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 of aerosol-light interactions in climate models. Both black carbon (BC) and brown carbon (BrC) can display a range of optical properties in ambient aerosol due to different sources and chemical transformation pathways. This study investigates the optical absorption properties of BC and BrC at an urban and a rural site in the Po Valley (Italy), a known European pollution hot spot. We observed spatial and seasonal variability of aerosol absorption coefficients, with the highest values measured in 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 experiments across the 370-880 nm wavelength range was 1.1 and 1.2 at the urban and the rural site, respectively. The observed AAE values in winter (the average AAE during the two winter campaigns was 1.2) are consistent with the contribution of wood burning BrC, as confirmed by macro-tracer analysis. The BC mass absorption cross section (MACBC) did not show a specific seasonal or spatial variability across the two sites and maintained an average value of 10 ± 5 m2 g-1 at 880 nm. The optical properties of BrC, investigated off-line after extraction of organic aerosol (OA) indicate that wood burning was the dominant BrC source in winter, while secondary organic aerosol (SOA) from other anthropogenic emissions was the main source of BrC in summer.