## 3 Results and Discussion
### Effect of Local Wind and Long Range Transport  
The dependency of visibility on local wind direction can be seen in both stations CB and LC. At CB (Figure 6a), relatively low visibility and high levels of PM\({}_{10}\) are seen in the northeastern, eastern, and southern directions, which is attributable to the advection of pollutants from the nearby industrial sources and highways (Table 1, and Figure S3 in Supplementary Materials). By contrast, relatively good visibility is mostly seen in the north given the fewer emission sources present upstream of the station. However, in the southwest and the west, visibility appears to be moderated by moisture from the sea, as seen from the \(VIS/VIS_{d}\) ratio being smaller than unity (here, 0.95). At LC (Figure 6b), visibility  
Figure 5: Average visibility, \(RH\)-corrected visibility and PM\({}_{10}\) under different \(RH\) intervals at (**a**) Chon Buri (CB); (**b**) Laem Chabang (LC); and (**c**) Rayong (RY). tends to be lower than CB, in general agreement with the higher level of PM\({}_{10}\). Poor visibility (5-6 km or less) is noticeable in the southeast direction due mainly to the presence of industries and highways. However, poor visibility is even more pronounced in the northwest due potentially to the combination of moisture and air pollutants generated from intense shipping activities in and around this coastal area. At RY (Figure 6c), both visibility and PM\({}_{10}\) levels appear to have lesser directional dependence, possibly because of the station having industrial facilities and highways nearby in various directions (Table 1, and Figure S3 in Supplementary Materials). The visibility reduction is most intensified from the east to the south given that moisture and emission sources tend to align in those directions. Back-trajectory modelling shows a total of five similar clusters at both stations: (1) northeesterly and high level (\"NE_H\") (with occurrences of 11.3% of total days at CB and 7.9% at RY); (2) northeesterly but low level (\"NE_L\") (33% at CB and 36.2% at RY); (3) south  
Figure 6: Polar plots of visibility (\(VIS\)), visibility/\(RH\)-corrected visibility (\(VIS\)/\(VIS_{\text{d}}\)), and PM\({}_{10}\) at (a) Chon Buri (CB); (b) Learn Chabang (LC); and (c) Rayong (RY). at CB and 18.7% at RY); (4) undefined and high level (\"U_H\") (11.5% at CB and 5.3% at RY); and (5) undefined and low level (\"U_L\") (17.5% at CB and 31.9% at RY) (Figure 7). Table 2 summarizes cluster-average visibility, _RH_, _RH_-corrected visibility, and PM\({}_{10}\). Using the Wilcoxon rank sum test (at a 0.05 level), most of the cluster pairs (>50%) were found to have significant difference in average visibility and average PM\({}_{10}\). In relative terms, low visibility generally occurs in the northeaesterly clusters, as seen in NE_H at CB (8.5 km) and NE_H and NE_L at both LC (6.1 and 6.3 km) and RY (7.3 and 7.4 km). A similar pattern was also seen for _RH_-corrected visibility. The northeaesterly clusters represent continental air masses and tend to be more polluted and less moist. Thus, it is possible to say that such low visibility is mainly influenced by pollution, causing relatively high PM\({}_{10}\) in these two clusters. In the dry season, biomass burning (forest and agricultural fires) is typically intensified over certain areas of the Indochinese Peninsula (e.g., Northern Thailand, Northern Laos, and Eastern Myanmar) and part of Southern China during March-April. Based on satellite fire hotspot data [62], many trajectories of these two clusters pass such fire-prone areas that could contribute smoke and other pollutants to the migrating air masses (not shown). The SE cluster tends to have high humidity since it is semi-maritime with many trajectories originating from and/or passing the sea (not shown), with its associated visibility, _RH_-corrected visibility, and PM\({}_{10}\) rank between the best and the third across the stations.