Some inferences on the mechanism of atmospheric gas/particle partitioning of polycyclic aromatic hydrocarbons (PAH) at Zaragoza (Spain)

Gas/particle partitioning of pollutants is an important mechanism determining atmospheric processing and its impact to environmental and human health. In this paper, the gas–particle partitioning of polycyclic aromatic hydrocarbons (PAH) has been studied with the aim of determining the main mechanism of PAH partitioning in Zaragoza (Spain) aerosols. To reach this goal, the ambient concentrations of PAH (gas and particle phase) collected in this city for one year period (2003–2004) have been analyzed. The partitioning between the particle and gas phases was studied according to three different models: the Junge adsorption model, the absorption into the organic matter model using the octanol–air ( K OA) partition coefficient and the absorption into the organic matter plus the adsorption onto the soot carbon model using the soot–air ( K SA) partition coefficients. Experimental gas/particle partition coefficients ( K P) correlated well with the subcooled liquid vapour pressures ( P L 0 ) of PAH but with slopes higher than the expected value of −1. Experimental K p values were well fit to the modelled ones when, in addition to absorption into organic matter, adsorption onto the soot carbon was considered. It could be concluded that the main partition mechanism in Zaragoza aerosols was explained by adsorption onto the soot carbon. However, K p modelled values were affected by the different thermodynamic parameters related to soot types. The influence of the organic matter and elemental carbon fractions on the K p modelling was also studied. The different particle characteristics, local factors, the presence of non-exchangeable fraction and non-equilibrium were considered like main keys to explain deviations of the experimental K p values from predictions according to models.