Spatial and Temporal Variability of Carbonaceous Aerosol Absorption in the Po Valley

Knowledge gaps in the optical properties of carbonaceous aerosols account for a significant fraction of the uncertainty ofaerosol - light interactions in climate models. Both black carbon (BC) and brown carbon (BrC) can display a range of opticalproperties in ambient aerosol due to different sources and chemical transformation pathways. This study investigates theoptical absorption properties of BC and BrC at an urban and a rural site in the Po Valley (Italy), a known European pollutionhot spot. We observed spatial and seasonal variability of aerosol absorption coefficients, with the highest values measuredin winter at the urban site of Milan (12 Mm-1 on average) and the lowest values in summer at the rural site of Motta Visconti(3 Mm-1 on average). The average aerosol Absorption Å ngström Exponent (AAE) measured during the two experimentsacross the 370-880 nm wavelength range was 1.1 and 1.2 at the urban and the rural site, respectively. The observed AAEvalues in winter (the average AAE during the two winter campaigns was 1.2) are consistent with the contribution of woodburning BrC, as confirmed by macro-tracer analysis. The BC mass absorption cross section (MACBC) did not show a specificseasonal or spatial variability across the two sites and maintained an average value of 10 ± 5 m2 g-1 at 880 nm. The opticalproperties of BrC, investigated off-line after extraction of organic aerosol (OA) indicate that wood burning was the dominantBrC source in winter, while secondary organic aerosol (SOA) from other anthropogenic emissions was the main source ofBrC in summer.