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Los Cerrillos, nestled within the sprawling Región Metropolitana of Chile, occupies a strategically important position just west of Santiago, the nation's capital. Its coordinates, -33.5000, -70.7167, place it within the Andean foothills, at an elevation of approximately 520 meters (1,706 feet) above sea level. The city’s terrain is characterized by gently sloping hills and valleys, a legacy of the region’s volcanic past. This topography significantly influences local air circulation patterns. Los Cerrillos sits on the western edge of the Santiago Basin, a naturally enclosed area bordered by the Andes Mountains to the east and the Chilean Coastal Range to the west. This basin effect traps pollutants, exacerbating air quality challenges. The surrounding landscape is a mix of urban development, agricultural land (particularly vineyards and fruit orchards in the Maipo Valley to the south), and increasingly, peri-urban sprawl. The urban–rural gradient is relatively sharp, with agricultural activities contributing to seasonal particulate matter, while the proximity to Santiago’s industrial belt, located further east, introduces a constant stream of industrial emissions. The Maipo River, flowing southward, provides a limited moderating influence on humidity, but its impact on air quality is secondary to the basin’s meteorological dynamics. The city’s location, therefore, creates a complex interplay of natural and anthropogenic factors that directly impact its air quality, making it particularly vulnerable to pollution episodes.

Los Cerrillos experiences a Mediterranean climate, dictating a distinct seasonal pattern in air quality. The dry season, spanning roughly May to September, is typically the most challenging. Characterized by minimal rainfall and strong, persistent high-pressure systems, this period frequently sees temperature inversions. Cold, dense air becomes trapped beneath a layer of warmer air aloft, preventing vertical mixing and effectively creating a lid that concentrates pollutants near ground level. This inversion layer is often exacerbated by the city’s topography, further hindering dispersion. During these months, particulate matter, originating from vehicle emissions, industrial sources, and wood-burning stoves, accumulates, leading to periods of reduced visibility and heightened respiratory concerns. Conversely, the wet season (October to April) brings increased rainfall and higher humidity, which naturally scavenges pollutants from the atmosphere, leading to improved air quality. Stronger winds, associated with frontal systems, also help to disperse pollutants. However, even during the wet season, localized pollution events can occur, particularly during periods of fog or calm conditions. November and December often see increased biomass burning in surrounding agricultural areas, contributing to particulate matter. Sensitive groups, including children, the elderly, and individuals with pre-existing respiratory conditions, should exercise caution during the dry months, limiting outdoor exertion and monitoring local air quality reports. The transition months of April and May are often periods of fluctuating air quality, requiring vigilance and adaptive behaviour.