## 3 Results
### Ambient Air Measurement with a HONO Collection System  
From 11 December to 25 December 2023, both PM2.5 and gaseous species such as HONO, NOx, and other gasses were measured. The average \(\pm\) standard deviation of the measured components throughout this period is summarized in Table 2. HONO was detected at an average of 0.31 ppb, peaking at 0.60 ppb (Figure 2a), which is comparable to previous measurements of HONO concentration in Seoul (0.36 ppb) recorded by [PERSON] et al. [16]. The mean concentration of O3 was 32.44 ppb, while NO and NO2 concentrations were 0.77 ppb and 3.28 ppb, respectively (Figure 2a,c). On the dates of 13, 19, and 24 December, O3 levels dropped below 20 ppb before rebounding and consistently remaining above 25 ppb for the rest of the measured period. Regarding PM2.5, the average concentration during this interval was 9.38 ug/m3, with the maximum concentration reaching 25.56 ug/m3 at 3 a.m. on 20 December (Figure 2f). Given the winter mean PM2.5 concentration from 2015 to 2022 in South Korea has been above 20 ug/m3, the study period exhibited notably lower particulate levels, although there were occasional increases potentially attributable to external influxes, as noted by [PERSON] et al. [45]. The mean concentrations of CO and CO2 registered as 212 ppb and 449 ppm, respectively, with a peak CO/CO2 ratio of 0.87 occurring at 3 a.m. on 20 December (Figure 2b). Significantly, the CO/CO2 ratio stood at 0.87 at the recorded time, which was higher than the mean value of 0.47 for the entire study period. This elevated CO/CO2 ratio may signify decreased combustion efficiency or substantial long-range transport from China to the Korean Peninsula, according to previous research [46, 47, 48, 49, 32]. Figure 2d presents the concentrations of ionic components (NH4\({}^{+}\), NO3\({}^{-}\), SO4\({}^{2-}\)) throughout the study period, with mean value of 0.99, 1.30, and 2.00 ug/m3, respectively. Notably, the concentrations peaked on 20 December and 24 December at 3.57 ug/m3 for NH4\({}^{+}\), 5.75 ug/m3 for NO3\({}^{-}\), and 7.09 ug/m3 for SO4\({}^{2-}\). The concurrent increase in particulate matter and concentrations of CO and CO2 on these dates indicates an external influx of polluted air.  
The mean concentration of SO2 was 0.79 ppb, and NH3 was measured at 4.33 ppb (Figure 2g). The concentration trend of NH3 levels paralleled temperature changes, which likely influenced ammonia release into the atmosphere [50, 51, 52]. HNO3 was measured at an average of 0.32 ppb, reaching a peak of 0.83 ppb during the PM2.5 increase period. Previous studies recorded winter HNO3 levels in Seoul that varied from 0.16 ppb (non-hazy days) to 0.34 ppb (hazy days). Concurrently, Daejeon recorded winter concentrations of 0.72 ppb  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Compounds** & **Unit** & **Mean \(\pm\) Std** \\ \hline PM2.5 & ug/m3 & 9.38 \(\pm\) 5.34 \\ NO3 & ug/m3 & 1.3 \(\pm\) 1.5 \\ SO4\({}^{2-}\) & ug/m3 & 2 \(\pm\) 1.16 \\ NH4\({}^{+}\) & ug/m3 & 0.99 \(\pm\) 0.76 \\ HONO & ppb & 0.31 \(\pm\) 0.1 \\ HNO3 & ppb & 0.32 \(\pm\) 0.19 \\ NO & ppb & 0.77 \(\pm\) 1.29 \\ NO2 & ppb & 3.28 \(\pm\) 2.22 \\ O3 & ppb & 32.44 \(\pm\) 7.97 \\ SO2 & ppb & 0.79 \(\pm\) 0.29 \\ NH3 & ppb & 4.33 \(\pm\) 3.26 \\ CO & ppb & 212.42 \(\pm\) 57.8 \\ CO2 & ppm & 448.62 \(\pm\) 7.35 \\ CO/CO2 & ppb/ppm & 0.47 \(\pm\) 0.12 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Results of mean concentrations of PM2.5 and gaseous compounds.  
and summer concentrations of 0.1 ppb, with the annual averages in Chuncheon and Seoul ranging from 0.60 to 1.06 ppb. These results imply that the observed concentrations in this study are influenced by factors such as temperature, seasonal variations, and haze phenomena [53; 54; 55].