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Pallejà, a municipality in the Baix Llobregat comarca of Catalonia, Spain, occupies a transitional zone between the coastal plains and the foothills of the Collserola mountain range. Situated at coordinates 41.4242, 1.9978, the town’s elevation is relatively low, averaging around 25-30 meters above sea level, contributing to potential temperature inversions. Its location approximately 15 kilometers southwest of Barcelona places it within the sphere of influence of the larger metropolitan area’s industrial and transport emissions. The surrounding landscape is a mosaic of agricultural fields – historically vineyards and now increasingly intensive farming – interspersed with residential areas and light industrial estates. The Llobregat River flows nearby, but its influence on local air circulation is limited by the topography. Pallejà experiences a typical Mediterranean climate, with warm, dry summers and mild, wet winters. The urban-rural gradient is pronounced; moving inland towards the Collserola range, air quality generally improves due to increased vegetation and reduced traffic density. However, prevailing winds often carry pollutants from Barcelona and the industrial corridor along the Llobregat Delta directly over Pallejà, impacting local air quality. The town’s compact urban form, while promoting walkability, can also trap pollutants during calm weather conditions.

Pallejà’s air quality follows a distinct seasonal pattern. Winter, from December to February, often sees poorer air quality due to temperature inversions – cold air trapped beneath warmer layers – preventing the dispersion of pollutants from heating systems and vehicle emissions. Calm, stable atmospheric conditions exacerbate this. Spring (March-May) brings improvement as temperatures rise and winds become more frequent, flushing out accumulated pollutants. However, agricultural activities, including fertilizer application, can contribute to ammonia levels. Summer (June-August) generally offers the best air quality, driven by strong sea breezes and high temperatures promoting pollutant dispersion, though ozone formation can be a concern during prolonged heatwaves. August, with reduced industrial activity due to holidays, is often the clearest month. Autumn (September-November) sees a gradual deterioration as heating demand increases and atmospheric stability returns. October and November are particularly susceptible to stagnant air masses. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during winter and autumn periods. Monitoring pollen levels is also crucial in spring. Avoiding peak traffic hours and staying informed about local air quality reports are recommended year-round.