Seasonal effects on the physico-chemical characteristics of PM2.1 in Rome: a study by SEM and XPS

A large body of epidemiological research has shown a significant statistical correlation between acute health effects and the fine fraction of urban particulate (PM2.5). There is still a wide debate over which characteristics of PM are responsible for the adverse health effects. The aim of this study was to evaluate seasonal effects on the composition and physico-chemical characteristics of PM2.1 (particulate with aerodynamic diameter<2.1 μm) in an urban area of Rome. The role as a “carrier” of carbonaceous particles was thoroughly studied using scanning electron microscopy with an EDS X-ray attachment and photo-electron spectroscopy (XPS). Atmospheric aerosol was analysed by ion chromatography to evaluate the soluble ions collected by an annular denuder. X-ray microanalysis data were subjected to hierarchical cluster analysis to classify the particles into groups (clusters) of similar chemical composition. We identified four clusters of particles in the PM2.1: carbonaceous particles, soil erosion particles, sulphates and metals. EDX spectra showed the presence of a surface coating containing S or sometimes S, K, Na on a variable percentage of carbonaceous particles. XPS data showed C, O, N, Si, S to be the most abundant elements with only a trace presence of Ca, Na and Fe. XPS spectra in the region of S2p peak confirm that the sulphur compounds in the PM2.1 are constituted by SO42− anion. The trend of the abundances in weight of S and N obtained by XPS showed an evident peak in summer. A similar summer peak was seen in the abundances of carbonaceous particles with S-coating obtained by SEM/EDX.