New insights on humic-like substances associated with wintertime urban aerosols from central and southern Europe: Size-resolved chemical characterization and optical properties

Although Humic-Like Substances (HULIS) are important contributors to the mass of organic aerosol in airborne particulate matter (PM), little is known about their chemical composition, while, their size resolved optical properties have not been studied yet. Here, HULIS fractions were isolated from size resolved aerosol samples (<= 0.49, 0.49-0.95, 0.95-3 and 3-10 mu m) collected in urban and suburban environments of four European cities during wintertime. The bulk (i.e., sum of all size fractions) concentration of HULIS ranged between 1.29 and 2.80 mu g m(-3) across sites with highest values in the <= 0.49 pm particle size fraction. The contribution of the carbon mass of HULIS (HULIS-C) to the watersoluble organic carbon content (WSOC) of PM was 32-43%, which is typical for urban sites affected by biomass burning. The Mass Absorption Efficiency (MAE), which characterizes the efficiency of absorbing solar energy per carbon mass of HULIS decreased with particle size, suggesting that the finest size fractions contain more light-absorbing chromophores, which could affect the light-absorbing ability of organic aerosols. The good correlation of HULIS with effective biomass tracers such as K+, as well as with secondary inorganic aerosol components, proposed that HULIS had both primary (i.e., biomass burning) and secondary sources. The Fourier Transfer Infrared coupled to Attenuation Total Reflectance (FTIR-ATR) spectra demonstrated prevalence of aromatic over carboxylic functional groups in most HULIS fractions, indicating contribution from coal combustion emissions in addition to fresh biomass burning