Factors, origin and sources affecting PM1 concentrations and composition at an urban background site
Introduction
In recent decades, the characterisation of airborne particulate matter (PM) has become an increasingly important topic of research since the epidemiological data have showed that PM has negative effects upon human health (Anderson et al., 2012, Rohr and Wyzga, 2012). Fine particles (with aerodynamic diameters of less than 2.5 μm, PM2.5, and 1 μm, PM1) may play an important role in affecting human health for a number of reasons: (i) they penetrate more effectively into the deep lung; (ii) they can penetrate more readily into indoor environments; (iii) they can remain suspended for longer periods of time in the atmosphere than coarse particles; (iv) they may be transported over long distances; (v) they tend to carry higher concentrations of the more toxic compounds, including acids, heavy metals and organic compounds; and (vi) they have a larger surface area per unit mass compared to larger particles and, thus, can absorb larger amounts of semi-volatile compounds (e.g., Pope and Dockery, 2006). Consequently, the study of levels, composition and emission sources of fine particles in densely populated areas is very important for health protection and to improve PM control strategies.
Fine particles are typically mainly composed of elemental carbon (EC), organic carbon (OC), inorganic ions and metals. Among these major components, some studies have associated carbonaceous particles with health effects (Rohr and Wyzga, 2012). These consist of compounds from combustion exhaust, soil, paved road dust, cooking and other sources (Harrison and Yin, 2008). EC comprises small (mainly sub-micrometre) graphitic particles which arise from primary emissions from combustion of various fuels, e.g., coal, wood, fuel oil and motor fuel, especially diesel. Organic carbon (OC) can be directly emitted into the atmosphere in the particulate phase or can originate in the atmosphere from gas-to-particle conversion processes (forming the so-called secondary organic aerosol, SOA) (Seinfeld and Pandis, 2006). Generally, EC lies in the submicrometre range, whereas OC exhibits wider size distributions (Pio et al., 2007).
Inorganic ions can be emitted from various primary sources, such as combustion, sea salt and crustal material. However, the main source of sulphate, nitrate and ammonium is the generation of secondary inorganic aerosol (SIA) through (photo-) chemical reactions of gaseous precursors (NOx, SO2, NH3) and O3 with atmospheric oxidants to form mainly ammonium nitrate (NH4NO3) and ammonium sulphate ((NH4)2SO4)) (Seinfeld and Pandis, 2006).
Although metals generally represent a small fraction of PM1 mass, their contribution to the overall toxicity of particles cannot be disregarded. The chemical and physical properties of some elemental species is size-dependent: for example, the solubility of Pb, Co and Cd increase in fine particles, making those elements more bioavailable (Birmili et al., 2006). The characterisation of elemental composition is also very important for source apportionment studies: the variable proportion of some well known elemental markers can help in the identification of potential sources using receptor modelling techniques.
Although it has been suggested that PM1 can provide a better estimation of anthropogenic particles than PM2.5 (Perrone et al., 2013), PM1 source apportionment studies are still limited in Europe as well as comprehensive information about its composition (e.g. Pérez et al., 2008, Vecchi et al., 2008, Theodosi et al., 2011, Perrone et al., 2013). Moreover, PM1 is not yet regulated in Europe and this is a major reason why there is a lack of available data and studies upon it. This is a serious gap, as some air pollution hotspots still remain in highly populated areas of Europe. Among others, the Po Valley (Northern Italy) deserves particular attention because of the frequent exceedance of guidelines and Limit Values fixed by EC Directives and international organizations such as WHO (Larsen et al., 2012). The present paper aims to investigate the composition of PM1 in Mestre-Venice, a large city on the eastern border of the Po Valley. Here, the highest concentrations of particulate matter and nitrogen oxides (NO + NO2 = NOx) are commonly recorded in winter, while high levels of ozone are measured in summer due to photochemical processes involving precursors of natural and anthropogenic origin (Masiol et al., 2014a, Masiol et al., 2014b).
PM1-bound elemental and organic carbon, water soluble inorganic ions and elements, have been analysed to determine the major contributors to PM1 mass and were then processed to (i) determine the seasonal cycles, (ii) estimate secondary inorganic and organic aerosol, (iii) assess the major components applying a mass closure model, (iv) identify and quantify the most probable sources by using a receptor modelling technique (positive matrix factorization, PMF) and (v) hypothesize their location applying a conditional probability function.
Section snippets
Measurement site
Venice is located between the eastern edge of the Po Valley and the Adriatic Sea. Along with the city of Mestre, they form a large coastal urban municipality hosting 270,000 inhabitants (~ 628 inhabitants km− 2) (ISTAT, 2011). The local emission scenario includes some major potential sources of PM: high density residential areas; heavily trafficked roads mostly congested during peak hours; a motorway and a motorway-link which are part of the main European routes E55 and E70; an extended
PM levels and main components
Table 1 summarizes the average seasonal concentrations of PM1 and its components, gaseous pollutants and meteorological variables. The average PM1 concentration over the whole period was 21 ± 22 μg m− 3. PM1 concentrations show the typical pattern of the study area, lower in warmer seasons and higher in the colder period (6.4 ± 2.2 μg m− 3 and 34 ± 24 μg m− 3 mean, respectively).
Measured levels are comparable with those observed by Pérez et al. (2008) (19 μg m− 3 annual mean) at an urban background site in
Conclusions
PM1 samples have been collected at an urban background site in Venice (Po Valley, Italy), within one of the most polluted areas in Europe. The main components of PM were determined: water soluble inorganic ions, elements, elemental and organic carbon. Collected data have been processed to (i) determine seasonal behaviour, (ii) estimate secondary inorganic and organic aerosol, (iii) estimate major components applying a mass closure model and (iv) identify and quantify most probable sources by
Disclaimer
The views expressed in this study are exclusively of the authors and may not reflect those of ARPAV.
Acknowledgments
The authors would to thank Prof. E. Argese, L. Gobbo for the analytical support. Ente della Zona Industriale di Porto Marghera supplied weather data.
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