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Sonpur, situated in the Saran district of Bihar at coordinates 25.7000°N, 85.1800°E, occupies a strategic position in the fertile Gangetic Plain of northern India. This urban settlement lies approximately 30 kilometres northwest of Patna, Bihar's capital, placing it within the broader Patna urban agglomeration while maintaining its distinct municipal character. The city's geography is defined by its low-lying terrain, with an average elevation of just 53 metres above sea level, making it part of the vast alluvial floodplain formed by the Ganges River system. Sonpur is positioned near the confluence of the Gandak and Ganges rivers, approximately 15 kilometres from the Ganges, which influences local humidity patterns and creates occasional fog banks during cooler months. The surrounding landscape is predominantly agricultural, with rice paddies, wheat fields, and sugarcane plantations extending in all directions, creating an urban-rural gradient where the city serves as a commercial hub for farm produce. This agricultural dominance means seasonal crop residue burning significantly impacts air quality, particularly during post-harvest periods. The city's location in the Indo-Gangetic Plain, often called India's 'air pollution belt,' subjects it to regional pollution transport from industrial clusters in nearby Patna and Hajipur, compounded by local urban emissions from vehicles, construction, and domestic fuel use. The flat terrain inhibits natural ventilation, allowing pollutants to accumulate, especially during temperature inversions common in winter. Proximity to the Himalayan foothills, approximately 200 kilometres north, contributes to occasional downslope winds that can temporarily clear pollutants but also bring colder air that exacerbates inversion conditions.

Sonpur's air quality follows a distinct seasonal rhythm shaped by the monsoon-dominated climate of eastern India. During winter (December-February), pollution reaches its peak as cold, dense air creates persistent temperature inversions that trap pollutants near the surface. Calm winds, frequent fog, and increased biomass burning for heating combine to create hazardous conditions, particularly in January when visibility often drops dramatically. Sensitive groups like children, elderly, and those with respiratory conditions should limit outdoor activities during morning hours when inversions are strongest. Spring (March-May) brings gradual improvement as temperatures rise and winds increase, though occasional dust storms from the Thar Desert region can cause sudden spikes in particulate matter. The pre-monsoon heat also accelerates photochemical reactions, potentially elevating ozone levels during afternoon hours. Summer (June-September) offers the cleanest air as the southwest monsoon arrives around mid-June, bringing vigorous winds and heavy rainfall that effectively scrub pollutants from the atmosphere. July and August typically have the most favourable conditions for outdoor activities. Autumn (October-November) sees deteriorating air quality as the monsoon retreats, winds weaken, and agricultural burning intensifies after the kharif harvest. The combination of crop residue smoke, festive fireworks during Diwali (typically October or November), and developing temperature inversions creates a sharp pollution increase that persists into winter. Throughout the year, morning hours generally have poorer air quality due to overnight pollutant accumulation and fog formation.