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Gapan City, nestled in the heart of Nueva Ecija province, Philippines, occupies a strategically important position within the Central Luzon region. Located at approximately 15.3122° N, 120.9489° E, the city sits on a gently sloping alluvial plain, a legacy of the Pampanga River basin’s sediment deposition. This terrain, generally between 10 and 30 meters above sea level, contributes to relatively stable atmospheric conditions, though it can also trap pollutants under certain meteorological circumstances. Gapan’s urban character is that of a rapidly developing commercial hub, serving as a vital trading center for the surrounding agricultural lands. The landscape surrounding Gapan is predominantly rural, characterized by extensive rice paddies and farms cultivating various crops, including corn, vegetables, and fruits. This proximity to intensive agricultural activity introduces potential sources of air pollution, such as pesticide drift and emissions from farm machinery. To the west, the Sierra Madre mountain range provides a partial barrier, influencing wind patterns and potentially limiting the dispersion of pollutants. The city’s location along major transportation routes, including the Pan-Philippine Highway, also contributes to vehicular emissions. The urban–rural gradient is relatively sharp, with the city’s built environment transitioning quickly into agricultural fields, impacting local air quality as pollutants from both sources interact. The lack of significant bodies of water within the immediate vicinity limits natural air purification processes, making the city’s air quality particularly susceptible to anthropogenic influences.

Gapan’s tropical climate dictates a distinct wet and dry season, profoundly influencing its air quality profile. The dry season, typically spanning November to May, often sees a gradual accumulation of pollutants. Reduced rainfall and lower humidity during these months contribute to a decrease in atmospheric cleansing, allowing particulate matter and gaseous pollutants to linger. Temperature inversions, common during the cooler months (December-February), trap pollutants near the ground, exacerbating localized air quality concerns. Agricultural burning, a traditional practice for land clearing and crop residue disposal, peaks towards the end of the dry season, significantly increasing particulate matter concentrations. The onset of the wet season (June to October), marked by the southwest monsoon, brings relief through increased rainfall and stronger winds, which effectively disperse pollutants. However, monsoon rains can also carry pollutants over long distances, potentially impacting air quality with regional haze. Fog, occasionally occurring during the cooler months, can further reduce visibility and trap pollutants. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should exercise caution during the dry season, particularly during periods of agricultural burning. Limiting outdoor activities during peak hours and utilizing air purifiers indoors can mitigate exposure. The transition months (May and November) often experience fluctuating air quality as the climate shifts, requiring adaptive strategies for managing potential pollution spikes.