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Katsuura, a coastal city in Chiba Prefecture, Japan, occupies a unique geographic position influencing its air quality. Situated at 35.1500°N, 140.3167°E, the city’s terrain is characterised by a blend of hilly landscapes descending towards the Pacific Ocean. This proximity to the ocean, specifically the Kuroshio Current, moderates temperatures but also introduces sea salt aerosols into the atmosphere, contributing to particulate matter. Katsuura isn’t heavily industrialised, but benefits from a thriving fishing industry, particularly tuna, and associated processing facilities. These contribute localised emissions. Surrounding Katsuura are agricultural zones, primarily cultivating rice and vegetables, with potential for seasonal burning impacting air quality. The urban-rural gradient is relatively sharp; the city centre is compact, quickly transitioning to forested hills and agricultural land. Prevailing winds generally blow from the northwest, carrying pollutants from the Kanto Plain, including the Tokyo metropolitan area, though the distance provides some dilution. The city’s elevation varies, with higher areas experiencing different wind patterns and pollutant dispersion compared to the coastal plains. The topography funnels winds through valleys, potentially creating localised areas of pollutant concentration. This coastal location and varied terrain create a complex interplay of factors affecting air quality in Katsuura.

Katsuura’s air quality follows a distinct seasonal pattern. Spring (March-May) often sees increased particulate matter from agricultural burning and dust storms originating from the Asian continent, carried by westerly winds. Pollen from local vegetation also contributes to discomfort for allergy sufferers. Summer (June-August) brings the humid subtropical climate, with frequent rainfall that generally washes pollutants from the air, resulting in relatively cleaner air. However, high temperatures can promote the formation of ground-level ozone. Autumn (September-November) is typically the clearest period, with stable atmospheric conditions and reduced agricultural activity. However, temperature inversions can trap pollutants near the ground, especially in the valleys. Winter (December-February) experiences the strongest northwesterly winds, potentially bringing pollutants from the Kanto Plain. Cold, dry air also exacerbates particulate matter concentrations. Sensitive groups, such as the elderly and those with respiratory conditions, should be particularly cautious during spring and winter. Fog, common in winter, can trap pollutants, reducing visibility and worsening air quality. Months to favour for outdoor activity are generally September and October, while March and January require greater precaution.