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San Julián, nestled within the Department of Sonsonate in El Salvador, occupies a geographically complex position. Situated at coordinates 13.6976, -89.5596, the city benefits from a moderate elevation within the country’s central plateau, though specific altitude data is limited. This location, approximately 15km inland from the Pacific coastline, influences its climate and, consequently, its air quality. The surrounding landscape is characterised by rolling hills and volcanic peaks, remnants of El Salvador’s volcanic origins. The terrain is predominantly agricultural, with coffee plantations and sugarcane fields dominating the rural areas immediately surrounding San Julián. This agricultural activity contributes to particulate matter pollution through land preparation, harvesting, and potential burning of agricultural waste. The urban-rural gradient is relatively sharp, transitioning quickly from denser residential areas within San Julián to expansive agricultural lands. The proximity to the Pacific Ocean introduces maritime influences, including humidity and potential salt spray, but also limits significant wind flushing of pollutants. There are no major industrial belts directly within San Julián, however, regional transport routes passing nearby contribute to vehicle emissions. The city’s valley location can also trap pollutants, exacerbating air quality issues during periods of calm weather.

San Julián experiences a pronounced wet and dry season, dictating the patterns of air quality throughout the year. The dry season, typically from November to April, sees a peak in particulate matter pollution. Reduced rainfall means less atmospheric cleansing, and agricultural practices like sugarcane harvesting often involve burning, releasing significant smoke and ash. Dust from unpaved roads also contributes. January and February are generally the months with the poorest air quality. The transition to the wet season, beginning in May, brings a dramatic improvement. Frequent rainfall washes pollutants from the atmosphere, and increased humidity suppresses dust. However, the wet season (May-October) can also see localised flooding, which can stir up mould and bacteria, impacting indoor air quality. September and October, during the peak of the rainy season, are generally the most favourable months for outdoor activity. Temperature inversions, common during the calmer mornings of the dry season, trap pollutants near ground level. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during dry season mornings and be mindful of agricultural burning schedules. Monitoring wind patterns is crucial, as easterly winds can carry pollutants from more industrialised areas further inland.