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Manihari, nestled in the Saharsa district of Bihar, India (25.3500° N, 87.6300° E), occupies a geographically significant position within the Gangetic plains. The city’s terrain is predominantly flat, characteristic of the Indo-Gangetic alluvial plain, resulting from centuries of sediment deposition by the Koshi River and its tributaries. This low-lying landscape, averaging around 125 meters above sea level, contributes to a tendency for stagnant air conditions, particularly during cooler months. Manihari’s proximity to the Koshi River, while providing a vital water source and supporting agricultural activities, also introduces humidity and potential for localized fog formation, which can trap pollutants. The surrounding landscape is a mosaic of fertile agricultural land, primarily dedicated to rice and maize cultivation, interspersed with small villages and scattered rural settlements. This creates a gradual urban-rural gradient, with agricultural practices contributing to seasonal particulate matter emissions, especially from crop residue burning. Further afield, the region is part of the broader North Bihar industrial belt, though Manihari itself lacks significant heavy industry. However, the transportation of goods to and from larger industrial centers generates vehicular emissions. The flat topography and prevailing wind patterns, often light and variable, limit effective dispersion of pollutants, making Manihari susceptible to localized air quality challenges. The lack of natural barriers further exacerbates this, as pollutants can easily spread across the expansive plains.

Manihari’s air quality follows a distinct seasonal pattern dictated by the region’s monsoon climate. The monsoon season, typically spanning June to September, brings relief from accumulated pollutants due to frequent rainfall and increased wind speeds, effectively flushing the atmosphere. However, the preceding pre-monsoon period (March-May) often witnesses a significant deterioration in air quality. Rising temperatures and decreasing humidity create stable atmospheric conditions, frequently leading to temperature inversions where cooler air is trapped beneath a layer of warmer air, preventing vertical mixing and trapping pollutants near the ground. This period is often characterized by hazy conditions and increased particulate matter. The post-monsoon season (October-November) presents a similar challenge, with cooler temperatures and calmer winds contributing to stagnant air and the potential for fog, further exacerbating pollution. Agricultural practices, particularly the widespread burning of crop residue after the harvest, significantly contribute to particulate matter pollution during this time. Winter months (December-February) are generally the most challenging, with cold temperatures, frequent fog, and minimal wind leading to prolonged periods of poor air quality. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should limit outdoor activity during these periods, particularly during foggy mornings. Maintaining indoor air quality through ventilation and air purifiers is also advisable. The monsoon provides a natural cleansing effect, making it the most favorable time for outdoor activities.