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. In this paper, a new methodology coupling aerosol optical andchemical parameters in the same source apportionment study is reported. Inaddition to results on source contributions, this approach providesinformation such as estimates for the atmospheric absorptionAngstrom exponent ( α ) of the sources and mass absorptioncross sections (MACs) for fossil fuel emissions at different wavelengths. A multi-time resolution source apportionment study using the Multilinear Engine(ME-2) was performed on a PM 10 dataset with different time resolutions (24, 12, and 1 h) collected during two different seasons in Milan(Italy) in 2016. Samples were optically analysed by an in-house polarphotometer to retrieve the aerosol absorption coefficient bap (inMm −1 ) at four wavelengths ( λ=405 , 532, 635, and 780 nm) and were chemically characterized for elements, ions, levoglucosan, andcarbonaceous components. The dataset joining chemically speciated andoptical data was the input for the multi-time resolution receptor model;this approach was proven to strengthen the identification of sources, thusbeing particularly useful when important chemical markers (e.g.levoglucosan, elemental carbon) are not available. The final solutionconsisted of eight factors (nitrate, sulfate, resuspended dust, biomassburning, construction works, traffic, industry, aged sea salt); theimplemented constraints led to a better physical description of factors andthe bootstrap analysis supported the goodness of the solution. As for bap apportionment, consistent with what was expected, biomass burning andtraffic were the main contributors to aerosol absorption in the atmosphere.A relevant feature of the approach proposed in this work is the possibilityof retrieving a lot of other information about optical parameters; for example,in contrast to the more traditional approach used by optical sourceapportionment models, here we obtained source-dependent α valueswithout any a priori assumption ( α biomass burning =1.83 and α fossil fuels =0.80 ). In addition, the MACs estimated for fossil fuelemissions were consistent with literature values. It is worth noting that the approach presented here can also be appliedusing more common receptor models (e.g. EPA PMF instead of multi-timeresolution ME-2) if the dataset comprises variables with the same timeresolution as well as optical data retrieved by widespread instrumentation(e.g. an Aethalometer instead of in-house instrumentation).

atmospheric chemistry and physics

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