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Somnāha, nestled in the fertile plains of Bihar, India (25.9518° N, 85.7042° E), occupies a geographically strategic yet challenging position for air quality. The city’s terrain is predominantly flat, characteristic of the Gangetic plain, with an elevation of approximately 58 meters above sea level. This low elevation contributes to atmospheric stability, hindering vertical mixing and trapping pollutants close to the ground, particularly during periods of calm weather. Somnāha is surrounded by a mosaic of agricultural land, primarily dedicated to rice and maize cultivation, which, while vital for the regional economy, introduces agricultural emissions like ammonia and particulate matter from crop residue burning, especially during the post-harvest season. To the west, the landscape gradually transitions into slightly undulating terrain, eventually merging with the broader Indo-Gangetic plain. The nearby Burhi Gandak River, though not directly adjacent to the city center, influences local humidity and can occasionally exacerbate fog formation, further trapping pollutants. The city’s proximity to industrial belts in surrounding districts, particularly those involved in brick kilns and small-scale manufacturing, adds to the pollution burden. The urban–rural gradient around Somnāha is relatively sharp, with a rapid transition from densely populated areas to agricultural fields, creating a localized concentration of emissions within the city limits. The prevailing wind patterns, often light and variable, further impede the dispersion of pollutants, making Somnāha susceptible to poor air quality episodes.

Somnāha’s air quality follows a distinct seasonal pattern dictated by the region’s monsoon climate. The dry season, spanning roughly October to May, presents the most significant air quality challenges. November and December often experience dense fog, a consequence of high humidity and low temperatures, which severely restricts pollutant dispersion and leads to a noticeable increase in particulate matter. Agricultural burning, a common practice after the harvest season (October-November), contributes significantly to this period’s elevated pollution levels. January and February are typically the coldest months, frequently accompanied by temperature inversions – a meteorological phenomenon where a layer of warm air sits above cooler air, preventing vertical mixing and trapping pollutants near the surface. The monsoon season (June to September) brings relief, with increased rainfall and stronger winds that naturally cleanse the atmosphere. However, even during the monsoon, localized pollution from construction activities and vehicular emissions can persist. The transition months of March-May see a gradual increase in temperature and decreasing rainfall, leading to a slow deterioration in air quality as the dry season approaches. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should exercise caution during the dry season, particularly during foggy periods and when agricultural burning is prevalent. Limiting outdoor exertion and wearing appropriate masks are advisable during these times. Indoor air quality should also be prioritized with proper ventilation and air purification.