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Longjia, nestled on the northeastern coast of Hainan Island in China (19.1487°N, 110.3209°E), is a small coastal city with a population of approximately 18,385. 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 Hainan, with a gentle slope inland towards the central mountainous spine of the island. This low elevation makes Longjia particularly vulnerable to marine influences and potential pollutant accumulation. The surrounding landscape is a mix of coconut groves, agricultural land – primarily rubber and tropical fruit plantations – and increasingly, tourism-related development. While not a major industrial hub itself, Longjia’s location downwind of larger industrial zones on the mainland and within Hainan province means it can experience transboundary pollution. The urban-rural gradient is quite sharp; the city centre quickly gives way to agricultural fields and coastal vegetation. 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 salt aerosol concentrations.

Longjia 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 promote 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 Asian landmass, often carrying dust and pollutants from the mainland. January and February are often the months with the lowest wind speeds, leading to stagnant air and pollutant build-up. Temperature inversions, common during the calmer dry season mornings, trap pollutants near the ground. Fog, prevalent during the transition periods, can exacerbate particulate matter concentrations. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during the dry season, particularly in January and February. Monitoring wind direction is key; periods with winds from the mainland are likely to bring poorer air quality. The wet season offers the most favourable conditions for outdoor activities, though localised flooding can present other hazards.