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Sant Feliu de Llobregat occupies a strategic position within the Baix Llobregat region of Catalonia, acting as a vital transition zone between the dense urban sprawl of the Barcelona metropolitan area and the protected natural landscapes of the Collserola mountain range. The city is nestled within the fertile Llobregat river basin, where the terrain shifts from the flat, alluvial plains of the river valley to the ascending foothills of the pre-coastal range. This specific topography creates a complex atmospheric environment; the surrounding mountains can act as physical barriers that hinder the dispersion of pollutants, particularly during periods of atmospheric stability. The urban character is a hybrid of residential neighborhoods and significant industrial belts, reflecting its history as a manufacturing hub. Furthermore, the city is intersected by major transport arteries, including high-capacity roads that funnel heavy vehicular traffic toward the coast. The proximity to the Mediterranean Sea introduces a moderating maritime influence, yet the urban-rural gradient is sharp, with the green lungs of the Collserola park providing a crucial ecological buffer. However, the combination of industrial emissions, intense traffic congestion, and the valley's tendency to trap air masses means that the city often contends with concentrated nitrogen dioxide and particulate matter. This geographic configuration makes Sant Feliu a focal point for studying the intersection of industrial heritage and modern urban air quality management within the wider Catalan landscape, where air quality is inextricably linked to regional industrial activity and natural ventilation.

Air quality in Sant Feliu de Llobregat follows a distinct Mediterranean cycle, heavily influenced by temperature and wind patterns. During winter, the city is prone to thermal inversions, where a layer of warm air traps cooler, polluted air near the ground. This phenomenon, coupled with increased heating demands, often leads to peaks in particulate matter and nitrogen dioxide, making January and February the most challenging months for those with respiratory conditions. As spring arrives, the atmosphere becomes more dynamic, though the transition can trigger ozone formation as sunlight intensifies. Summer brings the highest risk of ground-level ozone, driven by intense solar radiation and stagnant air masses. During July and August, the "sea breeze" effect typically helps clear the valley, yet peak afternoon heat can still trap pollutants, necessitating caution for sensitive groups during the hottest hours. Autumn typically marks the cleanest period, as increased rainfall washes particulates from the air and the prevailing winds shift, enhancing ventilation across the Llobregat basin. For residents and visitors, the late autumn months are ideal for outdoor activities. Health guidance suggests that asthmatics and the elderly should limit strenuous outdoor exercise during winter inversion events and summer ozone peaks. By monitoring the interplay between the Collserola range and the Mediterranean currents, one can navigate the city's seasonal shifts, favoring the crisp, rain-cleansed air of November for long walks through the urban-rural fringe while remaining vigilant during the stagnant summer heat waves.