## 2 Materials and Methods
### \(k_{0}\)-INAA Experiment  
NIST SRM 1648a urban PM, KRISS urban dust CRM candidate, and KRISS ash CRM candidate samples were analyzed in solid form using the \(k_{0}\)-INAA method at Jozef Stefan Institute (JSI), Slovenia [36; 37]. All the samples were made into pellets in cellulose paper (Whatman(r) 542, \(\varphi\) 55 mm) as shown in Figure 2a. Figure 1: Picture of (**a**) sieve shaker; (**b**) \(>\)100 \(\upmu\)m and \(<\)100 \(\upmu\)m sieved KRISS urban dust CRM candidate; (**c**) powder form of KRISS urban dust CRM candidate (\(<\)100 \(\upmu\)m) for analysis. About 100 mg of each sample was used. The thickness and diameter of each prepared sample as a pressed pellet were about 2.7 mm and 13 mm, respectively. For 12 h long irradiation, nine subsamples for NIST SRM 1648a, six for KRISS urban dust CRM candidate, three for KRISS ash CRM candidate, and five for blank cellulose paper were analyzed. For short irradiation, i.e., 2 min for the samples and 5 min for cellulose paper (blank sample), three additional subsamples were analyzed for each sample. In the case of the KRISS urban dust CRM candidate, the relative standard deviations (RSDs) of mass fractions between subsamples, which represent the homogeneity of the sample, were less than 10% for all elements, except for six elements, including Ag, Au, Cd, Hg, I, and Sn. Those elements with high RSDs were at a level similar to the limit of detection (LOD) of the analysis except for Au. In the case of NIST SRM 1648a and the KRISS ash CRM candidate, the RSDs of elements were less than 10%, excluding four (Au, Cu, W, and Zr) and three (Ba, Cu, and In) elements, respectively. The RSD and LOD of each element for all samples are shown in the Supplementary Materials (Table S1). Dry mass measurements were carried out using Mg(ClO\({}_{4}\))\({}_{2}\) desiccant for 48 h by employing 0.5 g of four replicates. The NIST SRM 1648a sample was dried to a constant mass before being weighed for analysis, whereas, for all other samples, a separate sub-sample was taken for dry mass correction from the bottle at the time of analysis [35]. In the case of cellulose paper, moisture content was measured by keeping it in a desiccator for 180 h to reach equilibrium. The dry masses of the samples were as follows: NIST SRM 1648a, 98.63% \(\pm\) 0.09% (unitless, f\({}_{\text{dry}}\) = 1.0139); KRISS urban dust CRM candidate, 95.60% \(\pm\) 0.11% (unitless, f\({}_{\text{dry}}\) = 1.0460); KRISS ash CRM candidate, 99.79% \(\pm\) 0.11% (unitless, f\({}_{\text{dry}}\) = 1.0021); and cellulose paper, 96.59% \(\pm\) 0.09% (unitless, f\({}_{\text{dry}}\) = 1.0353). All details of the experimental procedures are available in our previous literature [13; 38]. The short irradiation was performed for 2 min to analyze short-lived nuclides; in the case of cellulose paper, it was irradiated for 5 min, because the concentrations of the elements were mostly low. The long irradiation was performed for 12 h to analyze elements having medium/long half-life nuclides. NIST SRM 1648a was used as the quality control (QC) sample for each irradiation, and all samples were irradiated in the carousel facility of the 250 kW TRIGA Mark II reactor of the JSI with a thermal neutron flux of 1.1 \(\times\) 10\({}^{12}\) cm\({}^{-2}\cdot\)s\({}^{-1}\). The \(f\) value was 28.43 \(\pm\) 0.93 (unitless, \(k\) = 1), and the \(\alpha\) value was \(-\)0.0042 \(\pm\) 0.0020 (unitless, \(k\) = 1) [36; 37]. The \(f\) value is a thermal-to-epithermal neutron flux ratio and \(\alpha\) is a parameter describing the epithermal flux that deviated from the ideal 1/E distribution, which were obtained through the Cd-ratio method [32]. The _k_\({}_{0}\) constant, _Q_\({}_{0}\) (resonance integral to 2200 m\(\cdot\)s\({}^{-1}\) cross-section ratios), _E_\({}_{\text{r}}\) (effective resonance energy), and _T_\({}_{1/2}\) (half-life) were obtained from the literature [39; 40]. A single standard (IRMM-530R, Al-0.1%Au alloy in the form of a disc, 7 mm in dia.