Aliphatic and polycyclic aromatic hydrocarbons in urban rain, snow and fog

Abstract

Rain, snow and fog samples have been collected at an urban site in Switzerland. Concentrations of n-alkanes and polycyclic aromatic hydrocarbons (PAH) were determined in the particulate (⩾ 0.2 μm) and filterable phases. The n-alkanes in the particulate phase were characterized by a carbon preference index (CPI), in the range of C 21–C 33, of 7.1 in summer rain compared to 2.1 in winter rain. n-Alkanes with a CPI of 1.2 occurred in the filtrate of several samples. The presence of these highly water-insoluble n-alkanes in the filterable fraction suggests the formation of intermolecular associations (clusters, aggregates). The PAH patterns in the particulate phases resemble each other in snow, summer and winter rain, indicating similar sources and scavenging mechanisms. Variations in the concentrations of n-alkanes and PAH between seasons and between rain, snow and fog can be explained by the size of the associated particles and hydrometeors (rain droplet, snow flake, fog droplet) and the related scavenging efficiencies. The distribution coefficients obtained by field measurements have been applied to a scavenging model and to an equilibrium model taking hydrophobic partitioning into account.