## 4 Conclusions  
The KRISS urban dust CRM candidate and related samples were analyzed using the \(k_{0}\)-INAA and WD-XRF method. The proposed method was validated using NIST SRM 1648a urban PM, and the mass fractions of all of the elements obtained by \(k_{0}\)-INAA show good agreement with the NIST certified values within their specified uncertainties. The major source of KRISS urban dust was estimated to be traffic emissions, followed by industry emissions based on multi-elemental signature analysis, which showed a different characteristics of elemental composition compared with NIST SRM 1648a. KRISS urban dust CRM, which will be developed based on the results of this study, will provide useful information to CRM users who would prefer to verify their analytical method using CRMs.  
Previously, KRISS used only the SC-INAA method, which has the advantage of low uncertainty. However, it cannot correct the nuclear interferences of some elements (e.g., U and Al corrections) that are predominantly present in environmental samples. The \(k_{0}\)-INAA method seems to be useful for the analysis of environmental samples, such as airborne PMs, because more than 40 elements can be analyzed simultaneously and no complicated sample preparation is required. Therefore, the \(k_{0}\)-INAA method is one of the suitable methods for the analysis of complex matrix CRM, such as airborne PMs, which are challenging to analyze using other analytical methods.  
**Supplementary Materials:** The following are available online at [[http://www.mdpi.com/2076-3417/10/19/6649/s1](http://www.mdpi.com/2076-3417/10/19/6649/s1)]([http://www.mdpi.com/2076-3417/10/19/6649/s1](http://www.mdpi.com/2076-3417/10/19/6649/s1)), Figure S1. Gamma-ray spectra of KRISS ash CRM candidate after 2 min and 12 h irradiation, Figure S2. Gamma-ray spectra of cellulose paper after 5 min and 12 h irradiation, Table S1. Mass fractions, RSDs, and LODs of samples  
Figure 5: Comparison of mass fractions of REEs, U, and Th in NIST SRM 1648a urban PM and KRISS urban dust CRM candidate.  
obtained by \(k_{0}\)-INAA, Table S2. Mass fractions of elements in NIST SRM 1648a, KRISS urban dust CRM candidate, and KRISS ash CRM candidate analyzed by WD-XRF and \(k_{0}\)-INAA.  
**Author Contributions**: Conceptualization, H.C. and Y.-H.Y.; methodology, H.C., K.B.D., G.M.S., and R.J.; software, H.C., K.B.D., and R.J.; validation, H.C., K.B.D., and R.J.; formal analysis, H.C., K.B.D., M.C.L., and R.J.; investigation, H.C., K.B.D., and R.J.; resources, H.C., K.B.D., M.C.L., and R.J.; data curation, H.C., K.B.D., and R.J.; writing--original draft preparation, H.C.; writing--review and editing, H.C., K.B.D., M.C.L., G.M.S., R.J., and Y.-H.Y.; visualization, H.C.; supervision, H.C., G.M.S., R.J., and Y.-H.Y.; project administration, G.M.S., R.J., and Y.-H.Y.; funding acquisition, G.M.S., R.J., and Y.-H.Y. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The Slovenian co-author would like to thank the Metrology Institute of the Republic of Slovenia (MIRS), as his work contributes to MIRS/IJS Contract No. 6401-5/2009/27 for activities and obligations performed as a Designate Institute as an etalon for the amount of substance/chemical trace elements/in the organic and inorganic materials. This work was supported by KAERI through the Korea Government Project No. MSIT/1711078081, 2019. This work was also supported by the Korea Research Institute of Standards and Science (KRISS) under the project, \"Establishing National Metrology Infrastructure for Reliable Measurement of Particulate\" grant 19011010.  
**Acknowledgments:** The authors from KAERI & KRISS thank [PERSON], Head of the Department of Environmental Sciences at JSI, for her support of the experimental work at JSI funded by the Slovenian Research Agency (ARRS) programme P1-0143.  
**Conflicts of Interest:** The authors declare no conflict of interest.