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Ershui, nestled in Changhua County, Taiwan (23.8167° N, 120.6167° E), occupies a geographically significant position within the island’s western coastal plain. The city’s terrain is predominantly flat, characteristic of the alluvial plains formed by the Ershui River, from which it derives its name. This river, flowing westward into the Taiwan Strait, plays a crucial role in the local microclimate and influences air circulation patterns. Ershui’s location within the Changhua Coastal Industrial Belt places it in close proximity to a concentration of agricultural lands and light industrial facilities, a combination that significantly impacts air quality. The surrounding landscape is a mosaic of rice paddies, aquaculture ponds, and scattered rural settlements, gradually transitioning to more densely populated areas further inland. The urban–rural gradient is relatively smooth, with Ershui exhibiting a moderate level of urbanisation. Elevation is minimal, rarely exceeding 20 meters above sea level, which can exacerbate the trapping of pollutants, particularly during periods of stable atmospheric conditions. The proximity to the Taiwan Strait introduces a maritime influence, bringing in sea breezes that can disperse pollutants, but also occasionally carrying saline aerosols. The prevailing winds generally blow from the northeast during the winter months, impacting the city’s air quality from upstream sources, while summer monsoons bring moisture and potentially alter pollutant transport. The flat topography and coastal location combine to create a unique environment where local emissions and regional transport interact to shape Ershui’s air quality profile.

Ershui’s air quality experiences a distinct seasonal cycle heavily influenced by Taiwan’s monsoon climate. Winter (November to February) typically sees the poorest air quality, driven by the northeast monsoon. This wind pattern brings pollutants from industrial areas further north and inland, often trapped by temperature inversions that form under clear, calm conditions. These inversions, where a layer of warm air sits above cooler air near the ground, prevent vertical mixing and concentrate pollutants close to the surface. Fog, common during these months, further reduces visibility and exacerbates the issue. Spring (March to May) brings a gradual improvement as the monsoon weakens and winds become more variable. Summer (June to August) is generally the cleanest period, with frequent rainfall and strong convective activity dispersing pollutants. However, periods of intense heat can lead to stagnant air and ozone formation. Autumn (September to October) sees a transitional period, with increasing pollutant levels as industrial activity ramps up in preparation for the winter months. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should exercise caution during winter, limiting outdoor activities on days with poor visibility or noticeable haze. During the summer months, while generally cleaner, prolonged exposure to high temperatures and potential ozone levels should also be considered. Maintaining awareness of local weather forecasts and air quality reports is crucial for making informed decisions about outdoor activities throughout the year. The interplay of monsoon winds, temperature inversions, and rainfall dictates the rhythm of Ershui’s air quality.