## 2 Materials and Methods
### Monthly Variation, Seasonality and Relation with PM\({}_{10}\)
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Station a** & **Agency** & **Province** & **Variables b** & **Period** & **Background c** \\ \hline CB & TMD & Chon Buri & _VIS, RN, TEMP, RH, CC_ & 1981–2015 & Urban (coastal) \\ LC & TMD & Chon Buri & Same & 2001–2015 & Industrial (coastal) \\ ST & TMD & Chon Buri & Same & 1981–2015 & Suburban (coastal) \\ RY & TMD & Rayong & Same & 2007–2015 & Industrial (coastal) \\ CT & TMD & Chanthaburi & Same & 1981–2015 & Urban (coastal) \\ SK & TMD & Sa Kaeo & Same & 2001–2015 & Rural (inland) \\ PB & TMD & Prachin Buri & Same & 1981–2015 & Urban (inland) \\ P1 & PCD & Chon Buri & PM\({}_{10}\), SO\({}_{2}\), NO\({}_{x}\) CO, NMHC, & 2001–2015 & Urban (coastal) \\  & & _WS, WD_ & & \\ P2 & PCD & Chon Buri & PM\({}_{10}\), NO\({}_{x}\) & 2001–2015 & Industrial (coastal) \\ P3 & PCD & Chon Buri & PM\({}_{10}\), SO\({}_{2}\), NO\({}_{x}\), CO, NMHC, & 2001–2015 & Industrial (coastal) \\  & & _WS, WD, GR_ & & \\ P4 & PCD & Rayong & PM\({}_{10}\), SO\({}_{2}\), NO\({}_{x}\), CO, NMHC, & 2007–2015 & Industrial (coastal) \\ P5 & PCD & Rayong & PM\({}_{10}\), SO\({}_{2}\), NO\({}_{x}\), CO, NMHC & 2007–2015 & Industrial (coastal) \\ \hline \hline \end{tabular}  
* CB: Chon Buri (WMO484590), LC: Learn Chabang (WMO484630), ST: Sattabin (WMO484770), RY: Rayong (WMO484790), CT: Chanthaburi (WMO484800), SK: Sa Kaeo (WMO484400), PB: Prachin Buri (WMO484300), P1 (General Education Office), P2 (Sri Racha Juvenile Center), P3 (Kasestart University Sri Racha), P4 (Rayong Field Crops Research Center), and P5 (Map Ta Phut Health Center). Note that LC has only daytime monitoring from 0700 to 1900 LT. It is noted that P2 has been slightly relocated to a nearby site (<100 m away) since September 2013; however, it is still in the same background as its original site. b _VIS_: Visibility; _RN: Rain; TEMP_: Air temperature; _RH_: Relative humidity; CC: Cloud Cover; PM\({}_{10}\): Particulate matter with size smaller than or equal to 10 μm; SO\({}_{2}\): Sulphur dioxide; NO\({}_{x}\): Nitrogen oxides; CO: Carbon monoxide; NMHC: Non-methane hydrocarbon; _WS_ and _WD_: 10-mm wind speed and direction, respectively; and _GR_: Global radiation. Thai Meteorological Department (TMD) data are three-hourly while Pollution Control Department (PCD) data are hourly. c Background of each visibility station: (a) CB has a dense urban background. It is located close to the coast (0.8 km to the west) and near highways nos. 3 and 7 (0.25 km and 6 km to the east, respectively), with an Industrial Estate (IE) 15 km to the northeast. (b) LC has an industrial background. It is next to the coast to the west) and Laem Chabang Seaport (to the south) and separated from the two highways by 5 km and 13 km to the east, respectively. It is also close to the Laem Chabang IE (3 km to the east). (c) ST has a suburban background. It is close to highway no. 3 (0.5 km to the north) and the U-Tapao International Airport (2 km to the west). RY has an industrial background. It is close to highway no. 3 and the Map Ta Phut industrial complex (0.1 km and 3 km to the south, respectively). (d) CT is in a non-dense urban area near highway no. 3 (3 km to the east) and not far from the coast (16 km). (e) SK is in a rural area and near highway no. 33 (5 km to the north). (f) PB is in a non-dense urban area, located next to a major river and near highway no. 33 (8 km to the north).  
\end{table} 
Table 1: Monitoring stations and associated variables. class and the remaining classes based qualitatively on their frequency and separation. This procedure facilitates a set of common visibility classes for all stations, although with different thresholds. In this study, three classes were defined: I (good), II (moderate), and III (poor). The thresholds separating Classes I and II and Classes II and III were respectively assigned as follows: (a) Visibility of 14 km and 6 km for CB, RY, and ST; and (b) Visibility of 12 km and 6 km for the other stations.