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Santiago de Compostela, the culmination of the famed Camino de Santiago pilgrimage route, occupies a unique and historically significant position within Galicia, Spain. Nestled in the rolling hills of the O Morrazo region, approximately 13 kilometers inland from the Atlantic coast, the city’s geography profoundly influences its microclimate and, consequently, its air quality. The terrain is characterized by a series of gentle slopes and valleys, rising to an elevation of around 200-300 meters above sea level. This undulating landscape can contribute to localized air stagnation, particularly during periods of calm weather. The surrounding landscape is predominantly rural, a patchwork of small farms, vineyards, and eucalyptus plantations – a significant agricultural and forestry presence in Galicia. While not directly adjacent to major industrial belts, the region experiences agricultural burning practices, particularly during autumn, which can introduce particulate matter into the atmosphere. The urban–rural gradient is relatively smooth, with smaller villages and agricultural land gradually transitioning into the city limits. The proximity to the Atlantic Ocean moderates temperatures, but also introduces maritime influences like sea spray and occasional salt deposition. The prevailing westerly winds, typical of the Galician coast, generally provide ventilation, dispersing pollutants; however, their strength and direction can vary considerably, impacting local air quality patterns. The city’s historic urban fabric, with its narrow, winding streets, can also trap pollutants in certain areas, creating localized hotspots.

Santiago de Compostela’s air quality exhibits a distinct seasonal pattern dictated by its temperate oceanic climate. Winters, characterized by higher rainfall and cooler temperatures, generally see improved air quality due to frequent precipitation washing away pollutants and stronger winds dispersing them. However, periods of prolonged calm can lead to temperature inversions, trapping pollutants near the ground, especially in the valley areas. Spring brings a gradual increase in sunshine and a slight rise in temperatures, but also the potential for pollen-related air quality concerns. Summer is typically the driest season, and while temperatures are moderate, stagnant air conditions can occasionally develop, particularly during heatwaves, leading to a build-up of pollutants from local sources. Autumn presents a more complex picture. While the winds remain generally favourable, agricultural burning practices, a traditional method of land clearing and waste disposal, significantly contribute to particulate matter pollution. This is most pronounced in October and November. Fog, common during autumn and winter mornings, can also exacerbate air quality issues by trapping pollutants. Sensitive groups, such as children, the elderly, and individuals with respiratory conditions, should be particularly cautious during periods of agricultural burning and fog. During the summer months, limiting strenuous outdoor activity during the hottest part of the day is advisable. Spring requires awareness of pollen counts, and winter necessitates monitoring for potential inversion events and associated air quality degradation. Overall, the shoulder seasons (spring and autumn) present the greatest variability in air quality.