Many scientific papers, mostly published in last decade, have shown that underground metro systems generally show very poor air quality, particularly from the point of view of particulate matter (PM) and of elements contained therein (Nieuwenhhuijsen et al 2007). In this work, the air quality at the main station of the metro system of Rome (Termini hub) has been characterized by the point of view of particulate matter concentration and chemical composition. Indoor air in different environments (underground train platform and shopping center, metro carriages with and without air conditioning system) has been studied and compared with the air quality at the railway station, located outdoor at surface level, and with outdoor air at a nearby urban site. PM mass concentration was obtained by three methods: - determined on filter membranes by gravimetry; - estimated by number concentration measurements carried out by an optical particle counter (OPC); - calculated by adding individual chemical determinations (ions, elemental carbon, organic carbon, macro-elements and the bio-accessible and residual fractions of micro- and trace elements). Taking the gravimetric concentration as a reference, in all the subway environments the sum of the chemical analyses overestimated the gravimetric results, while the concentrations yielded by the OPC were remarkably lower (Figure 1a). These disagreements were mainly due to the prevalence of high-density iron particles in the subway samples, which was highlighted by light microscopy and scanning electron microscopy analysis. For the reconciliation between the gravimetric mass and the mass estimated from OPC data we modified the original OPC calibration factor. For the new calculations, we considered all particles having the shape of an ellipsoid, as observed by microscopy, and estimated the particle mass by multiplying the total particle volume by a factor ?, having the dimensions of a density, which was calculated as the weighted average of the densities of the main PM components in the subway indoor environments (3.4 g/cm3). To improve the performance of the chemical determinations we modified the algorithm used to obtain the mass closure, where macro-elements were all considered as oxides. The algorithm was modified by introducing elemental Fe, produced by wheels, rails and brakes abrasion, and considering as oxide only the amount of Fe produced by soil abrasion and soil re-suspension. The good agreement of the results obtained in this way by the three methods is shown in Figure 1b. The overall results of the study showed that train platform and carriages without air conditioning were the most polluted environments, with indoor/outdoor ratio up to two orders of magnitude for many components. The increase in the concentration of elements was mostly due to the residual fraction, indicating a low solubility of the chemical species in the PM samples. However, a significant increase was also observed for the bio-accessible fraction of some elements, particularly Ba, Zn, Cd and Ni. Macro-component concentrations were used to estimate the strength of the main macro-sources. The most significant contribution was found to derive from wheels, rails and brakes abrasion, (over 50% at the subway platform) from soil re-suspension and from organics. Metro system confirms to be a unique microenvironment, characterized by very high concentration and peculiar composition of atmospheric PM.
Reconciliation of PM mass values obtained from gravimetric measurements, chemical determinations and optical counter data in the metro system of Rome
Marcovecchio F;Perrino C
2015
Abstract
Many scientific papers, mostly published in last decade, have shown that underground metro systems generally show very poor air quality, particularly from the point of view of particulate matter (PM) and of elements contained therein (Nieuwenhhuijsen et al 2007). In this work, the air quality at the main station of the metro system of Rome (Termini hub) has been characterized by the point of view of particulate matter concentration and chemical composition. Indoor air in different environments (underground train platform and shopping center, metro carriages with and without air conditioning system) has been studied and compared with the air quality at the railway station, located outdoor at surface level, and with outdoor air at a nearby urban site. PM mass concentration was obtained by three methods: - determined on filter membranes by gravimetry; - estimated by number concentration measurements carried out by an optical particle counter (OPC); - calculated by adding individual chemical determinations (ions, elemental carbon, organic carbon, macro-elements and the bio-accessible and residual fractions of micro- and trace elements). Taking the gravimetric concentration as a reference, in all the subway environments the sum of the chemical analyses overestimated the gravimetric results, while the concentrations yielded by the OPC were remarkably lower (Figure 1a). These disagreements were mainly due to the prevalence of high-density iron particles in the subway samples, which was highlighted by light microscopy and scanning electron microscopy analysis. For the reconciliation between the gravimetric mass and the mass estimated from OPC data we modified the original OPC calibration factor. For the new calculations, we considered all particles having the shape of an ellipsoid, as observed by microscopy, and estimated the particle mass by multiplying the total particle volume by a factor ?, having the dimensions of a density, which was calculated as the weighted average of the densities of the main PM components in the subway indoor environments (3.4 g/cm3). To improve the performance of the chemical determinations we modified the algorithm used to obtain the mass closure, where macro-elements were all considered as oxides. The algorithm was modified by introducing elemental Fe, produced by wheels, rails and brakes abrasion, and considering as oxide only the amount of Fe produced by soil abrasion and soil re-suspension. The good agreement of the results obtained in this way by the three methods is shown in Figure 1b. The overall results of the study showed that train platform and carriages without air conditioning were the most polluted environments, with indoor/outdoor ratio up to two orders of magnitude for many components. The increase in the concentration of elements was mostly due to the residual fraction, indicating a low solubility of the chemical species in the PM samples. However, a significant increase was also observed for the bio-accessible fraction of some elements, particularly Ba, Zn, Cd and Ni. Macro-component concentrations were used to estimate the strength of the main macro-sources. The most significant contribution was found to derive from wheels, rails and brakes abrasion, (over 50% at the subway platform) from soil re-suspension and from organics. Metro system confirms to be a unique microenvironment, characterized by very high concentration and peculiar composition of atmospheric PM.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
