## 1 Introduction
Eastern Thailand is also well known for many tourist attractions (beaches, islands, national parks, eco-tourism, and historical sites) and is home to a major international airport. Visibility quality is thus valued, and its degradation can become a public concern [27]. Following the initialization of the Eastern Seaboard Development Plan by the government in the 1980s, as many as 30 industrial estates (IEs), together with two deep seaports, are currently present in the region [28], most of which are located near the coast. Road networks and shipping activities have been expanded to support the development. Shipping activities at the deep seaports can contribute to local air pollution [29]. The amount of industrial air pollutants emitted in this region was high, and had been ranked second among all of Thailand's administrative regions [30]. Recently, a major governmental initiative, named the Eastern Economic Corridor (EEC), has been implemented to expand the industrial, logistical, and transportation sectors of the country, which is likely to increase air pollution in the region. To our knowledge, studies on visibility in Upper Southeast Asian countries are still limited, and no dedicated effort has been found for long-term or climatological analysis over large urbanized/industrialized areas. In Thailand, [PERSON] and [PERSON] (2008) [31] applied multiple linear regression models to forecast dry-season visibility at a major airport in Bangkok (the capital, located in the central region) using surface meteorological variables as input. [PERSON] et al. (2009) [11] and [PERSON] et al. (2001) [32] examined the linear relationship between visibility and PM\({}_{10}\) in Chiang Mai (a major province located in the northern region of Thailand) and Bangkok, respectively. [PERSON] et al. (2003) [33] estimated atmospheric turbidity in the context of solar energy, with visibility data used complementarily. Given the importance of the eastern region and its ongoing economic development plan, visibility is an important issue that needs to be addressed and better understood. Figure 1: **(a)** Thailand; **(b)** eastern region and its provinces, and **(c)** monitoring stations. In **(b)**, built-up areas are shaded grey, representing urban, residential, industrial, and commercial areas combined based on Land Development Department (LDD) (2007) [34]. In **(c)**, for some Thai Meteorological Department (TMD) and Pollution Control Department (PCD) stations being very close to each other, only one filled circle is given. CB (Chon Buri), LC (Laem Chabang), and RY (Rayong) (in boxes) denote the most important TMD stations in the analysis, which are paired with nearby PCD stations for air quality data (here, P1 (General Education Office), P2 and P3 (Sri Racha Juvenile Center and Kasetsart University Sri Racha), and P4 and P5 (Rayong Field Crops Research Center and Map Ta Phut Health Center), respectively). Based on these motivations, we present the first ever work on long-term observation analysis of visibility and its relation with air pollutants and meteorological factors over Eastern Thailand. Here, we performed an analysis of daytime visibility observed at selected stations using long-term data by integrating several technical tools. Its seasonality and dependence on meteorological factors were discussed, with emphasis on the region's highly developed coastal areas and the dry season when visibility degradation is of more concern. Of policy relevance, we assessed the potential roles of air pollutants and secondary aerosol formation using a precursor-ratio method and estimated trends in visibility and PM\({}_{10}\). In the latter, meteorological adjustment was incorporated to exclude the effects of meteorology, enabling us to more robustly evaluate the anthropogenic influence and the contrasting nature between visibility and PM\({}_{10}\) trends.