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Onga, a small city nestled within Fukuoka Prefecture on the island of Kyushu, Japan, occupies a geographically significant position influencing its air quality. Located at approximately 33.85°N, 130.67°E, Onga sits within a relatively low-lying coastal plain, averaging around 20 meters above sea level. This proximity to Hakata Bay, part of the East China Sea, introduces a maritime influence, moderating temperatures but also potentially trapping pollutants under certain meteorological conditions. The surrounding landscape is a blend of gently rolling hills and cultivated fields, characteristic of Kyushu’s agricultural heartland. To the west, the landscape gradually transitions to more rugged terrain, while eastward lies the urban sprawl of Fukuoka City, a major industrial and commercial hub. This proximity to Fukuoka’s industrial belt, though distant enough to avoid direct industrial emissions within Onga itself, contributes to regional pollution transport. The urban–rural gradient is relatively smooth; Onga maintains a distinctly rural character, with rice paddies and small-scale agriculture dominating the immediate surroundings. The terrain, while generally flat, features subtle undulations that can influence local wind patterns and pollutant dispersion. The city’s location within a basin-like structure, formed by the surrounding hills, can exacerbate the effects of temperature inversions, particularly during the cooler months, hindering vertical mixing and trapping pollutants closer to the ground. The prevailing winds, generally from the southwest, carry moisture and occasionally pollutants from the Korean Peninsula and mainland Asia.

Onga’s air quality experiences a distinct seasonal cycle dictated by Kyushu’s climate and prevailing weather patterns. Spring (March-May) sees a gradual improvement as temperatures rise and winds become more frequent, dispersing accumulated winter pollutants. However, the season is also marked by occasional dust storms originating from the Gobi Desert, which can significantly degrade air quality, particularly in early spring. Summer (June-August) is typically the cleanest period, characterized by high temperatures, frequent rainfall, and robust sea breezes that effectively flush out pollutants. The monsoon season brings consistent moisture and wind, minimizing the potential for stagnation. Autumn (September-November) presents a more complex picture. While generally pleasant, the transition period can see periods of stagnant air, especially in October, as the monsoon weakens and a stable high-pressure system often settles over the region. This can lead to a build-up of pollutants from Fukuoka City and surrounding agricultural activities. Winter (December-February) is often the most challenging season. Cold temperatures and frequent temperature inversions trap pollutants near the ground, leading to periods of reduced visibility and potentially impacting respiratory health. Fog, common during these months, further exacerbates the problem by reducing dispersion. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should limit outdoor activity during periods of stagnant air and fog, particularly in winter. Increased ventilation indoors and awareness of regional air quality forecasts are recommended throughout the year.