Long - term monitoring of wet and dry depositions in a Po Valley City (Bologna, Italy)

Urban areas generally undergo high pollution loads, specially due to traffic and industrial plants, and particular microclimate conditions, depending on wind pattern and topographic structure. This results in higher contamination than in natural sites, with harmful and undesired effects on human health, ecosystem and materials, in particular those constituting cultural heritage. Sampling and analysis of wet and dry atmospheric deposition can provide a clear picture of the environmental quality of an ecosystem and can lead to actions aimed at reducing pollutant emissions in the atmosphere. This chapter describes the results of a nine years monitoring (1995-2003) of wet and dry depositions collected in the urban site of Bologna, with a DDAS (Dry Deposition on Aquatic Surface) sampler. Bologna is located in the Po Valley (Northern Italy), a heavily industrialized and densely populated region, characterized by a typical atmospheric circulation frequently associated with the air mass stagnation conditions. The purposes of this study were to investigate the chemical composition of both wet and dry depositions, with regard to the main cations (H+, Ca2+, Mg2+, Na+, K+, NH4+), anions (SO42–, NO3–, Cl–) and heavy metals (Cd, Cr, Cu, Ni, Pb, V, Zn), to examine the temporal trends of the main ions, to identify the contribution of different sources to the chemical composition, in particular distinguishing between natural and anthropogenic ones, and to point out local, regional and long-range influences. The study of correlation between ion pairs and data factor analysis represented a useful tool to this aim. Three main factors were identified which affected chemical composition in the 9 years survey: anthropogenic, marine and terrigenous contributions, acting respectively on a local, regional and both local and long-range scale. The separate study of wet and dry deposition revealed a predominance of anthropogenic species fallout in the former mechanism, due to their presence in the finest particles, compared to those from natural sources. Finally, a clear decrease of free acidity in atmospheric deposition was also observed, associated to the lowering of anthropogenic species, as nitrate and sulphate ions; this suggests an improvement of air quality in Bologna urban area, due to the recent advances in pollutant control.