**5. Conclusions**

This study analyzed changes in particulate matter concentrations and the spatial characteristics of the distribution of those concentrations according to time and land-use, using PM10 and PM2.5 big data measured in Daejeon by a private company's PAQMSSs from September-October 2019. The results are summarized as follows: first, the land-use types within the range of 650-m-diameter buffers based on 123 PAQMSSs were simplified to residences, commerce, industry, green, and roads, with ratios of 30.5%, 26.3%, 2.7%, 19.8%, and 24.7%, respectively. According to the grouping based on the ratios, four groups (residence, commerce, industry, and green) were classified. Analyses of the highest land-use ratio in each group identified residence (48.5%) in Group 1, commerce (46.7%) in Group 2, industry (45.7%) in Group 3, and green (42.1%) in Group 4. Then, the highest land-use ratio in each group was defined as being characteristic of the group.

Second, the PM10 and PM2.5 data showed moderate levels of particulate matter concentrations, and there were significant differences in the concentrations between groups over time and between groups at the same time (with the exception of during PM2). Particulate matter concentrations were high during all time periods in the order of the industry, residence, commerce, and green (with the exception of during PM2); however, concentrations in the industrial group were higher than those in the residential group. This may be a result of the increasing mixed land-use due to intensive zoning control. The weakening of zoning control has resulted in a large supply of residential areas in commercial areas, which has caused an increase in nighttime activities.

Third, the groups presenting the biggest concentration differences were both PM10 and PM2.5 in the residential and green groups. In addition, the particulate matter concentration in the green group (with high forest ratios) was low, and the concentration in the industrial group (with high industrial

and road ratios) was high. This indicates that the concentrations varied depending on the land-use ratio, which is in agreemen<sup>t</sup> with previous studies. Industrial areas use more fuel and have higher emissions of pollutants from combustion facilities and production processes than commercial and residential areas [49]. Moreover, PM10 showed the biggest di fferences during Noon, whereas PM2.5 showed di fferences during PM1. The reason for this di fference is believed to be that PM10 combined with O3 and water vapor and was transformed to PM2.5 by chemical reactions.

Fourth, PM10 and PM2.5 concentrations tended to be high in old and new urban areas (where commercial and business functions were concentrated), and where the industrial complex was located. Moreover, overall, the particulate matter concentrations were low in the morning (AM1 and AM2), highest in the afternoon (Noon), and gradually increased in the evening (PM1 and PM2). PM10 concentrations clearly showed variations over time, whereas the PM2.5 concentrations had distribution characteristics that remained stable in certain areas. The results of this study show that the PM10 concentration can be resolved naturally over time, however PM2.5 showed a stagnation phenomenon, whereby it was not easily diluted naturally from concentrated areas [50,51]. Addressing this problem involves promoting a long-term policy to reduce the occurrence of particulate matter pollution and, simultaneously, providing physical measures, such as parks and green areas, to minimize the e ffects of particulate matter on the human body. In other words, firstly, in order to prevent this phenomenon, a policy that minimizes air pollutants is needed. Secondly, green space is important for particulate matter management, as revealed in previous studies [26]. Su fficient parks and green areas should be provided to absorb fine dust generated in industrial, commercial, and residential areas.

This study examined whether particulate matter concentrations changed depending on time and land-use and analyzed the characteristics of the spatial distribution of particulate matter. The residential, commercial, and industrial areas representing urbanized areas were found to increase the particulate matter concentration and the green area was identified as a factor in decreasing the concentration [29,52]. This study provides guidelines for establishing particulate matter reduction policies since environmental policies are significant for pollution reduction. The limitation of the study was that the analysis only considered changes in the particulate matter concentrations in the city of Daejeon. However, the locations of sources of particulate matter vary and the various causes are complex and interwoven. Therefore, studies on a range of aspects are required, including the degree of influence of particulate matter between areas and the causes of occurrence, by expanding the current research scope.

**Author Contributions:** Data curation, S.S.J.; Formal analysis, S.S.J.; Writing—original draft, S.S.J.; Writing—review and editing, S.H.L. and Y.L.; Supervision, Y.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Research Foundationof Korea (NRF) gran<sup>t</sup> funded by the Korea governmen<sup>t</sup> (MSIT) (No. 2019R1F1A1062708).

**Acknowledgments:** The authors express their gratitude to the City of Daejeon for providing the material.

**Conflicts of Interest:** The authors declare no conflict of interest.
