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Nestled in the Fukushima Basin of central Fukushima Prefecture, Koriyama occupies a strategic position as the largest city in the Tōhoku region's interior, serving as a vital transportation and commercial hub. Situated approximately 200 kilometers north of Tokyo, the city lies at coordinates 37.4004°N, 140.3597°E, with an average elevation of 250 meters above sea level, creating a gently undulating urban landscape surrounded by the Ōu Mountains to the west and the Abukuma Highlands to the east. This basin topography significantly influences air quality through temperature inversions that trap pollutants, particularly during calm winter nights. Koriyama's location within Japan's 'Snow Country' belt means it experiences heavy snowfall that can temporarily suppress particulate matter, while its distance from major coastlines reduces direct maritime influences on pollution dispersion. The city forms part of an agricultural-urban mosaic, with surrounding rice paddies and fruit orchards contributing seasonal biomass burning emissions, though less intensively than in purely rural areas. Koriyama's urban character blends modern commercial districts with residential neighborhoods, lacking heavy industry but experiencing typical urban pollution from transportation (notably Route 4 and Tōhoku Expressway traffic) and residential heating. The city's position along the Abukuma River system provides some natural ventilation corridors, but the encircling mountains create a semi-enclosed basin effect that can concentrate pollutants when regional winds are weak, making geographic context crucial for understanding its air quality challenges.

Koriyama's air quality follows distinct seasonal patterns shaped by its inland basin climate and Japan's monsoon-influenced weather systems. Winter (December-February) brings the poorest air quality, with frequent temperature inversions trapping particulate matter from heating sources and vehicle emissions beneath cold, stagnant air layers; January typically sees peak pollution levels, making outdoor activities less advisable, especially for sensitive groups like asthmatics who should limit morning exposure when inversions are strongest. Spring (March-May) offers gradual improvement as increasing solar radiation breaks up inversions and southerly winds begin dispersing pollutants, though late March can still experience poor air quality during calm periods before the cherry blossom season brings cleaner conditions. Summer (June-August) provides the cleanest air despite higher ozone levels on hot afternoons, as the Pacific monsoon brings consistent southeasterly winds that ventilate the basin, with July and August being optimal for outdoor activities despite occasional humidity-related discomfort. Autumn (September-November) sees a transitional period where cooling temperatures and reduced wind speeds allow pollutants to gradually accumulate again, particularly in November when heating season begins but snow cover hasn't yet arrived to suppress dust; sensitive groups should monitor air quality forecasts during this transitional period. Throughout the year, fog formation in the basin can temporarily worsen pollution visibility and health impacts, while snowfall from December to March provides natural cleansing but also increases residential fuel burning.