Green Infrastructure and Urban Air Quality: A Semi-Systematic Review of Multiscale Evidence, Methodologies, and Policy-Relevant Insights

Abstract

Rapid urbanization has led to deteriorating air quality (AQ) in urban areas worldwide, creating a public health crisis as urban areas expand. Green infrastructure (GI), e.g., urban parks, street trees, and green roofs, has gained attention as a promising solution for mitigating air pollution and reducing urban heat. This paper reviews the impact of GI on urban AQ, focusing on its role in reducing air pollutants, e.g., particulate matter (e.g., diameter ≤ 2.5 µm) and gaseous pollutants (e.g., sulfur dioxide, nitrogen dioxide). A semi-systematic review was conducted, synthesizing research that employs both in-situ measurements and modelling approaches to assess GI’s effectiveness across different spatial scales, from street-level interventions to city-wide networks. Findings reveal that GI’s pollution-reducing effects are scale-dependent and shaped by urban spatial and functional form, GI configuration, and local environmental factors. Tree cover is notably effective in reducing airborne pollutants through deposition and dispersion mechanisms, with greater benefits observed in densely vegetated areas. However, GI shows limited efficacy in compact urban canyons, where restricted airflow limits pollution dispersion. Seasonal variations and spatial configuration also significantly influence GI’s ability to improve AQ. This review identifies gaps in current methodologies, particularly regarding long-term impacts and the scale-dependent performance of GI. It recommends that future research employ multi-scale, climate-sensitive analyses to enhance the understanding and implementation of GI as an urban AQ strategy. A more comprehensive approach, incorporating local climate factors and seasonal variations, is essential for optimizing GI’s effectiveness in urban environments.