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Hidaka, nestled in the southern expanse of Hokkaido, Japan, presents a unique geographic setting influencing its air quality. Situated at coordinates 42.4833, 142.0667, the city occupies a relatively flat coastal plain bordering the Pacific Ocean. This proximity to the ocean introduces a consistent maritime influence, moderating temperatures and contributing to higher humidity levels. The surrounding terrain is characterised by rolling hills and agricultural land, primarily dedicated to potato and beet farming, a significant economic driver for the region. Unlike many Japanese cities constrained by mountainous landscapes, Hidaka benefits from greater atmospheric dispersion. However, its location downwind of industrial areas further north and across the Tsugaru Strait from Honshu means it can occasionally receive pollutants transported by prevailing westerly winds. The urban-rural gradient is pronounced; the city centre is compact, quickly transitioning to expansive farmland. This limits localised urban heat island effects, but agricultural practices, including fertiliser use and seasonal burning of crop residue, can contribute to particulate matter. The relatively low population density (11,648) reduces overall emissions, but the open coastal location makes it vulnerable to long-range transport of pollutants. The city’s elevation is low, further enhancing the influence of marine air masses.

Hidaka’s air quality follows a distinct seasonal pattern dictated by meteorological conditions and human activity. Winter (December-February) often sees the clearest air, driven by strong Siberian high-pressure systems bringing cold, dry air and excellent atmospheric dispersion. However, temperature inversions can trap pollutants near the ground during periods of calm weather. Spring (March-May) brings increased agricultural activity, with potential for particulate matter from field preparation and burning. Winds become more variable, sometimes carrying pollutants from Honshu. Summer (June-August) is generally favourable, with prevailing south-easterly breezes flushing out pollutants. However, high humidity can promote the formation of secondary pollutants. Autumn (September-November) is often the most challenging period. The harvest season intensifies agricultural emissions, and the weakening of summer high-pressure systems leads to more stagnant air. Fog is common during autumn mornings, exacerbating particulate matter concentrations. Sensitive individuals – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during autumn and periods of stagnant air. Monitoring wind direction is crucial; westerly winds may bring pollutants from across the Tsugaru Strait. Overall, spring and autumn require heightened awareness, while winter and summer generally offer better air quality.