10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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The MWAA Model as a Tool for Carbonaceous Aerosols Apportionment and as an Input for the Improvement of TOT Measurements
DARIO MASSABÒ, Vera Bernardoni, Rosaria Erika Pileci, Silvia G. Danelli, Lorenzo Caponi, Gianluigi Valli, Roberta Vecchi, Paolo Prati, University of Genoa and INFN Genoa, Italy
Abstract Number: 1468 Working Group: Carbonaceous Aerosol
Abstract From the local to the global scale, carbonaceous aerosols (CA) play a crucial role on health and climate. In urban environments CA are a major component of PM: composed of Organic Carbon (OC) and Elemental Carbon (EC), are mainly found in PM finest fractions (PM2.5 and PM1). EC can strongly absorb light, which is why, when determined using optical methods, it is called Black Carbon (BC); the OC is instead characterized by a substantial transparency in the VIS range, except for a small fraction consisting of weakly absorbing and fairly refractory organic macromolecules: the Brown Carbon (BrC). The BrC absorption coefficient is characterized by a greater spectral dependence (babs ≈ λ-4) than the BC (babs ≈ λ-1).
Thanks to the optical analysis of PM samples, carried out thanks to the MWAA [1,2] (Multi-Wavelength Absorbance Analyzer – an instrument developed at the Dept. of Physics of the University of Genoa), it is possible to derive the CA source apportionment starting from the measurement of the aerosol absorption coefficient, determined at 5-λ. Through this methodology (MWAA model) it is possible to quantify EC and OC coming from the combustion of fossil material (ECFF and OCFF) and wood (ECWB and OCWB) [2]. The MWAA model has recently been refined and applied to more complex situations than rural/mountain, such as PM samples collected in the very high-polluted city of Milan, Italy [3]. As previously demonstrated for rural sites, also in this complex situation, given by a great number of pollution sources and very stable meteorological conditions causing a huge production of secondary aerosols, ECWB and OCWB apportioned by the model turned out to be very correlated with Levoglucosan, determined in the same samples by HPLC-PAD. The optical apportionment coming from the MWAA model also provides a direct measurement of the Ångström absorption exponent of the BrC (αBrC), which was found to be, also in this urban environment, 3.95 ± 0.20.
The thermo-optical techniques (TOT/TOR) currently represent the reference method for the quantification of EC and OC. Despite their diffusion, there is still a considerable disagreement between the results obtained, since the quantification of EC and OC varies according to the thermal protocol used and by the PM composition. Furthermore, the presence in the sample of BrC can alter the "split point", as it is able to absorb the laser light (635 nm) used to monitor the transmittance during the analysis [4, 5]. In order to investigate how the BrC affects the EC/OC separation, and with the objective of quantifying this important fraction of the OC, a Sunset EC/OC analyzer unit has been modified in order to make possible measurements also with a 407 nm laser. At this wavelength, BrC is much more absorbent and therefore able to influence deeply EC/OC separation.
Here we present the results of the refined MWAA model, applied to both rural and urban PM samples, and how the results obtained through the model can be implemented in the dual-λ TOT analysis to obtain valuable information on BrC concentration.
This work was supported by the INFN (MANIA and TRACCIA projects) and by the Provincial Administration of Genoa.
Bibliography [1] Massabò et al., (2013), Journal of Aerosol Science, 60, 34-46. [2] Massabò et al., (2015), Atmospheric Environment, 108, 1-12. [3] Bernardoni et al., (2017), Atmosphere, 8(11), 218. [4] Chen et al., (2015), Atmos. Meas. Tech., 8, 451-461. [5] Massabò et al., (2016), Atmospheric Environment, 125, 119-125.