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Hele, a coastal city in the northern part of Hainan Province, China, occupies a geographically significant position. Located at 18.9058°N, 110.4737°E, the city benefits from a tropical monsoon climate and a relatively flat coastal terrain. The surrounding landscape is characterised by lush vegetation, typical of Hainan’s tropical environment, transitioning inland to rolling hills and forested areas. Hele’s proximity to the Qiongzhou Strait, separating Hainan from Guangdong province, is a key factor influencing its air quality. The strait facilitates significant maritime traffic, contributing to particulate matter and sulfur dioxide emissions. While Hele isn’t a major industrial hub itself, its location within Hainan’s developing economic zone means it’s susceptible to regional pollution transport from larger industrial areas further inland and across the strait. The urban-rural gradient is relatively sharp, with agricultural land – primarily rubber and tropical fruit plantations – surrounding the city. This agricultural activity introduces ammonia and pesticide drift into the local atmosphere. The coastal location also means frequent sea breezes, which can either disperse pollutants or, under certain atmospheric conditions, trap them, exacerbating air quality issues. The elevation is low, increasing vulnerability to humidity and stagnant air masses.

Hele experiences a distinct wet and dry season, heavily influencing its air quality. The dry season, roughly from November to April, often sees poorer air quality. During this period, the northeast monsoon brings drier, cooler air, but also reduces atmospheric mixing, leading to pollutant build-up. Temperature inversions are common, trapping emissions near the ground. February and March are typically the months with the least rainfall and strongest monsoon winds carrying pollutants from the mainland. The wet season, from May to October, generally offers improved air quality due to frequent rainfall which effectively washes pollutants from the atmosphere. However, intense rainfall can also lead to localised flooding and the release of pollutants from agricultural runoff. August and September, peak monsoon months, are usually the clearest. Sensitive groups – children, the elderly, and those with respiratory conditions – should be particularly cautious during the dry season, limiting strenuous outdoor activity. The humid conditions throughout the year can also exacerbate respiratory issues. Monitoring wind direction is crucial; winds from the north and west are more likely to carry pollutants from industrial areas. Fog, common in the transitional seasons, can also trap pollutants, reducing visibility and increasing respiratory irritation.