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Haiwei, nestled on the northern coast of Hainan Island in China (19.4261°N, 108.8145°E), is a small coastal city with a population of approximately 25,568. Its geography is defined by its proximity to the Qiongzhou Strait, separating Hainan from Guangdong province. The terrain is relatively flat, typical of the coastal plains of northern Hainan, with a gentle slope inland towards the central mountainous spine of the island. This low elevation makes Haiwei particularly vulnerable to marine influences and potential air stagnation. The surrounding landscape is a mix of agricultural land – primarily rubber and tropical fruit plantations – and developing coastal tourism infrastructure. While not a major industrial hub itself, Haiwei’s location places it downwind of larger industrial zones on the mainland, particularly in Guangdong, meaning transboundary pollution is a significant factor. The urban-rural gradient is sharp; the city centre quickly transitions to agricultural fields and coastal mangrove ecosystems. The prevailing winds, influenced by the East Asian monsoon, play a crucial role in dispersing or concentrating pollutants. The coastal location also means high humidity and potential for sea breezes to impact local air circulation patterns, influencing pollutant dispersion and concentration. Understanding these geographical factors is key to understanding Haiwei’s air quality challenges.

Haiwei experiences a tropical monsoon climate, resulting in distinct wet and dry seasons rather than traditional four seasons. Air quality generally improves during the wet season (May to October) due to frequent rainfall which effectively scrubs the atmosphere of particulate matter and other pollutants. However, high humidity can also contribute to the formation of secondary pollutants. The dry season (November to April) typically sees poorer air quality. The northeast monsoon brings cooler, drier air from the mainland, often carrying pollutants from industrial areas across the Qiongzhou Strait. January and February are often the months with the least favourable conditions. Temperature inversions, common during the calmer dry season, trap pollutants near the ground, exacerbating air quality issues. Fog, prevalent in the cooler months, can also increase particulate matter concentrations. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during the dry season, particularly on calm, hazy days. Monitoring wind direction is crucial; winds from the north and northeast are likely to bring poorer air quality. The transition periods between wet and dry seasons can be unpredictable, with fluctuating air quality.