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San Felipe, Zambales, a coastal municipality in the Philippines, occupies a strategically important position on the western edge of Luzon Island. Its coordinates (15.0619, 120.0700) place it within a region characterized by a gradual transition from the Zambales Mountains eastward to the expansive South China Sea westward. The terrain is predominantly rolling hills and plains, sloping gently towards the coastline, influencing drainage patterns and local microclimates. Elevation varies, with higher ground inland contributing to localized temperature inversions, a key factor in air quality. The municipality’s proximity to the sea moderates temperatures but also introduces marine aerosols and potential salt spray, impacting particulate matter composition. Surrounding San Felipe is a mosaic of agricultural land, primarily dedicated to rice cultivation and fruit orchards, alongside scattered rural settlements. Further inland, the Zambales Mountain Range acts as a barrier, influencing wind patterns and potentially trapping pollutants. The urban–rural gradient is relatively smooth, with San Felipe exhibiting a small town character, lacking the dense industrial zones found in larger metropolitan areas. However, its coastal location makes it susceptible to transboundary pollution from shipping traffic and potential industrial emissions from nearby regions. The prevailing winds, generally from the southwest during the wet season, can carry pollutants from other parts of Luzon, while the easterly winds during the dry season offer some respite, dispersing accumulated emissions. The landscape’s undulating nature and coastal exposure combine to create a complex interplay of factors affecting San Felipe’s air quality.

San Felipe’s tropical climate dictates a distinct wet and dry season, profoundly influencing its air quality profile. The wet season, typically spanning from June to November, brings increased rainfall and humidity, generally leading to improved air quality due to atmospheric cleansing. However, this period also sees an increase in particulate matter from agricultural activities, including burning of crop residues, a common practice in the surrounding rural areas. The southwest monsoon (Habagat) dominates this season, bringing consistent winds that help disperse pollutants, although localized pockets of poor air quality can still form, particularly during periods of prolonged rainfall when visibility is reduced. The dry season, from December to May, presents a different challenge. Reduced rainfall and lower humidity contribute to a build-up of pollutants. Temperature inversions, common during the cooler months (December-February), trap emissions close to the ground, exacerbating air quality issues. Fog, particularly in the early morning, can also worsen conditions by reducing dispersion. March to May sees rising temperatures and increased dust from agricultural fields, further degrading air quality. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should exercise caution during the dry season, particularly during periods of fog or temperature inversions. Outdoor activities are generally more favourable during the wet season, although agricultural burning should be monitored. The interplay of monsoon winds, temperature fluctuations, and agricultural practices creates a dynamic air quality landscape throughout the year, requiring vigilance and proactive mitigation strategies.