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Saijō, nestled within Ehime Prefecture on the island of Shikoku, Japan, presents a unique geographic setting influencing its air quality. The city occupies a basin surrounded by rolling hills and mountains, a terrain characteristic of much of Shikoku. This topography, while aesthetically pleasing, can contribute to air stagnation, particularly during temperature inversions. Saijō is situated approximately 8km inland from the Seto Inland Sea, a significant body of water that moderates temperatures but doesn’t provide consistent flushing of pollutants. Historically, Saijō was a castle town and a key industrial centre, particularly for sake brewing and paper production. While heavy industry has declined, remnants of these activities, alongside modern manufacturing and agricultural zones in the surrounding plains, contribute to localised emissions. The urban-rural gradient is relatively sharp; moving quickly from the denser city centre to agricultural land and forested slopes. Elevation varies within the city limits, but generally remains below 200m, influencing dispersion patterns. The Niyodo River flows nearby, offering some natural ventilation, but its impact on widespread pollutant removal is limited. Understanding this interplay of terrain, proximity to water, and historical industrial activity is crucial for assessing Saijō’s air quality challenges.

Saijō’s air quality follows a distinct seasonal pattern. Spring (March-May) often sees increased particulate matter from agricultural burning as farmers prepare fields for planting, coupled with pollen from local flora. Winds are typically moderate, offering some dispersal, but temperature inversions can trap pollutants in the basin. Summer (June-August) brings the humid subtropical climate of Shikoku, with the rainy season initially improving air quality through wet deposition. However, stagnant air and high humidity can exacerbate ozone formation during periods of intense sunlight. Autumn (September-November) is generally the clearest period, benefiting from cooler temperatures, increased wind speeds associated with approaching weather systems, and the end of agricultural activities. Winter (December-February) presents the most challenging conditions. Cold, dry air masses from Siberia combine with the basin’s topography to create strong temperature inversions, trapping pollutants near the ground. Reduced sunlight limits photochemical reactions that break down some pollutants, leading to higher concentrations of particulate matter and nitrogen dioxide. Sensitive individuals – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during winter months and periods of stagnant air, regardless of season.