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Bontang, nestled on the eastern coast of Kalimantan Timur, Indonesia, occupies a unique and challenging geographic position significantly impacting its air quality. Situated at approximately 0.1333° latitude and 117.5000° longitude, the city lies on the Berau River delta, a low-lying coastal plain characterized by extensive mangrove forests and peat swamp ecosystems. The terrain is remarkably flat, with an average elevation of just a few meters above sea level, limiting vertical mixing of air pollutants and exacerbating ground-level concentrations. Bontang’s location within the broader East Kalimantan region places it near significant coal mining operations and associated processing plants, forming a crucial industrial belt. These industries, alongside the Bontang LNG plant – a major natural gas liquefaction facility – are primary contributors to the city’s air pollution profile. The surrounding landscape is a mosaic of dense tropical rainforest, interspersed with oil palm plantations, creating a complex interplay of biogenic emissions and agricultural practices that can influence local air quality. The urban–rural gradient is relatively sharp, with the city’s industrial core transitioning quickly into agricultural and forested areas. The proximity to the Makassar Strait influences humidity and coastal breezes, which can sometimes disperse pollutants, but also contribute to the formation of ground fogs, trapping emissions under stable atmospheric conditions. The flat topography and coastal location combine to create a vulnerable environment susceptible to both regional and local pollution sources.

Bontang’s tropical climate dictates a distinct wet and dry season, profoundly influencing its air quality patterns. The dry season, typically spanning from June to November, often witnesses a noticeable increase in air pollution. Reduced rainfall leads to lower atmospheric cleansing, allowing pollutants to accumulate. Furthermore, the drier conditions encourage agricultural burning in surrounding areas, contributing to particulate matter concentrations. Temperature inversions, common during the dry season, trap cooler air near the ground, preventing the vertical dispersion of pollutants emitted from industrial sources and residential activities. Coastal fogs, frequently occurring during the dry season mornings, further exacerbate this trapping effect, creating stagnant air masses. The wet season, from December to May, brings increased rainfall and higher humidity, generally leading to improved air quality as pollutants are washed from the atmosphere. However, periods of prolonged rainfall can also lead to increased methane emissions from the extensive peatlands surrounding Bontang, a potent greenhouse gas. During the transition months (May/June and November/December), fluctuating wind patterns and variable rainfall can result in unpredictable air quality episodes. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should exercise caution during the dry season, particularly during periods of fog or stagnant air. Limiting outdoor exertion and monitoring local air quality reports are advisable. The increased humidity during the wet season can, however, trigger mold growth, impacting indoor air quality and potentially exacerbating respiratory issues.