Size distribution of atmospheric particulate matter at traffic exposed sites in the urban area of Milan (Italy)

Abstract Atmospheric particulate matter (PM) presence at four urban sites in the city of Milan (Italy) is characterised in terms of particle size distribution (number, surface, volume) for the cold and warm season. Simultaneous monitoring of particle number concentration (from 300 nm up to 20 μm of diameter) has been performed between August 2002 and December 2004 by means of a low-volume particle size laser analyser. The monitoring sites are characterised by a different exposure to traffic emissions, enabling for the assessment of the role of this source on both PM concentration levels and on particle size distributions. Data from an urban background site, not directly exposed to traffic emission, a site in a residential area of the city, and two kerbside sites (one at open air, one in a road tunnel) directly exposed to the traffic emissions are compared. Weekdays’ and weekends’ data from the urban background site are analysed for assessing the effect of the reduced traffic circulation on Sundays. Particle concentration is higher at the traffic exposed sites, (where a large increase in particles concentration is observed), especially in the size range between 0.5 and 5 μm, as a consequence of the primary particulate traffic emissions and of the resuspension of soil dust, whereas below 0.5 μm the particle concentrations are similar to those measured in the ambient air. The comparison of seasonal data points out that in the cold season the total number of particles is about two times greater than in the warm season, both on weekdays and on Sundays. In both seasons, particle size distributions present similar patterns, with 99.5% of the total number of particles characterised by a diameter smaller than 1 μm and with Sundays’ distributions shifted towards lower values compared to weekdays’, especially in the coarse size range. According to all these findings, the traffic emissions of the area mainly affect the presence of supermicron particles, while they play only a limited influence on the finest size fractions, characterised by a large background presence, deriving from secondary formation processes in the atmosphere.