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San Alejo, nestled within the La Unión department of El Salvador, occupies a geographically significant position along the Pacific coast. Located at approximately 13.4333° N, 87.9667° W, the city’s terrain is characterized by a gently sloping coastal plain, rising gradually towards the foothills of the Sierra de Opico mountain range to the north. This proximity to both the Pacific Ocean and the mountains creates a complex interplay of environmental factors influencing air quality. The surrounding landscape is a mosaic of small-scale agriculture, primarily sugarcane and coffee cultivation, interspersed with pockets of coastal scrub and mangrove ecosystems. San Alejo’s location within the broader Salvadoran coastal belt places it within a region experiencing significant urban-rural gradients; while the city itself is relatively small, it’s part of a larger network of coastal communities and is within reasonable commuting distance of larger industrial zones further inland. The Pacific Ocean exerts a moderating influence on temperature, but also introduces marine aerosols and potential for coastal fog, impacting visibility and pollutant dispersion. The Sierra de Opico, while providing a scenic backdrop, can contribute to temperature inversions, particularly during the dry season, trapping pollutants closer to the ground. Agricultural practices, including the burning of fields after harvest, represent a localized source of particulate matter. The city’s relatively small size and limited industrial activity mean that regional transport of pollutants from larger urban centers like San Salvador is a more significant factor than local industrial emissions.

San Alejo’s tropical climate dictates a distinct wet and dry season pattern, profoundly influencing its air quality. The dry season, typically spanning November through April, is often characterized by lower rainfall and increased sunshine, but also by a higher propensity for temperature inversions. During these months, cooler air settles in the valleys and low-lying areas, trapping warmer air aloft. This inversion layer acts as a lid, preventing the vertical dispersion of pollutants, leading to periods of stagnant air and potentially elevated concentrations of particulate matter and other emissions. Agricultural burning, a common practice to clear land for planting, is most prevalent during this period, exacerbating the problem. The wet season, from May to October, brings frequent rainfall and increased humidity. These conditions generally lead to improved air quality as precipitation washes pollutants from the atmosphere and increased wind speeds promote dispersion. However, periods of heavy fog, particularly in the early morning hours, can temporarily reduce visibility and trap pollutants near the ground. The monsoon-like patterns of the wet season, with intermittent periods of intense rainfall, can also lead to localized flooding, which can stir up dust and sediments, contributing to particulate matter. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should exercise caution during the dry season, particularly during periods of agricultural burning or when temperature inversions are present. Outdoor activities are generally more favorable during the wet season, although vigilance regarding fog is advised.