Development of a dual-wavelength thermo-optical transmittance analyser: Characterization and first results.
Massabò D., Altomari A., Bernardoni V., Valli G., Vecchi R., Prati P.
Carbonaceous aerosol plays an important role in many different issues ranging from human health to global climate change. It mainly consists of organic carbon (OC) and elemental carbon (EC). Thermal-optical methods (TOT/TOR) are presently the most widespread approach to OC/EC speciation. Despite their popularity, there is still a disagreement among the results, especially for what concerns EC, as different thermal protocols can be used. In fact, the pyrolysis occurring during the analysis can heavily affect OC/EC separation, depending on PM composition and analysis protocol. The main hypothesis at the basis of the technique relies on the optical properties of EC and OC: while EC is strongly light absorbing, OC is generally transparent in the VIS. However, a fraction of light-absorbing OC exists: the brown carbon (BrC). The presence in the sample of BrC can affect EC/OC separation since it is slightly absorbing also at 635 nm, the typical wavelength used in TOT. At the Physics Department of the University of Genoa, a Sunset EC/OC analyser unit has been modified in order to perform thermo-optical analysis at two different wavelengths: 635 nm (original instrument wavelength) and 405 nm. The 405 nm transmittance measurement provides information about sample composition as well as on the pyrolytic carbon formation, both able to affect the moment of the analysis in which the laser transmittance is back to its starting value, thus defining EC/OC separation. We present the new instrument set-up, its full characterization with "synthetic" samples, and results obtained at 2-lambdas with both NIOSH and EUSAAR2 protocols applied to real PM samples collected in very different conditions and sites. Furthermore, we have recently introduced a new possibility, based on the apportionment of the absorption coefficient (babs) of particle loaded filters, for correcting the thermo-optical analysis of PM samples. The apportionment is based on the optical analysis performed by the Multi-Wavelength Absorbance Analyser (MWAA), an instrument developed at the Physics Department of the University of Genoa. The apportionment method uses the information gathered at five different wavelengths in a renewed and upgraded version of the approach usually referred to as Aethalometer model. We present here also the results of the thermo-optical analysis correction applied to the dual-analysis, which lead to a better homogeneity between the results obtained with different thermal protocols. This work was supported by INFN (grants MANIA-DEPOTMASS) and by Amministrazione Provinciale di Genova.