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\bibitem{Vallius_etal2005} {\textquotedblleft}\textit{Sources and elemental composition of ambient PM$_{2.5}$ in three European cities}{\textquotedblright}. Vallius M, Janssen NAH, Heinrich J, Hoek G, Ruuskanen J, Cyrys J, Van Grieken R, de Hartog JJ, Kreyling WG, Pekkanen J, The science of the total environment \textbf{337}, 147 (2005).   http://doi.org/10.1016/J.SCITOTENV.2004.06.018

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