Preprints
https://doi.org/10.5194/amt-2021-284
https://doi.org/10.5194/amt-2021-284

  27 Sep 2021

27 Sep 2021

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

Relative errors of derived multi-wavelengths intensive aerosol optical properties using CAPS_SSA, Nephelometer and TAP measurements

Patrick Weber1, Andreas Petzold1, Oliver Felix Bischof1,2, Benedikt Fischer1, Marcel Berg1, Andrew Freedman3, Timothy Onasch3, and Ulrich Bundke1 Patrick Weber et al.
  • 1Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research 8 – Troposphere (IEK-8), Jülich, Germany
  • 2TSI GmbH, Particle Instruments, Aachen, Germany
  • 3Aerodyne Research Inc., Billerica, MA 01821, USA

Abstract. Aerosol intensive optical properties like the Ångström exponents for aerosol light extinction, scattering and absorption, or the single-scattering albedo are indicators for aerosol size distributions, chemical composition and radiative behaviour and contain also source information. The observation of these parameters requires the measurement of aerosol optical properties at multiple wavelengths which usually implies the use of several instruments. Our study aims to quantify the uncertainties of the determination of multiple-wavelengths intensive properties by an optical closure approach, using different test aerosols. In our laboratory study, we measured the full set of aerosol optical properties for a range of light-absorbing aerosols with different properties, mixed externally with ammonium sulphate to generate aerosols of controlled single-scattering albedo. The investigated aerosol types were: fresh combustion soot emitted by an inverted flame soot generator (SOOT, fractal aggregates), Aquadag (AQ, spherical shape), Cabot industrial soot (BC, compact clusters), and an acrylic paint (Magic Black, MB). One focus was on the validity of the Differential Method (DM: absorption = extinction minus scattering) for the determination of Ångström exponents for different particle loads and mixtures of light-absorbing aerosol with ammonium sulphate, in comparison to data obtained from single instruments. The instruments used in this study were two CAPS PMssa (Cavity Attenuated Phase Shift Single Scattering Albedo, λ = 450, 630 nm) for light extinction and scattering coefficients, one Integrating Nephelometer (λ = 450, 550, 700 nm) for light scattering coefficient and one Tricolour Absorption Photometer (TAP, λ = 467, 528, 652 nm) for filter-based light absorption coefficient measurement. Our key finding is that the coefficients of light absorption σap, scattering σsp and extinction σep from the Differential Method agree with data from single reference instruments, and the slopes of regression lines equal unity within the precision error. We found, however, that the precision error for the DM suppresses 100 % for σap values lower than 10–20 Mm−1 for atmospheric relevant single scattering albedo. This increasing uncertainty with decreasing σap yields an absorption Ångström exponent (AAE) that is too uncertain for measurements in the range of atmospheric aerosol loadings. We recommend using DM only for measuring AAE values for σap > 50 Mm−1. Ångström exponents for scattering and extinction are reliable for extinction coefficients from 20 up to 1000 Mm−1 and stay within 10 % deviation from reference instruments, regardless of the chosen method. Single-scattering albedo (SSA) values for 450 nm and 630 nm wavelengths agree with values from the reference method σsp (NEPH)/σep (CAPS PMSSA) with less than 10 % uncertainty for all instrument combinations and sampled aerosol types which fulfil the proposed goal for measurement uncertainty of 10 % proposed by Laj et al., 2020 for GCOS (Global Climate Observing System) applications.

Patrick Weber et al.

Status: open (until 14 Nov 2021)

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  • RC1: 'Comment on amt-2021-284', Anonymous Referee #1, 16 Oct 2021 reply

Patrick Weber et al.

Patrick Weber et al.

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Short summary
In our laboratory closure study, we measured the full set of aerosol optical properties for different light absorbing aerosols using a set of instruments. Our key finding is that the extensive and intensive aerosol optical properties obtained, agree with data from reference instruments within the error margins except for the absorption Angström exponent. The reported uncertainty of the single scattering albedo fulfil the defined goals for Global Climate Observing System applications of 10 %.