Measurements and characterization by transmission electron microscopy of ultrafine particles in the urban air of Rome

Seasonal measurements of Organic Carbon (OC) and Elemental Carbon (EC) showed that the carbonaceous fraction in downtown Rome accounted for 30-40 % of the PM10 and was formed by 60-70% of EC and 30-40% of OC. Ultrafine Particle (UFP) concentrations followed a daily trend governed by both the evolution of the atmospheric mixing height and the variation of the autovehicular traffic intensity. The effect of radical photooxidative activity was evidenced by the evolution of particle number in the nucleation mode and nitrous acid concentrations: higher rates of particle formation from the gas phase were observed during periods of high solar radiation, when nitrous acid photolysis was maximum. Accordingly, the contribution of Secondary OC (OC sec) to OC was higher in summer (36%) than in winter (29%). UFP samples were collected and investigated by means of energy filtered transmission electron microscopy (EFTEM) in combination with energy dispersive X-ray spectroscopy. EFTEM analyses showed that 76% of particles were composed of carbon. Several particles showed the presence of semi-transparent part inside. The presence of semi-transparent part inside suggested that some volatile materials such as volatile organic compounds might have evaporated under high vacuum and electron beam in the EFTEM. TEM analysis of individual particle can provide useful information to toxicologists for the evaluation of the toxic effects increasingly identified with ultrafine and nanoparticulates in the air.