The Positive and Negative Effects of Green Space on PM2.5 Concentrations: A Review

Fine particulate matter (PM2.5) can have considerable negative effects on human health. An increasing number of scholars are finding that green space can not only decrease PM2.5 levels but also exacerbate PM2.5 levels. Few scholars have provided comprehensive reviews on this subject. This study reviews research from 1995 to 2024, including 118 studies based on a search of three databases (Web of Science, Engineering Village, and ResearchGate). We found that at the macro (e.g., city-wide) and meso (e.g., high-density built-up areas) scales, most studies report that green space can play a positive role in mitigating PM2.5 concentrations. However, at the micro-scale under specific temporal conditions, green spaces may increase PM2.5 concentrations in some micro-environments. Whether vegetation reduces or elevates local PM2.5 levels, these processes are influenced by various factors, including green space configuration, microclimatic conditions, built-environment characteristics, and emission source distributions. Mechanistically, vegetation can both decrease ambient PM2.5 levels through deposition, adsorption, and absorption and block its dispersion. In the process of exploring and optimizing the effect of greening on PM2.5, we should not only consider these factors in isolation but also account for the environmental factors that can significantly change the effect. Based on our review of a myriad of studies from different disciplinary backgrounds and scales, we propose an optimization strategy consisting of promoting ventilation through weakening sources and strengthening sinks.