Sources of high PM2.5 concentrations in Milan, Northern Italy: Molecular marker data and CMB modelling
Highlights
► High PM2.5 concentrations determine harmful health effects. ► Information on the impacting sources are needed for PM2.5 reduction strategies. ► Source apportionment for PM2.5 in the polluted Northern Italy was performed by CMB. ► Local and diffuse sources were estimated. ► The relevance of secondary aerosol in summer and winter is discussed.
Introduction
North Italy is one of the most industrialized and populated regions in Western Europe and is characterized by high levels of atmospheric pollution.
Harmful health effects have been associated with exposure to particulate matter, especially to fine particles (Schwartz et al., 2002, Anderson et al., 2001, Klemm et al., 2000). Thus concentrations of PM2.5, rather than PM10, are indicated as the principal means of assessing particle exposure. In Europe, the recent Air Quality Directive (2008/30/CE) establishes an annually averaged PM2.5 concentration to be reached by 2010 of 25 μg m− 3, which will be the legal limit value from 2015 onwards. PM2.5 accounts for a large part of PM10, and in Milan the annually averaged concentration of PM2.5 is 30 μg m− 3 (2009) (ARPA Lombardia, 2009). PM2.5 control is a challenging problem, especially in urban areas where large populations are exposed to high concentrations. In order to design effective PM2.5 reduction strategies, information on the strength of impacting sources is required.
Many studies have been conducted to investigate fine particle pollution in Northern Italy. Many studies have been made of PM2.5 and its chemical composition, especially the carbonaceous fraction elemental carbon (EC) and organic carbon (OC) (Lonati et al., 2005, Lonati et al., 2007, Lonati et al., 2008, Fermo et al., 2006, Giugliano et al., 2005), secondary inorganic ions (Lonati et al., 2005, Perrone et al., 2010) and elements (Lonati et al., 2005, Marcazzan et al., 2001, Vecchi et al., 2007, Vecchi et al., 2008). A question still remains regarding the chemical speciation of organic matter (OM), which accounts for a large part (20–40%) of fine PM mass (Putaud et al., 2004).
In order to understand the properties of OM, such as its toxicological effects, detailed knowledge of chemical composition is required. Some trace organic compounds, despite their low concentrations and their minor contribution to total PM mass, have been indicated as important in determining toxicological effects associated to PM2.5 exposure in polluted areas. For example, Hannigan and Cass (1998) acknowledged that polycyclic aromatic hydrocarbons (PAHs) account for most of the mutagenic potency that can be assigned to specific compounds in atmospheric particles in urban areas.
Some trace organic compounds are useful for source characterization. Molecular markers are individual organic species with a high degree of source specificity, making them suitable for use in source apportionment analysis (El Haddad et al., 2011, Yin et al., 2010, Chow et al., 2007, Sheesley et al., 2007, Fraser et al., 2003, Zheng et al., 2002 and many others). Other organic compounds are not directly emitted by a specific source, but they can be formed in the atmosphere as a result of chemical reactions (secondary source). This is true of carboxylic acids: in Europe, low-molecular-weight carboxylic acids (C2–C5) have been recognized as originating mainly from secondary photochemical formation (Legrand et al., 2005, Rohrl and Lammel, 2001).
In order to study PM2.5 mass concentrations and chemical compositions in Northern Italy, we conducted measurement campaigns at an urban site in Milan, a rural site and a remote site, over a three-year period (2006–2009). In Milan, PM10 was measured in conjunction with PM2.5 in order to characterize both particle-size fractions.
Chemical mass balance modelling (CMB) was applied to PM chemical composition data in order to quantify the major PM2.5 sources at the three sites during different seasons, and the major PM10 sources in Milan. Organic molecular markers were used in CMB modelling to estimate the contributions of primary sources to OC, after which the PM mass from each source was reconstructed by adding other primary components (e.g. EC and primary sulphate, nitrate and ammonium) to the OC, as also described in other works (El Haddad et al., 2011, Sheesley et al., 2007).
Results from CMB modelling were compared with results obtained using other methods employing molecular marker data (e.g. PAHs and the n-alkanes molecular-based approach) for the source apportionment of trace organic compounds. Many trace organic compounds in the particulate phase were included in the study i.e. carboxylic acids, PAHs, n-alkanes and levoglucosan. To our knowledge, there are no other studies providing such detailed characterizations of trace organic compounds and their sources in PM from this area. Our data provide new, important knowledge regarding the seasonal chemical speciation of OM in PM2.5 for sites subject to different anthropic influences.
Section snippets
Site locations
PM2.5 samples were collected at three locations in Northern Italy: at an urban site (MI), at a rural site (OB), and at a remote site (ASC) (Fig. 1). The MI site (MI; 45°31′19″N, 9°12′46″E) was set-up in the Milan area known as “Torre Sarca”, which is also the location of the Milan-Bicocca University; it is highly representative of Milan's characteristic traffic conditions. The OB site (OB; 45°08′40″N, 10°26′08″E) was located within the “Oasi Le Bine” nature reserve , far from any major city
Overview of PM2.5 concentrations and chemical composition
PM concentrations were higher during winter and lower during summer at the MI and OB sites. This seasonal trend is typical for Northern Italy, and is strongly influenced by seasonal meteorology: during the fall, and in particular during the winter, conditions of atmospheric stability with low mixing layers cause very high concentrations of atmospheric pollutants at ground level (Ferrero et al., 2010). On the contrary, during the summer, higher average wind speed and a broader mixing layer
Conclusions
In the present study we have estimated the contribution of major pollution sources to ambient particulate matter at three sites in Northern Italy. Our findings for the urban site in Milan, where air quality limits are frequently exceeded, provide important information for the design of efficient abatement policies. Traffic was found to be the strongest primary source (17–24%) for PM2.5, together with secondary inorganic and organic aerosol (21–54%), and biomass burning including residential
Acknowledgments
The authors would like to thank the Lombardy Region (VESPA project) and the Italian Space Agency ASI (QUISAT project) for the research funding received.
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