## 2 Materials and Methods
### Data Collection and Treatment  
In the initial step, meteorological data from all 15 available stations of the Thai Meteorological Department (TMD) and air quality data from six stations (located nearby these TMD stations) of the Pollution Control Department (PCD) located in the eastern region of Thailand were considered. The TMD data were requested from the agency directly and additionally compiled from the National Centers for Environmental Information ([[https://www.ncdc.noaa.gov/isd](https://www.ncdc.noaa.gov/isd)]([https://www.ncdc.noaa.gov/isd](https://www.ncdc.noaa.gov/isd))). Each of these stations has limited missing data for visibility (<10%). Given that human-observed visibility is based on a set of discrete targets (markers at different distances), inhomogeneity in visibility data can intrinsically arise when targets are changed or modified over time. However, no information of targets used in the past was available to us. Here, we proposed the use of a change point detection (CPD) technique [37; 38] to identify such potential inhomogeneity due to changed or modified targets at each TMD station, and, if found, the station was removed. To conserve space, our CPD procedure is presented in Section S1 of the Supplementary Materials. A field visit to several TMD and PCD (see Appendix A for all symbols and acronyms) stations was conducted to evaluate monitoring conditions. Based on the CPD, visibility data from eight stations were not found suitable for use. Accordingly, air quality stations close to any removed TMD station were then removed at this stage. Finally, a total of seven TMD stations--CB (Chon Buri), LC (Laem Chabang), ST (Sattahip), RY (Rayong), CT (Chanthaburi), SK (Sa Kaeo), and PB (Prachin Buri)--and five PCD stations--P1 (General Education Office), P2 (Sri Racha Juvenile Center), P3 (Kasetsart University Sri Racha), P4 (Rayong Field Crops Research Center), and P5 (Map Ta Phut Health Center)--were finally selected (Table 1 and Figure 1c). The temporal coverage of visibility data at the selected stations spans 1981-2015 (35 years) for four stations, 2001-2015 (15 years) for two stations, and 2007-2015 (nine years) for one station (Table 1). We placed emphasis on the three stations located in the developed coastal areas: CB (urban background), LC (industrial), and RY (industrial). Each of these stations has one or two PCD stations located nearby, feasibly allowing visibility and air quality data to be paired together for the analysis. Here, CB, LC, and RY were paired with P1, P2 and P3, and P4 and P5, respectively. The TMD data have a three-hourly resolution, reported at 0100, 0400, 0700,..., 1900, and 2200 local time (LT), and include visibility (_VIS_), air temperature (_TEMP_), relative humidity (_RH_), cloud cover (_CC_), rain (_RN_), and weather code (e.g., fog, haze, and smoke occurrence). The PCD data have an hourly resolution and include measurements of PM\({}_{10}\), sulfur dioxide (SO\({}_{2}\)), nitrogen oxides (as the sum of nitrogen oxide and nitrogen dioxide, i.e., NO\({}_{x}\) = NO + NO\({}_{2}\)), carbon monoxide (CO), non-methane hydrocarbons (NMHC), 10-m wind (_WS_: wind speed; and _WD_: wind direction), and global radiation (_GR_). All data are from near-surface monitoring, except for winds at about 10 m above ground level (AGL). It is noted that wind data at the selected TMD stations have the potential to be strongly affected by nearby objects or infrastructure. Thus, PCD wind data were used. Data completeness, unit consistency, and data ranges of the meteorological variables were examined based on the WMO guidelines [39]. Any improbable or spurious values were converted to missing. In addition to the conventional meteorological variables, recirculation factor (_RC_) and mixing height (_MH_) were also derived. As noted in Section 2.1, sea breeze is very present in the dry season during the daytime hours along the coasts in the region in question, potentially suppressing the ventilation of air pollutants out of the coastal areas due to horizontal wind circulation.