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Hiroshima, situated on the southwestern coast of Japan's Honshu Island within the Chugoku region, occupies a distinctive geographic position that significantly influences its air quality patterns. The city spreads across the delta of the Ota River, where it empties into Hiroshima Bay—an inlet of the Seto Inland Sea. This coastal location creates a complex interplay between marine and terrestrial influences on pollution dispersion. Hiroshima's terrain is characterized by low-lying urban areas flanked by forested hills to the north and west, with elevations generally below 50 meters in the city center but rising to 300-500 meters in surrounding areas like Mount Futaba. The city forms part of Japan's Setouchi Industrial Zone, though Hiroshima's industrial footprint is more moderate compared to other Japanese cities, with manufacturing concentrated in machinery, automotive, and food processing rather than heavy industry. The urban-rural gradient shows dense development along the river valleys transitioning to agricultural zones in the hinterlands, particularly rice paddies and citrus orchards. Hiroshima's position in the Seto Inland Sea basin creates a semi-enclosed environment where pollutants can accumulate under certain meteorological conditions, though the coastal breezes from Hiroshima Bay generally provide ventilation. The city's reconstruction following World War II resulted in a well-planned urban layout with extensive green spaces, including the Peace Memorial Park, which contributes to localized air quality benefits through urban vegetation.

Hiroshima's air quality follows distinct seasonal patterns shaped by Japan's temperate climate with four clear seasons. During spring (March-May), increasing temperatures and variable winds from the Pacific generally maintain good air quality, though occasional yellow dust (kosa) from continental Asia can cause temporary particulate matter increases in April. Summer (June-August) brings the rainy season (tsuyu) in June-July, with frequent precipitation effectively scrubbing pollutants from the atmosphere, resulting in excellent air quality despite higher humidity. The post-rainy season in August sees clearer skies but can experience photochemical smog formation on hot, stagnant days when temperatures exceed 30°C. Autumn (September-November) typically offers Hiroshima's best air quality, with crisp, clear days dominated by northerly winds that efficiently disperse pollutants, making October particularly favorable for outdoor activities. Winter (December-February) presents the most challenging conditions, as temperature inversions in the Seto Inland Sea basin trap pollutants near the surface, especially during calm, cold mornings. Heating demand increases particulate matter from residential sources, while reduced wind speeds limit dispersion. Sensitive groups including those with respiratory conditions should monitor air quality forecasts during winter inversions and limit strenuous outdoor activities on still winter mornings. The city's coastal location moderates extreme pollution episodes compared to inland Japanese cities, but winter remains the season requiring greatest awareness for vulnerable populations.