## 3 Results
### Time Series Results of Measured Concentrations
Ammonium sulfate, ammonium bisulfate, and ammonium nitrate are all SOAs. Their generations are dominated by NH\({}_{3}\) in the atmosphere. The content of NH\({}_{3}\) was analyzed using the mole ratio of NO\({}_{3}\), SO\({}_{4}\)\({}^{2-}\), and NH\({}_{4}\)\({}^{+}\) in PM\({}_{2.5}\). According to previous studies, the air can be defined as ammonium-rich if the mole ratio of NH\({}_{4}\)\({}^{+}\)/SO\({}_{4}\)\({}^{2-}\) in PM\({}_{2.5}\) is higher than 1.5 [35; 36]. If the mole ratio of NH\({}_{4}\)\({}^{+}\)/SO\({}_{4}\)\({}^{2-}\) in PM\({}_{2.5}\) is lower than 1.5, it is regarded that the contribution of SO\({}_{4}\)\({}^{2-}\) to SOA in PM\({}_{2.5}\) is significant. When the mole ratio of NO\({}_{3}\)\({}^{-}\)/SO\({}_{4}\)\({}^{2-}\) is high, the contribution of NO\({}_{3}\)\({}^{-}\) to SOA is dominant. Thus, the air can be defined as ammonium-rich. In this study, mole ratios of NH\({}_{4}\)\({}^{+}\)/SO\({}_{4}\)\({}^{2-}\) and NO\({}_{3}\)\({}^{-}\)/SO\({}_{4}\)\({}^{2-}\) were 4.75 and 2.90, respectively [37; 38]. Therefore, NH\({}_{4}\)\({}^{+}\) in the atmosphere was sufficient in the study area, and the generation of SOA would be relatively easy due to the high contributions of NO\({}_{3}\)\({}^{-}\) to SOA. Based on the results of WSOC, water-soluble ions, the ratio of K\({}^{+}\)/K, EC, and eBC, PM\({}_{2.5}\) had the following main causes: SOA generation, combustion of fossil fuels for heating, scattering of earth crustal materials, and so on. However, because WSOC and water-soluble ions accounted for 15.84% and 49.34% of total PM\({}_{2.5}\), respectively, it could be judged that SOA contributed to PM\({}_{2.5}\) the most in the study area. Figure 2 shows wind direction/speed and time-series concentrations. As shown in the graph, the temperature was \(-\)11-12 \({}^{\circ}\)C during the measurement period, and the relative humidity (RH) was 20-88%. From the measured concentrations, a gaseous material had a 7.55 times difference between minimum and maximum concentrations, and the average PM\({}_{2.5}\) had a 55.69 times difference between minimum and maximum concentrations. Even within the short period of this study, this displayed a significant difference between compositions of gaseous materials and PM\({}_{2.5}\). From results shown in Figure 2, significant differences in PM\({}_{2.5}\) were found during the following periods: 1 p.m. on 30 January-3 p.m. on 31 January, 9 a.m. on 1 February-3 p.m. on 2 February, and 9 a.m. on 6 February-3 p.m. on 7 February. The changes in concentrations during the above periods (event periods) might have been caused by external influxes due to several reasons. The first reason is that the CO concentration increased during the event period. The lifetime of atmospheric CO is about three months in winter. CO is normally an inert precursor for long-range transport for less than 10 days [39]. In addition, because CO reacts with OH radicals in photochemical reactions and produces CO\({}_{2}\), it is known that its daily concentration is very low from 1 p.m. to 4 p.m. when photochemical reactions are definitely active [35; 36]. In this study, CO concentrations during the event period and other periods were 283 ppb and 184 ppb, respectively, showing a higher concentration during the event period. CO concentrations by time indicated that its average concentration was about 180 ppb from 6 a.m. to 9 a.m. and about 150 ppb from 1 p.m. to 4 p.m. during other periods, showing lower concentrations when photochemical reactions were occurring. On the other hand, its concentration during the event period was 220 ppb from 6 a.m. to 9 a.m. and 270 ppb from 1 p.m. to 4 p.m. when photochemical reactions were dominating. Based on daily  
concentration changes in CO and CO concentrations by time at the event period and other periods, the study area might be affected by external influxes during the event period. The second reason is the increase in EC during the event period.