Exploiting multi-wavelength aerosol absorption coefficients in a multi-time source apportionment study to retrieve source dependent absorption parameters

In this paper, a new methodology coupling aerosol optical and chemical parameters in the same source apportionment study is reported. This approach gives additional relevant information such as estimates for the atmospheric Ångström Absorption Exponent (α) of the sources and Mass Absorption Coefficient (MAC) for fossil fuel emissions at different wavelengths. A multi-time source apportionment study using Multilinear Engine ME-2 was performed on a PM10 dataset with different time resolution (24 hours, 12 hours, and 1 hour) collected during two different seasons in Milan (Italy) in 2016. Samples were optically analysed to retrieve the aerosol absorption coefficient bap (in Mm-1) at four wavelengths (λ=405 nm, 532 nm, 635 nm and 780 nm) and chemically characterised for elements, ions, levoglucosan, and carbonaceous components. Time-resolved chemically speciated data were coupled with bap multi-wavelength measurements and introduced as input data in the multi-time receptor model; this approach was proven to strengthen the identification of sources being particularly useful when important chemical markers (e.g. levoglucosan, elemental carbon, …) are not available. The final solution consisted in 8 factors (nitrate, sulphate, resuspended dust, biomass burning, construction works, traffic, industry, aged sea salt); the implemented constraints led to a better physical description of factors and the bootstrap analysis supported the goodness of the solution. As for bap apportionment, consistently to what expected, the two factors assigned to biomass burning and traffic were the main contributors to aerosol absorption in atmosphere. A relevant feature of the approach proposed in this work is the possibility of retrieving many other information about optical parameters; for example, opposite to the more traditional approach used by optical source apportionment models, here we obtained the atmospheric Ångström Absorption Exponent (α) of the sources (α biomass burning = 1.83 and α fossil fuels = 0.80), without any a priori assumption. In addition, an estimate for the Mass Absorption Cross section (MAC) for fossil fuel emissions at four wavelengths was obtained and found to be consistent with literature ranges.