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Belas, a municipality within the greater Lisbon metropolitan area of Portugal, occupies a gently undulating landscape transitioning from the coastal plains to the foothills of the Sintra Mountains. Situated at approximately 150 meters above sea level, Belas benefits from a temperate Mediterranean climate, but its proximity to Lisbon and the Tagus Estuary significantly influences its air quality. The surrounding landscape is a mosaic of urban development, agricultural land – particularly vineyards and orchards – and patches of pine forest. The Tagus Estuary, a major waterway, provides some degree of ventilation, but can also trap pollutants during periods of atmospheric stability. While Belas itself isn’t heavily industrialised, it’s downwind of Lisbon’s industrial zones to the east and south, receiving transported pollutants. The urban-rural gradient is relatively sharp, with denser housing and traffic concentrated around the town centre, and more dispersed settlements and green spaces further afield. This gradient creates localised variations in pollution levels. The prevailing winds, generally from the northwest, can carry marine aerosols inland, contributing to particulate matter, while south-easterly winds can bring pollutants from Lisbon. The terrain’s gentle slopes can also contribute to temperature inversions, trapping pollutants near the ground.

Belas experiences a distinct seasonal air quality pattern. Summer (June-August) often sees improved air quality due to stronger winds dispersing pollutants and increased atmospheric mixing, though ozone levels can rise during prolonged sunny periods. However, wildfires in the surrounding regions can dramatically worsen particulate matter concentrations, particularly during hot, dry spells. Autumn (September-November) brings increased rainfall, which helps to wash pollutants from the atmosphere, but also a greater frequency of temperature inversions, trapping pollutants in the lower atmosphere. This can lead to elevated levels of particulate matter and nitrogen dioxide. Winter (December-February) is typically the period of poorest air quality. Reduced sunlight, weaker winds, and increased heating demand contribute to higher concentrations of particulate matter from wood burning and vehicle emissions. Fog, common during winter mornings, exacerbates the problem by trapping pollutants. Spring (March-May) sees a gradual improvement in air quality as temperatures rise and vegetation begins to grow, absorbing pollutants. Sensitive individuals – children, the elderly, and those with respiratory conditions – should be particularly cautious during winter and autumn months, limiting strenuous outdoor activity on high pollution days. Monitoring pollen levels is also important during spring.