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Kasuga, nestled within Fukuoka Prefecture on the island of Kyushu, Japan, occupies a geographically significant position at 33.5328° N, 130.4703° E. The city’s terrain is characterized by a gentle slope rising from the coastal plains towards the foothills of Mount Sefuri, contributing to a varied microclimate. Situated approximately 10 kilometers northwest of Fukuoka City, Kasuga benefits from, and is subtly influenced by, the larger urban agglomeration. The surrounding landscape is a blend of urban development and agricultural land, with rice paddies and small-scale farming prevalent in the areas further from the city center. The proximity to Hakata Bay, a significant inlet of the East China Sea, plays a crucial role in local weather patterns and influences air circulation. While not directly adjacent to major industrial zones, Kasuga’s location within the broader Fukuoka-Kitakyushu industrial belt means it experiences some secondary impacts from industrial emissions. The urban-rural gradient is relatively smooth, with suburban residential areas transitioning into agricultural zones within a short distance. Elevation varies from near sea level along the coastal fringes to around 200 meters at the base of the mountains, impacting local air flow and potential for temperature inversions. The prevailing westerlies, common to Kyushu, generally disperse pollutants outwards towards the sea, but can, under certain conditions, lead to localized accumulation, particularly during periods of calm weather. The city’s relatively small size and predominantly residential character mean that localized sources, such as vehicle traffic and residential heating, are the primary contributors to air quality concerns.

Kasuga’s air quality exhibits a distinct seasonal pattern dictated by Kyushu’s climate. Spring (March-May) is generally pleasant, with increasing temperatures and prevailing westerly winds that tend to dilute pollutants. However, the arrival of *tsuyu*, the plum rain season, in early summer (June-July) introduces high humidity and reduced wind speeds, which can trap moisture and, occasionally, pollutants near the ground. Summer (August-September) sees the strongest westerly winds, typically leading to good air quality, although heat stress becomes a significant concern. Autumn (October-November) is often considered the most challenging period. As temperatures cool, the likelihood of temperature inversions increases, particularly in the mornings, trapping pollutants released from residential heating and vehicle traffic. These inversions can persist for several hours, leading to localized air quality degradation. Winter (December-February) is characterized by cold temperatures and occasional fog, which further exacerbates the potential for pollutant accumulation. While snowfall is infrequent, the cold air can hold less moisture, increasing the relative concentration of particulate matter. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should be particularly cautious during autumn and winter mornings, limiting outdoor exertion and monitoring for any respiratory discomfort. During periods of stagnant air, especially in autumn, it’s advisable to remain indoors with windows closed and air purifiers operating.