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Heavy Metal Speciation Analysis and Distribution Characteristics in Atmospheric Particulate Matters

Anen He , Jiaojiao Xie , Chungang Yuan

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (9) : 1627 -1647.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (9) : 1627-1647 DOI: 10.7536/PC200836
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Heavy Metal Speciation Analysis and Distribution Characteristics in Atmospheric Particulate Matters
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Abstract

Heavy metals (HMs), as one of the important toxic components in atmospheric particulate matters (PMs), are closely related to human health. The health risk of HMs in particulate matters highly depends on their species and bioavailability. Heavy metal speciation analysis in PMs is significant for the study of air pollution environmental health. Based on related researches in recent years, this paper summarizes and discusses about HMs species in atmospheric particulate matters from four key aspects: (1) Synthetic body fluids and sequential extraction procedures (such as BCR, Tessier's, Chester) have been widely used to extract operationally-defined species of HMs; (2) Chromatography-mass spectrometry technology and new functional materials have been applied for specific-selective analysis, and in-situ speciation and atomic cluster structures of HMs can be characterized by XAFS (X-ray absorption fine structure); (3) Particle size distribution of HMs species in PMs is complicated, which is affected by many factors and tends to be concentrated in fine particles; (4) Spatial and temporal distribution characteristics of HMs in PMs are highly regional. Social development, industrial, and climate are the main factors. Health risks at summer and hazy days are relatively higher.

Contents

1 Introduction

2 Heavy metal speciation analysis methods

2.1 Operationally-define speciation

2.2 Specific-selective speciation

2.3 In-situ speciation

3 Heavy metal speciation distribution characteristics

3.1 Spatial distribution characteristics

3.2 Temporal distribution characteristics

3.3 Particle size distribution characteristics

4 Conclusion and outlook

Graphical abstract

Keywords

particulate matters / heavy metal / speciation / bioavailability

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Anen He, Jiaojiao Xie, Chungang Yuan. Heavy Metal Speciation Analysis and Distribution Characteristics in Atmospheric Particulate Matters. Progress in Chemistry, 2021, 33(9): 1627-1647 DOI:10.7536/PC200836

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1 Introduction

With the rapid development of social economy, the problem of air pollution has been widely concerned by scholars at home and abroad, especially in recent years, haze weather occurs frequently in China, and the environmental health effects are prominent[1⇓~3]. Atmospheric particulate matter has a wide range of sources, including natural sources (crustal weathering, sea salt aerosol generation, volcanic eruptions, etc.) And anthropogenic sources (fossil fuel combustion, industrial activities, solid waste incineration, vehicle exhaust, etc.)[4,5]. During the formation of atmospheric particles, a large number of toxic components are adsorbed and enriched, including heavy metals such As arsenic (As), lead (Pb) and chromium (Cr), soluble salts such as chlorides, sulfates and nitrates, organic compounds such as polycyclic aromatic hydrocarbons, carboxylic acids and quinones, and carbohydrates produced by biomass combustion, among which heavy metals are important toxic components in particles[6⇓⇓⇓~10]. The International Agency for Research on Cancer (IARC) classifies heavy metals in the environment into three categories: carcinogens, suspected carcinogens and non-carcinogens according to their potential hazards to human body. Arsenic (As), cadmium (Cd) and chromium (Cr) are Class I heavy metals, which are closely related to lung cancer, bladder cancer, kidney cancer, skin cancer, liver cancer and prostate cancer[11][12]. Nickel (Ni), lead (Pb) and mercury (Hg) are classified as Class II heavy metals[11,13]. At present, most of the studies on heavy metals in atmospheric particulates focus on the analysis and determination of total concentrations, and less attention has been paid to their speciation[14⇓⇓~17]. However, the environmental transport behavior, bioavailability and toxicity of heavy metals are closely related to their physical and chemical forms, so the health risk assessment based on the total concentration of heavy metals can not fully reflect the real situation[18⇓~20].
Air pollution is a long-standing environmental pollution problem in China, and it is also a high-risk lethal factor[21,22]. According to Lancet, a top international medical journal, more than 1.2 million premature deaths (coronary heart disease, stroke, chronic obstructive pulmonary disease, lung cancer) in China are related to air PM2.5[23]. The identification of toxic components in PM2.5 is an important part of understanding the environmental health effects of air pollution. In addition, the latest toxicological studies have pointed out that some toxic elements (As, Cd, Cr, Ni, Pb) in fine particulate matter are related to gene damage, cardiovascular disease and cancer induction, and the environmental toxicological mechanisms of different forms of pollutants are completely different, resulting in huge differences in environmental health effects[24][25][26,27][20,28,29]. Among them, the soluble elements of PMs are of priority concern because of their higher bioavailability and greater potential toxicity. A number of epidemiological and toxicological studies have reported various cardiopulmonary and toxicological effects associated with water-soluble heavy metals in PMs, most notably Ni, zinc (Zn), copper (Cu), vanadium (V) and iron (Fe)[30⇓~32]. Kodavanti et al. found that after lung exposure to HP-12 (PMs produced by fuel oil), many heavy metals will enter the systemic circulation and accumulate in extrapulmonary organs (such as heart and liver) within 24 hours, and water-soluble Zn will lead to the disorder of essential metals (Cu, selenium (Se))[33,34]. EHP, a leading journal in the field of environmental health, reported that exposure to particulate matter containing water-soluble Zn led to increased lung inflammation and mitochondrial DNA damage in rats.Moderate mRNA changes in genes involved in signal transduction, ion channel function, oxidative stress, mitochondrial fatty acid metabolism, and cell cycle regulation suggest that water-soluble Zn is associated with an increased risk of heart disease[35]. Wong et al. Found that soluble Ni and V can affect cardiovascular and respiratory systems by regulating oxidative stress-related genes and disrupting the expression of tyrosine protein kinases, respectively[36]. Studies by Kastury et al. Have shown that after inhalation of Pb-containing dust, the transformation of Pb forms may occur in the lungs and gastrointestinal tract, leading to chronic lung poisoning and fibrosis[37]. Moreover, soluble heavy metals (Cu, Fe (Ⅱ), Fe (Ⅲ), V, Zn) can also shorten the whole blood clotting time, and there may be a synergistic effect between different heavy metals, which may lead to thrombotic diseases such as atherosclerosis[38]. Hg is an important neurotoxin, and studies have shown that methylmercury can cross the placental barrier and cause damage to infant brain development, including motor, language and memory abilities, through mother-to-child transmission[39]. In addition, methylmercury and inorganic mercury can stimulate lymphocytes and lead to immune system disorders, increasing the risk of cardiovascular disease[40]. Water-soluble Hg (Ⅱ) is one of the strongest thiol binders, which causes apoptosis in renal tubular epithelial cells at low concentrations and has long-term effects on renal function[40]. Therefore, in the process of identifying the toxic components of atmospheric particulate matter, in addition to the analysis and determination of the total amount of elements, the more important thing is to obtain the information of their speciation distribution and bioavailability.
In this paper, the research progress on the speciation of heavy metals in atmospheric particulates was reviewed, and the speciation analysis methods and speciation distribution characteristics of heavy metals in atmospheric particulates were summarized in detail, which provided a reference for the prevention and control of air pollution and the study of environmental health effects.

2 Analysis method of heavy metal speciation in atmospheric particle

2.1 Operation-defined morphological analysis method

The "operation definition form" referred to here is to define the existence state of elements through artificial operation, generally using a single or mixed reagent to extract samples, and distinguish them according to the difficulty of extraction. This definition does not involve the characterization of physical parameters such as particle morphology and size[41,42].

2.1.1 One-step extraction method

At present, the extraction methods widely used for heavy metal speciation in atmospheric particles can be roughly divided into two categories: single-step extraction method and stepwise extraction method. In order to determine the bioavailability of heavy metals, in recent years, scholars at home and abroad have used different extractants to extract soluble heavy metals in a single step. The simplest extractant is a H2O, later modified to a buffered salt solution and a dilute acid. With the application of artificial simulated body fluid, serum, gastric juice and lung fluid, the bioavailability of heavy metals in atmospheric particulates can be assessed by simulating the internal environment of human body. Table 1 lists the extractants and their operating procedures for soluble heavy metals in atmospheric particulates in the literature.
Generally speaking, water-soluble heavy metal ions are most likely to migrate to organisms[45]. The water extraction process can well simulate the release of heavy metals from particulate matter under neutral conditions[47]. Water-soluble heavy metal ions, such as Cu, Mn and Ni, can induce the production of free radicals or reactive oxygen species (ROS) in vivo, resulting in inflammatory response in organisms[62⇓⇓~65]. Costa and Dreher found that water-soluble transition metal ions, rather than particulate matter itself, play a decisive role in the acute inflammatory response in rats[66]. Weber et al. quantified the ROS generation potential of atmospheric particulate matter in the southeastern United States using dithiothreitol (DTT) measurement, and linear regression showed that water-soluble organic carbon, biomass combustion markers, and water-soluble heavy metals with redox capacity were significantly correlated with the ROS generation potential (p < 0.01)[63]. Dunea et al pointed out that the PM2.5 produced by the steel and metal processing industry was directly related to the asthma of nearby children, and the increase of immunoglobulin E (lgE) and eosinophil levels in the body was significantly related to heavy metals such as Fe, Ni and Cd in fine particles (p < 0.01)[26]. Although this method has been widely used for a long time, the results provide limited information on the bioavailability of heavy metals in atmospheric particulates because water does not represent the real internal environment of human body fluids.
In order to improve the deficiency of pure water as a single extractant, salt solution, buffer solution and chelating agent were used to extract bioavailable heavy metals. Henirich et al. extracted soluble platinum (Pt) in mice with physiological saline (0.9% NaCl), and found that Pt was detected in lungs, kidneys, livers, spleens, stomachs, blood, feces and urine during an 8-day animal experiment. Through gel chromatography, low-molecular-weight complexed Pt was observed in the lungs of mice, while protein-bound Pt was found in the plasma[50]. Canepari et al. Used four different extractants to extract 18 elements such As As, Cd, Cr, Cu and Pb from PM10 samples, and studied and compared the extraction efficiency of acetate buffer, deionized water, dilute nitric acid and ethylenediaminetetraacetic acid (EDTA) for heavy metals in particulate matter. The results showed that acetate buffer was the most suitable for extracting heavy metals in PM10[49]. Voutsa et al. Used dilute HCl as an extractant to compare and analyze the bioavailability of Cd, Cu, Mn, Ni, Pb and Zn in particulate matter around cities and industrial sites in northern Greece, proving that urban particulate matter has a high potential health risk[53].
Compared with the in vivo test, the in vitro simulation test can provide toxicological information quickly, and the experimental process is simple and the cost is low. In order to obtain further information on the bioavailability of heavy metals in particulate matter, many studies have used simulated body fluid (SBF) to semi-quantitatively assess the extent to which heavy metals may be absorbed by the human body. Simulated body fluid is a reagent prepared based on the reaction conditions of the internal environment of the human body, and the operation is generally carried out at 37 ℃ in the dark. Atmospheric particles can enter the human body through inhalation, oral ingestion and skin contact, of which inhalation is the most important exposure route, so the lung is the most important target organ[56]. For the respiratory system, artificial simulated lung interstitial fluid (SLF), simulated lysosomal fluid (ALF), and Gamble's solution are commonly used extractants. Very low concentrations of HCO 3 - were maintained in the SLF to avoid changes in ionic composition and pH of the solution due to atmospheric loss of CO2, while 4-hydroxyethylpiperazine-ethanesulfonic acid buffer (HEPES) mimicked organic acids and metabolites in the lung fluid. The pH value of Gamble solution was 7.4, which could simulate the neutral condition of lung interstitial fluid. ALF is more acidic (pH = 4.5) and represents a pulmonary immune response and the presence of associated active macrophages. Gianfagna et al. Used SLF to study the dissolution of heavy metals in Italian urban PM2.5. Cd, Pb, Ni, As, Cu and Zn were detected at high frequency in the leaching solution[57]. Anastasio et al. Used SLF to study the key elements of particulate heavy metals in the formation of ROS in the lungs, and the results showed that Cu and 1,2-benzoquinone produced a large number of ROS at typical environmental concentrations[58]. Colombo et al. assessed the risk of respiratory ingestion of platinum group elements (PGE) in road dust and ground automotive catalysts (VECs) by Gamble solution and ALF[60]. The results showed that the concentration of PGE was higher in acidic ALF than in neutral Gamble solution, the percentage of soluble PGE in road dust was much higher than that in VECs, and the bioavailability of palladium (Pd) and rhodium (Rh) was higher than that of platinum. Hu and Lian compared the bioavailability of Pb in PM2.5 in vitro (Gamble, SLF, ALF and SELF (simulated epithelial lung fluid)) and in vivo. Among them, the results of Gamble solution in vitro were most similar to those of mice in vivo. DTPA, an amino acid in Gamble solution, was screened as an effective component for extracting Pb in vitro[59].
The exposure of atmospheric particulate matter through diet should not be ignored[67,68]. A physiological-based extraction test (PBET) was applied to simulate the digestive system. Cao et al analyzed the PBET of fine particulate matter in three functional areas (commercial, residential and industrial areas) in Shanghai. The dissolution fractions of Cu, Zn, Pb and Mn in PM2.5 and PM10 were 24.6% ~ 90.9% and 28.5% ~ 88.9% in the stomach environment, while those in the intestinal tract were 8.7% ~ 85.5% and 8.5% ~ 81.8%[56]. Among them, the risk of Pb intake exposure in children was higher than that of respiratory exposure.

2.1.2 Stepwise extraction method

The single-step extraction process is simple and has certain practicability and effectiveness, but the multi-form distribution information of heavy metals can not be obtained. The bioavailability of heavy metals represents the ease with which they enter the environment and organisms and the activity of their participation in biogeochemical cycles. The release characteristics of heavy metals in particulate matter are different due to their different physical and chemical States, and the order of ion exchange, physical desorption and dissolution release is also different, which can be used to distinguish the different forms of heavy metals. Sequential extraction protocols were first used for heavy metal speciation in soils decades ago[69,70]. In recent years, stepwise sequential extraction has also been successfully applied to the speciation analysis of heavy metals in atmospheric particulates, including the Tessier method and the BCR three-step method[71⇓~73][74][75,76]. After the Tessier method was proposed, researchers have continuously improved and optimized the step-by-step extraction scheme[77⇓⇓~80]. After years of development, the optimized sequential extraction scheme has been accepted by most scholars. The specific scheme and application are shown in Table 2.
It can be seen from Table 2 that the heavy metal forms extracted in the first step have the strongest correlation with bioavailability in different sequential extraction schemes.
Tessier method has been widely used in the speciation analysis of heavy metals in atmospheric particulates after continuous improvement[69,77,83,84]. The bioavailability of Cd, Pb and As in the atmospheric fine particulate matter in Guangzhou was more than 70%, and the carcinogenic risk of As, Cr and Cd was 3 to 33 times higher than the standard[84]. Barrag Barragán et al. combined multivariate statistical analysis with improved Tessier method to analyze atmospheric particles in Serbia, and the results showed that Cd mainly existed in exchangeable form, while Pb and Cu mainly existed in carbonate form[83]. Chester et al. Used acetate to determine the soluble state because of its better complexing power compared to water and simple inorganic salt solutions[91].
Arsenic is one of the carcinogenic heavy metals. The five-step extraction method proposed by Wenzel et al has been widely used in speciation analysis and bioavailability assessment of arsenic[104][97]. Yuan et al. studied the changes of As content in the atmospheric PM2.5 of five functional areas in Baoding before and after the implementation of the "coal to gas" policy in 2017, and found that the main forms of As distribution changed before and after the implementation of the policy[98]. "Coal to gas" has a positive effect on reducing the environmental and health risks of PM2.5. Zatka et al. Designed a separation and extraction scheme for Ni in atmospheric dust, which was later applied to analyze the imprinting of Ni compounds in production workshops and atmospheric particles[105][106,107].

2.2 Morphological selection analysis method

Speciation analysis (specific-selective) refers to the chemical speciation of heavy metals, including their valence state and combined state, which is usually separated and analyzed by chromatography-mass spectrometry (spectrometry), solid-phase extraction and specific adsorption of new materials[108]. The acute toxicity of trivalent arsenic is higher than that of pentavalent arsenic, and the acute toxicity of elemental mercury is much higher than that of divalent mercury. The chemical forms of elements will transform each other under the influence of different environmental media (atmosphere, water, sediment, etc.) And complex environmental factors (light, temperature, pH, etc.). Therefore, the effective separation and detection of chemical speciation of elements is an important part of understanding their environmental transformation and toxicological effects.
At present, high performance liquid chromatography-ultraviolet irradiation-atomic fluorescence spectrometry (HPLC-UV-AFS) has been widely used for speciation analysis of Hg. Jiang et al successfully separated and determined inorganic mercury (IHg), methyl mercury (MeHg), ethyl mercury (EtHg) and phenyl mercury (PhHg) in HPLC-AFS by using formic acid as the mobile phase and generating cold vapor (CVG) through ultraviolet induction[109]. The UV-CVG with formic acid greatly simplifies the instrument and the detection limit is at the level of 0.001 μg/L. Jiang et al. Further improved the system by adding L-cysteine to the chromatographic mobile phase for HPLC-CV-AFS separation and determination of Hg (II), MeHg and EtHg[110]. Compared with the previous method, the system has greatly improved simplicity and environmental friendliness due to the absence of strong oxidants, ultraviolet lamps and organic solvents. It is noteworthy that Hg (Ⅰ) is an important intermediate for the reduction and oxidation of Hg (Ⅱ) and Hg (0), because single-electron transfer is more likely to occur than multi-electron transfer in the environment, so the effective detection of Hg (Ⅰ) is an important link in the study of mercury biogeochemical cycle. Recently, Li et al. Developed a method using 2-mercaptoethanol extraction and HPLC-ICP-MS to successfully separate and identify Hg (I) in the environment, and reported for the first time that the content of Hg (I) in the atmospheric PM2.5 was 14.6~32.6 ng/m3, accounting for 14% -30% of the total dissolved mercury[111]. In addition to liquid chromatography, gas chromatography can also be used to separate different forms of Hg due to its volatility. Cai et al. Successfully distinguished and identified Hg (Ⅱ), MeHg and EtHg by GC-MS or GC-ICP-MS using NaBPh4 as a derivatization reagent combined with purge and trap, and confirmed the existence of EtHg in Florida swamp soil for the first time[112,113]. In addition to chromatographic techniques, Liu et al. developed a non-chromatographic method for the determination of IHg and MeHg by volatilization of substance-flameless/flame atomization-atomic fluorescence spectrometry (VSG-FL/FA-AFS)[114]. IHg can be determined without a flame, while MeHg can be converted to volatile hydride by KBH4 and then to elemental mercury in a reducing argon-hydrogen flame, so inorganic mercury and methylmercury can be distinguished by measuring total Hg and IHg.
The toxicity of inorganic arsenic compounds is very strong. The median lethal dose (LD50) of As (Ⅲ) in mice is 14 mg/kg, and the LD50 of As (Ⅴ) is 20 mg/kg. Long-term exposure to low doses can cause chronic arsenic poisoning, induce various skin diseases and cause renal function damage. In contrast, methyl arsenates (MMA (V) and DMA (V)) are less toxic, with a LD50 of G/kg, while more complex arsenic compounds, such as arsenobetaine (AsB), arseniccholine (AsC), arsenicsaccharide (AsS) and arseniclipid (AsL), which are widely distributed in aquatic organisms, are generally considered to be of low or no toxicity. As (Ⅲ), As (Ⅴ), MMA (V) and DMA (V) in atmospheric particles can be determined by hydride generation technique[115]. da Rocha et al. Used HG-AAS to determine the total As and As (Ⅲ) in TSP samples from Brazil. The concentrations were 3.8~20 ng/m3 and 2.7~10.5 ng/m3, respectively, and 58% -74% of the arsenic existed in the form of As (Ⅲ)[116]. Šlejkovski et al separated and detected As (Ⅲ), As (Ⅴ), MMA (V) and DMA (V) in Hungarian atmospheric aerosol by HPLC-anion exchange column-HG-AFS method, and only As (Ⅴ) was detected in the sample[117]. Farinha et al. Used the same method to analyze samples from Portuguese PM2.5 and PM10, and found As (Ⅲ) and As (Ⅴ), with As (Ⅴ) As the main component[118]. Xu et al used HPLC-HG-AFS system to analyze TSP in Beijing, and detected As (Ⅲ) and As (Ⅴ) in all samples, and As (Ⅴ) accounted for 81% -99% of extractable As, but no organic arsenic compounds were detected[119].
Microorganisms in the environment can significantly promote the biological methylation process of arsenic, resulting in the release of gaseous arsenic. Headspace sampling combined with GC-MS or GC-ICP-MS has been used to study volatile AsH3 formed by intestinal microorganisms and their stability.However, the direct headspace sampling method leads to higher detection limits.Pre-concentration methods such as solid phase microextraction (SPME) and cold trap can be used to improve the sensitivity[120,121]. Therefore, Yuan et al. developed a hydride generation-cold trap-gas chromatography-inductively coupled plasma mass spectrometry (HG-CT-GC-ICPMS) system for online analysis of volatile arsenic substances produced by bacterial culture, and separated and identified AsH3, (CH3)AsH2, (CH3)2AsH and (CH3)3As[122]. Feldmann et al. studied the stability of three volatile arsenic compounds ((CH3)AsH2, (CH3)2AsH, and CH3)3As) in the atmosphere and determined organoarsenic acid concentrations in PM10 at two sites in Argentina using HPLC-ICP-MS[123]. The results showed that the half-life of methyl arsenic in the dark was 2 ~ 133 d, and the half-life became shorter with the increase of methyl groups. In the daytime, the stability of methyl arsenic is significantly reduced, and the half-life of (CH3)3As is reduced by three orders of magnitude, indicating that methyl arsenic is unstable in the atmosphere. The results of HPLC-ICP-MS showed that organic arsenic acids, namely MMA (V), DMA (V) and TMAO (V), were detected in more than 90% of the sampling sites, of which TMAO was the main oxidation product, accounting for 66% and 69%, respectively. This study shows that the biovolatilization of methyl arsenic is a common phenomenon, and it will be oxidized to the corresponding organic arsenic compounds in the atmosphere. In addition, Pergantis et al. developed an anion-cation-ICPMS double chromatographic separation system to sensitively analyze five species of As (As (Ⅲ), As (Ⅴ), MMA (V), DMA (V), TMAO (V)) in fine particles[124]. Using this technique to detect atmospheric particles of different sizes in the Sahara Desert, it was found that inorganic As (Ⅴ) was the most dominant species, which was consistent with other atmospheric studies[125,126]. Tsopelas et al. Developed a set of anion exchanger (AG 1-X8), cation exchanger (Dowex 50WX8) and HG-ICP-OES to separate and determine As (Ⅲ), DMA (V), MMA (V) and As (Ⅴ) in particles of different sizes in industrial areas of Greece[127]. The detection limits of the method can reach 0.5, 0.4, 0.4 and 0.7 ng/m3 respectively. The results showed that As (Ⅴ) was the most dominant speciation, followed by As (Ⅲ), but MMA (V) and DMA (V) were not detected. The synthesis of the new material is suitable for environmental samples with low concentration and complex matrix. Hu et al. Synthesized a new adsorbent of mesoporous TiO2 modified by dimercaptosuccinic acid (DMSA), which was combined with ICP-OES for on-line separation of As (Ⅲ) and As (Ⅴ)[128]. Lian et al. synthesized thiol and amine functionalized mesoporous SiO2 and organic-inorganic hybrid monolithic column by one-pot method, and determined As (Ⅲ) and As (Ⅴ) in water by capillary microextraction (CME) and flame atomic absorption spectrometry[129,130].
Although chromatography-mass spectrometry (GC-MS) has the advantages of low detection limit, high sensitivity and simultaneous determination of multiple elements, it can not meet the requirements of in-situ, low disturbance and non-destructive, and may produce non-mass spectrometry/mass spectrometry interference due to complex matrix effects and polyatomic ions[131,132]. Solid phase extraction (SPE), solid phase microextraction (SPME), cloud point extraction (CPE) and liquid-liquid microextraction (LLME) have been successfully applied to the speciation analysis of heavy metals due to their simplicity, sensitivity, high enrichment factor and low cost[133][134]. Yan et al. Developed a SPE method based on ion-imprinted thiol-functionalized silica gel adsorbent and L-cysteine-functionalized multi-walled carbon nanotubes (MWCNTs-cysteine) for the on-line analysis of Cd (Ⅱ) in biological and environmental samples[135,136]. Cr (Ⅳ) is classified as a human carcinogen, while Cr (Ⅲ) has little toxicity, so the effective enrichment and separation of Cr species in the environment is of great significance for health assessment[137,138]. Yan et al. synthesized an amine-functionalized magnetic Fe3O4 microspheres (AF-MMPs), which was combined with flow injection on-line SPE system to analyze Cr (Ⅲ) and Cr (Ⅵ) in the environment by ICP-MS[139]. Lian et al. Have done a series of excellent work on Cr speciation analysis: Zincon-Si-MNPs coated with zinc immobilized SiO2 as SPE adsorbent can adsorb Cr (Ⅲ) at pH = 9.1, while total Cr can be adsorbed at pH = 6.5[140]. The novel organic-inorganic hybrid monolithic column (A & C-HMC) containing amino and carboxyl groups can be used for SPME. Cr (Ⅵ) and Cr (Ⅲ) can be completely adsorbed at pH = 3 and pH = 10, respectively. Only 150μL 10%HNO3 of eluent is needed to completely elute Cr (Ⅲ) and Cr (Ⅵ)[141]. Moreover, Cr (Ⅲ) can be specifically adsorbed by the carboxyl-modified mesoporous silica (CFMS) material and the organic-inorganic hybrid monolithic column (TMOS-co-CES) in a wide pH range (pH = 5-9), and the sample pretreatment steps are greatly simplified[142][143]. Recently, Lian et al. Developed an amino-functionalized hybrid capillary monolithic column (TEOS-co-AEAPTES) by a one-pot method, which can achieve the goal of simultaneous separation, enrichment and analysis of multiple elements[144].
Antimony (Sb) and tellurium (Te) may cause environmental pollution in the production and use of metallurgy, chemical industry and thermoelectric materials. Sb (Ⅲ) has been proved to be a lung carcinogen, while Te (Ⅳ) is 10 times more toxic than Te (Ⅵ)[145,146]. Therefore, it is very important to analyze the oxidation state and reduction state of Sb and Te rapidly, sensitively and reliably. On the basis of solid phase extraction technology, Hu et al. Synthesized and developed a variety of new functional materials based on the affinity difference of sulfhydryl (-SH) to inorganic substances containing Sb and Te[128]. A novel adsorbent of mesoporous TiO2 modified by dimercaptosuccinic acid (DMSA) can be used for the on-line separation and determination of Sb (Ⅲ) and Sb (Ⅴ). The modified sulfhydryl silica MNPs can realize the selective adsorption of Te (Ⅳ)[147]; Sb (Ⅲ) and Te (Ⅳ) can be selectively adsorbed by poly (SH-GMA-EDMA) capillary column[148]. Dispersive liquid-liquid microextraction (DLLME) can also be used for speciation analysis of As (Ⅲ), Se (Ⅳ) and Te (Ⅳ)[149,150]. In addition, Fe3O4@SiO2@ polyaniline (PANI) magnetic nanoparticles (MNPs) combined with magnetic solid phase extraction-ICP-MS can also realize the selective adsorption of Se (Ⅳ) and Te (Ⅳ)[151].

2.3 In-situ morphological analysis method

Synchrotron radiation is an important technical means for the study of environmental processes. X-ray absorption fine structure spectroscopy (XAFS) is an effective technique for in situ molecular morphology analysis. Precise measurements show that the fine structure of the X-ray absorption curve near the absorption limit is related to the arrangement of the atom and its surrounding atoms in the short range coordination (tetrahedron, octahedron). By measuring, analyzing and calculating the μ (E) curves of XAFS spectra, the information of cluster structure in a small range can be obtained, including the coordination number of neighboring atoms, the distance between atoms and other neighboring geometric structures around the absorbing atoms, as well as the morphological information of oxidation state and valence state of atoms. The in-situ speciation analysis of heavy metal elements can be realized by this technology. XAFS/EXAFS can be used to analyze the speciation of elements in solid samples nondestructively. This technique is mostly used for the chemical speciation analysis of Fe, As and other elements in geological samples, and in recent years, it has also been used for the speciation analysis of heavy metals in atmospheric particles. Li et al. Used XAFS technique to study the chemical speciation of Fe and Pb in tunnel PM10 samples[152]. The results show that Fe exists mainly in the form of FeO, Fe(NO3)3 and Fe3O4. Pb mainly exists in the form of PbSO4, PbO2, Pb3(PO4)2 and a small amount of PbO. Zhang et al. Used EXAFS technique to study the speciation of Fe in PM10 and PM2.5 in different functional areas of Shanghai[153]. The results show that Fe exists in the form of Fe2O3, Fe3O4 and Fe2(SO4)3 in all the samples, but the distribution of the three forms is different in different functional areas. The proportion of Fe3O4 was higher in the industrial area, while the proportion of Fe2(SO4)3 was higher in the downtown and suburban areas, and the proportion of Fe2(SO4)3 in the PM2.5 of the three functional areas was higher than that of PM10. The speciation of lead in PM10 was studied by XAFS. The results showed that lead mainly existed as PbCl2, PbSO4 and PbO[154]. The EXAFS technique was used to study the existing form of iron in particulate matter, and the results showed that the main existing form of iron in atmospheric particulate matter was α-Fe2O3, and there was a small amount of FeO[155]. Galbreath et al. Studied the PM2.5 produced by burning two kinds of coal, and analyzed the existing forms of As, Cr and Ni by XAFS technology. As and Ni existed in the form of As5+, O 4 - and Ni2+Ox, respectively[156]. The main existing forms of Cr are Cr3+ and Cr6+.
Different forms of heavy metals in particulate matter can be transformed into each other and have variability[98,103]. Due to improper analytical operation, the transformation between different forms of the same element often leads to obvious experimental errors. Therefore, in the process of experimental operation, we should avoid morphological changes in the process of sample analysis from the aspects of environmental physical chemistry and biological factors (temperature, light, pH and microorganisms, etc.). For the stepwise extraction method, the decrease of the pH of the extractant can lead to the increase of the content of acid-soluble heavy metals, and the volatilization and precipitation of the solvent can also cause the deviation of the results, so it is necessary to use a freshly prepared extractant[103]. Hg (Ⅰ) is an easily oxidized and reduced species. Studies have shown that L-cysteine, microwave radiation, ultrasonic extraction and heating can convert Hg (Ⅰ) to Hg (0)[111]. The increase of pH is beneficial to the increase of As (Ⅴ) concentration, while the acidic condition is beneficial to the existence of As (Ⅲ), and the methylation of inorganic arsenic will occur under the redox action of microorganisms[122]. Fe (Ⅱ) and Fe (Ⅲ) can transform into each other under different redox conditions. High temperature, high humidity, light and high pH can promote the transformation of Cr (Ⅲ) to Cr (Ⅵ). From the above discussion, quality control (QA/QC) of the morphological analysis process is particularly important[157].

3 Speciation Distribution of Heavy Metals in Atmospheric Particles

3.1 Spatial Distribution Characteristics of Heavy Metal Species in Atmospheric Particles

Using the key words "Atmospheric particulate matters", "Heavy metals", "Speciation", and "Bioavailability" or "Bioaccessibility" in databases such as ACS, Elsevier, Web of Science, Bing Scholar, and China National Knowledge Infrastructure (CNKI) from 2010 to 2020, the Speciation distribution data of Heavy metals in Atmospheric particulate matters in different regions of the world were obtained. For the single-step extraction method, the extraction concentration is taken as the bioavailability part for discussion; For those using the stepwise extraction method, the sum of the concentrations of the readily extractable species (i.e., water-soluble, exchangeable, acid-soluble, and reducible) is used as the bioavailability fraction for discussion[28,67]. If there are multiple sampling points in the same area, the average bioavailability concentration of multiple sampling points represents the real atmospheric health risk level of the area; If there are different particle sizes in the same area, calculate the bioavailability concentration of each part separately and then take the average value; If the sampling time spans different seasons, the bioavailability concentration of each time period is calculated separately and then the average value is taken. In addition, air pollution during special events in various countries (such as Diwali in India, dust storm events, summits, etc.) and heavy pollution industries (chimneys of thermal power plants, smoke outlets of iron and steel smelting, roadside dust, etc.) are not considered. The current standards of heavy metals in ambient air at home and abroad and the concentration limits of the World Health Organization (WHO) are shown in Table 3, and the bioavailability concentrations of heavy metals in atmospheric particulate matter in different regions of the world and cities in China are shown in Table 4 and Table 5.
Globally, there are significant differences in the bioavailability of heavy metals in atmospheric particulate matter across continents (Table 4). In Europe, with the exception of Fe, which reached the 133.25 ng/m3 in Poland and the 260.6 ng/m3 in Austria, the other heavy metals ranged from concentrations at which no 32.4 ng/m3 was detected (Table 4)[162][163]. Due to the high abundance of the crust, natural release is an important source of Fe in the environment. Compared with the WHO and EU standards, the pollution of heavy metals was at a low level except Cr. In three developing countries in Asia, heavy metal pollution in the atmosphere is widespread, and the concentration of Zn pollution in some countries is as high as 6380 ng/m3[164]. Zn is closely related to industrial processes and fracture friction sources, and high concentrations of Zn indicate serious industrial pollution in the region (Table 4). In general, the bioavailability of Cd, Cr, Pb and Ni exceeded the WHO standard. In Africa, air pollution is becoming more and more serious due to a large number of open burning and industrial pollution, as well as war and other adverse factors, which is characterized by high total amount of heavy metals and low soluble fraction. In Egypt, the bioavailability fraction of Cd, Pb, Mn and Fe in outdoor dust is only a small fraction[165]. In contrast, in the American continent, the concentration of heavy metals in the atmosphere is at a low level, and the bioavailability concentration is lower than the USEPA standard (Table 4)[166,167]. In addition, New Zealand is the only country located in Oceania in the search data, and its As pollution is serious, mainly from anthropogenic sources, exceeding the WHO standard by 18 times (Table 4)[168]. To sum up, from the perspective of large-scale spatial distribution, the bioavailability of heavy metals in atmospheric particles is in the order of Africa > Asia > Oceania > Europe > America; Developing countries > developed countries.
The level of economic development and energy use patterns in different regions determine the different sources of heavy metals. The chemical forms of heavy metals in particulate matter from different sources are different. Therefore, even in the same area, the bioavailability of different heavy metals will be quite different. For the same heavy metal, different results will be produced in different areas. Even for the same metal in the same area, different speciation information can be obtained by different analytical methods. For example, the bioavailability of heavy metals in PM10 was evaluated by simulated gastric fluid (SGF) and simulated lung fluid (SLF), and the bioavailability fractions were 1. 3% -56% and 0. 1% -79%, respectively. This is related to the pH value of the extraction solution and the extraction time, and is related to the chemical forms of heavy metals (metal sulfates are more soluble in acidic solutions than metal oxides)[169]. The sources of heavy metals in atmospheric particulates may be the same or similar in different regions. Furusj Furusjö et al. Observed that the main source of Cu, Fe, Zn, Cr, Mn, Ni and Pb in Stockholm, Sweden is fracture wear[170]. Similarly, W Wåhlin et al. Found that in Copenhagen, fracture wear was the main source of Cr, Fe, Cu, Zn, Zr, Mo, Sn, Sb, Ba and Pb[171]. The total amount of heavy metals represents the overall air pollution in the region, and the bioavailable forms are the key information reflecting the health effects. The heavy metal content in Table 4 and Table 5 is a subset of the total amount data, so if the heavy metal content in the table exceeds the limit standard, its potential environmental health threat should be paid enough attention.
There are also obvious spatial distribution characteristics of heavy metal pollution in China's atmosphere (Table 5). According to the statistics of the average bioavailability concentrations of heavy metals in atmospheric particulates in 13 cities in this paper, the concentrations of Zn and Al reached 674.4 ng/m3 and 625.9 ng/m3 respectively, followed by Fe pollution, with the concentration of 315.4 ng/m3 (Table 5). Compared with the national and WHO standards, Cr, As and Mn obviously exceed the standards, which is related to vehicle emissions, industrial processes and fuel combustion. Coal contains trace As and Cr, which will lead to heavy metal pollution in the atmosphere for a long time[172,173]. Cd, Ni, V and Pb are all within the limits of the secondary standard. If China is geographically divided into seven regions: East China, North China, Central China, South China, Southwest China, Northwest China and Northeast China for comparative analysis, the heavy metal pollution of atmospheric particulate matter in South China and Southwest China is relatively serious. Among the 12 heavy metals counted, the concentrations of bioavailable forms of 6 heavy metals, including Cd, Cu, Ni, Zn, Fe and V, are the highest in the atmospheric particulate matter in South China, which is closely related to the large number of automobiles due to the large population (Table 5)[84,174,175]. Although the total concentration of atmospheric PM2.5 in South China is not serious due to climate reasons, it is reported that the average proportion of water-soluble heavy metals in atmospheric particles in Dongguan City of Guangdong Province is 47%, and the proportion of water-soluble Cd, Pb and As in atmospheric particles in Guangzhou City is As high as 91%, 85% and 74%[174][84]. In the southwestern region, the high bioavailability concentrations of As, Al, and Mn due to abundant coal resources, mountainous terrain, inland location, rapid industrial progress, and high atmospheric stability contribute to the overall high level of pollution (Table 5)[176]. The bioavailability of heavy metals in North China is at a medium level, and the pollution of Pb and Zn is relatively high, and traffic emissions are the main source. As pollution has been improved due to the implementation of the coal-to-gas policy in Hebei Province, but the bioavailability is still 4 times higher than the national standard (Table 5)[29,98]. Northeast China is moderately polluted, with high concentrations of Pb and Cu (Table 5)[177]. The overall bioavailability pollution of heavy metals in East China is relatively light, which is the lowest in the country. Except for Cr, the concentrations of other heavy metals are within the national standard or WHO limits (Table 5)[56,178⇓⇓ ~181]. The order of bioavailability of heavy metals in atmospheric particles was South China > Southwest China > North China > Northeast China > East China. Eastern Region > Western Region.
According to the size of China's urban permanent population, China's cities can be divided into five categories and seven grades. Among the cities counted, Shanghai, Beijing and Guangzhou are mega-cities, Nanjing and Hangzhou are mega-cities, Hefei, Taiyuan, Jinzhou, Dongguan, Guiyang and Baoding are big cities, and Donghai and Macao are medium-sized cities. From the point of view of bioavailability of heavy metals, air pollution in big cities is the most serious, and the concentrations of Ni, Pb, Zn, Fe, As, Al, V and Mn are the highest. Cd, Cr, Ni, As and Mn all exceed the national standard (Table 5). Most of the big cities in this survey are inland cities. Climate and industrial development are the main factors leading to air pollution. Ni, As and Cd are closely related to industrial sources such As fuel combustion. The rational distribution of industrial enterprises and the construction of urban corridors are important measures to improve urban air quality. The concentrations of bioavailable fractions of heavy metals in atmospheric particulates in Shanghai, Beijing and Guangzhou were relatively low. Education, electronic technology and high-tech industries are the dominant industries in these cities, but because of the dense population and numerous vehicles, the heavy metal problems of Pb and Zn can not be ignored. The air pollution in Nanjing and Hangzhou is relatively light. The air quality in Macao and Donghai is the best, and the bioavailability concentrations of other heavy metals are within the national standard limits except Cr (Table 5). In addition, it is found from Table 5 that the top three cities in terms of Cr concentration are Dongguan, Macao and Jinzhou, all of which are eastern coastal cities. It is reported that the content of organic Cr in lakes, sea water and bay land water is high, so this may be one of the reasons why Cr pollution in coastal cities is more serious than that in inland cities[182]. In terms of urban population size, according to the data retrieved in this paper, the bioavailability of heavy metals in atmospheric particulates is in the order of large cities > megacities > megacities > medium-sized cities.
In different functional areas of a city, the bioavailability of heavy metals is different. Due to intensive human activities, the bioavailability of Cu, Zn, Pb and Mn in residential and commercial areas of Shanghai was higher than that in industrial areas, and Pb was the main contributor[56]. This is consistent with the findings of the New Zealand study[168]. For the industrialized city of Baoding, the bioavailability of Cd in the suburbs is higher than that in the industrial and residential areas, and the unorganized emission without treatment may lead to heavy metal pollution. The bioavailability of Cr was higher in residential areas than in suburban areas, indicating that household cooking was an important source of Cr. The pollution of Pb and As in industrial and suburban areas was more serious than that in residential areas[98,103]. This is consistent with the findings of the Indian study[183]. The comparison of As in road dust, household dust and residential PM2.5 in Guangzhou shows that the As (Ⅲ) in road dust is higher than that in household dust and residential area, while the As (Ⅴ) is relatively lower[125]. This may be due to the heavy rainfall in summer in Guangzhou City, and the mineral phase containing As (Ⅴ) is greatly leached. Taking into account the possible influence of the height of the sampling point on the bioavailability of heavy metals, the samples were collected from the ground, the roof of 15 m high building and three different residential floors, respectively.The results showed that the bioavailability of heavy metals had similar patterns and no significant difference[84,178]. To sum up, the bioavailability of heavy metals in the atmosphere varies in different functional areas of the same city. For developed cities, the health risk is greater in densely populated residential areas and roadsides, while for developing cities, the health risk is greater in industrial areas.
The bioavailability of heavy metals in atmospheric particulates had obvious regional differences in space. The total amount of heavy metals is not necessarily related to the content of their bioavailable forms. Generally speaking, air pollution in developing countries in Africa and Asia is more serious. In China, the bioavailability of heavy metals in particulate matter is higher in South, Southwest and North China due to industrial development patterns and climate factors. The health risks of heavy metals in different functional areas of the same city were different, but there was no significant correlation with the sampling height.

3.2 Temporal Distribution Characteristics of Heavy Metal Species in Atmospheric Particles

The statistical summary of published studies at home and abroad shows that the concentration of atmospheric particulate matter is generally more serious in winter and spring. Similarly, the total amount of heavy metals in winter is also higher than that in summer[188,189]. However, in different seasons, atmospheric temperature and diffusion conditions are different, humidity, wind speed and wind direction may have an impact on the form of heavy metals, and the chemical binding state of heavy metals determines the difficulty of their dissolution, so the change law of heavy metal forms in atmospheric particles will be different. The bioavailability of Cd in particulate matter was studied by using simulated lung fluid, and it was found that the extractable fraction of Cd in winter was greater than that in summer, while the results of Cu, Ni and Zn were opposite[57]. Gupta et al reported that in industrial and residential areas in India, the concentrations of soluble Zn, Pb, Cd, Mn, Ni and Fe were higher in winter than in summer, while the concentrations of heavy metals were lower in rainy season due to scouring[190]. The results showed that the concentrations of 12 water-soluble heavy metals in winter were higher than those in autumn, summer and spring[176]. In Macau, the researchers speculated that the marine climate and sampling sites would have an impact on the forms of heavy metals. The total water-soluble elements of heavy metals at Zhongshan Park sampling site adjacent to Zhuhai were greater in summer than in winter, while at the University of Macau and Causeway Road sampling sites, they were greater in winter than in summer[175]. The marine climate is characterized by stable annual precipitation and high temperature and humidity in summer, which may provide favorable conditions for the transformation of heavy metal forms. In a study of PM10 in Chile from 1997 to 2003 (sampling only in winter), it was found that the F1state of Pb decreased from 68.1% to 18.2%, while the F2 state increased from 25.5% to 52.3%, and the F3 and F4 States increased by 17% and 6.3% respectively[191].
This change is highly related to the "clean up plan" implemented in Chile in 1998, especially the elimination of gasoline containing Pb, which reduces the bioavailability of Pb. The total amount of As and Cu decreased due to the implementation of emission reduction measures, but their speciation did not change significantly from 1997 to 2003, which showed that the speciation proportion of heavy metals had little relationship with the total amount, but was related to the chemically bound state. Using the improved BCR method, Hu et al found that the F1 forms of Cu and Zn were significantly higher in winter than in summer, while the F1 form of Cd was significantly higher in summer than in winter[67]. The F1 form of Pb was significantly higher in summer than in winter, but the F2 form was opposite. High temperature and humid weather conditions may be conducive to the transformation of Pb to F1 form. The residual state of Co in winter was significantly higher than that in summer. However, As, Cr, Mn and Ni showed no significant seasonal variation. Yuan et al found that the bioavailability of Zn, Pb and Cu was high in summer, while that of Cd and Cr was high in winter[28]. The high bioavailability of Cd and Cr in winter was attributed to the extensive use of coal during the heating period in northern China and the unorganized emissions from the suburbs. In terms of climate, the northern spring and summer seasons are generally windy, which is conducive to atmospheric diffusion. In winter, it is cold and windless, the inversion phenomenon is common, the atmospheric stability is high, and the pollutants are not easy to diffuse. The F1 fraction of Cd in autumn and winter was significantly higher than that in spring and summer, while the F1 fraction of Cu in summer was significantly higher than that in other seasons. The F1state of Zn is high in all seasons, but it is significantly lower in winter than in other seasons. In winter, spring and summer, Pb was mainly in F1 form, and in autumn, Pb was mainly in F2 form. The F1state of Cr was significantly higher in winter than in other seasons. The bioavailability of heavy metals in autumn, winter, spring and summer was Zn ≈ Cd > Pb ≈ Cu > Cr, Cd ≈ Pb ≈ Zn > Cu > Cr, Zn ≈ Pb > Cu ≈ Cd > Cr and Zn ≈ Pb > Cu > Cd ≈ Cr. It can be seen that the main patterns (the top three) are almost the same in different seasons. Yuan et al. Further used the five-step extraction method to study As and found that the F1state of As was significantly higher in summer than in other seasons, and high temperature, high humidity and stronger solar radiation were the dominant factors, while in winter, it was mainly distributed in F1 and F4 States[29]. The bioavailability of As was in the following order: summer > spring > autumn > winter. In autumn in Guangzhou, more than 40% of Cd was in the F1 form, more than 70% of Pb, 40% of Zn, Cu and Cd were in the F2 form, more than 30% of Cu and Cr were in the F3 form, and more than 50% of Cr was in the F2 form[84].
To sum up, the changes of bioavailability of heavy metals in particulate matter in different seasons were mainly caused by pollution sources and meteorological factors. High temperature, high humidity and frequent precipitation in summer can activate heavy metals in metastable state (F2, F3) and transform them into unstable state (F1). In winter, the acid precipitation may be caused by heavy coal combustion, which promotes the transformation of Cd, Cr and As to highly active forms. Yuan et al. pointed out that due to the different composition of primary and secondary aerosols, the former is mainly black carbon and organic carbon, while the latter is mainly dissolved inorganic salts, and SO2, NOx and NH3 can be used as precursors of secondary aerosols, so the formation process of PM2.5 itself may also have an impact on the form of heavy metals[29]. In addition, for the same element in the same area, the main forms are basically the same, but the proportion of each form changes with the season. There is no uniform rule for the same heavy metal in different regions, which is related to the industrial situation, climatic conditions, anthropogenic emission sources, natural conditions and so on.
In addition, the prevailing monsoon in different seasons is different, and the wind direction affects the source of particulate matter, which plays an important role in the total amount and speciation of heavy metals. Foshan is the world's ceramic manufacturing center and the most polluted city in the Pearl River Delta. Under the action of the northwest wind in winter, the concentration of Cd in the air from the industrial area to the suburbs was as high as 220 ng/m3, which was 20.5 times higher than that in other wind directions. The reverse trajectory analysis also showed that the concentration of heavy metals was closely related to the atmospheric path[192]. Lee et al. Observed that the heavy metal (Pb, Cd, Mn, Fe) content of TSP in foggy days was significantly higher than that in sunny days, and the residential and industrial areas were similar[193]. This difference can be explained by the difference between the relative humidity and the ambient ventilation index in foggy and sunny days. Higher relative humidity is conducive to the condensation of particulate matter, poor ventilation will reduce the dilution effect, and will increase the accumulation of acidic pollutants in the low altitude[194].
In the past few years, the haze in the Yangtze River Delta and the Beijing-Tianjin-Hebei region has been serious. Haze is defined as a weather phenomenon with high concentrations of fine particulate matter, relative humidity < 80%, visibility < 10 km, and high concentrations of CO, SO2, NH3 gases, and organic carbon[195,196]. Fu and Chen et al. used TEM and ATOF-MS to study the chemical speciation distribution characteristics of heavy metals in atmospheric particulates during haze weather in Shanghai, and found that Fe, Pb, Zn and Sb were mainly sulfate, nitrate and oxide, and the proportion of Fe-carbon bound state and Fe-inorganic salt bound state reached 22. 3% and 19. 3%, respectively[197]. At the same time, sulfate and nitrate were more likely to combine with fine particles in the carbon matrix during the haze period than during the non-haze period[198]. Therefore, we have reason to believe that heavy metals during haze have greater health hazards. The contents of heavy metals were significantly higher in the haze period (winter) than in the non-haze period (summer)[199]. The carcinogenic risk (CR) of children and adults was greater than that of 10-6, and the health risk assessment in haze period was 50% higher than that in non-haze period, indicating that heavy metals in haze had a significant impact on human health[199]. Singapore haze study found that Cr, Cu, Ni, Pb and Zn accounted for more than 60% of the F3 and F4 States, while K and Mn mostly existed in the F1state[95]. Due to the high concentration of fine particulate matter during the haze period, the health risk is greater, and the bioavailability of heavy metals is significantly different between the haze period and the non-haze period.
To sum up, the temporal characteristics of heavy metal speciation in atmospheric particulates are as follows: In terms of season, the high concentration of heavy metals in winter leads to the corresponding increase in the effective concentration of heavy metals, and the high temperature and humidity in summer is conducive to the transformation of heavy metal to unstable speciation. The distribution of the main forms of the same heavy metal in the same area did not change significantly, and the seasonal changes in different areas were not obvious. Diurnal alternation may also lead to the difference of heavy metal forms, which needs further study. High temperature, high humidity and strong light are beneficial to the transformation of heavy metals to bioavailable forms. Wind speed and wind direction are also factors that can not be ignored affecting heavy metals in the atmosphere. There were significant differences in the forms of heavy metals between haze and non-haze periods.

3.3 Characteristics of heavy metal speciation and size distribution in atmospheric particles

Particulate matter of different sizes reaches the respiratory tract at different depths, resulting in different health effects. Particulate matter is associated with respiratory and cardiovascular diseases, and is closely related to particle size[200]. Heavy metals are important toxic components in particulate matter, and their speciation determines their bioavailability. At present, it is generally pointed out that the volume concentration of heavy metals in coarse particles is higher than that in fine particles, but the mass concentration of heavy metals in fine particles is higher than that in coarse particles, indicating that heavy metals tend to be enriched in fine particles[61,201][167,202,203]. The distribution characteristics of heavy metal forms (bioavailability) in atmospheric particles with different sizes are an important research direction. The following summarizes the size distribution characteristics of heavy metal forms in atmospheric particles by simulated body fluid and stepwise extraction methods respectively.
Physiology-based in vitro test can reflect the physiological activity of heavy metals in atmospheric fine particles by simulating the in vivo environment. Compared with in vivo test, it is simple, convenient and low-cost, so it has been widely used in recent years. Water and simple saline solutions are not generally used to simulate the in vivo environment because of their simple composition and are therefore not discussed in this section. This section will focus on the progress of studies on the bioavailability of heavy metals in particles of different sizes using simulated lung fluid and simulated gastrointestinal fluid. The speciation analysis of heavy metals in simulated body fluids has been reviewed, but the relationship between particle size and bioavailability has not been summarized[204,205]. Wiseman et al. Used ALF and Gamble solution to study the PM10, PM2.5 and PM1 of atmospheric particles at the same time, and found that the solubility of V, Pb, Cu, As and Sb decreased slightly with the decrease of particle size, but there was no significant difference[61,206]. For the PGE element in the catalyst of automobile exhaust emission, the solubility of Pd, Pt and Rh increases with the decrease of particle size. This shows that the effect of particle size on heavy metals has an element difference, the solubility of the former is about 60% and the latter is less than 40%. In addition, the pH value of the extractant is also an important factor, and the solubility of V, Pb, Cu, As and Sb in the more acidic ALF is significantly higher than that in Gamble solution[61]. However, when using simulated gastric juice to study roadside TSP and PM10, it was found that Pd and Pt had little relationship with particle size, which was attributed to the short sampling time (12 days), resulting in large changes during the day. Consistent with previous studies, Pt is more soluble than Pd[207]. It is also found that the coarse particles (> 5 μm) contain a higher proportion of high-abundance elements (K, Ca, Mg, Fe) in the crust, while the fine particles contain a higher proportion of trace toxic elements (As, Cd, Pb and Sb), which is related to the source of the particles. ALF and SLF were used to study the inhalational bioavailability of heavy metals in five classes of fine particles (< 0.4,0.4-0.7,0.7-1.1,1.1-2.1,2.1-3.3 μm), and it was found that there was no significant relationship between the inhalational bioavailability of 10 heavy metals, such As As, Ni, Cu, Zn and Pb, and the particle size[208].
Industrial sources often produce fine and ultrafine particles rich in toxic elements. A Gamble test was conducted in a typical industrial area in France to collect PM2.5, PM1 and PM0.1. The results showed that the bioavailability of Ba, Co, Cu, Mo, Mn, Ni, Rb, Sb and Ti increased with the decrease of particle size, and the bioavailability of Ba, Co, Cu, Mo, Mn, Ni, Rb, Sb and Ti was higher in submicron and ultrafine particles. This phenomenon is closely related to the formation process of particles: ultrafine particles (PM0.1) are usually produced by ultra-high temperature or calcination, which have high bioavailability (> 60%) for Mn and Ti, and submicron particles can fully react with Gamble solution and release heavy metals due to their high concentration and large specific surface area[209]. The analysis of TSP and PM2.5 by ALF also showed that the extracted concentrations of Mn and Zn were higher, indicating that automobiles were the main source of emissions, and the concentrations of As, Cd, Cr, Mn, Ni, Pb and Zn in fine particulate matter were higher[210].
It can be seen that the bioavailability of the same heavy metal in atmospheric particles may show an uncertain trend with the change of particle size. Firstly, different extractants (such as PBET, simulated lung fluid and simulated serum) have different chemical compositions and different complexing and dissolving abilities for heavy metals[211]. Secondly, the different pH value of simulated body fluid will also affect the degree of dissolution. Pretreatment parameters such as extraction time, temperature, shaking conditions, solid-liquid ratio and so on will also affect the final results[61,206][210]. Wragg et al proposed that Gamble solution needed 100 H to complete the reaction, but it was controversial[212][213,214]. Wiseman et al. compared the difference between 24 H and 30 d with ALF for PM10 samples. The dissolved fraction of Rh changed from 33% at 24 H to 17% at 30 d, and Pt also decreased by 9%. However, this result is contrary to the time-dependent increase in the bioavailability of PGE element in the milled catalyst observed by Colombo et al., which also shows that the heterogeneity of samples may have an important impact on the results[206][60]. Finally, the composition of secondary aerosols is complex, and various reactions in the atmospheric environment may cause heavy metals to change to unstable forms. For soil samples with the same complex composition, Li et al. Show that the composition of soil organic matter can have an impact on the bioavailability of toxic elements: as the particle size decreases, the total organic carbon (TOC) content increases, resulting in higher bioavailability of heavy metals[215].
There are few reports on the study of heavy metal speciation in atmospheric particles with different sizes by sequential extraction method. Yuan et al. analyzed As in TSP, PM10 and PM2.5 in Baoding City by five-step extraction method, and the results showed that the mass fraction of As in fine particulate matter was higher than that in coarse particulate matter in four seasons, and the proportion of As in F1state showed a PM2.5>PM10>TSP, while that in residual state was just the opposite, which was similar to the research results in Nanjing, Xiamen, Beijing and other places[216][67][217][218]. Further analysis showed that the bioavailability of Cd, Mn and Pb in PM2.5 was higher than that in TSP, while the bioavailability of Cu, Zn and Ni in coarse particles was slightly higher[67]. The bioavailability of Cu, Zn and Fe in the coarse particles was also higher in the samples from Beijing[218]. This may be related to the source and formation process of heavy metals. Cu, Zn, Fe, Ni and other lithophile elements are abundant in the crust, and their natural sources make most of their particle sizes distributed in coarse particles. Anthropogenic As, Cd, Mn, Pb and other elements mainly come from industrial processes and automobile exhaust, and are mainly distributed in fine particulate matter. This part of the literature data is summarized in Table 6.
To sum up, although there are few reports on the stepwise extraction method, it can be preliminarily concluded that the bioavailability of anthropogenic heavy metals in fine particles is higher, while the bioavailability of crustal elements in coarse particles is higher. However, the experimental study using simulated body fluid can not get consistent conclusions, which is related to the heterogeneity of samples, extraction agents, pH and so on. Therefore, a unified, accurate and recognized in vitro test method needs further study. Nevertheless, the conclusion that there is a correlation between the speciation of heavy metals in atmospheric particulates and their sources and particle size is acceptable. Firstly, it is a common phenomenon that crustal elements tend to enrich coarse particles and potentially toxic elements tend to enrich fine particles. Secondly, compared with coarse particles, fine particles have larger porous structure and specific surface area, which can absorb more toxic elements. Fine particles mainly come from industrial emissions and secondary aerosols formed by ammonium salts, sulfates and nitrates, which contain more toxic elements than coarse particles, and are mainly in inorganic chemical binding state (compared with organic complex state in nature). In a word, the potential health hazards of heavy metals in fine particles are greater than those in coarse particles, and the effects of particle size on the speciation of heavy metals are more complex, so more in-depth research is needed in the future.

4 Conclusion and prospect

It has been widely accepted that the form of pollutants determines the environmental effects. In order to obtain the form information, the form of heavy metals can be distinguished according to the difficulty of entering the human body to produce effects or environmental release through different exposure routes (breathing, ingestion, contact). Chemical speciation and in situ speciation analysis can be realized by using chromatography-mass spectrometry, new functional materials and synchrotron radiation. From the speciation analysis of heavy metals in atmospheric particles at home and abroad in the past decade, different spatial and temporal distribution characteristics and particle size distribution rules were obtained. In terms of space, the pollution in developing countries is more serious, while the situation in South and Southwest China is not so good. The bioavailability of heavy metals in winter and summer was higher than that in spring and autumn. Meteorological conditions such as temperature, humidity, acid gases, wind speed and direction, light and so on will have an impact on the form of heavy metals. The characteristics of particle size distribution are complex, and heavy metals tend to be enriched in fine particles, but different speciation analysis schemes can not reach a consistent conclusion, and the speciation of heavy metals is closely related to their sources and particle size.
Although some progress has been made in the speciation analysis of heavy metals in atmospheric particulates and their environmental health effects, there are still many aspects that need to be further studied. First of all, it is urgent to strengthen the research on the relationship between particle size and heavy metal forms. It is necessary to carry out a systematic study on the multi-size distribution characteristics of atmospheric particulate matter in typical areas, and to further study the mechanism of speciation distribution with size by using different speciation analysis schemes and combining with complex chemical composition. Secondly, the research scheme of simulated body fluid, especially simulated lung fluid, needs to be further unified, and the bioavailability test of simulated lung fluid should be carried out in combination with the particle size distribution, because it is not realistic to discuss the bioavailability evaluation of simulated lung fluid without particle size. Furthermore, it is of great practical significance to study the effects of meteorological conditions such as diurnal variation, temperature and humidity on the distribution of heavy metal forms. Finally, some new materials and new methods are constantly used in environmental water, but the research on new materials and new methods for heavy metal speciation analysis of atmospheric particulate matter is still very few, which needs to be strengthened urgently. Recently, Liu and Jiang first confirmed the existence of exogenous ultrafine particles in the blood circulation of the general population, and used multi-dimensional chemical fingerprinting technology to characterize heavy metals with different forms.In the future, the in vitro gas-liquid interface exposure system combined with genomics and metabolomics can be used to further study the toxic effects of different forms of heavy metals in atmospheric particulates on human cells[219]. In a word, heavy metal speciation analysis is an important branch of atmospheric particulate matter research, and its future development will be deeply integrated with different fields such as environmental science, public health, biology and so on.

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Funding

National Natural Science Foundation of China(91543107)

Fundamental Research Funds for the Central Universities(2017ZZD07)

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