Ultrafine particles (UFP, diameter < 100 nm), as reported in recent findings of toxicological and epidemiological studies, could represent health and environmental risks. Motor vehicle emissions usually constitute the most significant source of UFP in an urban environment. Number, surface and mass concentration of particles were determined at increasing distances from the most important Italian road: the ‘‘Autostrada del Sole’’ A1 highway. Particles in the size range from 0.0059 to 20 mm were measured with a Scanning Mobility Particle Sizer (SMPS) and an Aerodynamic Particle Sizer (APS) spectrometers. The A1 highway was selected because it is characterized by two different traffic conditions: a daily and a weekly traffic. During the weekdays the average traffic flow was about 50 vehicles min1 with more than 30% of vehicles being heavy-duty (HD) diesel trucks. The weekly traffic component is characterized by an increased traffic up to approximately 100 vehicles min1 during Monday mornings and Friday afternoons because of light-duty vehicles, with substantial reduction of the percentage of HD diesel trucks (typically only 10%). The purpose of this study is the characterization of the A1 highway in terms of evolution of particle size distribution (PSD) and total number concentration at different distances from the highway. This analysis is interesting because Italian traffic presents a higher i) percentage of diesel light-duty vehicles and ii) mean traffic speed in respect to US and Australian traffics. Particle number, surface and mass, exponentially decreases as one moves away from the freeway, whereas UFP number concentration measured at 400 m downwind from the freeway is indistinguishable from upwind background concentration.
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