Mechanisms of formation of particulate polycyclic aromatic hydrocarbons in relation to the particle size distribution; effects on meso-scale transport

Abstract

Study of the particle size distribution of an urban aerosol has revealed associations between polycyclic aromatic hydrocarbons (PAHs) and inorganic elements. Two particle size distribution families coexist: the first, having mean particle size less than 1 μm, corresponds to lead and to relatively non-volatile PAHs; the second, with mean particle size greater than 1 μm, corresponds to anthropogenic iron, anthropogenic manganese and to volatile PAHs except fluoranthene. Factor analysis shows that the existence of these two families implies two different mechanisms of particle formation, which both depend on the volatility of the compounds: adsorption of PAHs on preexisting particles corresponding to the fine fraction of aerosols (0.7 μm) and condensation of gaseous compounds leading to the formation of bigger particles (1.5 μm). These results are applied to the evaluation of the relative importance of parameters which determine the PAH concentration variations during meso-scale transport between the source zone and the western Meditteranean Sea: the effect of the specific reactivity of each PAH is predominant while the differences on the deposition rates related to particle size are of minor importance.