Supplementary material to "Determination of the multiple-scattering correction factor and its cross-sensitivity to scattering and wavelength dependence for different AE33 Aethalometer filter tapes: A multi-instrumental approach"

Abstract. Accurate measurements of light absorption by aerosolized particles, especially black carbon (BC), are of utter importance since BC represents the second most important climate-warming agent after carbon dioxide (CO2). Reducing the uncertainties related to the absorption measurement techniques will improve the global estimation of BC concentration and the radiative effects of light absorbing aerosols. Currently, one of the most widely used instruments for BC and absorption measurements is the dual-spot aethalometer, AE33, which derives the absorption coefficients of aerosol particles at 7 different wavelengths from the measurements of optical attenuation through a filter where particles are continuously collected. An accurate determination of the absorption coefficient relies on the quantification of non-linear processes related to the collection of sample on the filter. The multiple-scattering correction factor (C(λ)), which depends on the filter tape used and on the optical properties of the collected particles, is the parameter with the greatest uncertainty.An in-depth analysis of the AE33 multiple-scattering correction factor and its wavelength dependence for different filter tapes, i.e. the old most referenced known as TFE-coated glass and the current most widely used M8060, has been carried out by comparing the AE33 attenuation measurements with the absorption measurements from different filter-based techniques. Online co-located multi-angle absorption photometer (MAAP) measurements and offline PP_UniMI polar photometer measurements were used with this aim. We used data from three different measurement stations in North-East of Spain: an urban background station (Barcelona; BCN), a regional background station (Montseny; MSY) and a mountain-top station (Montsec d'Ares; MSA). The median C values (at 637 nm) measured at the three stations ranged between 2.29 (at BCN and MSY; lowest 5th percentile of 1.97 and highest 95th percentile of 2.68) and 2.51 (at MSA; lowest 5th percentile of 2.06 and highest 95th percentile of 3.06). The C factor was wavelength-dependent only at the mountain-top station, whereas at the urban and regional stations no statistically significant difference was found at the 7 different AE33 wavelengths. The wavelength-dependence of C at the mountain station was in part driven by the predominant effect of dust particles during Saharan dust outbreaks at this station. At the mountain station, neglecting the wavelength dependence of the C factor led to an underestimation of the Absorption Ångström Exponent (AAE) of 12 %. The analysis of the cross-sensitivity to scattering for different filter tapes revealed a large increase of the C factor at the three stations when the single scattering albedo (SSA) of the collected particles was above 0.90–0.95, with up to a 3-fold increase above the average values. The result of the cross-sensitivity to scattering displayed a fitted constant multiple scattering parameter, Cf, of 2.21 and 1.96 and a cross-sensitivity factor, ms, of 0.8 % and 1.7 % for MSY and MSA stations, respectively, for the TFE-coated glass filter tape. For the M8060 filter tape, Cf of 2.50, 1.96, 1.82 and a ms of 0.7 %, 1.5 %, 2.7 %, for BCN, MSY and MSA stations, respectively, were obtained. Differences in the absorption coefficient determined from AE33 measurements at BCN, MSY and MSA of around a 35–40 % can be expected when using the site-dependent C determined experimentally instead of the nominal C value.