Pollution Characterization, Source Identification, and Health Risks of Atmospheric-Particle-Bound Heavy Metals in PM10 and PM2.5 at Multiple Sites in an Emerging Megacity in the Central Region of China

Received: July 25, 2018
Revised: November 5, 2018
Accepted: November 21, 2018

Download Citation: RIS|BibTeX|https://doi.org/10.4209/aaqr.2018.07.0275  

Cite this article:

Jiang, N., Liu, X., Wang, S., Yu, X., Yin, S., Duan, S., Wang, S., Zhang, R. and Li, S. (2019). Pollution Characterization, Source Identification, and Health Risks of Atmospheric-Particle-Bound Heavy Metals in PM10 and PM2.5 at Multiple Sites in an Emerging Megacity in the Central Region of China. Aerosol Air Qual. Res. 19: 247-271. https://doi.org/10.4209/aaqr.2018.07.0275

HIGHLIGHTS

Keywords: Toxic elements; Coefficient of divergence; Enrichment factors; Principal component analysis; Carcinogenic risks; Potential source contribution function.

INTRODUCTION

 Over the past three decades, the rapid economic development, urbanization, and industrialization in China have caused atmospheric pollution to become a common phenomenon in megacities, and the primary pollutant is particulate matter (PM) (Ji et al., 2014). PM₁₀ (aerodynamic diameter ≤10 µm) and PM_2.5 (aerodynamic diameter ≤ 2.5 µm) have been attracting considerable attention in related research fields because of their important impacts not only on global and regional climate changes (Bytnerowicz et al., 2007), as well as reduced visibility (Moosmüller et al., 2009), but also in human cardiovascular diseases and wheezing (Brunekreef and Forsberg, 2005; Pope and Dockery, 2006; Bell, 2012; Dunea et al., 2016).

Toxic elements, such as As, Pb, Ni, Cd, Cu, Zn, and Cr, are important trace components in PM_2.5 and PM₁₀ with considerably higher concentrations in urban areas than natural background levels due to anthropogenic activities (Wu et al., 2007; Okuda et al., 2008; Fang et al., 2010; Charlesworth et al., 2011). As a result, for urban residents, relatively high potential risks to human health are caused by these elements in atmospheric particulates. For example, the multi-element risk of PM_2.5 through inhalation exposure was beyond the acceptable level in the metropolitan zone in Mexico (Diaz and Dominguez, 2009); for PM_2.5, Cr poses carcinogenic risks (CR), and As, Mn, and Cd, through direct inhalation, cause significant noncarcinogenic risks in Chengdu, a megacity of Southwest China (Li et al., 2016); adults and children in Zhengzhou were exposed to cancer risks of PM_2.5-bound As, Ni, and Pb in winter and summer, in which children were more sensitive (Jiang et al., 2018b). Moreover, the residents were also exposed to toxic elements through dermal absorption and ingestion due to PM deposition during indoor and outdoor activities. Therefore, further studies on human health risks should be calculated through three exposure pathways, i.e., inhalation, dermal absorption, and ingestion (Hu et al., 2012; Jiang et al., 2018e).

Zhengzhou, the capital of Henan Province, an emerging megacity in the central region of China, had a population of 9.72 million by the end of 2016, with a high population density of 1,306 persons per square km (Bureau of Statistics of Henan Province, 2017). In 2016, the consumption of coal and coke in above designated size industrial enterprises i.e., all industrial enterprises with revenue from principal business above 20 million RMB, by industrial sector amounted to 30.84 and 0.12 Mt, respectively; as a major transport hub of China, the freight traffic of 220.38 Mt and passenger traffic of 163.61 million persons were completed by all the transport modes of transportation, i.e., railways, highways, and civil aviation (Bureau of Statistics of Zhengzhou, 2017). The surrounding area of Zhengzhou has the highest highway density nationwide, and the ratio of freight traffic and passenger traffic by highways accounted for 87.4% and 67.3% of the total quantity of transport (data in 2016), respectively. Meanwhile, the length and area of paved roads increased rapidly, especially in 2014–2016, with an annual average growth rate of 8.3% and 10.2%, respectively (Fig. S1 in Supplemental Materials). By the end of 2016, Zhengzhou demonstrated an annual gross domestic product of RMB 811.4 billion (the exchange rate of RMB 6.95 to USD 1), which is an 8.5% increase from the previous year (Bureau of Statistics of Zhengzhou, 2017), and with the extensive economic growth mode, PM_2.5 and PM₁₀ pollution is serious in Zhengzhou (Ministry of Environmental Protection of the People’s Republic of China, 2017; Jiang et al., 2018d). However, a few studies focused on the chemical characteristics and sources of PM_2.5 at a single site in Zhengzhou (Yu et al., 2016; Jiang et al., 2017; Wang et al., 2017; Jiang et al., 2018c, 2019), with results only representing the surrounding region and hence, are not enough to explain the pollution level in Zhengzhou. There is only one study involving spatial-seasonal distribution and sources of PM_2.5-bound polycyclic aromatic hydrocarbons at multiple sampling sites in Zhengzhou (Li et al., 2019). However, no systematical study at multiple sites with various characteristics (i.e., rural, traffic, and urban sites) focuses on atmospheric-particle-bound heavy metals.

In this study, five typical sites were selected for PM_2.5 and PM₁₀ sampling in Zhengzhou for one year. The mass concentrations of PM and trace elements were determined, and the main sources of elements in PM_2.5 and PM₁₀ were identified by enrichment factor (EF), Pearson’s correlation analysis (CA), cluster analysis, and principal component analysis (PCA). In addition, health risks of local residents caused by toxic elements in the particle phase were estimated through the exposure pathways of inhalation, dermal absorption, and ingestion. The potential sources of PM_2.5, PM₁₀, and the toxic elements were also studied. The results of this study can provide the local government with important information for policy consideration of elements in PM prevention and control.

MATERIALS AND METHODS

Data and Sampling Site Information

The latitude of Zhengzhou ranges from 34°16′N to 34°58′N, and the longitude ranges from 112°42′E to 114°13′E. Fig. 1 shows the map of five sampling sites, including rural (ZM), traffic (HKG), and urban sites (GY, SSQ, and XM). The detailed information of the sites is shown in Table 1.

Fig. 1. Location of the sampling sites in Zhengzhou, China.

In this study, a total of 588 daily PM filters (285 samples for PM_2.5 and 303 samples for PM₁₀) in the ambient air for all 5 sites were collected using samplers (TH-150AII, Tianhong, China) with polypropylene filters (diameter: 90 mm; Tianjin Xinyao, China) at a flow rate of 100 L min^–1in each season from January 2016 to January 2017, and the sampling was conducted from 09:00 to 08:00 of the following day. All filters were conditioned in a clean room under a constant temperature of 20 ± 5°C and relative humidity of 50 ± 5% for 48 h of pre- and post-samples, and then weighed using a high-precision electronic balance (precision: 10 µg; XS 205, Mettler Toledo, Switzerland). Each filter was weighed at least twice, and the values were acceptable with the difference within 0.03 mg. Then the PM samples were stored in a freezer (–20°C) before elemental analysis.

 
Elemental Analysis

X-ray fluorescence (XRF) spectroscopy is a widespread method for elemental analysis with high stability (U.S. EPA, 1999; Mancilla and Mendoza, 2012; Hadley, 2017). A total of 21 elements in PM_2.5 and PM₁₀ were analyzed using an S8 TIGER wavelength dispersive XRF spectrometer (Bruker, Germany). The method detection limits range from 0.002 g m^–3 (Mg) to 0.021 g m^–3 (Fe). Field blank filters were analyzed to measure blank concentrations. More details are described by Wang et al. (2018).

Coefficient of Divergence

The coefficient of divergence (CD) was calculated to compare and evaluate the total deviations of elements in PM between different sampling sites, which is defined by the following formula (Wongphatarakul et al., 1998; Feng et al., 2007):

where j and k represent two sampling sites; x_ij represents the mean concentration of element i at site j; p is the total number of elements. When CD_jkapproaches zero, the result suggests that the two sites are similar for the chemical species, whereas when CD_jk approaches one, the two sites significantly differ. According to a previous study (Contini et al., 2012), CD lower than 0.2 is selected for representing a relatively similar chemical composition.

 
Source Identification

In order to qualitatively evaluate contributions of different sources, i.e., anthropogenic sources and crustal origins, EFs are calculated as follows (Mason, 1966; Chao and Wong, 2002):

where C is the concentration of trace element. Al is selected as the reference element (Hsu et al., 2016). (C/Al)_PM and (C/Al)_Crust are the ratios of trace elements to Al in each PM sample and the upper continental crust, respectively, and the data of average crustal abundances are shown in a study on the element background values of Chinese soil (Wei, 1990). EF > 10 indicates that the element was generated from an anthropogenic source (e.g., coal combustion and vehicle emission); when EF is approximately equal to one, crustal origin is indicated (Nolting et al., 1999).

Pearson’s CA and cluster analysis, two of the most popular methods, were chosen to detect linear relationship (Robin et al., 2013) and source contribution without source profiles (Viana et al., 2008) of trace elements in PM_2.5 and PM₁₀ at multiple sites through IBM SPSS for Windows, Version 22.0. Moreover, according to the species from the same source with similar characteristics (Ny and Lee, 2011), PCA was conducted for source identification of elements in PM by SPSS 22.0.

Health Risk Assessment

Exposure Assessment

Inhalation, ingestion, and dermal absorption are the main pathways of airborne toxic element exposure for local residents, i.e., children (< 15 years) and adults, divided by different respiration and behaviors (Hu et al., 2012). According to U.S. EPA (1989, 2004, 2009), exposure concentration (EC; µg m^–3), chemical daily intake [CDI; mg (kg d)^–1], and dermal absorption dose [DAD; mg (kg d)^–1] of toxic elements in PM were calculated as the following equations. All the relative parameters were obtained from the official website of U.S. EPA, unless otherwise specified.

where C is the upper bound of the 95% confidence limit of the toxic elements in PM, µg m^–3 or mg kg^–1; ET is the exposure time, 6 h d^–1; EF is the exposure frequency, 350 d year^–1; ED is exposure duration, 24 and 6 years for adults and children, respectively; AT₁ is the average time, AT₁(for carcinogens) = lifetime years × 365 d year^–1 × 24 h d^–1, AT₁ (for noncarcinogens) = ED year × 365 d year^–1 × 24 h d^–1; lifetime is 74 years, which is the life expectancy in Henan (National Health and Family Planning Commission of the People’s Republic of China, 2013); IngR is the ingestion rate, 100 and 200 mg d^–1 for adults and children, respectively; BW is the body weight, 59 (Wang et al., 2009; National Bureau of Statistical of China, 2015) and 15 kg for adults and children, respectively; AT₂ is the average time, AT₂ (for carcinogens) = 74 years × 365 d year^–1, AT₂ (for noncarcinogens) = ED year × 365 d year^–1; CF is the conversion factor, 10^–6 kg mg^–1; SA is the surface area, 5700 and 2800 cm² for adults and children, respectively; AF is the adherence factor, 0.07 and 0.2 mg (cm² d)^–1 for adults and children, respectively; and ABS is the absorption fraction, 0.001, 0.03, and 0.01 for Cd, As, and other toxic elements (Hu et al., 2012; U.S. EPA, 2017), respectively.

Risk Assessment

CR and hazard quotient (HQ) of toxic elements in PM were calculated for carcinogenic and noncarcinogenic risk assessment, respectively. The relative formulas are as follows (U.S. EPA, 1989, 2004, 2009): 

where all the upper parameter values for different elements were chosen according to U.S. EPA (2017); IUR is the inhalation unit risk, (µg m^–3)^–1; SF_O is the slope factor, [mg (kg d)^–1]^–1; GIABS is the gastrointestinal absorption factor; RfC_i is the inhalation reference concentrations, mg m^–3; RfDo is the oral reference dose, mg (kg d)^–1; and HI is the hazard index for the three exposure pathways. 

Trajectory Calculation

The hybrid single-particle Lagrangian Integrated Trajectory (HYSPLIT) model is a useful tool for assessing potential sources of pollutants. In this study, the model was used to obtain 48-h backward trajectories of air masses with an altitude of 500 m arriving at the urban site SSQ (34°46′24″N, 113°44′2″E) in Zhengzhou and with four trajectories each day (00:00, 06:00, 12:00, and 18:00). Then, the potential source contribution function (PSCF) model, based on the HYSPLIT results, was used to identify the potential source regions of PM_2.5, PM₁₀, and toxic elements. The study region is equally divided into i × j grid cells, and the PSCF value in the ij^th cell is calculated by m_ij/n_ij (Ashbaugh et al., 1985; Jeong et al., 2011). n_ij and m_ij are the number of trajectories whose endpoints fall in the ij^th cell and the concentrations higher than the pollution criterion, respectively. According to the previous studies (Hsu et al., 2003; Liao et al., 2017), 75, 150 µg m^–3, and the average concentration of As were chosen as the pollution criterion for PM_2.5, PM₁₀, and toxic elements, respectively. The details of these methods were described by Ashbaugh et al. (1985) and Hopke et al. (1995). Uncertainties exist due to the small values of n_ij. Thus, to reduce the uncertainties, an empirical weight function, W_ij, was defined in the following formula. Then, the PSCF results were multiplied by W_ij (Polissar et al., 1999; Zhang et al., 2013). The domain was approximately in the range of 25–40°N, 105–120°E, with the resolution of 0.3° × 0.3°.

RESULTS AND DISCUSSION

PM Mass Concentrations

Box-and-whisker plots for the concentrations of PM_2.5 and PM₁₀ in Zhengzhou are shown in Fig. 2. The daily PM concentrations varied substantially among the sampling sites, i.e., PM_2.5: from 18 µg m^–3 (XM) to 666 µg m^–3 (SSQ) and PM₁₀: from 32 µg m^–3 (ZM) to 819 µg m^–3(HKG). The average PM_2.5 concentrations decreased in the order of SSQ (143 ± 115 µg m^–3) > HKG (131 ± 91 µg m^–3) ≈ GY (130 ± 96 µg m^–3) > XM (113 ± 85 µg m^–3) ≈ ZM (112 ± 112 µg m^–3), whereas the mean PM₁₀ values showed the order of HKG (196 ± 172 µg m^–3) > GY (190 ± 130 µg m^–3) > SSQ (181 ± 109 µg m^–3) > ZM (171 ± 134 µg m^–3) > XM (162 ± 137 µg m^–3). Generally, among the sampling sites, the rural site, i.e., ZM, was relatively slightly polluted; the traffic site, i.e., HKG, and the urban site with high traffic load, i.e., SSQ, showed relatively high PM_2.5 levels. HKG displayed the highest PM₁₀ pollution because of its suburb-surrounded location with relatively high wind speed and poor dust control measures.

Fig. 2. Box-and-whisker plots for the PM_2.5 and PM₁₀ concentrations among the sampling sites in Zhengzhou in 2016.

According to the Chinese National Ambient Air Quality Standards (NAAQS; daily PM_2.5: 75 µg m^–3; daily PM₁₀: 150 µg m^–3), from 47% (ZM) to 79% (SSQ) and from 38% (ZM and XM) to 60% (SSQ) of the sampling days for PM_2.5 and PM₁₀, respectively, exceed the standard. Among the sampling sites, the annual mean values of PM_2.5 (112–143 µg m^–3) and PM₁₀ (162–196 µg m^–3) were 2.2–3.1 and 1.3–1.8 times higher than the NAAQS (annual PM_2.5: 35 µg m^–3; annual PM₁₀: 70 µg m^–3). In addition, these values were also found to be higher than megacities in the Yangtze River Delta region, the Pearl River Delta region and the Beijing-Tianjin-Hebei region in China, i.e., Shanghai (PM_2.5: 103 and 62 µg m^–3; PM₁₀: 149 and 97 µg m^–3) (Wang et al., 2013), Guangzhou (PM_2.5: 61 and 45 µg m^–3; PM₁₀: 93 and 75 µg m^–3) (Peng et al., 2011), and Beijing (PM_2.5: 102 µg m^–3; PM₁₀: 149 µg m^–3) (Guo et al., 2010), not to mention the levels in the US (PM_2.5: 2–15 µg m^–3; PM₁₀: 4–21 µg m^–3) (Eldred et al., 1997) and Europe (PM_2.5: 11–30 µg m^–3; PM₁₀: 19–38 µg m^–3) (Amato et al., 2016). All of these present more serious PM pollution in Zhengzhou.

From a seasonal perspective, the mean concentrations of PM among the sampling sites in Zhengzhou are exhibited in Table 2. Obvious seasonal variations were observed, with the highest average concentrations in winter (PM_2.5: 235 ± 126 µg m^–3; PM₁₀: 308 ± 162 µg m^–3) and the lowest average values in summer (PM_2.5: 48 ± 17 µg m^–3; PM₁₀: 62 ± 18 µg m^–3). The reasons for these variations of PM include not only the different sources of emissions, but also the variant meteorological conditions in four seasons. For example, a great deal of coal is combusted for heating in the northern cities in China in winter. Moreover, frequent stagnant meteorological conditions occur in this season, causing high PM pollution. In summer, generally, the planetary boundary layer (PBL) development is enhanced by high temperature and higher PBL influences the vertical dispersion of the pollutants (Yang et al., 2015).

Elemental Levels and Comparability among the Different Sites

Elemental Levels among the Different Sites

The mean concentrations of 21 elements in PM in Zhengzhou are summarized in Figs. 3, S2 and S3. In general, among the sampling sites, these concentrations of elements in PM_2.5 and PM₁₀ vary substantially. Crustal elements, i.e., Ca (from 1653 ± 1229 ng m^–3 to 4935 ± 6279 ng m^–3 and from 4357 ± 2110 ng m^–3 to 8949 ± 4670 ng m^–3, respectively), Si (from 1218 ± 949 ng m^–3 to 2925 ± 3298 ng m^–3 and from 3142 ± 1633 ng m^–3 to 5138 ± 3420 ng m^–3, respectively), Fe (from 766 ± 502 ng m^–3 to 2015 ± 2183 ng m^–3 and from 1900 ± 972 ng m^–3 to 3516 ± 2186 ng m^–3, respectively), K (from 1437 ± 1544 ng m^–3 to 1728 ± 1577 ng m^–3 and from 1712 ± 1143 ng m^–3 to 2424 ± 1682 ng m^–3, respectively) and Al (from 523 ± 395 ng m^–3 to 1408 ± 1240 ng m^–3 and from 1378 ± 789 ng m^–3 to 2326 ± 1210 ng m^–3, respectively) were the most abundant. According to the previous studies (Han et al., 2010; Li et al., 2017), these crustal element concentrations were considerably higher than those in Beijing, Shenyang, and Anshan in North China; meanwhile, the ratio of ∑crustal elements/PM (i.e., proportion of the five total crustal elements in PM) was approximately in the range of 7–10% and 9–13% for PM_2.5 and PM₁₀, respectively, in Zhengzhou and also much higher than those in Beijing (3–4%), Shenyang and Anshan (both approximately 1%). The elements associated with combustion, i.e., S (from 3150 ± 2171 ng m^–3 to 3965 ± 3205 ng m^–3 and from 3502 ± 3365 ng m^–3 to 5067 ± 4088 ng m^–3, respectively), and Cl (from 1051 ± 1268 ng m^–3 to 2981 ± 2502 ng m^–3and from 1259 ± 1493 ng m^–3 to 3160 ± 2681 ng m^–3, respectively) were also plentiful in PM. The concentrations of S associated with PM_2.5 in Zhengzhou were far beyond those in Beijing (from 1.1 ± 1.1 µg m^–3 to 1.3 ± 1.4 µg m^–3 in PM_2.5) (Li et al., 2017), with the former ratio of S/PM_2.5 (approximately 3–4%) also higher than the latter (approximately 1–2%). These results suggested that dust and combustion sources played more important roles in elemental levels of PM in Zhengzhou. Na, Mg, Zn, Ti, Pb, Mn, and Ba were present in moderate quantities, and Cu (except at the GY site), Sb, As, Se, V, Cr, and Ni were sparse in PM, with average concentrations almost lower than 50 ng m^–3 among the sites. Though the toxic elements accounted for only a small fraction of PM, they should also be paid enough attention for their adverse health effects, especially for As, with all mean concentrations in PM₁₀ (from 16 ± 15 ng m^–3 to 28 ± 28 ng m^–3) far exceeding the Chinese NAAQS (annual As: 6 ng m^–3) among the sampling sites.

Fig. 3. Annual concentrations of elements in PM_2.5 and PM₁₀ among the sampling sites in Zhengzhou in 2016.

For comparison, the total elemental concentrations varied obviously among different sites and decreased in the order: GY (18.70 µg m^–3) > SSQ (18.52 µg m^–3) > HKG (18.16 µg m^–3) > XM (14.91 µg m^–3) > ZM (11.28 µg m^–3) and SSQ (30.92 µg m^–3) > GY (26.34 µg m^–3) > HKG (23.33 µg m^–3) > XM (22.21 µg m^–3) > ZM (20.00 µg m^–3) in PM_2.5 and PM₁₀, respectively. Generally, the total elemental levels at the ZM site, defined as the rural site, are relatively low; in contrast to the variation tendency of PM, the total elemental concentrations at the GY site were high,especially Cl, Zn, Pb, and Cu, suggesting the important effects from combustion and vehicular sources (Zhang et al., 2009; Bhangare et al., 2011; Bozlaker et al., 2013; Bozlaker et al., 2014). High levels of crustal elements were observed at the SSQ and HKG sites, suggesting dust as one of main sources of elements in PM. Seasonal variations of element concentration were evident in Zhengzhou. Overall, among the sites, crustal elements, i.e., Ca, Si, Fe, K, and Al, demonstrated high levels in spring while elements from combustion presented relatively serious pollution in winter, and the elemental concentrations were low in summer. These phenomena are ascribed to the combined contribution of emissions and meteorological conditions. For example, lack of rain and high-speed winds are the typical spring climatic characteristics in North China (Zhang et al., 2013) that result in increasing crustal element levels in PM. By comparing the elements with different diameters, crustal elements were found to be more abundant in PM₁₀, with the sum concentration of Ca, Si, Fe, K, and Al accounting for 63–73% of the total elements among the five sites, which were higher than those in PM_2.5 (50–68%). However, elements from combustion, i.e., S, Cl, Pb, As, and Se were more gathered in PM_2.5, with an average ratio (element in PM_2.5/element in PM₁₀) from 77% (As) to 87% (Se).

 
Comparability among the Different Sites

The CD values of elements in PM_2.5 and PM₁₀ of the different sampling sites in this study are shown in Fig. 4. Among the sampling sites, the CD values obviously differed and ranged from 0.11 to 0.37 for PM_2.5 and 0.09 to 0.41 for PM₁₀ during four seasons, which were almost higher than the annual CD values. On the whole, the CD values for PM_2.5 are slightly higher than those for PM₁₀ from a seasonal perspective (except autumn), which may be due to the more complicated sources for fine particles than those for coarse particles. The CD values are not only related to discrepant spatial distribution of emission sources at the five sites but also attributed to different meteorological conditions in four seasons. Moreover, variant emission levels of elements have an effect on CD values. For example, a series of measures, including factories closed and production suspended, were conducted in the summer by the local government (Zhengzhou Municipal Environmental Protection Bureau, 2016). Measures were carried out to cooperate with the regional supervision centers, the organization set up by the Ministry of Environmental Protection of the People’s Republic of China to monitor the implementation of environmental regulations.

Fig. 4 Coefficient of divergence values of elements in PM2.5 and PM10 among the sampling sites in Zhengzhou in 2016 (PM2.5: bottom left, PM10: upper right).

Sources of Elements

The indicatory elements for various major sources, i.e., dust, coal combustion, traffic emission, industrial emission, oil fuel, and biomass burning, in previous studies are listed in Table 3. EFs, Pearson’s CA, cluster analysis, and PCA were conducted to identify the main sources of elements in PM.

Enrichment Factors of Elements

The calculated EFs of elements in PM among the sampling sites in this work are displayed in Fig. 5. Na, Sb, Pb, Zn, Cu, and As were significantly enriched at all sites with high EFs (PM_2.5: 60–12,125; PM₁₀: 44–8,032), indicating anthropogenic sources. In the previous study, Na is mainly derived from coke-making, and cold-forming and hot-forming processes in iron and steel industries (Tsai et al., 2007). Sb is emitted from industries, e.g., the steel or petrochemical industry (Querol et al., 2006), and also from vehicular emissions (Charlesworth et al., 2011). Smelting and coal combustion processes are considered to be the primary sources of Pb and As (Zhang et al., 2009; Bhangare et al., 2011). Zn and Cu are probably generated from abrasion of brake linings and tire tread wear (Bozlaker et al., 2014). Therefore, industries, vehicles, and coal combustion are the main anthropogenic sources of elements in PM in Zhengzhou. The EFs of Si, Mg, and Ti are close to 1, indicating that crustal origin is also an important source of elements in aerosol (Nolting et al., 1999). The EFs of other elements, i.e., K, Ca, Mn, Cr, Ba, V, Fe, and Ni, are between 1 and 10, suggesting the emission from anthropogenic and natural sources. For example, K is associated with dust (Jiang et al., 2018a) and biomass burning sources (Silva et al., 1999).

Fig. 5. Enrichment factors for elements among the sampling sites in Zhengzhou in 2016.

 
Pearson’s Correlation and Cluster Analysis

Correlation of different elements is an important basis of source identification that can help to confirm and obtain interpretation of PCA results. Table 4 shows the Pearson’s CA of elements in PM among the sampling sites in Zhengzhou. A significant correlation was observed among Na, Mg, Al, Si, Ca, Fe, and Mn in PM_2.5 and PM₁₀ during the sampling period at the five sampling sites, suggesting that these elements were probably emitted from a common source. As and Se are highly correlated (0.51–0.88 for PM_2.5 and 0.53–0.88 for PM₁₀), indicating a similar source. High correlation coefficients were observed not only between Zn and Cu but also between Ni and V, demonstrating similar sources.

The cluster analysis results of elements in PM are shown in Figs. S4 and S5 in Supplemental Materials. Three clusters of elements in PM_2.5 and PM₁₀ at the sampling sites, except for PM_2.5 at HKG and XM sites (four clusters), were chosen. Generally, Cluster 1 is characterized by Ni, Cr, V, Se, As, Cu, Sb, Mn, Na, Pb, Ti, Mg, Ba, and Zn; Cluster 2 contains partly or entirely crustal elements, i.e., Al, Fe, Si, Ca, K; other elements are distributed to Clusters 3 and 4 (if any). Similar clusters of elements exhibit good correlations, suggesting common sources (Sun et al., 2014). The sources can be identified by fingerprint elements (Table 3). The results suggested that vehicular emission, industrial emission, coal combustion, and oil fuel were probable important sources of PM-bound elements in Zhengzhou. In addition, dust and biomass burning also affected elemental levels in PM.

Principal Component Analysis

In this study, PCA with varimax rotation was conducted for identifying the sources of elements in PM by using the software (SPSS 22.0), and the retention of principal components with eigenvalues greater than 1.0 was used to identify the main pollutant sources (Winner and Cass, 2001). Tables 5 and 6 present the PCA results of elements in PM_2.5 and PM₁₀ in each sampling site, respectively. Elemental loadings higher than 0.6 are in bold and considered to be important. 

At ZM sampling site (rural site), for PM_2.5, Factor 1 accounts for 31% of the total variance of the data, with high loadings of Mg, Al, Si, Ca, Ti, Cr, Mn, and Fe, representing contributions from dust (Jiang et al., 2018a) and industrial emission (Taiwo et al., 2014); Factor 2 accounts for 27% of the total variance in the dataset, with high loadings of Cl, K, Cu, Sb, Ba, and Pb, which are representative of biomass burning and vehicular emission (Fang et al., 2006; Pan et al., 2013); Factor 3 (16%) is coal combustion (Bhangare et al., 2011), with a high content of S, As, and Se; Factor 4 (9%) is dominated by Zn, which is reported to originate primarily from the industrial emission (AEA, 2011). For PM₁₀, Factor 1 accounts for 31% of the total variance, with high loadings of Mg, Al, Si, Ca, Ti, Ni, Cr, Mn, and Fe that came from dust and industrial emission (Taiwo et al., 2014; Jiang et al., 2018a); Factor 2 accounts for 28% of the total variance in the data and has high loadings of Na, Mg, Al, Cl, K, V, Cu, Sb, and Ba, suggesting that dust (Jiang et al., 2018a) and vehicular emission (Charlesworth et al., 2011) are the major contributors; high loadings of S, Cl, As, Se, and Pb are shown on Factor 3 (21%), considered to originate from coal combustion (Bhangare et al., 2011).

At the HKG sampling site (traffic site), for PM_2.5, 49% of the total variance was observed in Factor 1, contributed by dust (Jiang et al., 2018a) and traffic emission, i.e., aircraft and vehicle sources (Charlesworth et al., 2011; Ren et al., 2012), because of the high loadings of Na, Mg, Al, Si, Ca, Ti, V, Ni, Sb, Cr, Mn, and Fe; Factor 2, with a high content of S, Cl, As, Se, and Pb, represents the contribution of coal combustion (Bhangare et al., 2011); Factor 3 (12%), with high loadings of K and Ba, represents biomass burning (Argyropoulos et al., 2013) in view of Cl loading; and Factor 4 (8%), with a high content of Cu and Zn, represents vehicular emission (Viana et al., 2006). For PM₁₀, Factor 1 (37%) is commonly related to dust (Jiang et al., 2018a) and industrial emission (Taiwo et al., 2014), characterized by high loads of Na, Mg, Al, Si, K, Ca, Ti, V, Ni, Sb, Ba, Mn, and Fe; Factor 2 (29%) is identified as coal combustion (Bhangare et al., 2011) and industrial emission (AEA, 2011), with high loadings of S, Cl, Cu, As, Se, and Pb; and Factor 3 (17%), with a high content of Ni and Cr, represents oil fuel (AEA, 2011).

At the GY sampling site (urban site), for PM_2.5, Factor 1 shows 45% of the total variance and contributions by dust (Jiang et al., 2018a) and vehicular emission (Charlesworth et al., 2011) due to high loadings of Mg, Al, Si, Ca, Ti, V, Ni, Sb, Cr, Mn, and Fe; Factor 2, with high loadings of S, Cl, Zn, As, Se, and Pb, shows 26% of the total variance and represents the coal combustion (Bhangare et al., 2011) and industrial emission (AEA, 2011); Factor 3 (13%) represents biomass burning (Argyropoulos et al., 2013), with high loadings of K and Ba. For PM₁₀, Factor 1 explains 40% of the total variance, with a high content of Na, Mg, Al, Si, Ca, Ti, V, Ni, Cr, Mn, and Fe, representing dust (Jiang et al., 2018a) and industrial emission sources (Taiwo et al., 2014); Factor 2 (26%) has high loadings of S, Cl, Zn, As, Se, and Pb, which originated from coal combustion (Bhangare et al., 2011) and industrial emission (AEA, 2011); and Factor 3 (18%) is strongly correlated with Na, K, and Ba and identified as biomass burning (Argyropoulos et al., 2013; Wang et al., 2016).

Given limited space, the details of PCA results in the other two urban sites are not shown with similar indicators. At the XM sampling site, for PM_2.5, Factor 1 (dust and vehicular emissions), Factor 2 (coal combustion and industrial emission), Factor 3 (biomass burning and vehicular emission), and Factor 4 (industrial emission) contribute 41%, 22%, 21%, and 7% of the variance, respectively; for PM₁₀, Factor 1 (dust and vehicular emissions), Factor 2 (coal combustion and industrial emission), and Factor 3 (biomass burning and industrial emission) contribute 43%, 26%, and 18% of the variance, respectively. At the SSQ sampling site, for PM_2.5, Factor 1 (dust and vehicular emission) and Factor 2 (coal combustion and industrial emission) account for 56% and 26% of the variance, respectively; for PM₁₀, Factor 1 (dust and vehicular emission), Factor 2 (coal combustion and industrial emission), and Factor 3 (biomass burning) contribute 44%, 24%, and 14% of the variance, respectively.

In the comparison of the source identification results among the five sites, the source contributions were found to be consistent with the category of sites. For example, ZM site is the rural site with low traffic effect. The main results included relatively high contributions of biomass burning, which is attributed to the use of straw as fuel for cooking and heating in winter, dust emission, and poor dust control measures in the country. HKG site is located near Xinzheng International Airport, and therefore, its traffic sources (i.e., aircraft and vehicles) contribute high proportion. SSQ site is an urban site with high traffic and is also affected highly by vehicle emission. Moreover, for comparison of PM_2.5 and PM₁₀ sources, in brief, combustion sources, including coal combustion, vehicular emission, and biomass burning, played more important roles in elements in PM_2.5, whereas dust source contributed more to PM₁₀-bound elements.

Health Risks Posed by Toxic Elements

The health risk values, including CR, HQ, and HI, of toxic elements in PM_2.5 and PM₁₀ through the inhalation, dermal absorption, and daily intake pathways are calculated in this study. For CR, 1 × 10^–6 < CR < 1 × 10^–4 indicates a tolerable risk for regulatory purposes, and CR > 1 × 10^–4 indicates intolerable risk (U.S. EPA, 1989). Moreover, for noncarcinogenic risk, HQ > 1 and HI > 1 suggests a significant risk of a single element and total toxic elements (U.S. EPA, 1989). The relative detailed data of health risk evaluation from toxic elements in PM through the three exposure pathways are presented in Tables S1–S3 in Supplemental Materials, and the carcinogenic and noncarcinogenic risks among the sampling sites in Zhengzhou are exhibited in Tables 7, S4 and S5. Meanwhile, the total CR and HQ of toxic elements in PM_2.5 and PM₁₀ are summarized in Fig. 6.

Fig. 6. Carcinogenic risk and hazard quotient of toxic elements in PM_2.5 and PM₁₀ among the sampling sites in Zhengzhou in 2016.

Overall, the CR values of As, Pb, and Ni were all higher than 1 × 10^–6, especially for As (PM_2.5: 1.2 × 10^–4–2.9 × 10^–4; PM₁₀: 1.0 × 10^–4–3.2 × 10^–4) and Ni (PM_2.5: 6.8 × 10^–5–1.9 × 10^–4; PM₁₀: 6.3 × 10^–5–1.8 × 10^–4), with CR values exceeding 1 × 10^–4, respectively. This result suggested that As and Ni caused intolerable risks and Pb showed tolerable risk in PM_2.5 and PM₁₀ in Zhengzhou. According to the previous study (AEA, 2011), metal production, and public electricity and heat production are the major sources of As; combustion of heavy fuel oil is the major source of Ni. Therefore, the local regulatory agency should strengthen management and control of coal-fired power plant and nonroad-mobile source emission to reduce As and Ni levels. The noncarcinogenic risks of As (PM_2.5: 1.03–8.33; PM₁₀: 1.01–9.37), Pb (PM_2.5: 0.31–6.24; PM₁₀: 0.30–4.59), and Sb (PM_2.5: 1.13–12.55; PM₁₀: 1.05–10.71) were higher than the threshold value, i.e., HQ = 1, indicating significant risks. In general, for comparison with adults, children are more sensitive, with higher CR and HQ values.

Among the three exposure pathways, the daily intake is the limiting one. Generally, the highest CR of As, Pb, and Ni existed through the daily intake pathway, especially for As, with CR values ranging from 9.8 × 10^–5 to 2.7 × 10^–4 in PM_2.5 and from 8.6 × 10^–5 to 3.0 × 10^–4 in PM₁₀. The noncarcinogenic risk of all toxic elements, i.e., HI, was also the highest through the daily intake pathway, with the values ranging from 0.9 to 21.4 in PM_2.5 and from 1.9 to 18.1 in PM₁₀, which are far beyond the limit (HI = 1). Moreover, in this pathway, Sb, As, and Pb exhibited high risks, with HQ values all exceeding 1 for children, thereby also indicating significant risks. By contrast, CR and HQ values from toxic elements in PM through inhalation and dermal absorption exposure were relatively low. However, significant noncarcinogenic risks still existed because of HI > 1, especially for children.

For comparison among the five sites, the health risks differed with various elemental concentrations. In general, the total CR values from As, Pb, and Ni in PM_2.5 ranged from 3.5 × 10^–4 (HKG) to 4.9 × 10^–4 (GY) and 2.0 × 10^–4 (HKG) to 2.8 × 10^–4 (GY) for children and adults, respectively; the CR values in PM₁₀ varied from 3.2 × 10^–4 (HKG) to 5.0 × 10^–4 (SSQ) and 1.9 × 10^–4 (HKG) to 2.8 × 10^–4 (SSQ) for children and adults, respectively. These results demonstrated intolerable CR from the three elements in PM at all the sites in Zhengzhou and thus should be given more attention. Moreover, the relatively obvious differences of CR at the sites, with values, i.e., the average ratio of difference between the lowest and highest risk, of 29% and 35% for PM_2.5 and PM₁₀. For noncarcinogenic risks, all total HI (i.e., ∑HI) of the sites were higher than 1, suggesting significant risks. For PM_2.5, the total HI values ranged from 16.6 (ZM) to 24.9 (GY) and 2.0 (ZM) to 4.1 (GY) for children and adults, with difference ratios of 34% and 51%, respectively. For PM₁₀, the total HI values were in the range of 18.2 (HKG)–25.4 (SSQ) and 3.3 (HKG)–4.5 (SSQ) for children and adults, respectively, both with difference ratios of 28%. Therefore, data at multiple sites are necessary for health risk assessment in the study area, especially for a large region. 

Source Regions of PM and Toxic Elements

Fig. 7 shows the 48-h back trajectories of air masses arriving in Zhengzhou during the sampling periods, with 1140 transport trajectories. Obviously, four types of air mass clusters influenced the pollution levels of the sampling site. Two northwestern airflows with long trajectories occurred. The first one was mainly from Kazakhstan, Xinjiang, or Gansu, passing through the northern part of Qinghai, Shaanxi, Shanxi, and Northwestern Henan. The second airflow came from Russia, Mongolia, or Inner Mongolia, then by way of the Beijing-Tianjin-Hebei region and northern Henan. These areas, i.e., Mongolia, Inner Mongolia, Xinjiang, Gansu, and Shaanxi, were commonly covered by the Gobi Desert and grasslands. The long-range transport brought the mineral aerosols, mixing with the local emission and aggravating PM, especially crustal materials and pollution levels, and changing the ratio of Ca to Al (Zhang et al., 2003), used to determine source area of dust, in the PM samples in this study area. Beijing-Tianjin-Hebei, characterized by huge consumption of coal and coal combustion as the predominant source of aerosols (Yao et al., 2009), is always one of the most deteriorated regions in China (Tao et al., 2016). Therefore, the second direction airflow probably increased the PM and elements relative to coal burning, i.e., S, Cl, As, Se, and Pb (Bhangare et al., 2011) concentrations, especially in winter, i.e., high emission season because of extra coal consumption for central heating, including the local emission in Zhengzhou and the emission from the airflow passing regions. The medium to long distance transport covered the eastern regions of Zhengzhou, i.e., the Yellow Sea, Jiangsu, Northern Anhui, Southern Shandong, and Eastern Henan; these inland areas featured flourishing agriculture (Kang et al., 2016). Hence, these trajectories from the east likely influenced the pollution level of PM, Na, Cl, and elements emitted from biomass burning, i.e., K and Ba (Argyropoulos et al., 2013; Wang et al., 2016), especially in harvest season. The short-distance transport from the southern areas of Zhengzhou covered Hunan, Hubei, and southern Henan. Generally, these regions present mitigating PM levels (Tao et al., 2016), suggesting possible decreasing PM and elemental values in this study region due to dilution function.

 Fig. 7. Forty-eight-hour back trajectories of air masses arriving at SSQ site in Zhengzhou in 2016.

In this study, source regions of PM₁₀, PM_2.5, and As, with the highest CR, were analyzed by PSCF model (Fig. 8). Generally, the potential source areas of the pollutants are almost distributed in the range of the surrounding regions of Zhengzhou in Henan Province. For PM_2.5, the northwestern and southeastern regions of Zhengzhou, i.e., Jiyuan, Jiaozuo, Xuchang, and Zhoukou, were likely potential source areas, with WPSCF values higher than 0.6. Moreover, the spatial source distributions of PM_2.5-bound As were confined in Jiyuan, Jiaozuo, Xinxiang, Anyang, and Kaifeng, which are the main industrial cities in Henan, with WPSCF values over 0.4. According to previous studies (AEA, 2011; Bhangare et al., 2011), industrial production (e.g., metal production as well as public electricity and heat production) and coal combustion are the major sources of As. Therefore, the relative activities in the five industrial cities influenced the pollution levels of PM_2.5-bound As in Zhengzhou. For PM₁₀, the potential source area was in the southwestern areas of Zhengzhou, mainly including Pingdingshan and Nanyang (WPSCF values above 0.4). Pingdingshan is the largest coal-producing city in Henan, with raw coal products amounting to 34.0 Mt in 2016 (Bureau of Statistics of Pingdingshan, 2017). Coal mining processes emitted huge amounts of coarse particles and accompanied with south air trajectories, the air mass carried a large amount of particles, thereby influencing the PM₁₀ concentration in Zhengzhou. Nanyang is a large agricultural city in Henan, with grain, oil-bearing crops, flue-cured tobacco, vegetables and edible fungus, and fruit outputs of 6.4, 1.4, 0.1, 10.5, and 1.6 Mt in 2016 (Bureau of Statistics of Henan Province, 2017). Agricultural activities and bare soil increased PM₁₀ levels in Zhengzhou when the south winds form. The WPSCF values of PM₁₀-bound As, which are generally less than those of PM_2.5-bound As, showed similar spatial distributions in fine particles due to the industries located in the surrounding cities. For comparison, fine particles with higher WPSCF values are more easily transmitted than coarse particles.

Fig. 8. Spatial distributions of WPSCF values of PM_2.5, PM₁₀, and As in Zhengzhou in 2016.

CONCLUSION

PM_2.5 and PM₁₀ filters were collected at five sites in Zhengzhou, and the concentrations, source apportionment, health risks, and source regions of the toxic elements were analyzed. The results indicated severe PM_2.5 and PM₁₀ pollution, with annual average concentrations that were considerably higher than the Chinese NAAQS. The rural site (ZM) exhibited only slight pollution, but the traffic site (HKG) and the urban site with high traffic (SSQ) showed relatively high PM_2.5 levels, and the highest PM₁₀ level was also observed at HKG. The highest and lowest mean levels were observed in winter and summer, respectively, for both PM_2.5 and PM₁₀.

Overall, high levels of PM-bound crustal elements and plentiful Cl indicated that dust and combustion sources played major roles. Furthermore, the PM₁₀-bound As greatly exceeded the Chinese NAAQS, posing a high potential risk. Generally, the total elemental levels were relatively low at ZM and high at GY, with high individual concentrations of Cl, Zn, Pb, and Cu, in particular, at the latter site. High levels of crustal elements were observed at SSQ and HKG, suggesting a significant influence from dust. High levels of crustal elements, which were more abundant in the PM₁₀, were observed in spring, whereas combustion-source elements, which were more abundant in the PM_2.5, displayed elevated levels in winter. The elemental concentrations were low in summer. In general, the CD values for the PM_2.5 were slightly higher than those for the PM₁₀. These results are not only related to discrepant spatial distributions of emission sources at the five sites but also attributable to different meteorological conditions across the four seasons.

The Na, Sb, Pb, Zn, Cu, and As were emitted from anthropogenic sources, whereas the Si, Mg, and Ti were crustal in origin. Pearson’s CA, cluster analysis, and PCA indicated that vehicles, industry, coal combustion, oil fuel, dust, and biomass burning were probably the main sources of PM-bound elements in Zhengzhou. The ZM site was characterized by low traffic and high contributions from biomass burning and dust emission, whereas the HKG site demonstrated high pollution from traffic sources, and the SSQ site was also highly affected by pollution from vehicles. Whereas elements in the PM_2.5 largely originated in combustion sources, those in the PM₁₀, by comparison, received greater contributions from dust sources.

The PM-bound As and Ni posed both intolerable carcinogenic risks and, in addition to Pb, significant non-CRs. In general, children, as shown by higher CR and HQ values, were more sensitive than adults to these risks. The daily intake pathway exhibited the highest CR and HI values. Obvious differences in the CR and HI values were detected between the various sites; hence, data from multiple sites in a study area are necessary for accurate health risk assessment, especially for a large region. Analysis of the source regions identified Jiyuan, Jiaozuo, Xuchang, and Zhoukou; Pingdingshan and Nanyang; and Jiyuan, Jiaozuo, Xinxiang, Anyang, and Kaifeng as the main potential source areas of PM_2.5, PM₁₀, and As, respectively. Furthermore, fine particles with higher WPSCF values were more easily transmitted than coarse ones.

ACKNOWLEDGEMENTS

The study was supported by the financial support from the National Natural Science Foundation of China (51808510, 51778587), National Key Research and Development Program of China (2017YFC0212400), Natural Science Foundation of Henan Province of China (162300410255), Foundation for University Young Key Teacher by Henan Province (2017GGJS005), Outstanding Young Talent Research Fund of Zhengzhou University (1421322059) and Science and Technology Planning Project of Transportation in Henan Province (2016Y2-2, 2018J3).

Aerosol Air Qual. Res. 19 :247 -271 . https://doi.org/10.4209/aaqr.2018.07.0275