## 1 Introduction  
The horizontal distance over which a human can observe and distinguish a dark object (or an object with sufficient visual contrast to the background) is termed as visibility or visual range [1, 2]. It is a fundamental parameter routinely observed at standard surface weather stations. Visibility is governed by optical properties of airborne particles and gases (apart from light intensity), physical properties of the target, and the threshold visual contrast of the observer. In a clean environment (in the absence of fog), visibility can be reduced mainly by molecular (i.e., Rayleigh) scattering and gas absorption;however, in a polluted environment, scattering and absorption of light by both particles and gases can reduce visibility [3; 4]. Thus, visibility is linked to air pollution and indicates its extent, especially for long-term studies when air quality data are unavailable [5; 6; 7; 8; 9]. Severe reduction of visibility can interfere with or disrupt transportation (e.g., traffic slowdown, delayed flights, and on-road accidents) and also discourage recreational activities and tourism [1]. Visibility is also dependent on several meteorological factors [10; 11; 12]. Most atmospheric aerosols are hygroscopic [6] (especially sea salt, and carbonaceous and inorganic aerosols), that is, they absorb water; this absorption in turn increases aerosol size and thus increases the light scattering [1]. The transport of pollutants from remote sources by winds and secondary aerosols through complex multi-phase atmospheric chemistry (especially inorganic components such as sulfate, nitrate, and ammonia) can enhance the visibility reduction over an area in addition to local pollutants emitted within the area [13]. Numerous studies have been done to understand visibility characterization and factors affecting it at both local and global scales. [PERSON] et al. (2009) [14] reported an increasing trend for visibility since the mid-1980s in Europe and North America, while the trend was found to be decreasing over South and East Asia, South America, Australia, and Africa. Another study by [PERSON] et al. (2012) [15] showed that, in spite of decreasing levels of PM\({}_{10}\) (particulate matter with a size smaller than or equal to 10 \(\upmu\)m) across the world, optical extinction has either increased or shown less decrease. [PERSON] et al. (2017) [16] studied the visibility trend in selected cities in the USA, India, and China during 1973-2015, and reported that visibility has improved in the USA and China (after an initial decrease until the 1990s) but was still a concern in India. Moreover, [PERSON] et al. (2017) [6] reported an increasing trend in visibility over the UK. Overall, it can be said that visibility has mostly improved in developed countries over the last few decades but that it is still a concern over developing countries due to rapid economic development [7; 10; 16; 17]. The impact of meteorological factors on visibility has also been reported in many studies [4; 6; 7; 8; 9; 10; 11; 12; 13]. Industrialization and urbanization in many areas across Thailand have resulted in a number of environmental issues, one of which is air pollution problems in the eastern region [18; 19; 20; 21; 22; 23]. Currently, the region comprises seven provinces (Chon Buri, Rayong, Chanthaburi, Trat, Prachin Buri, Sa Kaeo, and Chachoengsao) (Figure 1a,b), with the first two being the most industrialized provinces in the country. Eastern Thailand is economically important, accounting for 18% of national gross domestic product (GDP) (as of 2015), second only to the Bangkok Metropolitan Region (47%) [24]. In view of gross provincial product (GPP) (which is the same as GDP but scaled to a provincial level) and fuel consumption, Chon Buri and Rayong provinces both have an apparent growth, particularly after 1990 (although with a single drop in 2009 caused by the impact of the global financial crisis), and each has shown an overall positive population growth since 1981 [25; 26] (Figure 2).