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Merida, Leyte, a small city nestled on the eastern coast of the Philippines, occupies a geographically significant position within the Visayas region. Located at approximately 10.91° N, 124.54° E, its terrain is characterized by gently sloping coastal plains rising gradually towards the interior highlands of Leyte Island. The city’s proximity to the Philippine Sea directly influences 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 villages and scattered pockets of secondary forest. This agricultural activity, while vital to the local economy, contributes to localized sources of particulate matter, particularly during harvesting seasons. The urban–rural gradient is relatively smooth, with agricultural areas transitioning seamlessly into the city’s residential and commercial zones. Elevation is generally low, rarely exceeding 30 meters above sea level, which limits the potential for significant vertical mixing of air pollutants. The city’s coastal location also means it is susceptible to sea breezes, which can both disperse pollutants and, under certain conditions, trap them near the surface. While Merida isn't situated near major industrial belts, the presence of small-scale workshops and vehicle traffic within the city contribute to localized air pollution. The island's geology, composed primarily of sedimentary rocks, doesn't significantly impact air quality, but the prevalence of unpaved roads in surrounding rural areas can generate dust during the dry season. The overall geographic setting presents a complex interplay of natural ventilation and potential pollution accumulation.

Merida’s tropical climate dictates a distinct wet and dry season, profoundly impacting its air quality. The dry season, typically spanning from November to May, often sees a slight increase in particulate matter concentrations. This is partly due to reduced rainfall, which limits the natural cleansing effect of precipitation, and increased dust generation from unpaved roads and agricultural activities like land clearing and burning. Temperature inversions, though not as frequent as in colder climates, can occasionally occur during the cooler months (December-February), trapping pollutants closer to the ground, particularly in the mornings. The wet season, from June to October, brings frequent rainfall and increased humidity, generally leading to improved air quality as pollutants are washed from the atmosphere. However, periods of intense rainfall can also lead to localized flooding, which can temporarily release pollutants from inundated areas. Monsoon winds, particularly during the southwest monsoon (Habagat), can significantly influence wind patterns, sometimes dispersing pollutants offshore, but at other times, creating stagnant air conditions. Fog, though rare, can also contribute to pollutant trapping. During the peak of the dry season (March-April), outdoor activities are generally favorable, though sensitive individuals, such as those with respiratory conditions, should be mindful of potential dust exposure. Conversely, the transition months (May and November) can be unpredictable, with fluctuating rainfall and wind patterns. It’s advisable for vulnerable populations, including children and the elderly, to limit prolonged outdoor exposure during these periods and monitor local conditions.