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Ōta, nestled in the southeastern corner of Gunma Prefecture, Japan, occupies a geographically significant position within the Kantō region. The city’s terrain is characterized by a gradual transition from the rolling hills of the Chūbu Mountains to the flatter plains bordering the Tone River, a major waterway draining into Tokyo Bay. This location places Ōta at the edge of a substantial urban–rural gradient, with agricultural lands – primarily rice paddies and vegetable farms – dominating the surrounding landscape. The city itself is a blend of residential areas, light industry, and commercial zones, reflecting its role as a commuter town for Tokyo, approximately 150 kilometers to the east. Elevation varies, generally rising from around 30 meters near the Tone River to over 100 meters in the western, more mountainous districts. The proximity to the Tone River, while providing a vital water source, can also contribute to localized humidity and fog formation, impacting atmospheric dispersion. Ōta’s position within the Kantō Plain, a historically significant industrial belt, means it experiences some influence from industrial emissions originating from larger urban centers. The surrounding landscape, a mix of cultivated fields and forested slopes, plays a role in filtering pollutants, though the city’s connectivity to the wider metropolitan area means it is not entirely isolated from regional air quality challenges. The prevailing winds, generally from the west, carry air masses across the plains, influencing the distribution of pollutants within and around Ōta.

Ōta’s air quality experiences a distinct seasonal cycle heavily influenced by meteorological patterns. Spring (March-May) often brings a period of increased particulate matter, linked to agricultural activities like prescribed burning and dust from newly tilled fields, compounded by occasional temperature inversions trapping pollutants in the valleys. These inversions, common in the early spring, occur when a layer of warm air sits above cooler air near the ground, preventing vertical mixing. Summer (June-August) typically sees improved air quality due to increased rainfall and stronger winds, which effectively disperse pollutants. The humid subtropical climate ensures frequent precipitation, washing particulate matter from the atmosphere. Autumn (September-November) can be variable; while generally better than spring, the transition period can see stagnant air and localized pollution episodes. Winter (December-February) presents the most significant air quality challenges. Cold, stable air masses, coupled with frequent fog formation due to the Tone River’s moisture, create conditions conducive to temperature inversions and pollutant accumulation. Individuals with respiratory conditions, such as asthma, should exercise caution during these periods, particularly in the mornings when fog is most prevalent. Children and the elderly are also more vulnerable. Outdoor activities are generally best enjoyed during the summer months when air quality is typically at its best. While the city benefits from regional air quality management efforts, awareness of seasonal trends is crucial for minimizing exposure and protecting public health.