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Yi Xian, nestled within Hebei Province, China, occupies a crucial position in the North China Plain. Located at coordinates 39.3537°N, 115.4960°E, the city’s geography is largely defined by its flat, alluvial terrain, a consequence of sediment deposition from surrounding rivers. The terrain gradually rises to the west, approaching the foothills of the Taihang Mountains, creating a subtle elevation gradient. Yi Xian’s proximity to the Bohai Sea, approximately 150km to the southeast, introduces maritime influences, though these are moderated by the distance. The region is heavily agricultural, dominated by wheat, corn, and cotton cultivation, interspersed with smaller industrial zones focused on processing agricultural products and light manufacturing. This blend of agriculture and industry contributes significantly to the local air quality profile. The urban-rural gradient is pronounced; the city centre exhibits denser construction and traffic, while surrounding areas remain largely rural. This spatial arrangement impacts pollution dispersion, with pollutants tending to concentrate in the urban core. The North China Plain is known for its susceptibility to dust storms originating from the Gobi Desert, further complicating the air quality situation. The city’s location within a major transport corridor also increases pollutant loading from vehicle emissions.

Yi Xian experiences a typical temperate monsoon climate, profoundly influencing its air quality throughout the year. Winters (November to March) are cold and dry, often accompanied by temperature inversions that trap pollutants near the ground, leading to periods of poor air quality. Heating demands during these months, reliant on coal in many households and industries, exacerbate particulate matter concentrations. Spring (April-May) sees increasing temperatures and occasional dust storms originating from the northwest, impacting visibility and respiratory health. Summer (June-August) brings the East Asian monsoon, providing periods of cleaner air due to increased rainfall and wind speeds that disperse pollutants. However, high temperatures can also promote ozone formation. Autumn (September-October) is generally considered the most pleasant season with clear skies, but agricultural burning of crop residue after the harvest can contribute to localised spikes in particulate matter. Sensitive groups – children, the elderly, and those with respiratory conditions – should be particularly cautious during winter and spring. Avoiding strenuous outdoor activity during periods of high pollution is advisable. Monitoring local conditions and utilising air purification systems indoors can provide additional protection. The monsoon season offers the most favourable conditions for outdoor activities.