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Ratnapur, nestled in the central Terai plains of Nepal at coordinates 27.8500, 83.8667, presents a unique geographic context influencing its air quality. The city’s relatively low elevation, approximately 150 meters above sea level, contrasts sharply with the towering Himalayas to the north, creating a bowl-like topography prone to trapping pollutants. Ratnapur’s location within the Terai is characterised by fertile alluvial soils, supporting intensive agricultural practices, particularly rice and sugarcane cultivation. This agricultural activity contributes to seasonal biomass burning, a significant source of particulate matter. The city is not directly adjacent to major bodies of water, though the East Rapti River flows nearby, offering limited dispersion potential. While Ratnapur isn’t a major industrial hub, the increasing presence of brick kilns and small-scale industries along the East-West Highway, a crucial transport corridor, adds to localised emissions. The urban-rural gradient is relatively sharp; the city core quickly transitions to agricultural land, meaning pollutants generated within Ratnapur have limited opportunity to dilute before impacting the population. Dust from unpaved roads and construction further exacerbates the situation, particularly during the drier months. The surrounding landscape is predominantly flat, hindering natural ventilation and pollutant dispersal.

Ratnapur’s air quality follows a distinct seasonal pattern. The pre-monsoon period (March-May) sees a significant deterioration in air quality due to increased temperatures and dry conditions, fostering dust storms and encouraging agricultural burning in preparation for the rains. This is typically the least favourable time for outdoor activity, particularly for those with respiratory conditions. The monsoon season (June-September) brings temporary relief as rainfall washes away pollutants and increased humidity suppresses dust. However, stagnant air and high moisture can also trap pollutants near the ground. October-November sees a resurgence of pollution as the monsoon recedes, temperatures drop, and agricultural residue burning intensifies after the harvest. Temperature inversions, common during these cooler months, exacerbate the problem by preventing vertical mixing of air. Winter (December-February) often experiences foggy conditions, trapping pollutants and reducing visibility. Sensitive groups – children, the elderly, and those with pre-existing respiratory or cardiovascular illnesses – should limit strenuous outdoor activity during these periods. Monitoring local conditions and using appropriate respiratory protection when necessary is crucial. The best time for outdoor activity is generally during the monsoon season, though even then, localised pollution can occur.