Particle concentrations in urban microenvironments: a discussion upon the main influential parameters

In the last decades several scientific studies attempted to demonstrate adverse health effect caused by the exposure to aerosol particles. A debate on the worst particle property, both in terms of dimension and chemical composition, is still unresolved. Whichever the particle characteristics mainly related to the health risk, the short-term exposure to high particle concentrations represents a further important parameter to be considered in human exposure assessment studies. Transportation modes and urban microenvironments may significantly contribute to the daily total exposure through short-term exposure to particle concentrations elevated if compared to elsewhere. Hence, it is essential to be able to measure particle concentration variations through high-resolution time-measurements In the present study a discussion upon the influential parameters affecting particle concentration in urban areas is reported. Street geometry, traffic mode, wind speed and direction effects were analyzed through an experimental campaign performed in different streets of an Italian town. To this purpose a high-resolution time measurement apparatus was used in order to capture the dynamic of the freshly emitted particles. Number, surface area and mass concentrations and distributions were measured continuously along both the sides of street canyons and avenue canyons. The combined effect of street geometry and wind direction may contribute strongly to dilute the fresh particles emitted by vehicles. Higher wind speed was found to decrease concentrations in the canyon. Traffic mode also seems to influence exposure concentrations. In particular, submicrometer particle mass concentration was higher as the traffic is more congested; otherwise, coarse fraction dominates mass exposure concentration along street characterized by a more fluent traffic, showing a typical resuspension modality.