## Materials and Methods  
### Coal Properties  
A typical Zhundong coal had been used in earlier research ([PERSON] et al., 2016; [PERSON] et al., 2018), which was crushed, ground and sieved to obtain a narrow-sized coal sample in a range of 63-100 \(\upmu\)m. The proximate and ultimate analysis and the inorganic compositions of the Zhundong coal sample are shown in Table 2. It is a low-ash and low-sulfur coal which can be classified as a sub-bituminous coal. The results of the XRF (X-ray Fluorescence, EDAX EAGLE III, EDAX Inc., Mahwah, NJ, USA) analysis on low-temperature ash for coal show an abundance of Ca, S, Na and Mg.  
To distinguish between the occurrence forms of major inorganic elements, the coal was sequentially extracted with deionized water, 1 M ammonium acetate (CH\({}_{3}\)COOH\({}_{4}\), NH\({}_{4}\)OAc), 1 M hydrochloric acid (HCl) at a temperature of 60 \({}^{\circ}\)C for 24 h ([PERSON] et al., 2016). The ratio of solid to liquid was 1:50, i.e., each 1 g solid sample was mixed with a 50 mL solution. The water-soluble, ion-exchangeable (mainly organically bound) and acid-soluble inorganic elements were extracted sequentially, and the solid residue implies the HCl insoluble elements, which are mainly present as silicate or aluminosilicate. The solution and digested solid residue were analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS, ELAN DRC-e, PerkinElmer Inc., Hopkinton, MA, USA) to quantitatively determine the amount of inorganic elements. The chemical fractionation of alkali and alkaline earth metals (AAEM) in Zhundong coal is presented in Figure 1. The occurrence form of Na is mostly water-soluble and organically bound, and their total fractions reach nearly 90%. The occurrence forms of Ca and Mg are mostly organically bound and acid-soluble, with total fractions of about 80%. These elements in coal are expected to be the major contributors to inorganic PM, especially for ultrafine particles.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Zhundong Coal**} & \multicolumn{3}{c}{_Proximate analysis (wt \%, air-dried)_} & \multicolumn{3}{c}{_Ultimate analysis (wt \%, air-dried)_} \\  & M \({}^{1}\) & A \({}^{2}\) & VM \({}^{3}\) & FC \({}^{4}\) & C & H & O \({}^{5}\) & N & S \\  & 7.25 & 3.56 & 40.13 & 49.06 & 65.77 & 3.95 & 14.60 & 4.36 & 0.51 \\  & \multicolumn{3}{c}{_Chemical composition analysis of low-temperature ashes (wt \%)_} \\  & Na\({}_{2}\)O & K\({}_{2}\)O & SO\({}_{3}\) & CaO & Fe\({}_{2}\)O\({}_{3}\) & MgO & Al\({}_{2}\)O\({}_{3}\) & SiO\({}_{2}\) \\  & 4.30 & 0.51 & 26.92 & 37.96 & 3.89 & 10.66 & 8.76 & 7.00 \\ \hline \hline \end{tabular} \({}^{1}\) Moisture; \({}^{2}\) Ash; \({}^{3}\) Volatile matter; \({}^{4}\) Fixed carbon; \({}^{5}\) Counted by difference.  
\end{table}
Table 2: Proximate, ultimate and ash composition analysis of Zhundong coal.  
Moreover, the Zhundong coal sample was subjected to a CCSEM analysis (Computer-Controlled Scanning Electron Microscopy, GENESIS XM, EDAX Inc., Mahwah, NJ, USA), according to the detailed procedures presented in our prior studies [34; 35]. Though it cannot be used to analyze organically bound elements, CCSEM technology is beneficial for determining the types and contents of inorganic minerals and the mass fraction of major elements distributed in those minerals.