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Ōharu, nestled within Aichi Prefecture, Japan, presents a fascinating case study in urban-rural interaction and its impact on air quality. Situated at coordinates 35.1751, 136.8201, the city occupies a relatively flat coastal plain, a characteristic feature of the Nobi Plain, one of Japan’s most fertile agricultural regions. This proximity to extensive agricultural lands – primarily rice paddies and vegetable farms – influences local air quality through ammonia emissions from fertilizers. The city’s elevation is low, averaging around 10 meters above sea level, making it susceptible to temperature inversions, particularly during the colder months. Ōharu is part of the greater Nagoya metropolitan area, and while not a major industrial hub itself, it benefits from and is affected by the industrial activity concentrated in nearby Toyota City and Nagoya. The Ise Bay lies to the east, providing a moderating influence on temperatures but also potentially trapping pollutants under stable atmospheric conditions. The urban-rural gradient is relatively sharp, with quickly transitioning landscapes as one moves inland towards the forested hills. This gradient impacts pollutant dispersion, with rural areas acting as sinks or sources depending on prevailing winds and agricultural practices. The city’s compact size, with a population of approximately 32,318, means localized sources have a more pronounced effect.

Ōharu’s air quality follows a distinct seasonal pattern, heavily influenced by Japan’s climate. Spring (March-May) often sees increased particulate matter from *kaizuka* – the burning of agricultural waste – alongside pollen, impacting respiratory health. Winds from the Asian continent can also carry dust and pollutants during this period. Summer (June-August) brings the humid monsoon season, which generally improves air quality through rainfall washing away pollutants, but high humidity can exacerbate ozone formation. Autumn (September-November) is typically the clearest period, with stable high-pressure systems and cooler temperatures promoting good dispersion. However, increased heating demand towards November can lead to localized increases in smoke and particulate matter. Winter (December-February) is often the most challenging time. Temperature inversions are frequent, trapping pollutants near the ground, and reduced sunlight hinders their dissipation. The proximity to Ise Bay can also contribute to fog formation, further exacerbating pollution. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during winter mornings and periods of stagnant air. March and October generally offer the most favourable conditions for outdoor pursuits, with moderate temperatures and cleaner air.