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Simrāhi, a small city nestled in the northern reaches of Bihar, India, occupies a geographically significant position bordering Nepal. Located at approximately 26.3135° N, 86.8454° E, its terrain is predominantly characterized by the fertile Gangetic plains, a vast expanse of alluvial soil deposited over millennia by the Koshi River and its tributaries. The elevation is relatively low, averaging around 138 meters (453 feet) above sea level, contributing to a generally flat landscape. This flatness, while beneficial for agriculture, can exacerbate air quality issues by limiting atmospheric dispersion. Simrāhi’s proximity to the Indo-Nepal border means it’s influenced by transboundary pollution, with pollutants potentially originating from industrial areas and agricultural practices in Nepal. The surrounding landscape is overwhelmingly rural, dominated by agricultural fields cultivating crops like maize, wheat, and rice. This intensive agricultural activity, particularly the burning of crop residue after harvest, is a significant contributor to seasonal particulate matter pollution. The urban–rural gradient is gradual, with small villages and agricultural land seamlessly blending into the city’s periphery. While Simrāhi lacks major industrial zones within its immediate boundaries, it serves as a regional trading hub, with vehicular traffic contributing to localized pollution. The lack of significant natural barriers and the flat topography mean that pollutants tend to linger, especially during periods of stagnant air. The Koshi River, though not directly adjacent to the city center, influences regional weather patterns and can occasionally impact air quality through moisture content and wind direction.

Simrāhi’s air quality follows a distinct seasonal pattern dictated by its subtropical climate and agricultural practices. The dry season, typically spanning from October to May, presents the most challenging period. November and December often experience dense fog, trapping pollutants near the ground and significantly reducing visibility. This fog, combined with agricultural burning of crop residue immediately following the harvest season (October-November), leads to a sharp increase in particulate matter. January and February are often the worst months, with persistent fog and minimal wind leading to prolonged periods of poor air quality. The onset of the monsoon season (June-September) brings a welcome respite. The increased rainfall effectively ‘washes’ pollutants from the atmosphere, and the stronger winds facilitate dispersion. However, even during the monsoon, localized pollution from vehicular traffic and construction activities persists. The pre-monsoon period (April-May) can also experience elevated levels of pollutants as temperatures rise and winds become calmer, creating conditions conducive to stagnation. During the hottest months, temperature inversions can occur, trapping pollutants closer to the ground. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should limit outdoor activity during the dry season, particularly during foggy periods and immediately after harvest. Staying indoors with air purifiers and wearing N95 masks when venturing outside are recommended. The monsoon months offer a window for improved outdoor activity, but vigilance regarding localized pollution sources remains important.