Magnetic differentiation and quantification of airborne particulate matter from vehicular brake systems

Gonet, Tomasz and Maher, Barbara (2021) Magnetic differentiation and quantification of airborne particulate matter from vehicular brake systems. PhD thesis, Lancaster University.

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Abstract

Exposure to particulate air pollution poses a threat to pulmonary, cardiovascular and neurological health, being associated significantly with increased morbidity and premature mortality worldwide. Ultrafine, metal-rich particles (UFPs; < 0.1 µm) are especially hazardous to humans as they can reach major organs in the human body, including the heart and brain, and are highly bioreactive, leading to increased oxidative stress and inflammatory response in the human tissues. Fe-bearing (and especially Fe2+-bearing) UFPs might be especially important. They are reportedly associated with neurodegeneration and Alzheimer’s disease (AD). Moreover, they are usually co-associated with other toxic metals and organic species. It is thus both timely and important to identify and quantify the sources of Fe-rich UFPs in the urban environment. Here, the current knowledge regarding the sources of vehicle-derived Fe-bearing UFPs, their composition, particle size distribution and potential hazard to human health is first reviewed (Paper I). This chapter focuses on the data reported for the following sources of Fe-bearing UFPs: brake-wear emissions, engine-exhaust emissions (both diesel and petrol), tyre and road surface wear, resuspension of roadside dust, underground, train and tram emissions, and aircraft and shipping emissions. Brake-wear emissions were identified as one of the major sources of Fe-bearing UFPs in urban environments. While magnetite (Fe3O4 = FeO·Fe3O4) is known to be one of the most common Fe2+-bearing minerals in urban particulate matter (PM), its sources have not so far been precisely quantified. Here, a set of roadside dust samples (from roadside and urban background sites in Birmingham and Lancaster, U.K.), engine-exhaust emissions (both petrol and diesel), and dynamometer-generated brake-wear PM emissions were collected, to quantify the contributions made by specific traffic-related sources to the total airborne magnetite at the roadside (Paper II). The concentration of magnetic grains, as measured by magnetic remanence (SIRM), is notably higher (i.e. ~100 – 10,000 times higher) for brake-wear emissions, compared to other types of PM pollutants in most urban environments. A detailed AF demagnetisation of SIRM and magnetic component analysis allowed the separation of the magnetite signal from other contributing magnetic components, and the subsequent quantification of magnetite in the roadside dust and engine-exhaust PM samples. The mass concentration of magnetite in petrol-engine exhaust emissions is ~0.06 – 0.12 wt.%; in diesel-engine exhaust emissions ~0.08 – 0.18 wt.%; in background dust ~0.05 – 0.20 wt.%; and in roadside dust ~0.18 – 0.95 wt.%. In contrast, magnetite constitutes as much as ~20.2 wt.% of brake-derived PM10 (PM with aerodynamic diameter < 10 µm). Based on these calculations and reported source apportionment of PM10 at the roadside, I show that vehicle brake-wear is by far the most dominant source of airborne magnetite at the roadside of two U.K. cities (Lancaster and Birmingham), contributing ~77% and ~85% of total airborne magnetite at the Lancaster and Birmingham roadside sites, respectively. In comparison, petrol-engine exhaust emissions account for ~2 – 4%, diesel-engine exhaust emissions ~7 – 12%, and background dust ~6 – 10%. Paper III examines morphological, structural, chemical and magnetic properties of size-resolved brake-wear PM emissions. A set of dynamometer-generated brake-wear particulate emissions was collected, in a wide size range, from 16 nm up to 10 µm (in 14 size fractions). Using magnetic component analysis, low- and high-temperature magnetic measurements, and electron microscopy, the mass concentration of metallic Fe (α-Fe) was estimated to be ~1.6 wt.% in brake-derived PM10. Magnetite content reached levels of ~20 wt.% for particles > 0.600 µm and decreased to 2 – 15 wt.% for particles < 0.380 µm. Most brake-derived airborne particles are smaller than 200 nm (> 99% of particle number concentration). Moreover, even larger fractions (e.g. ~2.5 µm) are dominated by agglomerated UFPs ~10 – 50 nm in size, as demonstrated by electron microscopy. Overall, my thesis demonstrates that vehicle brake systems are one of the major sources of Fe-bearing UFPs in urban environments. Such UFPs (especially Fe2+-bearing magnetite, often co-associated with other potentially toxic metals and organic species) might pose a particular threat to neuronal and cardiovascular health. The observed abundance of magnetite in the ultrafine size fraction of brake-wear PM (estimated to be ~7.6 wt.% of PM < 200 nm) might be especially hazardous to the human brain, where ultrafine magnetite particles have been reported to have a causal link with neurodegeneration and Alzheimer’s disease (through generation of excess oxidative stress). Prospectively, given the potential risk to human health, the high concentrations of magnetite, specifically, in brake-wear PM might need to be reduced in order to mitigate such risk, especially for vulnerable population groups.