Characteristics, Sources and Transformation of Metals in Atmospheric Particulate Matter


Student thesis: Doctoral Thesis

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Award date26 Nov 2018


Atmospheric particulate matters (PM) have been reported to cause a variety of adverse health effects depending on their physical and chemical characteristics. Particle bound metals, especially transition metals can initiate respiratory disease through oxidative stress pathways and trigger systemic inflammatory disease response. In this study, we firstly developed a wet-chemical analytical method to quantify the concentration of various metals in atmospheric particulate matters. The method development and validation included the selective extraction method of total metals and water soluble analytes from acid and water matrixes applied to the instrumentation of Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). An investigation of the abundance and solubility of metals in size-segregated PM using the developed method was conducted at a typical urban site during the winter in Kowloon Tong, Hong Kong. The samples were extracted by both strong acid and water, and fourteen elements including Al, Ca, Cd, Cr, Co, Cu, Fe, Pb, Mg, Mn, Mo, Ni, V, and Zn were analyzed using ICP-OES and ICP-MS. The metals in PM showed distinctly different profiles of their distribution between coarse particles (2.5 μm<dp< 10 μm) and PM2.5 (dp<2.5 μm). The upper continental crustal enrichment factors (CEFs) of the measured metals for two particle size fractions showed that CEFs for nine of fourteen metals in PM2.5 were higher than 10 while Cd, Pb, Zn, Mn, and Cu were far above 100; whereas for coarse particles, the CEFs of most elements were lower than 10, except for Cd which was higher than 100. Water and acid extractable fractions of coarse PM and PM2.5 were analyzed and compared to investigate the solubility of transition metals. The water extractable fraction was found to be present mainly in the fine particles, whereas more of the coarse fraction mass remained as insoluble fraction. The results of this study demonstrated a large variation in water solubility of metals in urban aerosols in different size fractions and highlighted that solubility is an important metric for considering the relationship between metals and adverse health effects in epidemiological and toxicological studies.

After the winter study of PM in KLT in HK, we understand that different factors contributed to the characterization and distribution of atmospheric metals in urban environments and this may lead to uncertainty in determining their impact on public health. However, few studies have provided a comprehensive picture of the spatial and seasonal variability of metal concentration, solubility and size distribution, all of which have important roles in their contribution to health effects. This dissertation further presents an experimental investigation on the characteristics of metals in PM2.5 and coarse PM in two seasons from four urban sites in Hong Kong. The PM samples were extracted separately with aqua regia and water, and a total of sixteen elements were analyzed using ICP-MS and ICP-OES to determine the size segregated concentration and solubility of metals. The concentrations of major metals were distributed in similar patterns with the same order of magnitude among different urban sites. Source apportionment using Positive Matrix Factorization (PMF) indicated that three sources namely road dust, vehicular exhaust, and ship emission are major contributors to the urban atmospheric metal concentrations in Hong Kong with distinctly different profiles between coarse PM and PM2.5 fractions. The individual metals were assigned to different sources, consistent with literature documentation, except potassium emerging with substantial contribution from vehicle exhaust emission. Literature data from past studies on both local and other cities were compared to the results from the present study to investigate the impact of different emission sources and control policies on metal distribution in the urban atmosphere. A large variation of solubility among the metals reflected that the majority of metals in PM2.5 were more soluble than those in coarse PM indicating size dependent chemical states of metals. The data from this study provides a rich dataset of metals in the urban atmosphere and can be useful for targeted emission control to mitigate the adverse impact of metallic pollution on public health.

In addition to the atmospheric PM metal characteristics, different urban traffic microenvironments also have their significance in public health implications for daily commuters. To investigate the chemical properties of particulate matter (PM) in different public transport microenvironments in Hong Kong, coarse and fine PM samples were collected from three different types of transport modes including Mass Transit Railway (MTR)-Aboveground (AG), MTR Underground (UG) and Bus routes from October 2013 to April 2014. Average PM2.5 concentrations through the UG, AG and Bus routes were 47.9, 86.8 and 43.8 μg m-3, respectively, whereas the coarse PM concentrations were 4-5 folds less. The total metal concentrations in PM2.5 of AG route were 2.3 and 3.7 times that of UG and BUS routes, respectively. The most abundant metals at three stations in PM2.5 and coarse PM were quite similar and mainly generated by frictional processes of wheels, rails, and brakes of the system as well as by the mechanical wearing of these parts. The most abundant PAH in three routes in PM2.5 was ATRQN, followed by 2-MNA, and the sum of them contributed to 35% and 42% of total PAHs in coarse PM and PM2.5, respectively. Crude oils, lubricant oil, and diesel emissions would be the major sources of PAHs from MTR aboveground stations. The relative abundance of the n¬-Alkanes among different samples was similar to the PAHs and the carbon preference index (CPI) values of the whole n-alkanes range were consistently from 0.99 to 1.04 among all samples, indicating the significant contribution from the vehicle exhaust and fossil fuel burning. The concentrations of hopanes and steranes were higher in PM2.5 than in coarse PM due to diesel and coal burning. These results may provide a unique opportunity to investigate the source-specific contribution of the PM pollutants to the commuter exposure in public transport.

In the previous section, we found that iron is one of the most abundant transition metals in PM in both the urban environment and MTR system in HK. The oxidation state of iron affects the PM toxicity and water solubility to organisms depending on the reactivity and solubility of iron species. A high precision and accuracy method for iron detection is very necessary. However, the previous detection methods of iron in other studies are not selective to the oxidation state of iron and are costly. Moreover, they cannot be directly applied to iron speciation in atmospheric aerosols due to challenging to detection limit at a low concentration level of the particle bound iron in the atmosphere. We therefore present the development and evaluation of a dispersive absorption spectroscopic technique for trace level soluble ferrous detection. The technique makes use of the broadband absorption spectra of the ferrous-ferrozine complex with a novel spectral fitting algorithm to determine soluble ferrous concentrations in samples and achieves much improved measurement precision compared to conventional methods. The developed method was evaluated by both model simulations and experimental investigations. The results demonstrated the robustness of the method against the spectral fluctuation, wavelength drift, and electronic noise, while achieving an excellent linearity (R2 > 0.999) and a low detection limit (0.06 mg L-1) for soluble ferrous detection. The developed method was also used for the speciation of soluble iron in size segregated atmospheric aerosols. The measurement was carried out during Spring and Summer in the typical urban environment in Hong Kong. The measured total iron concentrations are in good agreement compared to conventional ICP-OES measurements. 

Investigation of ambient particulate matter samples shows the size dependent characteristic of iron speciation in the atmosphere with a more active role of fine particles in transforming between ferrous and ferric. The method demonstrated in this study provides a cost and time effective approach for the speciation of iron in ambient aerosols. The investigation on soluble iron speciation, lab simulation of transformation between Fe(III) and Fe(II) and oxidative potential of particle iron were carried out based on the air particle samples collected from Hong Kong, Xi’an and Beijing using a customized PM sampler designed in this study. The PM extracts were obtained from both water extraction using a multi-tube vortex mixer, and acid extraction using a microwave digester. The concentration of total iron in acid based samples were determined using ICP-OES. The concentration of Fe(II) and the proportion of Fe(III) that transformed into Fe(II) were determined using a LED-based liquid waveguide capillary cell hyphenated spectrometer. Photo-irradiation was conducted in a UV chamber to simulate the photo-transformation reaction between Fe(III) and Fe(II). Water soluble PM extract was used for the analysis of iron concentration and the measurement of reactive oxygen species (ROS).Our results revealed that the average percentage of total soluble iron in the daytime samples was 2.7% higher than that in the nighttime samples based on the data analysis of water soluble iron speciation due to a longer period of exposure to solar radiation in daytime. The ratios of soluble Fe(II) to total soluble iron fell by an average value of 63.7%, indicating that soluble Fe(II) dominated in water soluble iron, while the average ratio was 8.1% higher in daytime than in nighttime. The change in the percentage of soluble Fe(II) followed the aging time as indicated by the lab simulation of transformation between Fe(III) and Fe(II), where an increasing trend of Fe(II) was obtained in terms of UV irradiation compared to that without UV irradiation. It was confirmed that Fe(II) and Fe(III) underwent chemical change in a short period of time since UV was the key factor in influencing the transformation between Fe(III) and Fe(II). Our results further exhibited that after 48 h of UV irradiation, only a small part of the soluble Fe(III) transformed to soluble Fe(II) whereas most of the soluble Fe(II) were from the transformation of Fe(III) and Fe(II) in the solid phase depending largely on the form and structure of iron in the PM samples. On average, ROS production was ~34.3% higher in samples collected during daytime than nighttime due to the higher UV index and longer UV irradiation in daytime. By considering the Fe(III) reduction is a slower process compared to Fe(II) oxidation, the amount of ROS measured in PM samples could be mainly from Fe(II) oxidation. The result of ROS measurement for Fe(II) in Mill-Q water further proved that after UV irradiation ROS intensity increased with the increase in concentration of Fe(II). In addition, other soluble formation of chemicals in PM may be considered as the minor contributors. Our experimental data also confirmed that almost all the ROS detected were induced by transition metals, i.e. Fe in this study, and UV irradiation can induce the transformation to generate soluble Fe(II) from solid phase to liquid phase and thus contribute the increased formation of ROS.