1 Marilena Teri et al.
  • 1University of Vienna, Faculty of Physics, Aerosol Physics and Environmental Physics, Vienna, 1090, Austria
  • 2Leibniz-Institute for Tropospheric Research, Tropospheric Aerosols, Leipzig, 04318, Germany
  • 3Dipartimento di Fisica “A. Pontremoli”, Università degli Studi di Milano & INFN-Milan, 20133 Milano, Italy

Abstract. Aerosol particles in the atmosphere interact with solar radiation through scattering and absorption. Accurate aerosol optical properties are needed to reduce the uncertainties of climate predictions. The aerosol optical properties can be obtained via optical modeling based on the measured particle size distribution. This approach requires knowledge or assumptions on the particle refractive index and shape. Meanwhile, integrating nephelometry provides information on the aerosol scattering properties directly. However, their measurements are affected by angular non-idealities, and their data need to be corrected for angular truncation and illumination to provide the particle scattering coefficient. We performed an extensive closure study, including a laboratory and a simulated experiment, aiming to compare different nephelometer angular truncation and illumination corrections (further referred to as "angular corrections"). We focused on coarse mode irregularly shaped aerosols, such as mineral dust, a worldwide abundant aerosol component. The angular correction of irregular particles is found to be only ~2 % higher than the angular correction of volume equivalent spheres. If the angular correction is calculated with Mie theory, the particle size distribution is needed. Our calculations show that if the particle size distribution is retrieved from optical particle spectrometer measurements and the irregular shape effect is not considered, the angular correction can be overestimated by about 5 % and up to 22 %. For mineral dust, the traditional angular correction based on the wavelength dependency of the scattering coefficient seems more accurate. We propose a guideline to establish the most appropriate angular correction depending on the aerosol type and the investigated size range.

Marilena Teri et al.

Status: open (until 25 Jan 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2021-369', Anonymous Referee #1, 10 Jan 2022 reply
  • RC2: 'Comment on amt-2021-369', Anonymous Referee #2, 13 Jan 2022 reply

Marilena Teri et al.

Marilena Teri et al.

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Short summary
We performed an extensive closure study, including a laboratory and a simulated experiment, to evaluate and compare different angular corrections for the Aurora 4000 polar nephelometer, focusing on mineral dust aerosol. We describe the impact of particle size, refractive index and shape on the determination of the particle scattering coefficient, and we propose a guideline to establish the most appropriate angular correction depending on the aerosol type and the investigated size range.