Characteristics of an aged organic "brown" aerosol in the urban Po Valley atmosphere (original) (raw)

We characterize the atmospheric nondust aerosol having the strongest spectral dependence of light absorption (as indicated by the Absorption Angstrom Exponent, AAE) at visible wavelengths in the urban Po Valley. In situ ground measurements of aerosol spectral optical properties, PM 1 chemical composition (HR-ToF-AMS), and coarse and fine size distributions, were carried out in Bologna, and data statistically analysed. Findings prove that a "brown" aerosol (AAE from 2.5 to 6) in the ambient atmosphere is composed by "droplet" mode particles enriched in aged organic aerosol (OA) and nitrate. We provide a comprehensive physico-chemical characterisation of this brown aerosol, including its spectral optical signature, and possible sources. To our knowledge, no previous work has considered these issues in the ambient atmosphere. We compared to literature to put findings in a broader perspective. There is consistency with recent "diluted" urban observations (airborne, and AERONET), and combustion chamber observations. Our study adds to these previous ones that the high AAE values featuring the "brown" aerosol depend on the OA to Black Carbon (BC) ratio more than on OA, and that the link between AAE and OA-to-BC (already observed for freshly emitted primary aerosols from biomass burning) does exist in the ambient atmosphere for this aged "brown" aerosol, as well. The comparison with studies on the composition evolution of OA in the atmosphere strengthens the result that this "brown" aerosol is an aged OA, and provides experimental evidence for the aged "brown" OA formation in the ambient atmosphere. Findings will have important atmospheric implications for modeling studies, and remote sensing observations, as regards the parametrization and identification of Brown OA, and Brown Carbon in the atmosphere. 1 Introduction Aerosol has an important role in the Earth's climate with both direct and indirect effects; beside that, it affects air quality and atmospheric chemistry. At present, our understanding of the light-absorbing aerosol types is very incomplete (see reviews by Laskin et al., 2015; Moise et al., 2015). An important absorber of solar radiation in the visible region is the atmospheric carbonaceous aerosol (IPCC 2013). Visible-light absorbing properties of this aerosol type vary between two extremes. On one side, there is Black Carbon (BC) that strongly absorbs light over a broad spectral range. On the other side, there is the colourless Organic Carbon (OC) with no absorption or little absorption in the UV spectral range. Between these extremes, there