Urban trees: Sink or source for particulate matter (PM)? A review of PM reduction and biogenic volatile organic compounds (BVOCs) emissions

Urban air pollution is a critical environmental issue impacting both ecosystems and human health. Among key pollutants, particulate matter (PM) poses a particular concern in East Asia. Urban trees serve as both mitigators and contributors to urban air quality challenges. Their capacity to reduce particulate matter (PM) through various deposition and physiological mechanisms underscores their importance in urban greening strategies. However, their concurrent emission of biogenic volatile organic compounds (BVOCs), which can react with atmospheric pollutants, complicates their role in mitigating air pollution. This review provides a comprehensive overview of PM properties, tree traits affecting PM removal, and the physiological and biochemical responses of trees under pollution stress. It further examines how BVOC emissions are influenced by environmental conditions and how they participate in atmospheric reactions. Special attention is given to leaf morphological and physiological features such as trichomes, waxes, and stomatal behavior. The review underscores the importance of considering feedback loops between PM stress and BVOC emissions. Urban planning and greening policies must balance the pollutant-removal capacity of trees with their potential to exacerbate air pollution through reactive emissions to effectively improve urban air quality.