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Waiyuanshan, nestled within Yilan County in northeastern Taiwan, presents a unique geographic setting influencing its air quality. Situated at coordinates 24.7443, 121.6684, the city occupies a valley floor transitioning into the foothills of the Central Mountain Range. This terrain creates a natural bowl effect, potentially trapping pollutants during periods of stagnant air. The elevation, while not extreme, contributes to cooler temperatures and increased humidity compared to coastal plains. Waiyuanshan benefits from proximity to the Lanyang River, which provides some natural ventilation, but also experiences localized humidity that can exacerbate particulate matter issues. Surrounding the city are extensive agricultural lands, primarily tea plantations and fruit orchards, alongside pockets of smaller-scale industry. The urban-rural gradient is relatively sharp; the city itself is compact, quickly giving way to forested slopes and agricultural fields. This proximity to natural areas generally contributes to better baseline air quality than larger, heavily industrialized Taiwanese cities. However, agricultural practices, including seasonal burning of crop residue, can introduce localized pollution events. The prevailing wind patterns, influenced by the mountainous terrain and seasonal monsoon systems, play a crucial role in dispersing or concentrating pollutants within the valley.

Waiyuanshan’s air quality follows a distinct seasonal pattern. Spring (March-May) often sees increased particulate matter from agricultural burning as farmers prepare fields for planting, coupled with occasional dust storms originating from mainland China. Summer (June-August) brings the southwest monsoon, providing generally good ventilation and washing away pollutants with frequent rainfall. However, high humidity can promote the formation of secondary pollutants like ozone. Autumn (September-November) is typically the clearest period, with stable high-pressure systems and cooler temperatures. This is the most favourable time for outdoor activities. Winter (December-February) presents the most challenging conditions. Northeast monsoon winds bring cold, dry air, and temperature inversions frequently trap pollutants in the valley, leading to reduced visibility and increased concentrations of particulate matter. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during these periods. Fog is also common in winter, further exacerbating particulate matter issues. Monitoring wind direction and avoiding activity during calm, cold mornings is advisable. The transition periods between seasons often experience fluctuating air quality, requiring increased vigilance.