3.1 Atmospheric Hg concentration
The wind direction was northwesterly during the entire sampling period, which is consistent with the perennial wind direction of the region, with a wind speed range of 0.6–5.2 m s− 1. To evaluate the level of atmospheric mercury pollution in the Wuda region, the GEM content at the coal field boundary northwest of the Wuda District was measured for use as the background value. The data were recorded every 5 s, and the real-time measurement lasted 3 min. According to the on-site monitoring data, the background GEM concentration ranged from 13 to 29 ng m− 3, with an average value of 22 ng m− 3, which far exceeds the global atmospheric mercury background value (1.5–1.8 ng m− 3) (Jaffe et al., 2005). The average GEM concentrations in the Wuda Coalfield and the adjacent urban area were 80 ng m− 3 (65–90 ng m− 3) and 52 ng m− 3 (25–95 ng m− 3), respectively, which was 3.6 and 2.4 times the background level of the area, respectively. The concentrations were far higher than those in other typical industrial cities or regions, such as Guiyang (9.72 ng m− 3) (Fu et al., 2011), Guangzhou (5.4 ng m− 3) (Wang et al., 2007), Changchun (18.4 ng m− 3) (Fang et al., 2001), and Taiwan (6.3–9.4 ng m− 3) (Kuo et al., 2006).
The Hg content of TSP in the Wuda Coalfield and adjacent urban area were 25–45 ng m− 3 and 14–29 ng m− 3, with average contents of 33 ng m− 3 and 21 ng m− 3, respectively. The average PHg concentration in the TSP samples from the Wuda Coalfield was 28, 77, and 33 times that of Beijing (1.18 ng m− 3) (Liu et al., 2002), Shanghai (0.429 ng m− 3) (Xiu et al., 2003), and Changchun (0.022–1.98 ng m− 3) (Fang et al., 2001), respectively, indicating significant localized atmospheric mercury pollution caused by coalfield fires.
Using a scanning electron microscope (SEM-EDX) to analyze the morphology and chemical composition of 1,600 single atmospheric particles in the Wuda urban area, Wang et al. (2018) found that mineral particles, combustion particles, and sulfur-containing particles were the main particle forms in the region, which contained abundant weathered coal gangue and raw coal dust particles. This suggests that the atmospheric pollutants from the coal field fires were transmitted to the downwind urban area. It should be noted that this study omitted the contribution of other potential mercury pollution sources (such as transportation, biomass combustion, industrial waste gas, and other long-distance transportation of particulate matter). The Wuda area has significant atmospheric mercury concentrations, despite its much lower population size and total gross domestic product (GDP) than those of the other cities listed in this manuscript. This suggests that the coal fires are the highest mercury contributors in the area, with other source factors contributing significantly less in comparison.
3.2 Correlation between PHg content and OC/EC content in TSP
OC and EC are mainly derived from coal combustion, motor vehicle emissions, and biomass combustion. Moreover, EC is more stable and is principally sourced from the primary combustion emissions of coal (Gray et al., 1986; Salma et al., 2004; Viana et al., 2006). The total carbon (TC) concentration in TSP in the Wuda urban area during the sampling period varied from 59.57 to 134.49 µg m− 3, with an average value of 87.73 µg m− 3. The OC and EC contents measured by the TOT method were 48.88 µg m− 3 (30.08–64.95 µg m− 3) and 41.95 µg m− 3 (29.50–69.54 µg m− 3), respectively. Using the TOR method, the OC and EC contents were 57.25 µg m− 3 (29.2–96.56 µg m− 3) and 30.58 µg m− 3 (17.53–48.05 µg m− 3), respectively. The TC content of TSP in the Wuda urban area was 1.9 and 4.4 times that of the typical Chinese industrial cities of Tangshan (47 µg m− 3, 30–64 µg m− 3) (Guo et al., 2013) and Harbin (19.81 µg m− 3, 9.35–35.12 µg m− 3) (Huang, 2013), respectively.
We observed no correlation between PHg content and OC/EC content in TSP. The correlation coefficients (r) did not exceed 0.1 in the TOR and TOT modes (Fig. 2). This suggests that mercury in atmospheric particles is less likely to exist in its organic state and is also unlikely to be adsorbed on the surface of carbon particles in its elemental state. Therefore, mercury in atmospheric particles is more likely to occur in the form of inorganic compounds.
3.3 Correlation between PHg content and water-soluble ion content in TSP and PM 2.5
Water-soluble ions are an important component of atmospheric particles (Behrooz et al., 2017a; Chakraborty and Gupta, 2010). They are mostly composed of cations such as NH4+, K+, Ca2+, Na+, and Mg2+, and anions such as SO42−, NO3−, Cl−, and F−. In coastal cities, SO42−, NO3−, Cl−, and Na+ are largely derived from sea salt particles, whereas in inland cities they are largely derived from fossil fuel combustion, the chlor-alkali industry, and other production activities. Moreover, K+ is mostly derived from biomass combustion (Alastuey et al., 2006; Behrooz et al., 2017b; Bisht et al., 2015; Zhang et al., 2013). The content ranges of F−, Cl−, NO2−, NO3−, and SO42− in TSP in the Wuda urban area were 0.08–1.41, 15.48–32.94, 0.09–0.38, 6.73–12.38, and 17.85–40.7 µg m− 3, respectively, and their average values were 0.83, 22.75, 0.24, 9.50, and 25.15 µg m− 3, respectively. In PM2.5, the same ions yielded contents of 0.09 µg m− 3 (0.02–0.21 µg m− 3), 10.69 µg m− 3 (6.43–18.56 µg m− 3), 0.13 µg m− 3 (0.02–0.26 µg m− 3), 8.71 µg m− 3 (5.93–11.90 µg m− 3), and 10.291 µg m− 3 (7.11–15.01 µg m− 3), respectively, accounting for 10, 47, 56, 92, and 41% of the corresponding ion concentrations in TSP, respectively. Moreover, SO42−, Cl−, and NO3− were the dominant anions, accounting for approximately 43, 39, and 16% of the total anion concentration, respectively.
Based on Pearson’s correlation analysis, the PHg content was significantly correlated with Cl− and NO3− in PM2.5, and the corresponding r values were 0.854 and 0.745, respectively (Table 1). This indicates that Hg tends to combine with Cl− and NO3− to form mercury particles, likely as HgCl2 and Hg(NO3)2. We also observed a high correlation between PHg and NO3− in TSP (r = 0.643). The Cl− in TSP showed no clear correlation with PHg, but was closely related to Na+ (r = 0.890) (Table 2). This suggests that the association between Cl− and PHg in TSP may be masked by the interference of NaCl particles. In addition, PHg was correlated with Ca+, Na+, and Mg2+ contents in PM2.5 (Table 1). However, this may be an indirect effect caused by the correlation between Ca+, Na+, Mg2+, and NO3−/Cl− (Table 1). The contents of Cl− and NO3− in PM2.5 accounted for 47 and 92% of the corresponding ion concentrations in TSP, respectively, indicating that PHg predominantly occurs as fine particulate matter.
Some studies have shown that burning coal produces SO2 and NO2, which subsequently forms H2SO4 and HNO3 through oxidation and moisture absorption (Fang and Liu, 2010; Pathak et al., 2009; Tatu et al., 2006). As shown in Fig. 3, the TSP sampling volume from the coalfield fire area ranged from 11,476 to 25,490 L (average 17,849 L), with corresponding solution pHs of 3.28–4.81; the TSP sampling volume from the urban area was 43,859–45,533 L (average 44,515 L), with corresponding solution pHs of 5.43–6.41. The sampling volumes from the coalfield fire area were smaller but produced more acid; this suggests that the acidic gas produced by the combustion of underground coal fires was the main source of acid in this area and may also be transported to other areas through airflow migration. We observed a significant negative correlation between PHg content and acidity in TSP (r = -0.906), as PHg more easily accumulates in acidic environments.
3.4 Analysis of PHg speciation based on AAS-TPD
Figure 4 shows the thermal decomposition curves of Hg in the TSP samples collected from the Wuda mining and urban areas under an argon atmosphere. Mercury decomposition predominantly occurs in the range of 150–450°C. Owing to the coexistence of various mercury species with different thermal stability characteristics, multiple Hg signal peaks appear in this temperature range (Cao et al., 2019a; Cao et al., 2019b). To clear the individual mercury peak ranges, Origin 6.0 software was used to deconvolve the mercury decomposition curves and obtain four peaks (A, B, C, and D) (Fig. 4). The starting temperature ranges of the four TSP peaks from the coalfield area were 173–253, 276–335, 320–380, and 367–379°C, with the center peaks at 213, 306, 350, and 373°C, respectively. The corresponding TSP ranges from the urban area were 185–268, 275–335, 265–412, and 367–389°C, with the central peaks at 223, 305, 340, and 378°C, respectively. The thermal decomposition characteristics of PHg in the two regions were consistent, indicating the same species of Hg.
Compared with the thermal decomposition characteristics of standard mercury compounds (Table 3), the characteristic peaks of HgCl2, HgS, HgO, and Hg(NO3)2·H2O occurred near 212, 310, 340, and 373°C, respectively (Liu et al., 2013; Lopez-Anton et al., 2011; Lv, 2017; Meng and Wang, 2012; Wang, 2016). The starting ranges of the characteristic peaks are highly consistent with peaks A, B, C, and D. The mercury in atmospheric particles in the study area therefore likely exists mainly in the forms of HgCl2, HgS, HgO, and Hg(NO3)2·H2O. This is consistent with the high correlation between PHg, Cl−, and NO3− obtained by water-soluble ion analysis.
Based on the deconvolution results, the contents of HgCl2, HgS, HgO, and Hg(NO3)2·H2O in atmospheric particles from the coalfield area accounted for 9, 23, 58, and 10%, respectively. The proportions in the urban area were similar, with corresponding rates of 15%, 30%, 41%, and 14%.