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Pedestrian Exposure to Size-Resolved Particles in Milan

ABSTRACT Measurement campaigns for airborne particles along a pedestrian route in the city center of Milan were performed by means of a portable instrument consisting of an optical particle counter and a global positioning system (GPS) signal receiver. Based on the size-resolved particle number concentration data and on proper density factors experimentally determined for Milan urban area, the mass concentrations were calculated in terms of particulate matter with aerodynamic diameters ≤10 μm (PM10), ≤2.5 μm (PM2.5), and ≤1 μm (PM1). Besides directly measuring the personal exposure to PM throughout the route, the measurement campaigns pointed out small spatial and temporal variations of the concentration ranges in the different urban microenvironments visited along the route as well as very peculiar features of the particles levels in the underground subway. These findings suggested that the personal exposure of pedestrians in the city center could be estimated by simply taking into account the exposure at the open air and in the subway. The comparison between measured and calculated exposures according to the microenvironment-based estimation results in reasonable accordance, even though the estimations tend to slightly underestimate (12%) the actual measured exposure. IMPLICATIONS This work demonstrates that the actual exposure of pedestrians in Milan city center can be estimated by exposure modeling based on ambient data from an urban background site, provided that high-time-resolution data are available. This suggests that instruments such as the optical particle counters (OPCs) should be also included in the air quality monitoring network, together with conventional monitors used for air quality standard verification, in order to allow personal exposure assessment at open-air urban sites. However, because OPCs provide indirect information on the particle mass concentration, local field tests are required for establishing the proper algorithm to obtain mass concentrations from particle counting.