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Marcos, situated in Ilocos Norte, Philippines (18.0444°N, 120.6786°E), is a coastal municipality deeply influenced by its geographical setting. The city’s terrain is generally flat, characteristic of the Ilocos coastal plain, gradually rising inland towards the Cordillera Central mountain range. This proximity to the mountains offers some shielding from direct easterly winds, but also creates potential for localized temperature inversions. Marcos directly borders the South China Sea, experiencing a strong maritime influence on its climate and air circulation. The urban area is relatively small, with a population of approximately 18,010, exhibiting a clear urban-rural gradient as it transitions into surrounding agricultural lands – primarily rice paddies and vegetable farms. While not a major industrial hub, the presence of smaller-scale agricultural processing and fishing activities contributes to localized emissions. The prevailing winds are generally from the southwest during the wet season and northeast during the dry season, impacting pollutant dispersion. The coastal location means salt spray is a constant atmospheric component, potentially influencing aerosol formation and impacting air quality monitoring. Understanding this interplay of coastal, agricultural, and limited industrial factors is crucial for assessing Marcos’s air quality profile.

Marcos experiences a distinct wet and dry season, heavily influencing its air quality. The dry season, typically from November to May, sees lower rainfall and stronger northeast winds. These winds can carry dust and particulate matter from inland areas and potentially from across the South China Sea, leading to increased PM2.5 and PM10 concentrations. February to April are often the driest months, and consequently, periods of poorer air quality. The wet season, from June to October, brings the southwest monsoon, which effectively washes pollutants from the atmosphere, resulting in generally improved air quality. However, increased humidity can also promote the formation of secondary pollutants. September and October, during the peak of the monsoon, often experience the cleanest air. Temperature inversions, particularly during calmer periods in the dry season, can trap pollutants near the ground, exacerbating air quality issues. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during the drier months. Monitoring wind patterns and rainfall is key to predicting air quality fluctuations in Marcos.