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Bagan Si Api-api, a coastal city in Riau province, Indonesia, occupies a low-lying position on the eastern coast of Sumatra, at coordinates 2.1667°S, 100.8167°E. Its geography is fundamentally shaped by its proximity to the Strait of Malacca, a crucial international shipping lane. The terrain is predominantly flat, consisting of peat swamp forests and alluvial plains, typical of coastal Sumatra. This landscape, while ecologically significant, contributes to unique air quality challenges. The city’s population of approximately 73,360 is concentrated around the port and along the coastline. Surrounding Bagan Si Api-api are extensive areas of oil palm plantations, a major economic driver for the region, and mangrove forests, which offer some natural air filtration but are increasingly threatened. The urban-rural gradient is relatively sharp, transitioning quickly from the city centre to agricultural land. Industrial activity, primarily related to port operations and palm oil processing, is concentrated near the harbour. The flat topography and frequent humidity can exacerbate pollution by limiting atmospheric dispersion, trapping pollutants close to the ground. The prevailing winds, influenced by the monsoon seasons, play a critical role in pollutant transport, sometimes carrying haze from land and forest fires in neighbouring provinces.

Bagan Si Api-api experiences a tropical climate characterized by distinct wet and dry seasons, rather than traditional four seasons. Air quality fluctuates significantly with these shifts. The dry season, typically from February to October, sees a marked increase in particulate matter due to land and forest fires, often originating from peatlands in Riau and neighbouring provinces. These fires, frequently linked to agricultural practices, release substantial smoke containing PM2.5, impacting visibility and respiratory health. August and September are particularly problematic months. The wet season, from November to January, generally brings improved air quality as rainfall washes pollutants from the atmosphere. However, even during the wet season, localised pollution from port activities and palm oil processing can persist. Temperature inversions, common during calmer periods, can trap pollutants near the ground, worsening conditions. The monsoon winds, shifting direction with the seasons, influence the dispersal of pollutants. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit outdoor activity during the peak of the dry season and monitor conditions closely. Staying indoors with windows closed and using air purifiers are recommended during periods of haze.