Source-specific light absorption by carbonaceous components in the
complex aerosol matrix from yearly filter-based measurements
Authors: Moschos Vaios , Gysel-Beer Martin , Modini Robin L. , Corbin Joel C. , Massabò Dario , Costa Camilla , Danelli Silvia G. , Vlachou Athanasia , Daellenbach Kaspar R. , Szidat Sönke , Prati Paolo , Prévôt André S. H. , Baltensperger Urs , El Haddad Imad 
Published: December 2020

Abstract. Understanding the sources of light-absorbing organic (brown) carbon (BrC) and its interaction with black carbon (BC) and other non-refractory particulate matter (NR-PM) fractions is important for reducing uncertainties in the aerosol direct radiative forcing. In this study, we combine multiple filter-based techniques to achieve long-term, spectrally-resolved, source- and species-specific atmospheric absorption closure. We determine the total aerosol absorption at seven wavelengths based on the Aethalometer attenuation measurements, calibrated with the multi-wavelength absorption analyzer. We measure the spectrally-resolved imaginary part of the refractive index, k, for methanol extracts and determine the source-specific k for organic aerosol (OA) fractions using positive matrix factorization. The average k at 370 nm is found to be 0.06 for fresh biomass smoke, 0.03 for winter-oxygenated OA, and 0.006 for other less absorbing OA, which translates to corresponding mass absorption efficiencies (MAE) in dilute bulk solutions of 1.4, 0.7 and 0.13 m2 g−1. We apply Mie calculations to estimate the contributions of these fractions to total aerosol absorption. While enhanced absorption in the near-UV has been traditionally attributed to primary biomass smoke, here we show that anthropogenic oxygenated OA may be as important for BrC absorption during winter, especially at an urban background site. We demonstrate the absence of tar-balls in residential biomass burning particles of this study, and attribute the totality of the NR-PM absorption at shorter wavelengths to methanol-extractable BrC. As for BC, we show that the mass absorption cross-section (MAC) of this fraction is independent of its source, while we observe evidence for a lensing effect associated with the presence of NR-PM components. We find that bare BC has a MAC of 6.3 m2 g−1 at 660 nm and an absorption Ångström exponent (AAE) of 0.93 ± 0.16, while in the presence of coatings its absorption is enhanced by a factor of ~1.4. This falls within the global average enhancement factor of 1.5 ± 0.3 from filter-based techniques. Based on Mie-calculations of closure between observed and predicted total light absorption, we provide the first experimental evidence for a suppression of the lensing effect by BrC. The total absorption suppression remains modest, ~10–20 % at 370 nm, and is restricted to shorter wavelengths where BrC absorption is significant. Our long-term observations at locations representative of European urban background and rural Alpine sites show that extractable particulate BrC accounts for ~30 % of the absorption in the near-UV range (370 nm) on a yearly basis. However, if lensing suppression occurred due to internal mixing of BC and BrC as apparently the case for many samples in our study, then the additional absorption by BrC (vs transparent OA) would be partially compensated by a concurrent BC lensing factor reduction, which manifests as lower than expected total aerosol AAE values. Overall, when integrated with the solar spectrum at 300–900 nm, bare BC is found to contribute around two thirds of the solar radiation absorption by total carbonaceous aerosols, amplified by the lensing effect (with an interquartile range, IQR, of 8–27 %), while the IQR of yearly average contributions by particulate BrC is 6–13 % (13–20 % at the rural site during winter).