Two-wavelength thermo-optical determination of Black and Brown Carbon in atmospheric aerosols

Carbonaceous aerosol (CA) 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) although a minor fraction of carbonate carbon could be also present. Thermo-optical analysis are widely adopted for the quantitative determination of total, TC, organic, OC and elemental, EC, Carbon in aerosol samples collected on quartz fibre filters. Nevertheless, the methodology presents several issues in particular about the artefacts related to the formation of pyrolytic carbon. It is usually neglected the uncertainty due to the possible presence of brown carbon (BrC) in the sample under analysis, i.e. the optically active fraction of OC produced by biomass burning and with characteristics intermediate between OC and EC. The Sunset EC/OC analyser unit at the Physics Department of the University of Genoa has been modified, making possible the alternative use of the standard laser diode at λ = 635 nm or of a laser diode at λ = 405 nm, to monitor the optical transmittance during the thermo-optical analysis. The additional use of the 405 nm transmittance measurement provides valuable information about the composition of the sample (i.e. the presence of Brown Carbon (BrC), that is part of OC but also a light absorbing species and can shift the split point (Andreae & Gelencsér, 2006)) as well as on the pyrolytic carbon formation, both able to affect the instrumental “split point” (i.e. the moment of the analysis in which the laser transmittance is back to its starting value, thus defining EC/OC separation). We present here the new instrument set-up, tested using both synthetic and real (urban and rural) aerosols, collected on quartz fibre filters. The first test was the analysis of synthetic samples prepared with a solution of Aquadag (i.e. a water-based colloidal graphite suspension) which can be considered as composed by EC/BC only. Samples were analysed with both NIOSH5040 and EUSAAR_2 protocols. Then other samples prepared with a solution of ammonium sulphate (NH4)2SO4 in Aquadag were analysed as well to mimic the behaviour of real-world aerosol samples (i.e. with a scattering compound mixed to the absorbing Aquadag spherules). This second set of samples was analysed through the EUSAAR_2 protocol only. Finally, a set of real PM10 samples from an urban area of the city of Genoa (IT) whit negligible contribute of biomass burning to PM composition (Bove et al., 2014) was used. After this characterization phase, the modified Sunset set-up was used for the first time, in conjunction with the MWAA instrument and apportionment methodology (Massabò et al., 2015), to retrieve the MAC (Mass Absorption Coefficient) of Brown Carbon at the two wavelengths of λ = 635 nm and λ = 405 nm, in a set of samples collected wintertime in a mountain site. Results turned out to be well correlated with Levoglucosan concentrations determined on the same aerosol samples.