## 3 Results and Discussion  
### Annual Levels of Atmospheric Aerosols and Gaseous Species  
Table 1 shows the annual mean concentrations of major air pollutants measured in Seoul and Baengnyeong Island between 2012 and 2019. In addition, Figure 1 shows the annual mean (red circle), median, minimum, maximum and the interquartile ranges of PM2.5, SO\({}_{4}\)\({}^{-2}\), NO\({}_{3}\), NH\({}_{4}\)\({}^{+}\), EC and OC. Most aerosol species have shown a general decreasing trend over the past eight years. The aerosol concentrations in Seoul increased until 2013 and then started to decrease since 2014. Unlike other components, OC did not decrease and showed a slight increasing trend of around 3-4 \(\mu\)g m\({}^{-3}\) in average.  
On Baengnyeong Island, the maximum annual mean PM2.5 concentration was 28.7 \(\mu\)g m\({}^{-3}\) in 2014 and has decreased since 2015. The annual mean concentrations of SO\({}_{4}\)\({}^{-2}\) and NH\({}_{4}\)\({}^{+}\) kept decreasing from 2012 to 2018. Unlike other pollutants, the NO\({}_{3}\)\({}^{-}\) concentration is increasing steadily. The annual mean NO\({}_{3}\)\({}^{-}\) concentration was 1.86 \(\mu\)g m\({}^{-3}\) in 2012 and kept increasing to 4.92 \(\mu\)g m\({}^{-3}\) in 2019. The annual mean EC concentration was on a declining trend from 0.97 \(\mu\)g m\({}^{-3}\) in 2014 to 0.51 \(\mu\)g m\({}^{-3}\) in 2019, although the concentration briefly increased in 2016. The annual mean OC concentration remained stagnant at around 3 \(\mu\)g m\({}^{-3}\) and decreased slightly after 2018. The annual mean concentrations of aerosol species were higher in Seoul than Baengnyeong Island. Furthermore, the deviations of the annual mean concentrations of each pollutant were larger in Seoul.  
Figure 2 shows the annual levels of gaseous species measured in Seoul and on Baengnyeong Island. The annual mean SO\({}_{2}\) concentration in Seoul showed a slight decreasing tendency around 5 ppb. In the case of NO\({}_{2}\), the annual mean concentration showed a maximum of 32.5 ppb in 2014 and showed a decreasing trend to 23.9 ppb in 2019. The annual mean O\({}_{3}\) concentrations increased by 22.3 ppb in 2012 to 29.6 ppb in 2019. On Baengnyeong Island, the annual mean SO\({}_{2}\) concentration was 5.54 ppb in 2013 and has been continuously decreasing, reaching 1.66 ppb in 2019. In contrast, NO\({}_{2}\) showed a clear increasing trend. The annual mean concentration of NO\({}_{2}\) was 2.99 ppb in 2012 and increased to 3.96 ppb in 2019. In the case of O\({}_{3}\), the annual mean concentration increased  
\begin{table}
\begin{tabular}{c c c c c c c c|c c c c} \hline \hline  & \multirow{2}{*}{**Year**} & \multicolumn{6}{c}{**Atmospheric Aerosols (\(\mu\)g m\({}^{-3}\))**} & \multicolumn{6}{c}{**Gaseous Species (ppb)**} \\ \cline{3-11}  & & **PM2.5** & **SO\({}_{4}\)\({}^{-2}\)** & **NO\({}_{3}\)** & **NH\({}_{4}\)\({}^{+}\)** & **EC** & **OC** & **SO\({}_{2}\)** & **NO\({}_{2}\)** & **CO** & **O\({}_{3}\)** \\ \hline \multirow{8}{*}{Seoul} & 2012 & 28.8 & 6.07 & 5.36 & 3.70 & 1.52 & 3.13 & 5.37 & 26.4 & 545 & 22.3 \\  & 2013 & 38.3 & 8.88 & 9.28 & 5.19 & 1.76 & 3.80 & 4.97 & 31.1 & 608 & 20.9 \\  & 2014 & 37.3 & 7.43 & 7.93 & 4.95 & 1.56 & 3.82 & 5.11 &significantly from 2012 to 2014, from 23.3 ppb to 49.1 ppb, and since then, the concentration has been maintained at over 40 ppb.  
Figure 1: The annual levels of major atmospheric aerosols. The annual mean (red circle), minimum, interquartile range (Q1), median, interquartile range (Q3) and maximum values are shown.