AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Experimental Evidence of the Lensing Effect Suppression for Atmospheric Black Carbon Containing Brown Coatings
VAIOS MOSCHOS, Robin Modini, Joel Corbin, Dario Massabò, Silvia G. Danelli, Camilla Costa, Athanasia Vlachou, Kaspar Daellenbach, Paolo Prati, Martin Gysel, Andre S.H. Prévôt, Urs Baltensperger, Imad El Haddad, Paul Scherrer Institute
Abstract Number: 657 Working Group: Carbonaceous Aerosol
Abstract Accounting for the wavelength- and source-dependent optical absorption properties of the abundant light-absorbing organic (brown) carbon (BrC) and the mixing state of atmospheric black carbon (BC) are essential to reduce the large uncertainty in aerosol radiative forcing. Estimation of BrC absorbance online by subtraction is highly uncertain and may be biased if not decoupled from the potential BC absorption enhancement (lensing) due to non-refractory (organic and inorganic) coating acquisition.
Here, the reported total particulate absorbance is based on long-term, filter-based seven-wavelength Aethalometer (AE33) data, corrected for multiple scattering effects with Multi-Wavelength Absorbance Analyzer (5λ MWAA) measurements. Using ultraviolet-visible spectroscopy absorbance measurements along with particle size distributions obtained by a scanning mobility particle sizer, we have conducted optical closure calculations to assess the importance of source-specific extractable particulate BrC1 versus BC absorbance. The wavelength dependence of bare BC absorption is estimated using MWAA measurements upon successive filter extractions to remove the influence of BrC/coatings. The lensing contribution, supported by observations from field-emission scanning electron microscopy, is estimated at longer wavelengths using a proxy for the BC coating thickness.
Based on the observational constraints and independent modeling approaches, we demonstrate that the interplay between lensing and BrC absorption results in lower than expected BC absorption at shorter wavelengths. This indicates that the volume additivity assumption is not valid for particulate absorption by internally-mixed heterogeneous atmospheric aerosol populations. These comprehensive experimental analyses verify the lensing suppression predicted for core-shell structures containing moderately absorbing BrC2. The implications discussed in this presentation are relevant for co-emitted species from biomass burning or aged plumes with high BrC to BC mass/absorbance ratio.
[1] Moschos V, et al. (2018) ES&T Lett.5, 302. [2] Lack DA, Cappa CD. (2010) ACP.10, 4207.