## 3 Results and Discussion
### Characteristics of Pollution Episodes during Summer and Winter  
The PM\({}_{2.5}\) at the BKO and CNU sites exceeded the 24 h Korean PM\({}_{2.5}\) standard (35 \(\upmu\)g/m\({}^{3}\)) seven and five times, respectively, during the summer. More such episodes occurred during the winter, with 16 and 20 events at the BKO and CNU sites, respectively, accounting for 38% and 48% of all measurements. As shown in Table 1 and Figure 4, these PM\({}_{2.5}\) episodes during the summer were associated with a highly enriched SO\({}_{4}\)\({}^{2-}\) concentration with local and/or regional haze pollutants from China [11; 12; 28]. Our previous studies indicated that high PM\({}_{2.5}\) pollution episodes during the summer in Gwangju were strongly associated with highly elevated concentrations of SO\({}_{4}\)\({}^{2-}\), which was regionally transported from polluted regions in China, rather than local production [10; 12; 74]. During the summer pollution episodes, the SO\({}_{4}\)\({}^{2-}\) and SIS contributions increased, but the OM contribution decreased. For the period from August 14 to 16, the average contributions of \(\sum\)SIS, SO\({}_{4}\)\({}^{2-}\), and OM to the PM\({}_{2.5}\) were 66.7%, 47.1%, and 19.5% at the CNU site, and 62.2%, 42.6%, and 27.9% at the BKO site. During the pollution episode observed on August 22-23, the \(\sum\)SIS, SO\({}_{4}\)\({}^{2-}\), and OM concentrations contributed 67.7%, 43.6%, and 21.0% to the PM\({}_{2.5}\) at BKO and 66.8%, 41.0%, and 17.7% to the PM\({}_{2.5}\) at CNU. The wind speed and RH were 1.0-1.2 m/s and 75-81% on August 14-16, and 0.9-1.1% and 81-89% on August 22-23, respectively. A high daytime O\({}_{3}\) (which is not shown here), high temperature, and low daytime RH values are likely important factors in the photochemical formation of SO\({}_{4}\)\({}^{2-}\) aerosols. Moreover, high RH and low WS conditions could cause the local production of secondary SO\({}_{4}\)\({}^{2-}\) particles through aqueous-phase oxidation in fog and cloud and the heterogeneous oxidation of SO\({}_{2}\) on aerosol surfaces. The afternoon O\({}_{3}\) levels and meteorological conditions during the summertime PM\({}_{2.5}\) episodes imply that the gas-phase and aqueous-phase SO\({}_{4}\)\({}^{2-}\) chemistry was, to some extent, one possible factor leading to the local SO\({}_{4}\)\({}^{2-}\) formation. However, during the winter pollution episodes, the \(\sum\)SIS and NO\({}_{3}\)\({}^{-}\) contributions increased compared to those on non-polluted days, but the SO\({}_{4}\)\({}^{2-}\) and OM contributions were typical for the sampling periods. The average \(\sum\)SIS, NO\({}_{3}\)\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), and OM contributions were 56.8% (40.2-72.5%), 25.7% (19.4-44.7%), 19.0% (10.0-25.3%), and 27.4% (21.4-37.5%) at BKO, and 59.0% (41.1-73.1%), 27.4% (20.5-44.1%), 19.8% (10.8-25.0%), and 25.4% (16.6-34.0%) at CNU, respectively. The 24 h average ambient temperature and WS and RH values during the winter episodes were in the range of 1.5-7.8 \({}^{\circ}\)C, 0.4-1.4 m/s, and 62-88%, respectively. Notably, on December 20 and 26, when the PM\({}_{2.5}\) values were 73 and 93 \(\upmu\)g/m\({}^{3}\), the WS and RH were 0.4 m/s and 88% on December 20 and 1.2 m/s and 67% on December 26. In order to identify the influence of either long-range transport or local pollution on the PM\({}_{2.5}\) pollution episodes during the summer and winter intensive measurement periods, MODIS (Moderate Resolution Imaging Spectroradiometer) images ([[http://lance-modis.eosdis.nasa.gov/cgi-bin/imagery/realtime.cgi](http://lance-modis.eosdis.nasa.gov/cgi-bin/imagery/realtime.cgi)]([http://lance-modis.eosdis.nasa.gov/cgi-bin/imagery/realtime.cgi](http://lance-modis.eosdis.nasa.gov/cgi-bin/imagery/realtime.cgi))) around the Korean peninsula and the transport pathways of air masses that reached the sampling site (CNU, 35.18N, 126.91E) were examined.