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@ARTICLE{Weber:903141,
      author       = {Weber, Patrick and Petzold, Andreas and Bischof, Oliver
                      Felix and Fischer, Benedikt and Berg, Marcel and Freedman,
                      Andrew and Onasch, Timothy and Bundke, Ulrich},
      title        = {{R}elative errors of derived multi-wavelengths intensive
                      aerosol optical properties using ${CAPS}_{SSA},$
                      {N}ephelometer and {TAP} measurements},
      journal      = {Atmospheric measurement techniques discussions},
      issn         = {1867-8610},
      address      = {Katlenburg-Lindau},
      publisher    = {Copernicus},
      reportid     = {FZJ-2021-04865},
      year         = {2021},
      abstract     = {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.},
      cin          = {IEK-8},
      ddc          = {550},
      cid          = {I:(DE-Juel1)IEK-8-20101013},
      pnm          = {2111 - Air Quality (POF4-211)},
      pid          = {G:(DE-HGF)POF4-2111},
      typ          = {PUB:(DE-HGF)25},
      doi          = {10.5194/amt-2021-284},
      url          = {https://juser.fz-juelich.de/record/903141},
}