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Hexiang, nestled on the southwestern coast of Hainan Island, China, occupies a unique geographic position significantly influencing its air quality. Located at approximately 19.53°N, 109.64°E, the city’s terrain is characterized by a gently sloping coastal plain transitioning into low hills inland. This topography, combined with its proximity to the South China Sea, creates a maritime climate with high humidity and relatively consistent temperatures throughout the year. Hexiang’s location within the broader Hainan Free Trade Port zone means it experiences increasing economic activity, though industrial development remains comparatively limited compared to other Chinese coastal cities. The surrounding landscape is predominantly tropical rainforest and agricultural land, primarily focused on fruit cultivation (particularly lychees and mangoes) and rubber plantations. This agricultural activity, while generally low-emission, can contribute to localized particulate matter during harvesting and processing. The urban–rural gradient is relatively smooth, with small villages and agricultural areas blending seamlessly into the city’s periphery. The prevailing winds generally blow from the sea towards the land, which can initially dilute pollutants but also transport maritime aerosols. However, during periods of calm weather, particularly during the transition seasons, stagnant air conditions can exacerbate pollution accumulation. The city’s elevation, averaging around 10-30 meters above sea level, further limits vertical dispersion of pollutants, contributing to a localized impact. The lack of significant mountain ranges to the west also means that pollutants are not easily carried away, potentially trapping them within the coastal plain.

Hexiang’s tropical climate dictates a distinct wet and dry season pattern, profoundly impacting air quality. The wet season, typically spanning from May to October, brings frequent rainfall and high humidity, naturally cleansing the atmosphere and generally resulting in improved air quality. Monsoon rains are a dominant factor, effectively scrubbing pollutants from the air and reducing particulate matter concentrations. However, heavy rainfall can also lead to increased levels of ozone due to increased sunlight and stagnant air conditions following intense downpours. The dry season, from November to April, presents a different challenge. Reduced rainfall and lower humidity contribute to a build-up of pollutants, particularly during periods of calm winds. Temperature inversions, where cooler air is trapped beneath a layer of warmer air, are more common during these months, preventing vertical mixing and further concentrating pollutants near the ground. November and December often experience the most persistent periods of stagnant air. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should exercise caution during these months, limiting prolonged outdoor activity and avoiding strenuous exercise. While the overall air quality is generally good compared to many other Chinese cities, the dry season necessitates heightened awareness. The increased sunlight during the dry season can also contribute to photochemical smog formation, although this is typically less severe than in more temperate climates. The transition months of April and October can be unpredictable, with fluctuating wind patterns and rainfall, leading to variable air quality conditions.