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Castilla, Sorsogon, nestled on the southeastern coast of Luzon Island in the Philippines, occupies a geographically significant position within the Bicol Region. Its coordinates (12.9553, 123.8764) place it within a gently sloping coastal plain, transitioning to rolling hills inland. The municipality borders the Pacific Ocean, directly influencing its climate and, consequently, its air quality. The surrounding landscape is a mosaic of agricultural lands – primarily rice paddies and coconut plantations – interspersed with small-scale fishing villages and patches of secondary forest. This agricultural activity, while vital to the local economy, contributes to localized sources of air pollutants, particularly during burning seasons. The urban character of Castilla is predominantly rural, with a small central market area and residential zones radiating outwards. The urban–rural gradient is gradual, with agricultural land quickly enveloping the built environment. Elevation varies slightly, generally remaining below 100 meters above sea level, minimizing the potential for significant terrain-induced air stagnation. The proximity to the Pacific Ocean introduces a constant maritime influence, with prevailing winds often dispersing pollutants, but also bringing in moisture and occasionally, sea salt aerosols. Further inland, the gradual rise in elevation and the presence of forested areas can act as natural filters, though their effectiveness is dependent on wind patterns and the scale of pollution sources. The region lacks major industrial belts, reducing large-scale industrial emissions, but localized agricultural practices remain a key factor in air quality dynamics.

Castilla’s tropical climate dictates a distinct wet and dry season, profoundly impacting its air quality profile. The dry season, typically spanning from November to May, often sees a slight increase in particulate matter due to agricultural burning practices. Farmers clearing land for planting or managing crop residue frequently employ controlled burns, releasing smoke and soot into the atmosphere. While these burns are often localized, prevailing winds can carry the pollutants over wider areas, particularly during periods of lower wind speed. The wet season, from June to October, brings frequent rainfall and increased humidity, generally leading to improved air quality as precipitation washes away particulate matter and dilutes gaseous pollutants. However, periods of prolonged rainfall can also lead to stagnant air conditions, especially in low-lying areas, potentially trapping pollutants near the ground. Fog, though infrequent, can exacerbate this effect, further reducing visibility and increasing the concentration of airborne particles. Temperature inversions, where cooler air is trapped beneath warmer air, are less common but can occur, particularly during the transition periods between seasons, leading to a temporary build-up of pollutants. During the dry months (February-April), sensitive individuals, such as those with respiratory conditions or young children, should limit prolonged outdoor exposure, especially during early morning hours when burning activities are often most prevalent. The wet season generally offers more favourable conditions for outdoor activities, though vigilance regarding potential flooding and waterborne illnesses remains important.