Relative errors of derived multi-wavelengths intensive aerosol optical properties using CAPS_SSA, Nephelometer and TAP measurements
Authors: Weber Patrick , Petzold Andreas , Bischof Oliver Felix , Fischer Benedikt , Berg Marcel , Freedman Andrew , Onasch Timothy , Bundke Ulrich 
Published: September 2021

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.