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Birendranagar, the capital of Karnali Province in Nepal, occupies a strategically important location within a rugged, mountainous landscape. Situated at approximately 28.6°N, 81.63°E and an elevation of around 1,300 meters (4,265 feet) above sea level, the city’s geography profoundly influences its air quality. The surrounding terrain is dominated by the Himalayan foothills and the Bheri River valley, creating a natural basin that can trap pollutants. The Bheri River itself, while providing a vital water source, can also contribute to humidity and localized fog formation, impacting dispersion. The urban area is relatively compact, reflecting its smaller population size compared to Nepal’s major metropolitan centers. The urban–rural gradient is sharp; Birendranagar transitions quickly into agricultural lands and forested slopes. While large-scale industrial activity is limited within the city itself, the broader Karnali region features agricultural practices, including burning of crop residue after harvest, which can contribute to transboundary pollution events. The proximity to the western edge of the Mahabharat Range further restricts airflow, particularly during certain times of the year. The prevailing winds, often channeled through the valley, can concentrate pollutants within the basin. The city’s location, nestled within this geographically constrained environment, makes it particularly susceptible to episodes of poor air quality, demanding careful monitoring and mitigation strategies.

Birendranagar’s air quality follows a distinct seasonal pattern dictated by Nepal’s monsoon climate. The pre-monsoon season (March-May) often witnesses a gradual increase in particulate matter as agricultural burning intensifies in surrounding areas, preparing fields for planting. Temperature inversions, common during this period due to stable atmospheric conditions, trap pollutants close to the ground, exacerbating the problem. The monsoon season (June-September) brings relief, with frequent rainfall effectively washing away pollutants and strong winds dispersing them. Humidity increases significantly, which can affect respiratory comfort for some individuals. The post-monsoon season (October-November) is arguably the most challenging. Following the monsoon rains, a period of drier weather coincides with the burning of crop residue, leading to elevated levels of smoke and particulate matter. Fog, frequently forming in the Bheri River valley due to cooler temperatures and high humidity, further restricts air movement and concentrates pollutants. Winter months (December-February) are generally colder, with periods of calm winds and temperature inversions, potentially leading to stagnant air and localized pollution. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should exercise caution during the pre-monsoon and post-monsoon seasons, limiting outdoor activity during peak pollution periods and wearing appropriate masks when necessary. Maintaining indoor air quality through ventilation and air purification can also provide some protection.