Caoayan, a municipality nestled in the province of Ilocos Sur, Philippines (17.5469°N, 120.3831°E), presents a unique geographic setting influencing its air quality. The town is situated within a relatively flat coastal plain, bordering the South China Sea. This proximity to the ocean introduces a constant supply of sea breezes, which can both disperse and transport pollutants. The terrain gently rises inland, transitioning towards the Cordillera Central mountain range, creating a complex interplay of air currents. Caoayan’s urban character is largely defined by its agricultural base, primarily rice paddies and small-scale farming, alongside emerging commercial activity concentrated around the national highway. The surrounding landscape is dominated by these agricultural zones, interspersed with residential areas. The urban-rural gradient is relatively sharp, with quickly transitioning land use. While not a major industrial hub, the presence of smaller-scale processing facilities for agricultural products contributes to localised emissions. The coastal location means susceptibility to salt spray, which can affect corrosion and potentially influence aerosol formation. Understanding these geographical factors is crucial for interpreting air quality patterns in Caoayan, as wind direction, temperature, and humidity all play a significant role in pollutant dispersion and concentration.

Caoayan experiences a distinct wet and dry season, heavily influencing its air quality. The dry season, typically from November to May, sees a peak in particulate matter pollution. Reduced rainfall means less atmospheric cleansing, and agricultural activities like land preparation and harvesting generate dust. Stronger winds during this period can also carry dust from surrounding areas. February to April are generally the months with the least favourable air quality for sensitive individuals. The onset of the wet season, around June, brings significant improvement. Frequent rainfall effectively washes pollutants from the atmosphere, and increased humidity suppresses dust. However, the southwest monsoon (Habagat) can also bring in pollutants from other regions. September and October, during the tail end of the monsoon, can experience localised flooding, potentially releasing stagnant pollutants. Temperature inversions, though less frequent than in larger cities, can occur during calmer periods, trapping pollutants near the ground. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during the dry season and monitor conditions during monsoon transitions. Fog, common during the cooler months, can exacerbate respiratory issues by trapping pollutants.