## 3 Results and Discussion
### Effect of Secondary Aerosols on Visibility  
Based on the diurnal variation of PM\({}_{10}\), NO\({}_{x}\), SO\({}_{2}\), NMHC, and CO levels, their average concentration over 0600-1600 LT were computed to convert to daily data. These selected daytime hours include the early morning, when pollutants can reasonably be assumed to represent fresh anthropogenic emissions (e.g., from rush-hour activities), and the later hours when complex photochemical reactions evolve. This is generally the time when most of the pollutants tend to concentrate with relatively low mixing height (Supplementary Materials, Figures S1 and S4). It is noted that the peak in SO\({}_{2}\) level occurs in the afternoon at LC and RY but in the early morning and evening at CB. [PERSON] et al. (2014) [63] also reported such afternoon peaks of SO\({}_{2}\) in the North China Plain and suggested several possible causes: down-mixing of pollutant layers aloft, plume transport, local winds, and high humidity. As seen in Figure 8, the PM\({}_{10}\)/CO ratios generally increase from Class I (good visibility) to Class III (poor visibility), although for LC its value in Class II is slightly larger than that in Class III. This suggests that more secondary PM is formed in poor visibility conditions. On the other hand, CO normalized gaseous pollutants showed the opposite pattern, with decreasing levels with decreasing visibility. This can be explained with more gas-phase to particle-phase conversion in lower visibility conditions. Gaseous SO\({}_{2}\) is oxidized to sulfate by hydroxyl radical or by dissolution into cloud, fog, or rain water, and subsequent aqueous phase oxidation. Nitrate is formed by the oxidation of NO and NO\({}_{2}\) by the hydroxyl radical during the day and by reacting with O\({}_{3}\) and water molecule at night. Most of the secondary organic aerosols are formed by aromatic VOCs [1]. It should be noted that the ratio SO\({}_{2}\)/CO decreases rapidly from visibility Class I (good) to visibility Class III (poor) particularly at CB and RY, as compared to NO\({}_{x}\)/CO and NMHC/CO. This suggests the relative importance of SO\({}_{2}\) gas-to-particle conversion to the low visibility condition. Given that PM\({}_{10}\) is positively correlated with every precursor (not shown), SO\({}_{2}\) is likely the gaseous pollutant that influences visibility most. Based on the gridded MIX emission inventory [64], industry and power plants are the major SO\({}_{2}\) emission sources in Eastern Thailand. SO\({}_{2}\) is also expected from diesel-powered heavy and light duty trucks dominantly used in land and marine transportation. Controlling emissions from these sources can benefit visibility improvement. Secondary aerosols can contribute several chemical species to the composition of fine PM, varying the light-extinction capability of fine particulate matter [2]. These results suggest the important role of secondary aerosols in visibility reduction, and hence necessary attention should be given to secondary aerosols as well as their precursors for visibility management. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Station** & **\% of Days** & _VIS (km)_ & _RH (\%)_ & _VIS\({}_{\textbf{d}}\) (km)_ & PM\({}_{\textbf{10}}\) (\(\textbf{\SIUnitSymbolMicro g}\) m\({}^{-3}\))_ \\ \hline \multicolumn{6}{l}{**CB**} \\ NE\_H & 11.3 & 8.5 & 45.5 & 8.6 & 29.0 \\ NE\_L & 33.0 & 8.8 & 47.8 & 8.9 & 25.9 \\ SE & 26.7 & 8.6 & 56.7 & 9.0 & 23.8 \\ U\_H & 11.5 & 8.5 & 46.9 & 8.6 & 25.4 \\ U\_L & 17.5 & 8.9 & 57.8 & 9.4 & 26.2 \\ \hline \multicolumn{6}{l}{**LC**} \\ NE\_H & 11.3 & 6.1 & 48.8 & 6.2 & 57.6 \\ NE\_L & 33.0 & 6.3 & 53.1 & 6.5 & 53.7 \\ SE & 26.7 & 8.1 & 66.9 & 8.9 & 50.9 \\ U\_H & 11.5 & 6.4 & 52.3 & 6.6 & 57.3 \\ U\_L & 17.5 & 7.5 & 65.8 & 8.2 & 49.5 \\ \hline \multicolumn{6}{l}{**RY**} \\ NE\_H & 7.9 & 7.3 & 53.4 & 7.6 & 45.0 \\ NE\_L & 36.2 & 7.4 & 53.2 & 7.6 & 46.7 \\ SE & 18.7 & 7.7 & 58.1 & 8.1 & 42.5 \\ U\_H & 5.3 & 9.4 & 59.3 & 9.8 & 29.4 \\ U\_L & 31.9 & 7.9 & 55.4 & 8.2 & 44.1 \\ \hline \hline \end{tabular}
\end{table}