Coupling outdoor air quality with thermal comfort in the presence of street trees: a pilot investigation in Shenyang, Northeast China

Together, the heat island effect and air pollution pose a threat to human health and well-being in urban settings. Nature-based solutions such as planting trees are a mitigation strategy to improve outdoor temperatures (thermal comfort) and enhance air quality in urban areas. In this study, outdoor thermal comfort, and particulate matter levels were compared between treeless and treed areas to provide a better understanding of how street trees improve thermal comfort and air quality. Street trees decreased the physiological equivalent temperature from 46.3 to 44.2 degrees C in summer but increased it from 36.4 to 37.5 degrees C in autumn. Air temperature and relative humidity contributed more in summer while wind speed contributed more in autumn. Particulate matter concentrations were negatively correlated with physiological equivalent temperature in summer but not in autumn. The presence of trees decreased concentrations of fine particulate matter in hot summer conditions but increased in hot autumn conditions. The presence of trees increased coarse particulate matter in very hot summer conditions in summer and in hot autumn conditions. Overall, the layout of trees in urban street canyons should consider the trade-off between outdoor thermal comfort and air quality improvement.