Characteristics of an aged organic "brown" aerosol in the urban PoValley atmosphere

摘要

We characterize the atmospheric nondust aerosol having the strongest spectral dependence of light absorption (asindicated by the Absorption Angstrom Exponent, AAE) at visible wavelengths in the urban Po Valley. In situ ground measurementsof aerosol spectral optical properties, PM1 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 theambient atmosphere is composed by "droplet" mode particles enriched in aged organic aerosol (OA) and nitrate. We providea comprehensive physico-chemical characterisation of this brown aerosol, including its spectral optical signature, and possiblesources. To our knowledge, no previous work has considered these issues in the ambient atmosphere. We compared toliterature to put findings in a broader perspective. There is consistency with recent "diluted" urban observations (airborne, andAERONET), and combustion chamber observations. Our study adds to these previous ones that the high AAE values featuringthe "brown" aerosol depend on the OA to Black Carbon (BC) ratio more than on OA, and that the link between AAE andOA-to-BC (already observed for freshly emitted primary aerosols from biomass burning) does exist in the ambient atmospherefor this aged "brown" aerosol, as well. The comparison with studies on the composition evolution of OA in the atmospherestrengthens the result that this "brown" aerosol is an aged OA, and provides experimental evidence for the aged "brown" OAformation in the ambient atmosphere. Findings will have important atmospheric implications for modeling studies, and remotesensing observations, as regards the parametrization and identification of Brown OA, and Brown Carbon in the atmosphere.