Magnetic- and particle-based techniques to investigate metal deposition on urban green

Highlights

• Leaf biomonitoring is a suitable tool to investigate metal pollution.

• Ivy leaves can be used for the identification of different land use classes.

• Leaf SIRM increased in the order forest < rural < roadside < industrial < train.

• Leaf SIRM relates with Fe, Zn, Pb, Mn, and Cd, particulate content.

• Combining magnetic- and particle-techniques may facilitate PM source attribution.

Abstract

Urban green works as a recorder of atmospheric PM. This paper reports on the utility of combining magnetic- and particle-based techniques to investigate PM leaf deposition as a bio-indicator of metal pollution. Ivy (Hedera helix) leaves were collected from five different land use classes, i.e. forest, rural, roadside, industrial, train. Leaf magnetic measurements were done in terms of saturation isothermal remanent magnetization (leaf SIRM), while ca. 40,000 leaf-deposited particles were analyzed through SEM/EDX to estimate the elemental composition. The influence of the different land use classes was registered both magnetically and in terms of metal content. Leaf area-normalized SIRM values ranged from 19.9 to 444.0 μA, in the following order forest < rural < roadside < industrial < train. Leaf SIRM showed to be significantly correlated (p < 0.01) with the content in Fe, Zn, and Pb, followed by Mn and Cd (p < 0.05), while no significant correlation was found with the metals Cr and Cu. Although presenting a similar metal content, roadside and train were magnetically very distinct. By exhibiting a very high content in Pb, and with an Fe content being comparable to the one observed at the forest and rural land uses, the industrial leaf-deposited particles showed to be mainly due to industrial activity. While SEM/EDX is a suitable approach for detailed particle analysis, leaf SIRM of ivy can be used as a rapid discriminatory tool for metal pollution. Their complementary use delivers further knowledge on land use classes reflecting different PM conditions and/or sources.

Introduction

Trace metals of anthropogenic origin are of particular interest within airborne particulate matter (PM), given its non-degradability in the environment (Qian et al., 2014). However, only a small number of air monitoring stations routinely measures metal concentrations (EEA, 2013). As well as polluting the air, metal particulates can get deposited on terrestrial and water surfaces, building up in soils or sediments, and ultimately leading to bioaccumulation in food chains (EEA, 2013). Therefore, the transfer of metals to the biosphere (Kocić et al., 2014) as constituents of PM is one of the most complex issues within the air pollution problem.

In a Flemish study on the chemical composition of PM₁₀, the elements Cr, Mn, Fe, Cu, Zn, and Pb, were identified as traffic-related species (Vercauteren et al., 2011). Several trace metals are emitted through the abrasion of tires (Cu, Zn, Cd) and brake pads (Cu), corrosion (Fe, Cu, Zn, Cd, Cr), lubricating oils (Cu, Zn, Cd) or fuel additives (Zn, Cd, Pb) (Tomašević and Aničić, 2010, and references therein). Pb is strongly associated with historic leaded-fuels usage, while Cu, Cd, and Zn, can also be identified as industrial or incinerator emissions, in addition to traffic (Zhang et al., 2012). Deposition of airborne metals (through wet or dry processes) is a major source of e.g. soil contamination, with urban soils serving as useful indicators of environmental pollution (Wang et al., 2012). The monitoring of soil pollution through geochemical methods (e.g. AAS and ICP-AES) involves the collection and processing of soil samples, which is laborious and time-consuming, making it difficult to perform large scale pollution mapping or monitoring (Dankoub et al., 2012, Xia et al., 2014). In the last decades, magnetic measurements have been increasingly used as a more simple, robust and cost-effective method to investigate soil contamination, allowing the study of extensive areas in short periods. The concept of environmental magnetism as a proxy for air pollution monitoring was first reported based on the analysis of soils, sediments and street or roof dust (e.g. Jordanova et al., 2003, Lu and Bai, 2006, Muxworthy et al., 2003, Petrovský and Ellwood, 1999, Thompson and Oldfield, 1986, Wang et al., 2012). Positive correlations between soil metal content and magnetic susceptibility have been established (Dankoub et al., 2012, Jordanova et al., 2003), and in some countries soil magnetic susceptibility mapping was applied for evaluating anthropogenic soil pollution (Kapička et al., 1999, Lecoanet et al., 2001). Similar correlations were observed by Hunt et al. (1984) between the saturation isothermal remanent magnetization (SIRM), which measures the ferro(i)magnetic fraction, and the metal component of atmospheric particulates, suggesting the use of magnetic parameters as facilitators on the identification and discrimination of PM sources. Because sources of magnetic particles (such as Fe oxides and/or sulfides, derived from e.g. combustion processes (Matzka and Maher, 1999)) and trace metals have shown to be closely related (e.g. Lu et al., 2007, Norouzi et al., 2016), their joint characterization may enable the identification of specific pollution sources.

When studying urban environments, where metal and PM pollution are of critical interest, the accessibility to soil samples can be highly hampered due to soil coverage by roads, pavements and buildings. Nonetheless, urban vegetation is usually widespread in cities, providing natural surfaces for deposition and immobilization of small atmospheric particles (Freer-Smith et al., 2005, Kardel et al., 2012, Litschke and Kuttler, 2008, Mitchell et al., 2010, Weber et al., 2014) by leaf deposition or in-wax encapsulation (Kardel et al., 2011, Terzaghi et al., 2013). Therefore, several studies have used vegetation samples (such as plant leaves) as magnetic bio-indicators of air pollution (e.g. Hofman et al., 2013, Kardel et al., 2011, Matzka and Maher, 1999, Mitchell et al., 2010, Moreno et al., 2003, Norouzi et al., 2016, Sagnotti et al., 2009, Vuković et al., 2015, Yin et al., 2013). A review on environmental magnetic studies of PM, with particular focus on magnetic biomonitoring using roadside plant leaves, was also made available (Rai, 2013). The use of e.g. plant leaves as indicators of urban PM, which are then submitted to magnetic techniques, provides a rapid and robust PM monitoring. On the other hand, particle analysis such as in terms of composition and size should not be overlooked, as these factors closely influence the PM impacts on human health. Scanning electron microscopy (SEM) can be used as a particle-based technique for such an investigation on leaf-deposited PM, as it allows the examination of plant surfaces at high resolution (Pathan et al., 2008).

The enrichment of magnetic particles has been associated with trace metals such as Cr, Mn, Fe, Cu, Zn, Cd, and Pb (Lu et al., 2007). However, depending on the local conditions not all metals will be present or occurring in the same proportion, which happens similarly for magnetic particles. Because the fractions of magnetic and metal particles tend to reflect the local conditions in terms of anthropogenic pollution, magnetic analysis as a measure of trace metal content was here applied to ivy leaves sampled from different land use classes. This paper reports thus on the magnetic and metal analyses of ivy leaves collected from different land uses (forest, rural, roadside, industrial, and train) in Antwerp, Belgium. The main objectives here addressed are as follows: (a) to discriminate the influence of different land use classes on PM leaf deposition, according to its magnetic behavior and metal content, (b) to examine the relationship between the magnetic behavior of sampled leaves and the trace metal content from the leaf-deposited PM, and (c) to illustrate the combined use of magnetic analysis with a particle analysis-based technique for a more integrated study of urban leaf-deposited PM.