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Baiheqiao, nestled within Yunnan Province in southwestern China, occupies a unique geographic position at coordinates 22.9711°N, 103.7964°E. The city’s terrain is characterised by rolling hills and valleys, typical of the Yunnan-Guizhou Plateau, with an elevation around 1,900 meters. This altitude significantly influences atmospheric conditions, impacting pollutant dispersion. Baiheqiao is situated near the Nanpan River, a major waterway in the region, but its influence on local air quality is more related to humidity and fog formation than direct industrial discharge. The surrounding landscape is predominantly agricultural, with terraced fields cultivating crops like rice and tea, alongside pockets of karst topography. While not a major industrial hub itself, Baiheqiao is within reach of larger industrial zones in nearby cities like Mengzi, contributing to regional background pollution. The urban-rural gradient is relatively sharp; the city centre is compact, quickly transitioning to agricultural land. This proximity to agricultural practices introduces potential sources of particulate matter from burning crop residue and fertilizer use. The prevailing winds, influenced by the complex topography, play a crucial role in pollutant transport and accumulation, often channeling air masses through the valleys. This geographical setting creates localized air quality challenges, demanding careful monitoring and mitigation strategies.

Baiheqiao experiences a distinct wet and dry season, heavily influencing its air quality. The dry season, typically from November to April, sees consistently clearer skies but also increased dust and particulate matter from agricultural activities and construction. Temperature inversions are common during these cooler months, trapping pollutants near the ground, particularly in the valleys. January and February often present the poorest air quality due to stagnant air and increased coal burning for heating in surrounding areas. The wet season, spanning May to October, brings monsoon rains that effectively wash pollutants from the atmosphere, leading to significantly improved air quality. However, high humidity can also promote the formation of secondary pollutants. August and September, while generally cleaner, can experience localised haze from regional wildfires. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during the dry season, particularly in January and February. During the wet season, while air quality is generally good, monitoring for mould and humidity-related respiratory issues is advisable. The monsoon’s arrival provides a natural cleansing effect, but vigilance remains important throughout the year.