Review on the multi-scale interactions of urban forests and atmospheric particles: Affecting factors are scale-dependent among tree, stand and region

Increasing studies worldwide have examined the impacts of urban forests on mitigating atmospheric particulate matter (PM) over the past decades. These scattered studies revealed the aerodynamics of atmospheric PM deposited on urban forests, as well as the various factors influencing the PM capture and removal by urban forests. However, these evidences are varying and even contradictory, and the affecting factors do not follow a universal pattern. In addition, these studies generally have been conducted in a specific scale such as leaf, stand and city without considering the multi-scale associations and incorporations. This literature review tried to address the associations of urban forest and PM removal across single tree, stand and regional scales, and summarized the confounding factors for PM capture and removal within each scale. Particle size and local meteorology have significant impacts across scales. For an individual tree, PM capture and removal capacity are largely determined by the leaf morphology and epidermal structures, but at the stand scale, the biophysical characteristics and configurational designs of urban forests are the essential factors. At the city and regional scale, the determinants are the fraction of forest coverage, as well as background pollution levels. The literature collation emphasizes the necessity of concerning the appropriate factors responding to the specific scale when quantifying and evaluating PM capture and removal by urban forests, and warrants a multi-scale research paradigm and inclusive modeling evaluation incorporating the confounding factors from multiple scales for PM capture and removal by urban forests. • The multi-scale interactions between urban forest and PM removal were reviewed. • Local climate and meteorology are main drivers through tree, stand and regional scales. • The vegetation-related parameters for PM removal vary and dependent across scales. • Epidermal structures are concerned for tree scale, bio-physicals for stand scale, and surface area for region scale. • A research paradigm integrating multi-scale affecting factors should be established.