## 4 Conclusions  
Observed long-term visibility in the eastern region of Thailand was studied, with the focus on its urbanized/highly industrialized coastal areas, using weather data from seven stations with a temporal coverage of 35 years (1981-2015) for four stations, 15 years (2001-2015) for two stations, and nine years (2007-2015) for one station. The relationship between visibility and air pollutants was also investigated using air quality data from five stations (15 years for three stations and seven years for two stations). Visibility shows strong seasonality and its degradation intensifies in the dry season due to increased air pollution as well as due to certain favorable meteorological conditions such as low wet scavenging, reduced mixing height, and increased recirculation factor. Visibility showed negative correlation with PM\({}_{10}\) which is evident with correlation analysis, class-wise comparison and weekday-weekend differences. Visibility showed negative correlation coefficient of \(-0.29\) (\(-0.22\)), \(-0.32\) (\(-0.23\)) and \(-0.51\) (\(-0.64\)) with PM\({}_{10}\) on daily (monthly scale) for CB, LC and RY, respectively. Mean level of PM\({}_{10}\) under poor visibility condition (Class III) was around 1.6-1.7 times higher than in good visibility condition (Class I). Average visibility was 9.1 km, 7.1 km and 8.2 km during the weekend and 8.6 km, 6.8 km and 7.6 km during the weekdays for CB, LC and RY due to a higher mean level of PM\({}_{10}\)  
Figure 10: Meteorologically adjusted (red) and original or non-adjusted (blue) trends in visibility and PM\({}_{10}\) at (**a**) Chon Buri (CB); (**b**) Laem Chabang (LC); (**c**) Rayong (RY). The underlined values are of statistical significance at a 0.05 level against the null hypothesis of no trend. At each station, the trend period was determined by the time coverage of the visibility and air quality data available and considered. during the weekdays. The results from the equal step-size method showed a negative relation between visibility and \(RH\) with the effect of hygroscopic growth of aerosol being a more dominating process as compared to a change in size distribution of dry particles. The strong dependency of visibility on wind direction with relatively lower visibility with wind prevailing from sources of local pollution is indicative of the influence of local pollution sources. The back-trajectory results showed relatively lower visibility with the northeasterly clusters, which represent continental air masses and brings polluted air from inland areas. The effect of air pollution and humidity is not readily discerned for undefined clusters. These evidently suggest the important influences of the long-range transport of pollutants and humidity on visibility. The precursor ratio method showed an increase in the formation of sulfate, nitrate and secondary organic aerosol under low visibility conditions, suggesting their potential role in visibility degradation. Among these precursors, the formation of sulfate from SO\({}_{2}\) seems to play the most important role in visibility degradation. This highlights the importance of controlling the gaseous precursors of secondary aerosols for visibility improvement in the region. Trend analysis of average visibility shows a negative shift in trend, decreasing trend, or persistence of relatively low visibility over most of the stations, raising an environmental concern and suggesting a need for more attention from policymakers and air quality workers. The contrasting trends in visibility and PM\({}_{10}\) level after the GLM-based meteorological adjustment are partly attributed to the role of fine particles, to which visibility is more sensitive. This study provides empirical evidence that visibility is impacted by secondary aerosols, which would be warranted concretely by taking into account fine particulate matter (e.g., PM\({}_{2.5}\)) data and their chemical components, which were not available in this study.