Evaluation of the absorption Ångström exponents for traffic and wood burning in the Aethalometer-based source apportionment using radiocarbon measurements of ambient aerosol

摘要

Equivalent black carbon (EBC) measured by amulti-wavelength Aethalometer can be apportioned to traffic and woodburning. The method is based on the differences in the dependence of aerosolabsorption on the wavelength of light used to investigate the sample,parameterized by the source-specific absorption Ångström exponent(). While the spectral dependence (defined as values) ofthe traffic-related EBC light absorption is low, wood smoke particlesfeature enhanced light absorption in the blue and near ultraviolet. Sourceapportionment results using this methodology are hence strongly dependent onthe values assumed for both types of emissions: traffic , and wood burning . Most studies use a single and pair in the Aethalometer model,derived from previous work. However, an accurate determination of the sourcespecific values is currently lacking and in some recentpublications the applicability of the Aethalometer model was questioned.Here we present an indirect methodology for the determination of and by comparing the source apportionment of EBCusing the Aethalometer model with C measurements of the EC fractionon 16 to 40 h filter samples from several locations and campaigns acrossSwitzerland during 2005–2012, mainly in winter. The data obtained at eightstations with different source characteristics also enabled the evaluationof the performance and the uncertainties of the Aethalometer model indifferent environments. The best combination of and (0.9 and 1.68, respectively) was obtained by fitting theAethalometer model outputs (calculated with the absorption coefficients at470 and 950 nm) against the fossil fraction of EC (EC ∕ EC) derivedfrom C measurements. Aethalometer and C source apportionmentresults are well correlated (  =  0.81) and the fitting residuals exhibitonly a minor positive bias of 1.6 % and an average precision of 9.3 %.This indicates that the Aethalometer model reproduces reasonably well theC results for all stations investigated in this study using our bestestimate of a single and pair. Combiningthe EC, C, and Aethalometer measurements further allowed assessing thedependence of the mass absorption cross section (MAC) of EBC on its source.Results indicate no significant difference in MAC at 880 nm between EBCoriginating from traffic or wood-burning emissions. Using EC ∕ EC asreference and constant a priori selected values, was also calculated for each individual data point. No clearstation-to-station or season-to-season differences in wereobserved, but and values areinterdependent. For example, an increase in by 0.1 resultsin a decrease in by 0.1. The fitting residuals of different and combinations depend on EC ∕ ECsuch that a good agreement cannot be obtained over the entire EC ∕ ECrange using other pairs. Additional combinations of   =  0.8, and 1.0 and   =  1.8 and 1.6,respectively, are possible but only for EC ∕ EC between  ∼  40 and 85 %. Applying values previously used in theliterature such as of  ∼  2 or any in combination with   =  1.1 to our data setresults in large residuals. Therefore we recommend to use the best combination as obtained here (  =  0.9 and   =  1.68) in future studies when no or only limited additionalinformation like C measurements are available. However, these resultswere obtained for locations impacted by black carbon (BC) mainly fromtraffic consisting of a modern car fleet and residential wood combustionwith well-constrained combustion efficiencies. For regions of the world withdifferent combustion conditions, additional BC sources, or fuels used, furtherinvestigations are needed.