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Obihiro, nestled in the Tokachi Plain of Hokkaido, Japan, occupies a geographically significant position within a region characterized by expansive agricultural lands and a relatively low-lying terrain. Located at approximately 42.9167° N, 143.2000° E, the city sits at an elevation of roughly 28 meters (92 feet) above sea level, contributing to a generally flat landscape. The Tokachi Plain itself is a vast alluvial plain formed by the Tokachi River and its tributaries, providing fertile ground for intensive agriculture, primarily dairy farming and the cultivation of potatoes, wheat, and barley. This agricultural activity, while vital to the regional economy, can contribute to localized air quality concerns, particularly related to ammonia emissions from fertilizer use and livestock operations. Obihiro’s proximity to the Tokachi River and smaller streams influences local humidity and microclimates. The surrounding landscape is dominated by rolling hills and mountains to the east and west, which can influence wind patterns and trap pollutants under certain meteorological conditions. While not directly adjacent to a major industrial belt, Obihiro serves as a regional hub for agricultural processing and related industries, introducing some localized industrial emissions. The urban–rural gradient is gradual, with agricultural fields and smaller villages blending seamlessly into the city limits, a characteristic feature of Hokkaido’s urban development. The city’s location within a temperate climate zone, with distinct seasons, further shapes its environmental profile and potential air quality challenges.

Obihiro’s air quality experiences a distinct seasonal cycle heavily influenced by its climate and surrounding environment. Winter, spanning December to February, often presents the most challenging conditions. Cold temperatures and frequent temperature inversions, where a layer of warm air traps cooler air near the ground, lead to pollutant accumulation. Reduced wind speeds during this period exacerbate the problem, limiting the dispersal of emissions from agricultural activities and localized industry. Fog, common in winter, further restricts visibility and traps pollutants. Spring (March-May) brings a gradual improvement as temperatures rise and winds increase, dispersing accumulated pollutants. However, agricultural activities ramp up during this period, potentially increasing ammonia emissions. Summer (June-August) generally offers the best air quality, with consistent winds and higher temperatures promoting pollutant dispersion. Occasional rainfall also helps to cleanse the atmosphere. Autumn (September-November) sees a transition period, with decreasing temperatures and increasing humidity. While winds remain relatively strong, the potential for temperature inversions begins to reappear, leading to occasional periods of poorer air quality. Sensitive groups, such as children, the elderly, and individuals with respiratory conditions, should be particularly mindful during winter months, limiting prolonged outdoor exposure on days with stagnant air or dense fog. During the agricultural seasons (spring and autumn), minimizing exposure near fields where fertilizer application is occurring is advisable. Overall, Obihiro’s air quality is generally good, but seasonal variations and localized sources require awareness and proactive health measures.